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III.-I I.I IIIIII. ,. II .’ z'l‘EI'I I 0 II‘IviiI‘II. \I..IIII.II.III II . .IIIIIIIIIIIIII.IIIIII IIIII IIIIIIIIlzIIIHIltI I IIIII ...II ..I III . I .IIOIQIIIIIII.‘III .IUCHIIOII I a I Iv I. I. I‘LIIIII.III. . II. I .u .1 It.JIII.lv-Ivn IwIHIruIHI.Inm I 1 II II.‘ I I .IIII ' . I ..I.II‘ I'lleIb II I Ir~ a . I . I,IIIII2..I..I . vi .. I INVOIWII I III. I . , .. «(ptI . I I .u .I III l\\\\\\\ \\ \\ ‘lllll will \\\\\\l - £14402:qu 8 1i i r. I». -.-- s ' ‘4" .“Ua l r4._4. 9’}. I 9 34—... ! This is to certify that the thesis entitled Feeding, Filtering, Fecundity, and Growth Kinetics as Limits to Population Dynamics of Daphnia Rolex presented by Mike Allan Nessels has been accepted towards fulfillment of the requirements for Master of Science degree in Fisheries and Wildlife \Q Major professor Date May 16, 1986 0-7639 MS U is an Affirmative Action/Equal Opportunity Institution MSU LIBRARIES m. RETURNING MATERIALS: Place in book drop to remove this checkout from your record. FINES will be charged if book is returned after the date stamped below. 'JQYHQIB ”’23; ‘6‘ FEEDING, FILTERING, FECUNDITY, AND GROWTH KINETICS AS LIMITS T0 POPULATION DYNAMICS OF DAPHNIA PULEX By Mike Allan Wessels A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Fisheries and Wildlife 1986 ABSTRACT FEEDING, FILTERING, FECUNDITY, AND GROWTH KINETICS AS LIMITS TO POPULATION DYNAMICS OF DAPHNIA PULEX BY Mike Allan Wessels Laboratory determined feeding, filtering, fecundity, and individual and population growth rate kinetics of Daphnia pulex fed Chlamydomonas reinhardi at 27 C were used to explain the mechanisms which allow these filter-feeding cladocerans to simultaneously maintain both viable populations and reduced algal densities in eutrophic lakes. Survival of the cladoceran population at suppressed food levels was dependent on allocation to the neonates of maternal energy reserves which in turn were dependent on the ratio of the mass of algae avail- able to the total daphnid population biomass. Variation in food to mass ratios and the related variation in fecundity combined to yield a final state of oscillating dynamic population equilibrimm where the population possessed significant unrealized grazing, growth, and reproductive potentials which would prevent proliferation of any phytoplankton which could be assimilated by the cladocerans. ACKNOWLEDGMENTS I express my sincere appreciation and gratitude especially to my graduate advisor Dr. Darrell L. King and to my graduate committee members Dr. Donald J. Hall and Dr. William.W. Taylor, for their contributions in assisting me complete my Master's research and thesis. This study was supported in part by the Michigan State University Institute of Water Research (USGS 14-08-001-6913) and by the Michigan State University Agricultural Experiment Station (Project — MICL 01387). 11 II. III. IV. LIST OF CONTENTS INDUCTION . C O O O O O O O O O O O O O O O O O 0 MATERIALS AND METHODS . . . . . . . . . . . . . . . A. COMMON EXPERIMENTAL METHODS . . . . . . . . . . B. FEEDING AND FILTERING RATE DETERMINATION EXPERIMENTAL METHODS . . . . . . . . . . . . . C. INDIVIDUAL GROWTH AND FECUNDITY RATE DETERMINATION EXPERIMENTAL METHODS . . . . . . . D. POPULATION GROWTH RATE DYNAMICS DETERMINATION EXPERIMENTAL METHODS . . . . . . . . . . . . . . RESULTS AND DISCUSSION . . . . . . . . . . . . . . . A. FEEDING AND FILTERING RATE TRIALS . . . . . . . B. INDIVIDUAL GROWTH AND FECUNDITY RATE EXPERIMENTS C. POPULATION GROWTH RATE STUDIES . . . . . . . . CONCLUSIONS . . . . . . . . . . . . . . . . . . . . iii . 52 . 69 .107 LIST OF TABLES Table 1. Kinetic rate constants derived from observed daphnid feeding rates as a function of algal cell concentration provided for both empirical feeding rate hyperbolic models (Equation 5 and 6) as differentiated for the eight experimental daphnid size classes. Also given are the coefficients of determination (r2) obtained for each model as fit to the measured data. . . . . . . . . . . . . . . 2. Regression coefficients derived from observed and calculated filtering rates as a function of algal cell concentration provided for Equation 11 (the negative exponential function developed from Equation 6) and for Equation 12 (the negative exponential function constructed from observed filtering rates), as differentiated for the eight experimental daphnid size classes. Also given are the coefficients of determination (r2) obtained for each model as fit to the observed and calculated data and the P values associated with the two Student's t-tests for coincidence for two straight line regression models. Significance level 8 0.05. 3. Average measured daphnid specific growth rates calculated including and excluding egg biomass contribution observed at the seven experimental algal cell concentrations provided with the ._ estimated standard error of the mean (X i S.E. (X)). 4. Kinetic rate constants derived from.measured daphnid specific growth rates, including and excluding egg biomass, as a function of algal cell concentration provided for the threshold food concentration corrected hyperbolic empirical specific growth rate model (modified Equation 5, u=u (C-C )/(C+K -2C )). Also given are the assggIated qcoeffigiengs of determination (r2). iv LIST OF TABLES (Continued . . .) Table Regression coefficients for daphnid fecundity empirical models derived from observed daphnid clutch sizes as a function of carapace length for the six discrete experimental algal cell concentrations. Also given are the respective coefficients of determination (r2) for the linear regression models. . . . . . . . . . . . . . . . . 63 Statistical analysis results for the two Student's t-tests for coincidence from separate straight-line fits using the regression coeffi- cients given in Table 5 for each algal cell concentration. Also provided are the correspond- ing P values for the individual t-tests for common y-intercept (a) and parallelism (b). Significance level - 0.05. . . . . . . . . . . . . . . . 64 Population maintenance threshold food per mass ratios derived from measured daphnid population specific growth rates as a function of log algal biomass supplied per daphnid population biomass using Equation 8 for the duplicate 40,000 and 80,000 algal cells/ml food concentration daphnid population cultures. Also provided are the associated coefficients of determination (r2) . . . . . . 86 Kinetic rate constants derived from observed daphnid population specific growth rates as a function of algal biomass supplied per daphnid population biomass obtained from the duplicate 40.000 and 80,000 algal cells/ml food concentra- tion daphnid population cultures provided for the threshold corrected hyperbolic empirical model (modified Equation 5, ‘M . M AB/DB — AB/DBq max. AB/DB + RAB/DB (AB/DBq)). Also given are the coefficients of determination (r2) calculated for the model as fit to the measured data . . . . . . . ... . . . . . . . 89 - 2 Measured cultured daphnid population parameters and statistics obtained from representative census dates when population biomass levels were high (apex) and low (trough) for both 40,000 and 80,000 algal cells/ml food concentrations. . . . . . . . . . . . 92 LIST OF TABLES (Continued . . .) Table 10. Calculated maximum potential and observed real- ized daily total food consumption using daphnic population composition size distribution data presented in Table 9 for high (apex) and low (trough) population biomass levels applying derived feeding rate kinetic constants given in Table 1 for Equation 6 . . . . . . . . . . . . . . . . . . . vi 0 O 102 5a. & 5b. LIST OF FIGURES Relationship between in body weight and in cara- pace length for 2, pulex. Equation 2; w-7.69X10'3L 2.39, is indicated by a solid straight line . . . . . . . 15 Functional response filtering rate curves for 2, pulex as related to algal cell concentration. Line A illustrates filtering rate response des- cribed by Equation 10, line B depicts filtering rate response expected from Equation 11, and line C exemplifies filtering rate response as predicted by Equation 12 . . . . . . . . . . . . . . . . . . . . . . . 24 Relationship between daphnid feeding rate (algae/ daphnid/hour) as a function of algal cell concen- tration (algal cells/ml) for the 1.76-2.00 mm daphnid size class. Line A represents the func- tional response curve for Equation 5 (threshold food concentration corrected) and Line B repre- sents the functional response curve for Equation 6 (non-threshold food concentration corrected), using kinetic rate constants given in TAble l . . . . . . 31 Relationship between daphnid filtering rates (ml/ daphnid/hour) as a function of algal cell concen- tration (algal cells/ml) for the 1.76-2.00 mm daphnid size class. Line A represents the func- tional response curve for Equation 10 (threshold food concentration corrected), line B represents the functional response curve for Equation 11 (non-threshold food concentration corrected), and line C represents the functional response curve for Equation 12 (non-threshold food concentration orrected), using kinetic rate constants given in Table 1. (Equations 10 and 11) and regression coefficients presented in Table 2 (Equation 12). . . . . 32 Relationship between the regression coefficients y-intercept (Figure 5a) and slope (Figure 5b) as a function of carapace length (mm) obtained from the daphnid filtering rate regression models vii LIST OF FIGURES (Continued . . .) Figure 10. given in TAble 2. In Figure 5a, the solid straight line represents Equation 16; as-O. 49+l. 01(L), r28 0.87. In Figure 5b, the solid parabolic curve represents Equation 17; b--S. 82X10'6+l. 65X10‘5(L)- 5. 29x1o-6(L)2, rZ-o. 58. . . . . . . . . . . . . Daphnid filtering rate (ml/daphnid/hour) as a multivariate functional relationship between algal cell concentration (algal cells/ml) and carapace length (mm) as predicted by Equation 18; V/D/Ta-O. 49+1. 01(L)e-(- -s. 82x1o-6+1. 65x1o-5(L) -s. 29 XIO'6 (L)2 (C). . . . . . . . . . . . . . . . . . Relationship between daphnid filtering rate (m1/ daphnid/hour) as a function of algal cell concen- tration (algal cells/ml) for a 2.00 mm daphnid applying Equation 12 and the empirical filtering rate models presented by Knoechel and Holtby (1986a,b). Line A represents the functional response curve for Equation 12 (Table 2), line B represents6 the functional response for V/D/T- 5.105L2 176 (food type, bacteria), line C re resents the functional response for V/D/T-7. 396L2°4 3 (combined models), line D regresents the functional response for V/D/T-7. S34L3 0 2(food type, large algae), and line E represents the functional res- ponse for V/D/T-ll. 695L2 480 (food type, yeast) Relationship between discrete mass-specific daphnid respiration rate (ul 0 /mg daphnid/hour) as a function of carapace lengéh (mm). . . . . . . . Relationship between mass normalized daphnid feed- ing rates (mg algae/mg daphnid/hour) as a function of carapace length (mm) calculated for five algal cell concentrations (algal cells/ml). . . . . . . Relationship between discrete mass-specific daph- nid respiration rate standardized for the amount of algae consumed (ul 0 /mg algae) as a function of carapace length (mm) calculated for a high and low algal cell concentration (algal cells/ml). . viii 37 . 40 42 44 34 . 48 LIST OF FIGURES (Continued . . .) Figure 110 12a. & 12b. 13. 14. 153. & 15b. Relationship between discrete mass-specific daphnid respiration rate standardized for the amount of algae consumed and adjusted for daphnid biomass (ul 02/mg algae/mg daphnid) as a function of carapace length (mm) calculated for a high and low algal cell concentration (algal cells/ml) . . . . . . . . . . . . . . . . . Relationship between average measured daphnid specific growth rate (based on daphnid culture biomass, u day-1), calculated exclusive (Figure 12a) and inclusive (Figure 12b) of egg biomass as a function of algal cell concentra- tion (algal cells/ml). The continuous hyper- bolic curves in Figures 12a and b are diagrammed as given by Equation 5 (Table 4). The vertical bars indicate the standard error of the mean for the averagg_measured daphnid specific growth rates (X‘* S.E.(X). . . . . . . . . . . . Relationship between daphnid fecundity (eggs/ female) and carapace length (mm) as observed for the individual algal cell concentration culture suspensions (algal cells/ml) . . . . . . Daphnid fecundity empirical linear regression models (Table 5) graphed as eggs/female as a function of carapace length (mm) for each indi- vidual algal cell concentration culture suspension (algal cells/ml) Line A910,000 algal cells/ml, line B-20,000 algal cells/ml, line C-40,000 algal cells/ml, line D-80,000 algal cells/ml, line E-100,000 algal cells/ml, line F-200,000 algal cells/m1. . . . . . . . . . . Relationship between the regression coefficients y-intercept (Figure 15a) and slope (Figure 15b) as a function of algal cell concentration (algal cells/ml) obtained from the daphnid fecundity regression models given in Table 5. In Figure 15a, the solid straight line repre- sents Equation 21; a=-10.38-l.72X10'4(C), r2=0.92. ix 50 56 61 62 LIST OF FIGURES (Continued . . .) Figure l6. 17. 18. 19a. & 19b 0 20a. & 20b. In Figure 15b, the solid straight line represents Equation 22; b-7.45+1.08X10‘4(C), r2-0093 O O O O O O O O O O O O O O O I O O O O O O O 0 6S Daphnid fecundity (eggs/female) as a multi- variate functional relationship between algal cell concentration (algal cells/ml) and daphnid carapace length (mm) as predicted by Equation 23; E/r-(-1o.38-1.72x1o-4(C))+(7.45+ 1.08x1o-4(C))(L). . . . . . . . . . . . . . . . . . . . 67 Observed culture daphnid population size measured as total population numbers as a function of time (days) for all experimental algal cell food concentrations (algal cells/ml) . . . . 71 Observed cultured daphnid population size measured as summed population biomass as a function of time (days) for all experimental algal cell food concentrations (algal/cells/ml) . . . . 72 Relationship between measured daphnid popula- tion specific growth rate (u day‘l) and food per mass (mg algae/mg daphnid) obtained from the duplicate 40,000 (Figure 19a) and 80,000 (Figure 19b) algal cells/ml food concentration daphnid population cultures. The continuous hyperbolic curves were drawn using Equation 5 and the kinetic rate constants given in Table 8. Curve A represents the series A population duplicate and curve B represents the series B population duplicate in each figure . . . . . . . . . . 83 Relationship between calculated daily daphnid food consumption in both conventional (mg algae/daphnid/day, Figure 20a) and mass normal- ized (mg algae/mg daphnid/day, Figure 20b) feed- ing rate units as a function of carapace length (mm) determined using Equation 6 and the kinetic rate constants given in Table l for duplicate high and low cultured daphnid population biomass levels (Table 9) supplied 40,000 and 80,000 LIST OF FIGURES (Continued . . .) Figure algal cells/ml each day. Curve A represents low population biomass supplied 40,000 algal cells/ml, curve B represents low population biomass supplied 80,000 algal cells/ml, curve C represents high population biomass supplied 40,000 algal cells/ml, and curve D represents high population biomass supplied 80,000 algal cells/ml, in each respective figure . . . . . . . . . . . . . . . . . . . . . . xi . . . 94 INTRODUCTION Cultural eutrophication of freshwater lakes is a primary environmental concern. It represents a serious threat to the main- tenance of acceptable water quality standards. Cultural eutrophica- tion simply defined, involves the detrimental unnatural increase in plant production of a lake as a consequence of human activities. This human induced increase in plant production in most cases can be related directly to an excessively high input of a previously limit- ing essential nutrient. 0f the potentially limiting nutrients, phosphorus most often has been identified as the primary causative factor (Vollenweider 1968; Schindler and Fee 1974; Smith 1982; Premo et.al. 1985). The most conspicuous and objectionable symptom of such phosphorus enrichment in most lakes, is increased phytoplankton abundance and the resulting decrease in water clarity. Conventional physical and chemical lake management techniques implemented to permanently regulate and rectify cultural eutrophica- tion by effectively moderating phosphorus inputs have included; waste nutrient diversion (Edmondson 1972 a,b), advanced wastewater treatment (Malueg et.al. 1973), preventitive watershed land manage- ment practices (Christensen and Wilson 1976; Goldman 1981), dredging (Bengtsson etl. al. 1975), and aluminum sulfate application (Soltero et.al. 1981). However, in lake watersheds where nutrient deposition originates from many uncontrollable, diffuse, nonpoint sources or, 1 2 where diversion or reduction of nutrient loading is impractical or cost prohibitive or, in instances where internal lake nutrient load- ing rates exceed defined standards making nutrient control impossible, other management methods are required. In this case the methods utilized treat the symptoms of cultural eutrophication rather than the base cause and include; artificial aeration (Bernhardt 1967; Pastorok et.al. 1981), hydrologic flushing (Welch et.a1. 1972), algal harvesting (Oswald 1976), and algacide application (Horne and Goldman 1974). Biomanipulation has been suggested as a feasible alternative technique to such chemical and physical procedures applied for the purpose of reducing undesirable plant production in eutrOphic lakes (Fott et.al. 1974; Shapiro et.al. 1975; Andersson et.al. 1978; 0.8. Environ. Prot. Ag. 1979, 1981; Shapiro 1980; Goad 1984; Shapiro and Wright 1984). The potential of biomanipulation as an alternative management technique originated from studies of the alterations of fish populations and the resulting changes in zooplankton and phyto- plankton communities. Hrbacek et.a1. (1961) observed this inter- active biological phenomena in his seminal investigation, while other researchers (Grygierek et.al. 1966; Burlbert et.al. 1972; Losos and Hetesa 1973; Helfrich 1976; Stenson et.al. 1978; Lynch and Shapiro 1981; Hurlbert and Mulla 1981; Spencer and King 1984), subsequently substantiated the beneficial practicality of this technique. Biomanipulation involves the control of natural limnetic trophic levels, primarily upper order trophic levels, to regulate algal biomass. Biomanipulation relies on biological factors to 3 control nuisance algal blooms, as opposed to the more conventional dependence on control of physical and chemical factors. The presence of significant populations of large filter- feeding cladocerans appears to be pivotal to successful biological control of nuisance phytoplankton populations. The positive correla- tion that exists between low phytoplankton biomass and uninhibited grazing intensity characteristic of a viable, large, herbivorous filter-feeding cladoceran species has been documented by Lampert (1978a), Edmondson and Litt (1982), Elliot et.al. (1983), Osgood (1984), Schoenberg and Carlson (1984), Vanni (1984), and Sterner (1986). In aquatic ecosystems where this cladoceran control of phyto- plankton is functioning, zooplanktivorous fish predation pressure on the cladocerans has been reduced, usually by piscivorous fish. This balanced equilibrated biological state can persist only if piscivorous fish densities are not critically reduced and catastrOphic physical, chemical, or biological events do not decimate the cladoceran populations. However, even if predation on the cladocerans is relaxed by maintenance of large numbers of piscivorous fish, control of algal abundance by cladocerans involves consideration of a great many inter- acting rate functions. Clearly, Cladocera population dynamics involve interactions and feedbacks between feeding and filtering rate kinetics, individual growth and fecundity rates, cladoceran population dynamics,- and the production rate of the algae. Feeding and filtering rate kinetics of Daphnia have been studied extensively for various purposes ranging from physiological 4 inquiries to incorporation of descriptive relationships into simula- tion models of trophic dynamics. To obtain such information on daphnid feeding and filtering rate kinetics, experimentation has involved both laboratory trials with controlled environmental variables-(Ryther 1954; Rigler 1961; Mc Mahon and Rigler 1963, 1965; Me Mahon 1965; Burns and Rigler 1967; Burns 1968a, 1968b, 1969; Arnold 1971; Berman and Richman 1974; Hayward and Gallup 1976; Lampert 1977a, Watts and Young 1980; Porter et.al. 1982, 1983; Paloheimo et.al. 1982; Richman and Dodson 1983; Holm et.al. 1983; Muck and Lampert 1980, 1984; Meise et.al. 1985), and field investigations with relatively uncontrolled variables (Haney 1973; Crowley 1973; Haney and Hall 1975; Chow-Fraser and Knoechel 1985; Knoechel and Blair 1986), to mention just a few. Also, Lehman (1976) and Peters and Downing (1984) present articles containing a survey of published literature regarding this tapic. Some ofthe variables and reference parameters examined and utilized include; _animal species composition and abundance, diet resource effects, 7 particle size and volume effects, behavioral versus mechanistic feed— ing processes, temperature, blue-green algal toxicity and manage- ability, diet resource concentrations, animal size relationships, diel vertical migration, and photoperiod. Generally, from the reviewed literature, empirical models, describing the feeding rate functional response curves for Daphnia are of three basic types (Mullin et.al. 1975; Lehman 1976; Lampert 1977a; Porter et.al. 1982). These are; the rectilinear model, the Ivlev model, and the Michaelis-Menten model. Regardless of the model utilized, the shape of the feeding rate curve as a function of food 5 _goncentration is uniformly depicted as a hyperbolic function assuming _afigonstant rate after the discrete incipient limiting food concentra- fltion is exceeded. (Conceptual mechanistic models describing filtering rate processes and kinetics are usually expressed as a function of food concentration and body length. However, filtering rate functional response curves and derived empirical and theoretical models vary con- siderably. Me Mahon (1965), Burns and Rigler (1967), Burns (1969), Paloheimo et.al. (1982), Chow-Fraser and Knoechel (1985), and Knoechel and Blair (1986), all empirically model daphnid filtering rate functional responses as a power function of daphnid body size. Hayward and Gallup's (1976) filtering rate relationship as a function of algal concentration for two daphnid species was determined to be the inverse obtained for their feeding rate data and is equivalent to a negative exponential function of food concentration. Rigler (1961), Me Mahon and Rigler (1963, 1965), Crowley (1973), and Porter et.al. (1982), concluded that daphnid filtering rates increased with decreasing food concentrations becoming constant at food concentra- tions below the discrete incipient limiting food concentration. Henry and Peters (1984) suggested that cladoceran filtering rates can best be computed by utilizing a multivariate empirical model with filtering rate calculated as a function of animal dry weight, food concentration, experimental volume, and experimental volume per animal. “Finally, Lehman (l976)_hypothesized_that an appropriate theoretical model capable of predicting filtering rates for filter-feeding zoo- plankton, would include the fundamentals of energy optimization. 6 Lehman's model is predicated on theoretical filtering behavior of a zooplankter to maximize its net gain of energy in a particulate suspension of known composition. His model depicts a filtering rate functional response, in relation to food concentration, that increases _ with decreasing food concentration, attains a physiological maximum, _and then declines proportionately with decreasing food concentration, ultimately intersecting at the origin. Lehman also provides a feeding rate model based on the same precepts as his filtering rate model. In this instance, his theoretical feeding rate functional response to that presented by Mullin et.al. (1975), Lampert (1977a), and Porter et.al. (1982). The general concensus from the reviewed literature is that the filtering rate of cladocerans increases with increased body size of the organism, and decreases with increased food concentration,_al- though there is some debate about the functional response at low food concentrations. The main point of debate in the literature concerning filter- feeding zooplankton filtering and therefore, feeding rate functional responses at decreased food concentrations, revolves around the existence or non-existence of a threshold food concentration. A food concentration threshold is defined as the food concentration where filter-feeding activities of zooplankton cease as a physiological reaction to minimize energy expenditures during periods of depleted food resources. The base of this argument revolves around whether or not filter-feeding cladoceran behavior is characteristic of 7 theoretical optimal foraging and energy optimization models. Accord- ing to Parson et.al. (1967), Mullin et.al. (1975), Frost (1975, 1980), Lehman (1976), and Lam and Frost (1976), filter-feeding zooplankton do indeed, comply with the theoretical energy optimization models and exhibit a threshold food concentration. However, Crowley (1973), Muck and Lampert (1980, 1984), and Porter et.al. (1982, 1983), provide experimental data supporting a diametrically opposite position, suggesting no need to correct empirical filtering and feed- ing rate models for threshold food concentrations. From the literature, it appears that the primary factors affegting growth and fecundity rates of individual daphnids, are “temperature and food concentration (Richman 1958; Kryutchkova and Sladecek 1968; Arnold 1971; Weglenska 1971; Vijverberg 1976; Lampert l977a,b,c; Porter and Orcutt 1980; Neill 1981; Paloheimo et.al. 1982; Lynch and Ennis 1983; Orcutt and Peter 1983; Porter et.al. 1983; Holm and Shapiro 1984; Stich and Lampert 1984; Orcutt and Porter 1984; Meise et.al. 1985). kWhile food and temperature have been demonstrated to be two very significant factors affecting daphnid growth and fecundity rates, Goulden et.al. (1982), Tessier et.al. (1983), and Goulden and Henry (1984), suggest that the_ability ofDaphnia to accumulate lipid energy reserves in a manner which allows these lipid deposits to be allocated to their progeny is of equal importance in determining the growth rate potential of cladocerans. Additionally, the study con- ducted by Lynch and Ennis (1983) demonstrated that such maternal energy reserves, as a function of maternal nutritional environment, 8 are of import to neonate physiology, morphology, and potential fecundity rates, as well as potential growth rates. Of the thirty-one papers reviewed dealing with zooplankton population dynamics, fifteen were studies of naturally occurring zooplankton assemblages (Hall 1964; Wright 1965; Tappa 1965; Clark and Carter 1974; George and Edwards 1974; Kwik and Carter 1975; Allan 1977; Lynch 1978; Threlkeld 1979; Goldman et.al. 1979; Lynch 1979; Brambilla 1980; Taylor and Gerking 1980, 1985, De Mott 1983), while the remainder were laboratory population studies of which two were conducted on rotifer populations (Ingle et.al. 1937; Anderson 1937; Dunham 1938; Pratt 1943; Frank 1952; Slobodkin 1954, 1959; Frank et.al. 1957; Smith 1963; King 1967; Neill 1975; Goulden and Hornig 1980; Elliot et.al. 1983; Romanovsky 1984; Stemberger and Gilbert 1985). Regardless of the approach taken toward the study and analysis of zooplankton populations, it appears that the primary factor limiting . these assemblages is food availability. Prominent in Slobodkin's (1954) classical work, is the remarkably linear relationship obtained when mean number of organisms at equilibrium is graphed as a function of the relative quantities of food provided to each population. This linearity was later substantiated and verified by King (1967), with his laboratory cultured rotifer populations. umln_addition to food availability, other, often synergistic, elements impacting on zooplankton populations include vulnerability Wto size-selective predation and temperature influences (Hall 1964; Wright 1965; Allan 1977; Goldman et.al. 1979; Threlkeld 1979; Elliot et.al. 1983). 9 Emanating from the available and voluminous quantity of zooplankton literature previously reviewed, are excellent but, diverse examples of significant applied research. However, due to the heterogeneous composition of these studies when collated, the /compilation of the requisitefdata for purposesof_deriving quanti- tetive_kineticrate relationships between_feod availability and .eaphnid feeding,filtering, fecundity,-and-individualvand population fifi- ._.....__ -—_..—_-. growth dynamics, is impossible.’ And yet, such kinetic rate relation- ships are absolutely necessary to develop a mechanistic understanding of the feedback limits which allow cladocerans to simultaneously maintain both their population growth and the control of phytoplankton biomass. This interaction is essential for successful biomanipulative ..l.__.ej‘ management efforts of eutrophic lakes. The objectiveefiof this conducted research therefore, are to #-‘ __ —¥ ~—-——~.. deyelop such quantitative kinetic rate relationships for a single cladoceran fed one alga at a constant temperature. The experimental temperature chosen was 27 C. The daphnid species utilized was Bephnia pulex and the diet algal species provided as food was Chlamy- domonas reinhardi. The objectives included formulation of quantitative relationships between rates of feeding, filtering, fecundity, and_ individual and population growth all as a function of food availability at one constant temperature. MATERIALS AND METHODS As defined in the introduction, the objectives of this thesis required three distinct but, complementary experimental phases. Despite the existence of some differences among the various experi- mental phases, there are several commonalities presented as follows. COMMON EXPERIMENTAL METHODS Daphnia pulex was collected from the permanent ponds south of the Michigan State University main campus at the Inland Lakes Research and Study Center. Stock daphnid populations were cultured in the laboratory in a 25 liter aquarium filled with a mixture of aerated, aged tap water, algal nutrient media, and pond water. Air temperature varied between 19 and‘28 C. The daphnid culture water mixture was replenished periodically and all cultures were supplied regularly with the experimental diet algal species. The original stock culture from which daphnid clones were selected for the required laboratory experiments, were raised from two female daphnids obtained from the initial zooplankton field sample. Species identification was verified according to Edmondson (1959). The diet algal species used in this study was Chlamdomonas reinhardi. The strain, wild type (+), was purchased from the Carolina Biological Supply Company. This alga was cultured in a standard defined inorganic nutrient medium and maintained at the ambient 10 11 environmental laboratory conditions described for the daphnid stock cultures. The chemical composition of the algal nutrient media is identical to that used by Spencer and King (1985), except that vitamin supplements were not included for this algal media formula. Algal cultures were decanted, replenished with media, and reseeded regularly to avoid excessive bacterial contamination (Stemberger and Gilbert 1985). The alga was cultured in constant illumination and continuously aerated. Individual daphnid growth and fecundity, feeding and filter- ing, and population growth kinetic rate experiments were conducted at four to six food concentrations ranging from zero to 400,000 algal cells/m1. Experimental daphnid cultures were supplied daily with fresh cell suspensions utilizing the renewed batch culture method. All feeding, filtering, individual growth and fecundity, and population growth kinetic rate studies were performed at 27 C. in a LAB-LINE AMBI-HI-LO CHAMBER 3550 incubator. Experiments were conducted in the dark. All study organisms were acclimated to the experimental conditions prior to experimental analysis for a time period ranging from 12-16 hours. An experimental temperature of 27 C. was chosen because it was typical of mean summer temperatures experienced by the natural daphnid populations in the M.S.U. ponds from.which the study organism was obtained (Spencer and King 1984). Additionally, by main- taining a relatively high but, tolerable, experimental temperature, daphnid population kinetic rate dynamics can be accelerated; thereby, reducing the time required for the population study. The dilution water used for all experiments was aged, aerated Millipore membrane filtered (0.45 um), tap water. Glass distilled 12 water was utilized for the dilution of the algal nutrient media. Preparation of the appropriate algal food suspensions for the individual and population growth rate experimental cultures and the feeding and filtering kinetic rate experimental trials proceeded as follows. An adequate experimental volume of concentrated algal cell suspension was removed from the established stock algal culture and placed in a clean volumetric flask. Algal stock cultures were come posed of fresh, exponentially growing algae in early to mid log growth phase. The volumetric flask containing the concentrated stock algal culture was diluted and allowed to stand overnight in the dark under constant aeration. The next day, a 10.1 ml aliquot of diluted stock algal cell solution was removed and preserved with 1 drop of Lugol's solution. From this preserved sample, 1.0 ml was transferred by graduated pipet to a Sedgewick-Rafter algal counting cell. Algal cells were enumerated using a phase contrast Leitz compound microscope equipped with a calibrated Whipple-grid algal counting field at 100x or 210x. Algal cell concentration was calculated from a series of algal cell counts (n = 40). The equation used to calculate algal cell concentration is presented as Equation 1. algal cell concentration (algal cells/ml) a (mean number of algal cells counted/Whipple-grid) (1000 mmzlml, Sedgewick— Rafter cell area per unit volume of the algal sample)/ (Whipple-grid area (mm2)) (1) Following algal cell enumeration of the diluted stock algal cell solution, desired serial algal cell dilutions were prepared to provide greater accuracy. Verification of the desired algal cell l3 suspensions was accomplished by counting each dilution once more. The algal food suspensions were then transferred to the experimental culture vessels, and the daphnids were added. Algal cell suspensions were subjected to constant aeration without illumination for a 24 hour period prior to serial dilution because this treatment enhanced algal motility, and suppressed algal reproduction through nutrient and light deprivation. Effectiveness of this technique was verified by algal cell enumeration and direct observation. Inhibited algal cell growth and motility stimulation was further augmented by decreasing the nitrogen reagent weight when preparing the algal nutrient culture media (12.5 mg KN03/L). The experimental advantage of culturing a motile Q; reinhardi algal popu- lation, lies in the ability to maintain a homogeneous algal food suspension without having to resort to physical and, potentially disruptive, circulation techniques to prevent algal cell sedimentation. Daphnids utilized for any of the experimental analyses in these study phases were selected based on subjective physiological appearance and behavioral criteria. Physiologically impaired or morphologically damaged organisms were disposed. Adult daphnids utilized were all in a non-gravid state prior to the initiation of an experiment. Eight daphnid size classes were defined: 0.69-0.82 mm, 0.89- 1.00 mm, 1.01-1.25 mm, 1.26-1.38 mm, 1.39-1.50 mm, 1.51-1.75 mm, 1.76-2.00 mm, and 2.01-2.25 mm. These length defined size classes were utilized to examine individual and population kinetic rate dynamics as a function of daphnid size. l4 Daphnids were transferred individually at random from the daphnid culture aquarium with a large mouthed pipet into a beaker, and then placed on a glass depression plate cell. Excess water was removed to immobilize the daphnids for length measurement. Lenth was measured using an American Optical binocular dissecting microscope equipped with a calibrated occular micrometer at 15X. Daphnix carapace length was determined by measuring from the anterior most apex of the head to the point of inflection on the posteroventral margin of the carapace. An allometric length-weight relationship index equation was developed for the Q. pgle§_strain used. The technique consisted of removing healthy non-ovigorous daphnids from the stock cultures and placing them in a beaker containing prepared algal cell suspension dilu- tion water. Daphnids remained in the beaker for approximately two hours. This was done primarily to rinse the organism and allow for the egestion of their gut contents before weight determination (Paloheimo et.al. 1982). Daphnids were then rinsed with distilled water. Following the second rinsing, the daphnids were transferred to a glass depression plate cell and measured to the nearest 0.025 mm. Following carapace length measurement, the experimental organisms were placed on a tared weighing pan constructed from thin aluminum foil. The organisms were then dried to constant weight in a drying oven for 24 hours at 60 C., cooled in a dessicator, and weighed with a Cahn Model G electrobalance sensitive to 0.05 ug on the 0-to-5-mg range setting. A double log plot was made relating weight vs. carapace length of the organism (Figure 1). A linear regression analysis of 15 -4 p. I A 0 ..° ,7, 3 3 C >- 2’. 8 e m v 5 '5 - l l l ‘0.5 0.0 0.5 |.0 In CARAPACE LENGTH (mm) Figure 1. Relationship between in body weight and in carapace length for _Q. pulex. Equation 2; W87.69X10‘3L2-39, is indicated by a solid straight line. 16 ln (weight) against 1n (length) yielded a transformed power equation describing the length per dry weight index relationship. The equa- tion fit to the resulting power curve is a function expressed by Equation 2 with a coefficient of determination of 0.96. w a 7.69 x 10'3 L2°39 (2) where, W - weight of daphnid in mg L - carapace length of a daphnid in mm This transformed expression provided a convenient and expedient conver- sion method for systematically calculating either individual or population daphnid biomass as a function of carapace length. The weight of the experimental diet alga Chlamydomonas reinhardi was determined in multiple replicates (h = 5) by filtering, drying, and weighing a known volume of an algal cell suSpension at specific concentrations. A stock solution of algae was prepared as previously described, a subsample aliquot pipeted into a receiving flask and preserved with 1 drop of Lugol's solution, and algal cell concentration determined by obtaining a mean value from a series of Sedgewick-Rafter algal cell counts. Ten to 100 mls of the algal cell suspension were filtered through a tared 0.45 um sterilized gridded Millipore filter, dried at 60 C. for 24 hours, weighed, and the weight per algal cell calculated. Algal cell weight, determined regularly throughout the period of laboratory experimentation, was calculated to be 3.00 x 10.8 mg dry weight per algal cell (S.E. = 1.75 x 10-9). 17 FEEDING AND FILTERING RATE DETERMINATION EXPERIMENTAL METHODS Feeding and filtering kinetic rates were estimated utilizing the "differential algal cell count" method, the general format of which has been applied and described previously by Gauld (1951) and Ryther (1954). Typically, the differential algal cell count method involves microscopically estimating algal cell concentrations from the experimental cultures before, during, and after a defined period in which selected known experimental daphnid size classes were allowed to graze in suspensions of known algal cell concentration. This tech- nique consisted of pipeting a two, three, or five ml aliquot of sample from the experimental filter-feeding flask, preserving it with one drop of Lugol's solution, and then determining the algal cell concen- tration at each time period. Prior to any feeding and filtering rate experiment, daphnids of the approximate size class range desired were transferred individually at random from the stock culture with a large mouthed eye dropper into a beaker containing algal cell suspension dilution water. Study organisms were then measured according to the previously described technique to the nearest : 0.05 mm. After appropriate experimental size selection, the daphnids were placed in variable dense concentra- tions of algal cell suspensions and left overnight without illumination at the experimental temperature (27 C.). Before the actual feeding and filtering kinetic rate experi- ments, the daphnids were re-measured for length and then transferred into aerated, aged tap water for approximately one hour. This was done to rinse the organisms and to allow for egestion prior to the experimental trial. 18 Following preparation of the serial algal cell dilutions for the feeding and filtering kinetic rate trials, the daphnids were transferred from the aerated, aged tap water into their respective experimental containers. A volume of 50 mls was used for all feeding and filtering kinetic rate experiments which were conducted in 50 m1 Erlenmeyer flasks. Feeding and filtering kinetic rate experiments were performed for all eight daphnid size classes. The number of daphnids per experimental vessel varied for each size class, depending on the size of the daphnid under investigation and the algal cell suspension concentration. The experimental feeding and filtering flasks were swirled gently periodically to resuspend any sedimenting cells and create a homogeneous algal cell distribution. Sample ali- quots from the experimental feeding and filtering kinetic rate trial containers for algal cell enumeration were obtained with a graduated pipet at regular intervals as necessitated by experimental design requirements. For the feeding and filtering kinetic rate trials, daphnids were allowed to graze in the experimental cultures for time periods ranging from one to 12 hours, depending upon algal cell suspension concentration, experimental objectives, and daphnid size and number. Beginning experimental algal cell concentrations utilized for the feeding and filtering kinetic rate experiments ranged from 7,500 to 400,000 algal cells/ml. Filtering rate expresses the volume of algal cell suspension passed through the filtering appendages per organism per unit time, assuming a 100% capture efficiency of algae. Feeding rate provides 19 the estimation of the quantity of algal cells removed from the algal suspension per organism per time. The calculation utilized for measuring observed daphnid feeding rates is described by Equation 3. C1 ' C2 ‘f-f:fif— (V) l 2 A/D/T (3) where, A/D/T = algal cells consumed per daphnid per hour (daphnid feeding rate) = algal cell concentration at time 1 (algal cells/ml) = algal cell concentration at time 2 (algal cells/ml) - experimental time 1 (hours) 8 experimental time 2 (hours) NI—‘NH - experimental filter-feeding volume (mls) Uaw oma< .mommmao mafia cascade AmocoEwuoaxo uswfio «no How woumfiuoouommqe mm Ac one m cowumoomv maooos ofiaoouoozs moon woweoom Hmowuaaao soon now oooa>oue coaumuuoooaoo Haoo Hmwam mo cowuoosw m we wanna meaooou ofionooo oo>uomoo Scum eo>fiuoo mucoumooo moon oauoofix .H manna 31 .H oHomH ca om>Hw muemumooo some oauocfix moan: .Awouoouuoo coaumuucoocoo ooo~ oaonmoosulcoov o coaumsom new o>u=o mmcoemou Hmcoquuoau ecu mueomouaou m mafia one Aomuoouuou coaumuucouaoo m00u maonmounuv m :ofiumacm now o>u=o mmcoamou Hchfiuocaw mnu muaomonaou < ocag .mmdao oNHm mficnemo ES oo.~tom.a onu “cu AHE\mHHoo Ammaov cofiumuucoocoo Haoo Human mo :ofiuocau m we Ausos\oacnamo\mmwamv sump wcfimoow oficnmmo somzuon owzmcowomaom .m muswfim Ano_x: Etmqqmo 4<04< 00¢ oon CON oo. _ a _ _ o C) If) l/O/V (EOI x I ‘Jnou/muudop/aoblo) 1 C3 9 L00. 32 .ANH cowumaomv N manna ca woucomouo muaofiofimuooo scammouwou can AHH one OH macaumavmv H manna ca co>ww musmumaoo oumu ofiuoaax wean: .Aoouoouuoo oowumuucooooo moow oaoemousulcocv NH coaumoom now o>u=u uncommon Hmcoauocom onu muoomoueou 0 mafia one .Aoouuouuoo :oHumuucoocoo o00m vaonmounu loony NH cowumavu new o>u=o uncommon Hm:o«uo::m onu muaomouaou m mafia .Aoouomuuoo coaumuuaoocoo ooow oaonmounuv 0H coaumavm you o>u=o omaoamou HchNuoesm osu mucomouoou 4 mafia .mmmao moan ofianamu Be oo.Niom.H onu you Aaa\mHHoo Howamv coaumuucooooo Haoo Human mo aowuoasm m on Anso:\owcnano\dav momma wawuouawm oficnemo :oo3uoo mesmGOfiuoHom .q muswfim Ame. x 3 259.150 44044 00? con CON .oo. . . _ _ AV 0 O C .I. ” ‘..l -.O O 0 o O O e on. 0.. C O \l o o m. ./ coo W 0 .d O u. co m. ‘I.’ w 0 Au.N M O u. 0 O. .L/CI/A 33 and the third uncorrected for threshold food concentration (Equation 12) as derived from the measured filtering rate data. As discussed earlier there is a disagreement in the literature concerning the shape of the predicted functional response filtering and feeding rate curves, primarily at low food concentrations. The debate revolves around the dispute as to whether the predicted feeding and filtering rate curves should be corrected for a threshold food concentration, with threshold food concentration indicating resource conditions where cladoceran filter-feeding activities cease. Presently, this is regarded as a hypothetical behavioral response for zooplankton when basal metabolic demands are not energetically satis- fied due to a severely food limited environment. Threshold food concentration correction of empirical feeding and filtering rate models, originates from researchers who assume that filter-feeders comply with theoretical optimal foraging and energy optimization models (Parson et.al. 1967; Mullin et.al. 1975; Frost 1975, 1980; Lehman 1976; Lam and Frost 1976). These models imply that a filter-feeder maximizes the net rate of energy gained from its food as a function of energy expended in filtering food particles. This theory fits well with predicted relations between filtering rate, feeding rate, and abundance of food with empirical models at high resource concentrations, but not at low particle densities, hence, the debate as suggested earlier. However, those authors espousing theoretical foraging and energy optimization models apply them almost exclusively to a different type of organism, the calanoid copepods. These organisms, as opposed 34 to a continuous filter-feeding cladoceran, have been shown to employ alternative feeding and filtering mechanisms and strategies (Richman and Dodson 1983). Therefore, the suitability of the foraging and energy optimization models, exemplified by Equations 5 and 10, may not fit well when applied to filter-feeding cladocerans. In addition, the inhalant current produced by movement of the thoracic appendages subserves respiration as well as feeding in filter-feeding zooplankton (Jorgensen 1966; Crowley 1973). Therefore, it is essential that filtering continue even if net energy expenditure is greater than net energy intake during the filter-feeding process at low food densities. This demand for respiration suggests that filter- feeding continues and that no threshold food concentration is reached. Supplemental evidence substantiating this position is pro- vided in studies conducted by Muck and Lampert (1980, 1984) and Porter et.al. (1982, 1983) in direct refutation of Lehman (1976). In fact, Muck and Lampert (1980) failed to demonstrate a threshold food concen- tration feeding or filtering behavior even for Eudiaptomus, a calanoid copepod. Upon examination of Figures 3 and 4, it is apparent that there is little difference between the feeding and filtering rate functional response curves, suggesting little real difference in the various empirical models fit to the measured data. Discrepancies are only observed to exist at the lowest algal cell concentrations which, especially for Figure 4, is the region demonstrating the most data point scatter and variability. The variability is of sufficient magnitude at the low concentrations that no clear choice of model can be justified. 35 Thus, there is no distinct evidence for a threshold correction and models represented by Equation 5 and 10 do not seem appropriate for filter-feeding cladocerans. Calculation of filtering rate for the various size classes of daphnids using Equation 11 and the kinetic constants for the non- threshold food concentration corrected model given in Table 1 yields filtering rate values which appear to decline with increased food concentration in a negative exponential manner similar to that of Equation 12 (Figure 4). Separate exponential relationships were determined for both such calculated filtering rates and observed filtering rates for each size class of daphnids. The constants for these equations are given in Table 2. To determine if the two negative exponential curves were coincident, two separate statistical Student's t-tests were utilized. These two tests consisted of a test for parallelism of slope and a test for common y-intercept (Kleinbaum and Kupper 1978). In all instances the null hypotheses were not rejected (significance level 8 0.05), indicating coincidence of the two curves (Table 2). Therefore, either Equation 11 or 12 can be used to calculate filtering rates of daphnids as a function of food concentration. Regardless of the model chosen, daphnid filtering rates for any size class varies as a function of algal cell concentration. How- ever, the regression coefficients (slope and y-intercept, Table 2) vary as a function of carapace length of the organism (Figures 5a and b). From Figure 5a it is apparent that the y-intercept (a) in the negative exponential filtering rate model (Equation 12) is a function of carapace length. The relationship between the y-intercept 36 00.0Am 00.0vmv0N.0 n0.0 000.0: N0¢.H 00.0 N0H.¢I 00¢.H mN.NIH0.N 00.0vmv00.0 00.0Am 00.0 «00.0: 00¢.H 00.0 N00.0I 000.H 00.N10~.H 00.0A0 0N.0vmv0H.0 00.0 H00.0t 00H.H H0.0 H00.mt 00H.H mm.Hth.H 0N.0vmv00.0 00.0vmv00.0 00.0 5H0.mt 000.H mmmmo .00.0 n Ho>mH moomoHMHome .mHoooa :OHmmouwou oeHH uanmuum o3u How ooooeHocHoo now momenta m.u:o0=um can on» nuHa wouoHoommm moaHm> m on» one mode ooumHaono one eo>ummoo onu ou uHm mm Hovoa some now ooonuoo ANuy oOHumzHEuouoo we mucoHonmooo onu one ao>H0 ooH< .mommmHo oNHo ochemv HmuooaHuoexo uszo use you ooumHuaouo00H0 mm .Amouou 0cHuouHHm 0o>uomoo Boom oouosuumaoo :OHuoaam HwHuoocoaxo o>Humwoc onuv NH :OHDmavm How one A0 :OHumavm scum onOHo>oo GOHuocam HoHuoocoaxo o>Hum0oo onuv HH :OHumsum you 0o0H>ou0 eOHumuueoocoo HHoo HomHo mo :OHuoaam m we moumu 0oHuouHHm wouwH90Hmo 0cm wo>uomno scum 0o>Huoo muaoHonmooo aOHmmouwom .N magma 37 .mm.ou~u .maaveuoaxm~.m tAvatonme.H+010HxN0.mauo "NH :OHDmavm muoomouoou o>uao oHHoomuma endow and .am manage aH .km.ouwu .AavHo.a+ms.o-um “0H coaumacm mucomouoou ocHH nemeuum 0HHom ecu on muome oH .N oHomH :H co>H0 mHoeoE QOHmmouwou mama 0oHuouHHu ochaoo ozu Eouw vochDno A580 nuweoH oomomumo mo GOHuocam o no Ann oustmv oQOHm 0cm Amm ouamev unmoumucHtm muooHonmooo GOHmmoumou oSu ooozuon 0H5m00HumHom .o 000 mm moustm it. 0...... :hOZUJ U0‘t18‘0 :bOZUJ woou0 msOHumuuooocoo HHoo Homwm HmuooaHammxo co>om one on vo>uoono =0Hu=nHuucou mmmaoHo 000 0cHo=Hoxo one 0sHo=HocH oouoHauHuo mouou eusouw onHuoao ochauv venomous omauo>< .n oHomH 55 Goulden et.al. (1982), Tessier and Goulden (1982), Tessier et.al. (1983), and Goulden and Henry (1984). Negative average measured daphnid specific growth rates were obtained upon the death of the entire inoculated daphnid cohort. To predict daphnid specific growth rates as a function of algal cell concentration, average measured daphnid specific growth rates (including and excluding the egg biomass contribution), were fit to the hyperbolic threshold food concentration corrected empirical model, Equation 5. This fit was completed in relation to the algal cell concentration from which the average measured daphnid Specific growth rates were obtained. Though the mathematical structure of Equation 5 was unmodified for this application, it was necessary to substitute for the correct terminology; (u) for (AID/T) and (“max.) for (AID/Tmax.)' The linear transformation presented in Equation 9 was used to calculate the kinetic rate constants, “max. and KC' A single threshold food concentration quantity (Cq) required for the computation of the kinetic rate constants and subsequent implementation of Equation 5, was determined graphically from the data presented in Table 3 and Figures 12a and b for daphnid specific growth rates measured with and without the contribution of egg biomass. The individual daphnid threshold food concentration was estimated to be 7,250 algal cells/ml for a daphnid specific growth rate of zero. Calculated kinetic rate constants for Equation 5 are presented in Table 4, along with their corresponding coefficients of determination. The curves illustrating predicted daphnid specific growth rate (including and excluding egg biomass contribution), calculated as a function of algal cell concen- tration utilizing Equation 5, are provided in Figures 12a and b. 56 .Axv.m.m H xv moueu zuzoum oHuHoeae ochoeo mouseeea e0eao>e ecu now seem.enu mo.mouuo eueoseue ego eueoHecH eueo HeoHuue> one .Aq eHoeav n :oHuezvm an ce>H0 me moaaeuweHo one a wee eNH eeuamHm :H eo>uao oHHoou000£ esoanHunoo use .AHa\eHHeo HemHev :oHoeuuaoonoo HHeo HewHe mo QOHDonam e we eeeBOHA 000 no AoNH ousmHmv o>HezHonH one AeNH ouszmv o>HeaHoxo voueHaoHeu .Atheu : .meeEOHn ouauHau oHonaem no eemeov eueu nuaoum onHoooe oHenaeo vouaeeea o0eue>e consume 0HeeGOHueHom .0 use eNH eeuame #25. 2.3.33 43.? . 2.2. .5343 .25.: 8e co. .n _ com 8. _ _ _ 9.0- s . .d a II 0.0 m A1 m m, a .. 2.6 a o .u m 1 1| I. H and N I. a. o 1 and I T .. cod 1 020. O. o ‘2 10 O “Q 0 n§oon1 31.73 HIMOUD OIJIOEdS O n O OCAV 57 emeEOHm 00.H 00N.N 0H0.0H 0m00.0 000 00H: eeeaoH0 00.H om~.e mom.mH Aqm~.o mam “gonna: Nu AHE\0HHoo HemHev AHaerHou He0Hev Eaaera: a 0 m M 0 0 0 .MMMV :0Hue0Hauou00 00 euceHoHuueoo neueHuoeee enu one ne>H0 oeH< .AA 0N1 M+00\A 0100 an: .m :0Hue000 eeHmHuoav H0008 euen suaoum oHuHuoae HeoHuHaam oHHoauoazs eouoouuoo noHueuueoonoo 0000 0Hoeeounu ecu you 000H>0ua noHueuueeoeou HHeo HewHe 00 :OHuoeau e we .eeeaoHn 000 00H00Hoxe tee mnHvzHocH .eeueu susouw onHoeae 0H0£0e0 veuneeea Scum 00>Huee euaeuenou eueu uHuoon .0 oHoeH 58 Three important features are illustrated in Figures 12a and b. The first is the establishment of an existing threshold food concen- tration. This food quantity indicates the minimum limit, on an individual daphnid basis, at which an organism is just able to equalize its metabolic losses by assimilation of food so that the body mass of the organism remains constant. Under these conditions biomass production or daphnid specific growth rate is zero. Therefore, if food conditions were to be suppressed below the threshold food concentration, daphnid survivorship would be imperiled. Threshold food concentrations for individual _D. 132135 pre- viously have been observed by Lampert (l977c) and Lampert and Schober (1980), and for a variety of rotifer species by Stemberger and Gilbert (1985). Additionally, Lampert (1978) and Lampert and Schober (1980) have identified and estimated a reproductive threshold food concentra- tion defined as the food concentration that just maintains egg produc- tion in zobplankton. Such a threshold food concentration required to sustain egg production is defined more in the context of zooplankton population level dynamics where population biomass production must exceed mortality and not just compensate for metabolic losses. In this study, egg production was observed at all algal cell concentrations. This implies, that individual and reproductive threshold food concen- trations, as estimated, could have coincided. However, neonate survivorship as a function of algal cell concentration was not evaluated, this could potentially influence the correct estimation of a reproductive threshold food concentration at which viable progeny are produced. This criticism extends also to the data of Lampert (l977c) and Lampert and Schober (1980). 59 The second important distinctive feature in Figures 12a and b, is the significant relative impact that the contribution of egg bio- mass has upon calculated daphnid specific growth rates. Including egg biomass in addition to total daphnid body biomass increased calculated specific growth rates by approximately 202 at the maximum point of divergence between the curves presented in Figures 12a and b. The coordinate where curve separation occurs, indicates the point when primiparous females were observed in the culture beakers. Divergence of the two curves in Figures 12a and b, represents the amount of pro- ductive energy, measured as specific growth rate, that a gravid female allocates towards reproduction as opposed to the energy partitioned for somatic growth as a function of algal cell concentration. The final relationship given in Figures 12a and b, is the demonstration of the affect that food availability has upon individual daphnid growth. Daphnid individual specific growth rates increase with increasing algal cell concentration approaching an asymptote at 100,000 algal cells/ml beyond which the rate of daphnid specific growth as a function of increasing food concentration decreases. In- creased cladoceran and rotifer growth as a function of increasing food concentration expressed as a hyperbolic functional response in the manner given in Figures 12a and b, has been observed by King (1967), Vijverberg (1976), Lampert (l977b), Porter and Orcutt (1980), Porter et.al. (1983), Orcutt and Porter (1984), and Stemberger and Gilbert (1985). Daphnid fecundity, measured as eggs per female per clutch, was obtained for all experimental algal cell concentrations and 60 plotted as a function of carapace length yielding Figure 13. Separate straight line regression models were fit to these data by applying the least squares statistical analysis method (Kleinbaum and Kupper 1978) to predict daphnid fecundity as a function of carapace length at each of the six algal cell concentrations used. Figure 14 illustrates the discrete predictive empirical regression models fit to the fecundity parameters in Figure 13 for each algal cell concen- tration. Table 5 contains the estimated regression coefficients for the separate straight line regression models corresponding to algal cell concentration and their associated coefficients of determination. A statistical test for coincidence was performed as described earlier (Student's t-test for parallelism of slope and common y-intercept), to determine if significant differences existed among the various straight line regression models as a function of algal cell concen- tration. Results for the tests for coincidence between the models are presented in Table 6 along with the determined P values. While some of the fecundity regression models did exhibit coincidence, it still can be seen in Figures 13 and 14, that daphnid fecundity, as expressed by eggs per female per clutch, increases with increased carapace length and food concentration. From Table 5, it is apparent that the regression coefficients (slope and y-intercept), estimated for the daphnid fecundity regres- sion models, vary as a function of algal cell concentration as illus- trated in Figures 15a and b. The association between the y-intercept (a) and algal cell concentration can be fit to a straight line regres- sion model by the least squares method yielding Equation 21. 61 .AHE\eHHoo HewHev 0030000000 ouauHso =0Huenuoeocou HHeo HewHe He=0H>H00H 050 now me>uemno we 055 euweeH eueaeueu Hie Amen—0300000 00235000 30:02. 0003000 300003300 .0H 0.330 055 :00sz u0H00H noee you A350 zuwceH eoeeeueo 00 ooHuocam e we eHeaew\0000 ee 00:00pm An oHnesv eHeooa 00Heeoumeu weenHH HeoHuHeae aquoaoem 0H0£0e0 2:5 . .zhozua uoHuoeaeou 020 one 00>H0 oeH< .eHevoa 00Heeeu00u ueoaHH 0:0 mom ANHV .ecoHueuuoeonoo HHoo He0He HeuneeHueexe oueuoeHv 0H0 030 you someoH eoeeeueo 00 ooHuonam e 0e eeeHe nouaHo 0H0£0e0 ee>ueeoo scum 00>Huoe eHeeoa HeoHuHeee 00chaoem 0Hanee0 H00 eueoHonweoo aoHemouwem -m eHneH 64 mH ouamoamuawwm Hmsua>avaw mnu yaw mm=Hm> m wawvcommwuuou mnu mum vmvfi>oun omH< <2 H00.0 m unacfioafioo <2 no.0vmv~0.0 acavfiocfioo H00.0vm H00.0vm unavaoawoo 0c uoz aw mucuauummmou :oammmuwmu oau mafia: muwm mafialuswfimuum mumumamm aoum mucmvauafioo you mummqu m.uaovaum oau msu How muaammu mammamam Hmuaumaumum .o «flame 65 .ma.ou~u .onquoaxmo.a+mc.hun “mm :ofiumaam mucmmmuamu mafia unmfimuum uaflom an“ .nma muswfim sH .Nm.ou~u .onquoaan.a Imm.0HIIm “Hm coaumavm mucmmmuamu mafia uzwamuum vwaom man .mnH muswam cH .n manna ca cm>ww mamvoa coammmummu huwvasumm vfianamv onu Baum 0maamuno Aaa\maamu Hmemv coaumuucmucoo Hamo Human mo coauoasm m an Anna muawfimv macaw 0cm Amma unawwmv unmuumuaHI> mucmauammmou :oammmuwmu mnu cmmaumn nasmGOfiumHmm .nO. x 3 EQJJUO 44644 SN 00. OON q — A (1) 3&18 LN a van mma mouswfim .nO. x 3 ix» 4.50 410.: 00. _ 0v- 0 ? m LdBSUILNI-A 66 4 2 —10.38 - 1.72 x 10‘ (C) r = 0.92 (21) 0.) II where, y-intercept (regression coefficient) algal cell concentration (algal cells/ml) Similarly, the relationship between slope (b) and algal cell concentration can be fit to a straight line regression model by the least squares method producing Equation 22. b - 7.45 + 1.08 x 10‘4 (c) r2 - 0.93 (22) where, b - slope (regression coefficient) 'C - algal cell concentration (algal cells/ml) By combining Equations 21 and 22, it is possible to construct a single empirical linear regression model that predicts daphnid fecundity as a function of both carapace length and algal cell con- centration as presented in Equation 23. E/F = (-10.38 - 1.72 x 10‘“ (C)) + (7.45 + 1.08 x 10'“ (C)) (L) (23) where, C - algal cell concentration (algal cells/ml) L - daphnid carapace length (mm) E/F - eggs per clutch (predicted daphnid fecundity) Figure 16 graphically depicts the functional relationships described in Equation 23. As seen in Figures 12a and b, daphnid growth rate clearly varies as a function of food concentration. Exhibited at the higher algal cell concentrations, is the potential for 2, pulex to express 67 .. ",a I?’ ‘ 3‘ Figure 16. Daphnid fecundity (eggs/female) as a multivariate functional relationship between algal cell concentration (algal cells/ml) and daphnid carapace length (mm) as predicted by Equation 23; E/F-(-lO.38-l . 72x10-4(C) )+(7 .45+1.08x10-4(C)) (L). 68 their innate capacity for relatively rapid post embryonic development and attainment of primiparous size (approximately 1.50 mm), while experiencing little mortality. Daphnid fecundity also is observed to vary as a function of food concentration, as well as with carapace length, as demonstrated in Figures 13 and 14. At all food concentrations, there is a posi- tive linear relationship between daphnid carapace length and fecund- ity, with greatest size and fecundity observed at the highest food concentration. The functional relationship between daphnid carapace length and fecundity as related to food concentration was conveniently reduced to a single mathematical statement (Equation 23). Illus- trating Equation 23 yielded Figure 16, which succinctly demonstrates the prolific reproductive potential of 2. Luigi with increasing carapace length at high food levels. Combining the results presented in Figures 12a and b, 13, 14, and 16 for 2. pulex, there exists a definite correlation between accelerated daphnid growth and realized prolific fecundity (as a function of carapace length), at elevated food concentrations. These inherent abilities possessed by 2. ‘p_u_l_e_x; to exploit favorable food conditions and to rapidly incorporate net energy gain into bio- mass production (somatic and reproductive tissue formation), are very beneficial life history traits considering the ephemeral and dynamic environment in which the population naturally occurs. Of equal importance as demonstrated in this experimental phase, is the observed potential for daphnids to exist under less conducive food conditions which is also characteristic of the fluctuating environment the natural daphnid population is exposed to. 69 POPULATION GROWTH RATE STUDIES As indicated in the introduction, the concluding research experimental phase involved the study of the dynamics of daphnid populations. To conduct this study, duplicate daphnid populations were cultured at algal cell concentrations of 0, 10,000, 20,000, 40,000, and 80,000 algal cells/ml at a constant temperature of 27 C., with the algal cell suspensions being renewed daily. All daphnid population cultures were inoculated with five neonates less than eight hours old produced by a single isolated female maintained in a dense algal cell suspension. The study was terminated after 60 days, during which time viable daphnid populations experienced three oscillating growth and decline cycles. During the entire study period, only dead organisms were removed from.the experi- mental population cultures. Duplicate daphnid population cultures were censused on alternate days for the population parameters described in the materials and methods section. All surviving daphnids in each popula- tion culture were measured for length and size specific daphnid body mass was calculated with Equation 2. Following length to weight con- version, individual daphnid biomass was summed for each population culture yielding total daphnid population biomass for that census period. Daphnid population specific growth rates were calculated for each population culture applying Equation 15 using summed daphnid population biomass values obtained from two consecutive censuses. Al- so calculated at each census was a food per mass ratio of biomass of algae supplied to a population culture each day (mg) per daphnid population biomass existing when the census was taken (mg). Additional 70 technical procedures used to perform this experimental laboratory study of daphnid population dynamics were described previously in the methods and materials section. Daphnid population size observed for the duplicate (A and B Series) population cultures maintained at the five algal cell concen- trations are provided in Figures 17 and 18. Figure 17 illustrates daphnid population size expressed in units of number of daphnids in the population as a function of time. Figure 18 illustrates daphnid population size expressed in units of total daphnid population bio- mass (mg) as a function of time. It was observed throughout the course of the‘daphnid papula- tion growth rate experiment, that analyzing viable daphnid population dynamics based on population numbers proved to be too variable. This is especially evident when daphnid population numbers were categorized into the eight different size classes for each census period and stable size-age frequency distributions were never attained for any culture population supplied any algal cell concentration. Also encountered were appreciable fluctuations in total population numbers conforming to no apparent pattern when the duplicate populations were compared. This made impossible data analyses and relevant biological conclusions concerning daphnid population numbers as a function of food availability over time. Moreover, the magnitude of the mass differences exhibited and the unique size specific growth, reproductive, respiration, feeding, and filtering rate kinetics as a function of daphnid body mass noted previously for the eight daphnid size classes argues against using total population numbers. Thus, total population biomass rather than NUHUHI 71 5 O x —ASERIES 33 __ \\ " 3 SERIES 6?- g \ \ 2 II- \ E4— ‘ O \ a F G l I o I I I l J 2 4 2 4 DAYS I I I I I I "O I I I III I I I I I I Table 17. 40,000 Observed culture daphnid population size measured as total population numbers as a function of time (days) for all experimental algal cell food concentrations (algal cells/ml). 72 o”: I0,000 0.05)- 0 O.l2*— ’\ ._ ASERIES '- 3 -- asemes 30’”: 0.0I 0.04— , o ‘L l 2 4 ° 5 I2 I5 0515 nus 0.525 0.20)- 0.300 . 20,000 0.I5-— _ ,’\ 30.575 gone- — ,r\ 0250 0.05p- ,I \ ‘ _I I \l‘ 0.04 4 \‘ 0.I25 °_LLllllLllIr1\I olllllLlLlJllllJ 3 It 2| 30 39 I2 24 55 45 50 05v: 05v: .34. 50,000 \ r' 1‘ 1" , I.00I— 20.75 0.50 0.25 0 45 0 05?: Figure 18. Observed cultured daphnid population size measured as summed population biomass as a function of time (days) for all experimental algal cell food concentrations (algal cells/ml). 73 numbers appears to be a more biologically meaningful unit upon which to express population dynamic phenomena in these unstable daphnid assemblages. The use of total population biomass units yielded much more regularly oscillating populations which enhanced population dynamic analyses as a function of food availability. As depicted in either Figure 17 or 18, the daphnid population cultures supplied the three lowest algal cell concentrations (0, 10,000, and 20,000 algal cells/ml), all became extinct, with daphnid popula- tions perishing in relative order of the algal cell concentration suSpension provided to each culture. Despite the extinction of these daphnid population cultures, population growth, measured in units of accrued biomass, occurred prior to culture population death as indicated in Figure 18. In fact, except for the zero algal cell concentration suspension, inoculated daphnid cohorts were observed to attain reproductive size (1.50 mm) and to produce eggs. However, neonates released by the inoculated female daphnid cohorts at these algal cell concentrations were unable to survive to reproductive size thereby, preventing the perpetuation of the population beyond the lifespan of the second generation daphnids. Apparent neonate growth in the zero algal cell concentration, the growth of the inoculated neonate cohorts to a reproductive size in the 10,000 and 20,000 algal cell concentration suspension, and the- failure of the neonate cohorts to produce viable progeny at the same algal cell concentrations can be explained in terms of the relative quantity of accumulated and maternally allocated energy reserves (Lipid deposits) available to each daphnid as suggested by Goulden and Hornig 74 (1980), Goulden et.al. (1982), Tessier and Goulden (1982), Tessier et. al.(1983), and Goulden and Henry (1984). These residual energy reserves, maternally allocated to the neonates as a function of the abundant food available to the seed female, can be metabolized to temporarily sustain basal metabolism and support body growth for several days even at a zero food concentration level. However, when these maternally allocated lipid deposits are completely metabolized and available body mass catabolized, the neonate expires quickly, since there is no food obtainable to assist in supplementing the re- quired energy necessary to sustain bodily functions. At the food concentrations of 10,000 and 20,000 algal cells/ml food availability was apparently sufficient for inoculated neonatal filter-feeding activities to supplement maternally allocated energy reserves in suppoying the necessary energy required to support neo- natal basal metabolism and growth to reproductive size. However, the substantial impact that adequate maternally allocated energy reserves contribute to potential neonatal growth and survival at low food con- centrations is not fully realized until the viability of the inoculated cohort's progeny is assessed. Apparently, maternal energy reserves possessed by the original inoculated neonates allocated to them from the well fed seed female provided sufficient additional energy to sustain basal metabolism and growth through the most vulnerable daphnid size classes. Once this early critical size class was passed the relative mass normalized filter- feeding and adjusted respiration rate kinetics of the daphnids improved and the energetically superior intermediate sized daphnids could then 75 fully ruly upon their own abilities at these low food concentrations to obtain the necessary energy to sustain basal metabolism, accumulate energy reserves, support body growth, and initiate reproduction. However, as stated earlier, neonates produced by the females compos— ing the original inoculated cohort at either 10,000 or 20,000 algal cells/ml never survived beyond the neonate and early juvenile size classes. The death of these second generation daphnids before they reproduced at 10,000 and 20,000 algal cells/ml can be attributed to the small quantities of maternal energy reserves allocated to them by their parents existing under conditions of low food availability. Thus, daphnid population cultures maintained at 0, 10,000, and 20,000 algal cells/ml became extinct following the death of either original inoculated daphnid cohort members or second generation daphnids. Extinction occurred primarily because inoculated females could not allocate sufficient maternal energy reserves to their pro- geny at these low food levels. Adequate allocated maternal energy reserves are necessary at limiting food concentrations to sustain the relatively inefficient neonate until it develops into a more energetic— ally efficient and competitively superior intermediate daphnid size class. The existence of such a threshold food concentration required to sustain a viable daphnid population culture was not encountered in the reviewed literature. Previous laboratory studies conducted on zooplankton p0pu1ation dynamics (e.g., Ingle et.al. 1937; Slobodkin 1954; Smith 1963; King 1967; Goulden and Hornig 1980; Stemberger and Gilbert 1985) have all included food concentration ranges sufficient to sustain experimental culture populations where p0pu1ation biomass 76 production exceeded population metabolic losses and mortality. However, several field investigations did report zooplankton popula- tion extinction as a consequence of interspecific competition for a suppressed food base but, they failed to suggest a mechanism respons- ible for population extinction as a function of food availability (George and Edwards 1974; Kwik and Carter 1975; Allan 1977; Lynch 1978; De Mott 1983; Romanovsky 1984). Since viable daphnid populations were cultured at a food con- centration of 40,000 algal cells/ml and daphnid populations became extinct at a food concentration of 20,000 algal cells/ml (Figures 17 and 18), it can be inferred that the critical population threshold food concentration for Q3nglgx_fed‘§, reinhardi at 27 C. lies between 20,000 and 40,000 algal cells/ml. This population threshold food concentration (perhaps approximately 30,000 algal cells/ml), is con- siderably higher than that obtained in the preceding section from Figures 12a abd b and Table 3 (7,250 algal cells/ml). The reason for MR this discrepancy is associated with the allocation of maternal energy) _rer-_ reserves_£elative to food availability: In this study phase, second generation neonates were not supplied sufficient maternal energy reserves to survive at food concentrations below the threshold. There- fore, by continuously removing released progeny from the culture con- tainer and conducting an abbreviated experiment as was done in the individual growth and fecundity rate experiments, a population thres- hold food concentration was considerably underestimated. Lampert (1977, 1978b) and Lampert and Schober (1980) defined population threshold food concentration as the quantity of available organic carbon sufficient to induce egg production. However, the 77 correlation of egg production to minimum resource concentration (Lampert and Schober's estimated reproductive threshold food concen- tration), is not necessarily an accurate determination of the ability of a daphnid p0pu1ation to persist at specific levels of food avail- ability. N0t only do eggs have to be produced but, neonates must also be able to survive given the existing limiting environmental con- ditions from which they were conceived. Therefore, a more biologic- ally tenable estimation of a population threshold food concentration would include egg production and neonate survival. Neonate survival at limiting food concentrations is dependent upon the maternal energy reserves provided by the female. Maternal energy reserve allocation by the female is related to the energy available to her prior to egg extrusion. This interactive relationship between available energy, maternal energy reserve, and neonate survival is particularly important at lower resource concentrations. To provide an accurate estimate of a population's threshold food concentration, it is not enough to measure only egg production, for to maintain the population, the egg must hatch and the neonates must be able to survive and in turn, pro- duce viable neonates. Viable daphnid populations, capable of sustained reproductive activities, were cultured at food concentrations of 40,000 and 80,000 algal cells/ml for the entire study period as indicated in Figures 17 and 18. Both populations exhibited typical oscillating growth and decline cycles as observed by numerous researchers conducting similar laboratory zooplankton population studies. The moderating effect that population biomass has in assisting to dampen erratic daphnid population 78 fluctuations and to indicate the interacting relationships between daphnid population biomass, food availability, and time is apparent in these figures. Oscillating daphnid population cycles have been discussed by Pratt (1943), Frank (1952), Slobodkin (1954), Smith (1963), and Goulden and Hornig (1980). Fundamental to the understanding of cyclical population oscillations, is the intrinsic biological mechanistic phenomena responsible for producing them. This phenomena has been attributed to a time period delay between an organism's present physiological state and the environmental conditions which provoked that state. Implied is an innate physiological mechanism that makes it impossible for daphnids to instantaneously respond to ephemeral environmental conditions when existing in a dynamic environment. Therefore, an absence of synchronization exists between the organism and its physiological state and the vagaries and vicissitudes of the environ- ment . In both the 40,000 and 80,000 algal cells/m1 food concentrations, the general dynamic growth pattern exhibited was rapid biomass in— crease, followed by a decline in population biomass, and two subse- quent population biomass oscillations of similar magnitude (Figure 18). A sustained, stable, steady-state population biomass equilibrium was never attained for any experimental culture. From the data presented in Figure 18 it appears that a continuing population biomass oscilla- tion is as close as this organism gets to a steady-state equilibrium. Upon inoculation of the initial five neonates in each culture, algal cell densities were sufficient and population biomass small enough that rapid neonate growth and development was promoted. 79 Reproductive size was attained quickly with sexually mature adults possessing large quantities of visible lipid deposits as they did throughout their life history. Fecundity rates were high during the early growth phases of each culture and neonate survival was en- hanced by the ready availability of maternal energy reserves. With the elevated constant experimental temperature and continued abundant food availability, the population quickly expanded in the cultures fed at 40,000 and 80,000 algal cells/ml. Since the amount of food fed was constant in each culture, such increases in biomass led to decreases in the food available per milligram of daphnid population biomass. This decrease in the food per mass ratio was accompanied by reductions in the visible oil accumula- tion within the daphnid hemocoel. This trend continued until the population biomass reached a maximum. .As a consequence of this reduction in food per mass, the amount of lipid deposits accumulated per individual daphnid decreased and the quantity of maternal energy reserve partitioned by ovigorous females to their progeny decreased considerably. Because of the reduced maternal energy reserves, suppressed food availability, and their inefficient mass normalized rate kinetics (Figures 9 and 10) the mortality of the smaller daphnid size classes increased appreciably. The adult size classes, having allocated all their residual energy reserves into reproduction, also experienced high mortality rates at this time as is suggested by Figure 10. Therefore, with the existing daphnid population biomass apparently exceeding the energy capacity of the system, the population 80 suffered a crash in numbers as reflected in substantial mortality rates of both the largest and smallest daphnid size classes. A sharp decrease in daphnid fecundity rates occur simultaneously with in- creasing daphnid mortality rates during this period. The decreased population biomass associated with such accelerated mortality leads to increased food per mass ratios and in- creased visible oil accumulation in the surviving daphnids. This assumulation of lipid deposits allows increased growth and increased allocation of maternal energy reserves to neonates in amounts which ensure their survival. The growth of these neonates leads to declining food per mass ratios thereby initiating the next oscillation. As seen in Figure 18 this process appears to be continuous. Applying simple population dynamics theory to this study, an assumption can be made that an equilibrium biomass exists relative to the supply of available food, the mass of individuals in the population, and the birth rate and death rate of the population. As observed in this study, when food available exceeds population food demand, birth rate will be high and the biomass of the population will increase. As the biomass of the population increases the amount of food per individual decreases, birth rate correspondingly decreases and at some equilibrium population biomass the birth rate should approximately equal the death rate. However, in the populations of _12. p_u_l§§ studied here, a constant population biomass equilibrium is not realized because there is a time lag between the decrease of food concentrations and the delayed response of decreasing birth rates. Because excess fecundity is supported by residual energy reserves, 81 the daphnid population biomass will exceed the theoretical equili- brium biomass. As a result, the daphnid population starves and ex- periences high mortality rates. After this population biomass equili- brium point has been exceeded, the population declines in size coin- cidental to a decrease in population biomass. The mechanism responsible for the time lag provoking popula- tion oscillations has been identified as accumulated or maternally allocated lipid deposits. These residual energy reserves accumulated and allocated during dynamic population phases when food is abundant can be metabolized at high population biomass levels when food is un— available to temporarily sustain daphnid population activity and reproduction. A population culture system overstressed for energy ensues and the population reacts to deteriorating ambient resource conditions by sacrificing the most vulnerable fraction of its assem- blage. Only the most energetically superior and physiologically effi- cient intermediate sized individuals emerge to perpetuate the population. Responding to improved food conditions following the period of population biomass loss, the troughs in the population biomass curves in Figure 18, daphnids began forming lipid deposits necessary to supply the required energy to sustain continued growth, reproduction, and basal metabolism. This is indicative of another p0pu1ation time lag as the population continues its cyclical oscillation around the theoretical equilibrium population biomass (Figure 18). The relationship between two of the population parameters measured at each census, population specific growth rate and the ratio of total mass of food supplied to existing total daphnid 82 population biomass is illustrated in Figures 19a and b. Observed throughout the oscillating population growth and decline phases of positive and negative rates of population biomass change (Figure 18), is a continuous fluctuation of the population specific growth rate above and below the theoretical population equilibrium value. Below this theoretical population equilibrium, the trouth in population curves in Figure 18, population specific growth rates are positive and above, the apex on the population curves in Figure 18, they are negative. Using Figures 19a and b, it is possible to illustrate the biological relationship that exists between calculated population specific growth rates and the measured food per mass ratio. Upon inoculation of the initial daphnid cohort, calculated population speci- fic growth rates are high as are the measured food per mass ratios. This implies excellent growth potential since there is sufficient algal biomass availability to promote and support high daphnid popula- tion biomass production rates. However, as the daphnid population biomass increases and the measured food per mass ratio decreases, the accompanying population specific growth rates decrease concurrently. Once the population begins its typical oscillating growth and decline cycle, population specific growth rate alternatives between positive and negative rates. Negative specific growth rates occur as the popu- lation biomass curve descends from apex to trough and positive speci- fic growth rates occur as the population biomass curve ascends from the trough to the next apex. Alternating positive and negative population specific growth rates also appear to be confined within a distinct food per mass range 83 .000000 0000 00 000000000 0000000000 0 000000 000 0000000000 0 0>000 000 000000000 0000000000 0 000000 000 0000000000 0 0>000 .0 00009 00 0030» 000000000 0000 0000003 000 000 m 00000000 00000 03000 0003 00>000 00000 I00000 0000000000 009 .00000000 0000000000 0000000 0000000000000 0000 Ha\00000 00000 .000 00:000. ooo.om can .000 00:000. ooo.o0 000000050 000 5000 00000000 A0000000 ma\00w00 000 0000 000 0000 000 a0I000 00 0000 003000 00000000 0000000000 0000000 00000000 0003000 000000000000 .0 000 000 0000000 .02.... 25.8... 0... 8.5.8.. 252...... as. . 00$. 5.. 000... 005.. ...... 000... o 0 0.. o.“ o. _ 0 . _ _ _ . 0 ... l 2.0. s muEmm OI. .- muEum OI. o 000.00 <-. 000.00 ..-. I o 9‘. o l C) "I O I 509 "I aim HlMOUQ amass I 07.0 ONwOI .ufioI ( m n) alvu HlMOHQ olsloads 84 characteristic for the experimental algal cell concentration fed each population culture (Figures 19a and b). As proposed earlier, population dynamics theory assumes that an equilibrium biomass exists between the supply of available food and population biomass. Though this equilibrium biomass is never attained indefinitely for any daphnid culture assemblage exhibiting an oscillating population growth pattern, due to time lag effects, there is a maximum population biomass that can be supported provided a constant rate of energy availability. What eventuates, because of experimentally imposed energy constraints, is a daphnid population biomass incessantly fluctuating around this theoretical population biomass equilibrium point. Above which the population culture system is energy stressed and population biomass declines. Below which the population culture system has sufficient resources and population bio- mass increases. Therefore, as presented, the theoretical daphnid population equilibrium biomass is defined by the amount of energy available to the culture system. The theoretical equilibrium biomass is the food per mass ratio where biomass of algae supplied to the system per maximum daphnid population biomass that can be sustained is equivalent to a population specific growth rate of zero. At this specific growth rate, daphnid population biomass production equals population mortal- ity and metabolic losses; and the population exists at equilibrium. This food per mass ratio, where the daphnid population specific growth rate equals zero can be referred to as the population mainten- ance threshold. Whenever the daphnid population biomass and its 85 exerted energy demands increase beyond the capacity of the culture system's available energy supply, yielding a food per mass ratio below the threshold, negative specific growth rates occur. This indi- cates an energy deficient system and the daphnid population responds by experiencing high mortality rates, reduced fecundity rates, and decreasing population biomass. Completing the downward cyclic phase, the reduced daphnid population biomass and its associated decreased energy demand is underutilizing the culture system's capacity to supply available energy. The food per mass ratio increases to levels above the threshold and positive specific growth rates occur. This indicates the culture is able to support additional daphnid biomass production and the population responds by increasing its intrinsic birth rate and body growth and population biomass increases. The population maintenance threshold food per mass ratios were calculated with Equation 8. Prior to estimating the population main- tenance threshold food per mass ratios, necessary terminology substitu- tions in Equation 8 were made; (0) for (AID/T) and log (food per mass) for log (C). The population maintenance threshold food per mass ratios were computed for the duplicate 40,000 and 80,000 algal cells/ml cultures and are given in Table 7 with their respective coefficients of determination (r2). Theoretically, it would be expected that the population main- tenance threshold food per mass ratios calculated for each daphnid population supplied either algal cell food concentration would be equivalent. However, as observed in Table 7, the population maintenance threshold food per mass ratios calculated for the 40,000 algal cells/ml food concentration cultures are slightly higher than those calculated 86 00.0 00.0 00.0 00.0 N0 0000.0 0000.0 0000.0 0000.0 00\0< 08\00000 00000 000.00 08\00000 00000 000.00 08\00000 00000 000.00 08\00000 00000 000.00 0 000000 0000000 0000000000 0 000000 00000000 0000000000 .AN0V 0000000800000 00 000000000000 0000000000 000 000 0000>o00 0004 .00000000 0000000000 0000000 0000000000000 0000 08\00000 00000 000.00 000 000.00 000000000 000 000 0 00000000 00000 0008000 0000000000 0000000 000 00000000 0008000 00000 000 00 00000000 0 00 00000 003000 00000000 0000000000 0000000 00000008 8000 00>0000 000000 0008 000 0000 000000000 00000000008 0000000000 .0 00008 87 for the 80,000 algal cells/ml cultures. This measured discrepancy might occur because of possible increased daphnid population energy requirements necessary to sustain elevated filtering activities at a more dilute algal cell suspension (40,000 algal cells/m1) than a more dense algal cell suspension (80,000 algal cells/m1). Regardless, the daphnid population cultured at either algal cell suspension appear to require approximately 29 percent of the population's body biomass in food per day when the populations are existing at theoretical equilibrium conditions. Other suggestions as to why a slight differ- ence exists between the two sets of calculated population maintenance threshold food per mass ratios involve technical laboratory experi- mental error or fundamental experimental aberrations due to inherent biological variability. The empirical threshold corrected hyperbolic model (Equation 5) was used to predict daphnid population specific growth rate as a func- tion of algal biomass supplied per existing daphnid population biomass. To this model were fit measured daphnid population specific growth rates in relation to observed food per mass ratios obtained for both the 40,000 and 80,000 algal cells/m1 cultures at each census. To correctly apply Equation 5 for this fit, it was necessary to substitute for the appropriate terminology in the model; (u) for (A/D/T), (“max.) for (AID/Tmax.)’ (AB/DB) for (C), (AB/DBq) for (Cq), and (RAB/DB) for (KC). AB/DB was defined as the food per mass ratio. The linear transformation given by Equation 9 was used to cal- culate the kinetic rate constants; (“max.) and (RAB/DB). The popula- tion maintenance threshold food per mass ratio values used were those 88 in Table 7. Kinetic rate constants for Equation 5 calculated for both duplicate sets of daphnid population cultures are given in Table 8, along with the associated coefficients of determination (r2). The population maintenance threshold corrected hyperbolic curves illustrat- ing predicted daphnid population specific growth rates computed as a function of algal biomass supplied per daphnid population biomass are presented as the curves in Figures 19a and b. Unique to this method of predicting daphnid population specific growth rates, is the ability to relate daphnid population specific growth rate to some function of resource availability. In this context, daphnid population dynamics expressed as specific growth rates are related to potentially controlling environmental factors such as the relative supply of some identified limiting resource. For this study, food was established as the limiting factor regulating daphnid popula- tion growth or biomass production. In the presented population speci- fic growth rate model however, the limiting resource term is seen to be the food per mass ratio rather than absolute food concentration. Demonstrating the importance of this relationship between available food and daphnid population biomass as observed for the viable daphnid populations, a population maintenance threshold food per mass ratio (AB/DBq) was estimated for the daphnid population cu1= tures existing at equilibrium (specific growth rate equalling zero). This ratio represents maximum daphnid population biomass that can be sustained given a defined concentration of food supplied daily to the daphnid population cultures (Table 7). Whenever total daphnid popula- tion biomass increased to cause food per mass ratios to fall below the 89 00.0 00.0 00.0 00.0 N a m~0~.o Hoom.o Noo~.o 0000.o ma\m< «NN.0 m¢~¢.o o00.0 00¢.0 mo\m<0 . 0005 00.0 0o.o ma.o 00.o : He\m000u 00000 ooo.o0 0a\00000 00000 ooo.oq 0a\00000 00000 ooo.om 0a\0000u 00000 ooo.oq 0 000000 0000000 0000000000 0 000000 0000000 0000000000 .0000 00000008 000 00 000 00 00008 000 000 0000000000 .Aficmn\m00 0000 00\0< I 00\0< z I z .0 00000000 000000080 00008 000000080 0000000000 000000000 000000000 000 000 0000>000 00000000 0000000000 0000000 0000000000000 0000 00x00000 00000 000.00 000 000.00 000000000 000 8000 00000000 0000000 0000000000 0000000 000 00000000 0008000 00000 00 00000000 0 00 00000 003000 00000000 0000000000 0000000 00>00000 8000 00>0000 000000000 0000 0000000 .m 00000 90 theoretical threshold, negative population specific growth rates were calculated and observed (Figures 19a and b). When total daphnid population biomass decressed yielding food per mass ratios above this threshold value, positive population specific growth rates were calculated and observed (Figures 19a and b). This dynamic pattern results in a continuous oscillating p0pu1ation growth and decline cycle with daphnid population biomass fluctuating around the theoreti- cal population biomass equilibrium quantity represented by the popula- tion maintenance threshold food per mass ratio derived for each population culture system for a certain level of food availability (Figure 18). Therefore, the food per mass ratio represents the primary controlling factor existing in this experimental phase. Integral to this discussion of daphnid population dynamics as a function of food availability, is the quantification of the amount of food consumed by each mass specific daphnid size class in a day at various phases of the population cycle utilizing Equation 6 and the filter-feeding kinetic rate constants obtained in the first experimental section for both conventional and mass normalized filter-feeding rate units (Table 1). Of equal value is the relative daphnid size specific mass normalized food consumption efficiencies at suppressed food con- ditions affecting size specific daphnid survival expressed by calcu- lated relative mass normalized respiration rate efficiencies (Figure 10). To perform this exercise, a representative population biomass apex and trough (Figure 18) were selected for the 40,000 and 80,000 algal cells/ml cultures after the culture populations had attained their oscillating steady state existence. 91 Table 9 gives the relative statistics, parameters, and size- specific daphnid population composition distributions obtained from the respective census dates when daphnid population biomass is high (apex) and when it is low (trough) for all four daphnid population cultures used in this analysis. For each individual daphnid size lass present in the population for either biomass level, the amount of algae consumed per hour was determined with the kinetic rate con- stants. Following the discrete measurement of algal cell consumption for all individual daphnid size classes multiplied by the number of daphnids in each size class, total algal cell consumption for the population per hour was obtained and subtracted from the original algal cell concentrations supplied to each population. This adjusted food concentration represents the quantity of algae available for consump- tion beginning the second calculated hourly feeding interval. This iterative feeding rate process was continued until the algal cell concentration was reduced to less than 1,000 algal cells/ml. Iterative algal cell consumption computation below this food concentration was demonstrated to have a minimal quantitative effect on established daphnid size class consumption values. Concluding the final iterative food consumption calculation, discrete algal cell consumption for each daphnid size class per hour was summed for the entire feeding interval, divided by the number of daphnids composing each individual size class, and algal cell concen- tration converted to mass (3.00.X 10"8 mg[§.‘£ginhardi algal cell). A subsequent mass conversion involved normalizing discrete daphnid algal cell consumption per day by mean individual daphnid size class 111388 . 92 m~.o O \‘f OOMHNO‘QH mm ma ma mm.H mH.H \TINO‘O Na ma NH Hm.a mm.o HHMCNQMO H Hm mm.H mm.o O‘ N NOQONQNH HA m~.H Away mamaofin coaumaaaoa noses: coaumasaoa SESEEEEE Hones: can mmmHo mufim AmHmBQM\mmwov mafia nouaao cmuofivmua wwwm Hmuou mafia m>wuo=coummu madmamw Hones: moamamm maouoww>o “wasps Adamauw\mmwmv mnan nousao some Aaav madmamm msouomw>o nuwcma momamumo some anmsouuv ooo.om Ammamv ooo.om anwsouuv ooo.os Axoamv ooo.o¢ .naowumuucoocoo boom Ha\nHHmo Human ooo.om paw ooo.oq nuon now Asmaouuv 30H com Axoamv swan mums mau>ma mmmaown coaumasaoo cons momma mamamo m>wumucmmounmu aouu vocawuao muaumuumum can «woumawumn coaumasaoa wasnamv vmusuaso womammo: .m manna 93 Illustrated in Figure 20a, is the mass of algae consumed per daphnid per day for the categorized daphnid size classes. As would be expected, the largest daphnid size classes because of their quantitatively superior filter-feeding rate capacities on an indivi- dual basis consumed more algal biomass than the smaller daphnid size classes at either high or low population biomass levels. However, all daphnid size classes are observed to consume a larger biomass of algae when population biomass levels are low (troughs in the popula- tion curves in Figure 18), than when population biomass levels are high (apexes in the population curves in Figure 18), regardless of supplied food concentration. Normalizing the mass of algae consumed per daphnid size class individual per day by the mass of the average daphnid size class individual for both high (apex) and low (trough) population biomass levels, yields Figure 20b. By normalizing food consumption by daphnid body biomass, the relative competitive efficiencies of the various daphnid size classes can be exhibited when food is suppressed at higher levels of population biomass and when food is abundant at lower levels of population biomass. As observed in Figure 20b, the intermediate daphnid size classes possess the most competitive and efficient daily mass normal- ized food consumption rates regardless of population biomass levels at either food concentration. Also demonstrated in Figure 20b, is the considerably increased quantity of food consumed by each daphnid size class when population biomass is low as compared to when population biomass is high. This difference in food consumption rates, as 94 .munwfim m>auomammu sumo Ga .Ha\maaoo Howam ooo.om vmfiHaasm momaoan coaumasaom swag mucmmmuawu a m>uno mam .HE\mHHmo Human ooo.oq nmfiaaanm mmmaoan coaumasnoo swan musmmouamu u o>usu .Ha\maaoo Amman ooo.ow vwaaamam mmmaoan cofiumasmoa sod mucomouamu m m>uso .Ha\maamo Amman ooo.oc umfiaaaom nmmaoan cowumasaom 30H mucmmmuamu < m>u=o .hmv comm Ha \maaou Human ooo.om can ooo.os uoaflaaam as magmas mao>ma mamaoan aosumaaaoa cascamv vouauaao 30H was swan mumowaasv you A manna ca sm>ww mucoumsoo mumu ouumsax was com o cowumaum mafia: vocfiahmumv Aaav :uwoma mommmumo mo coauocaw m on mugs: mums wcwvmow Anew unawam .hmu\c«:5amp wa\mmwam wav vmuwamahoa mama can Amom ouawam .%mv\v«:5amv\mmwam may Hmcowuam>aou :uon :« soauaasmcoo coon vacnamv madam tonnaooama cmmzumn mfinmcowumamm 2:... .55 .n can now newsman ...—.0qu uoHw musmumcoo oHuoaHx mum» wsHvoom om>Huwo waHmHaam mHm>oH mmoEOHn =0HumHanoa H.4w50puv aoH can Axmamv an5 now a oHan cH wouommwua mono oOHunnHuumHo muHm coHuHmoaaoo GOHuoHsnoa vHsnamv wcha :OHuaaamooo ooou Hmuou zHHmv omuHHmmu wo>umwno can HmHusouom anameE ooumHaoHoU .OH oHan 103 mm~.Ho~ mom.HmH.m H = «mm.mmm mmo.a~m.s N o NHH.os~.H osm.mmm.aH m a soc.om~ sH~.m-.s N m omm.mom moo.ooa.~ o a ~mm.m- «sm.ms~.m m o o m moo.moH on.cm~.H N 4 Axmv\mHHoo HmMHov Azmu\mHHmo Homev Honaaz oHsnmmn AEEV ouHm H\a\< omuHHmoa . ”N5 a\n\< HmHuaouom meamv Ha\mHHmo Hmem ooo.oq . o z meq.HmH.H ~mo.mm~.o m o o~m.~mH.H omo.Haa.m s m mo~.mmm oH~.m-.s N m o a NoH.~nm qo~.nom.~ m o eNH.Hm mm~.mha H m som.mHs mms.mo~.o HH < Azmu\mHHmo HmmHmv Azmv\mHHoo Hmemv umnaaz vHaann AESV ouHm a\n\< ooNHHmum .xaa H\G\< HmHucouom Hamaounv Ha\mHHou HmmHa ooo.oq A. . emaeHucoov oH «Hams 104 biomass. Clearly, all daphnid population parameters censused were influenced by food concentration. Two indices indicative and sensitive to food availability were measured daphnid population Speci- fic growth rate and the observed relative quantities of accumulated or maternally allocated residual energy reserves possessed by the various experimental daphnid size classes. Since daphnid population specific growth rates were calculated in terms of population biomass production rates in a culture system where supplied food quantities were constant, it was possible to relate p0pulation specific growth rate as a function of food per mass. As presented earlier, when positive rates of population biomass changes were recorded (reflective of low population biomass and abundant food availability), food per mass ratios measured exceeded the esti— mated theoretical population maintenance threshold food per mass ratio. During this period of positive rates of daphnid population biomass pro- duction changes, daphnids of all size classes were observed to possess increased amounts of visible residual energy reserves (lipid deposit formation). This physiological phenomenon is characteristic of popula- tion biomass growth intervals when food is abundant. However, when daphnid population biomass production surpassed the culture system's carrying capacity (indicating high population biomass and scarce food availability), measured food per mass ratios descended below the estimated theoretical population maintenance thres- hold food per mass ratio. This suggests that the food mass supplied to the culture system is insufficient to maintain the existing excess daphnid population biomass and negative rates of population biomass production occur as a consequence of population starvation. 105 Providing energetic support to daphnid population biomass overproduction when food availability per daphnid population biomass was progressively decreasing, was the metabolization and subsequent depletion of previously accumulated and maternally allocated residual energy reserves. Utilization of stored or conferred lipid deposits desensitizes the daphnid population to rapidly declining food condi- tions and provides energy to stimulate excess reproduction and grazing pressures beyond the culture system's limited energy capacity. This temporary support of excess daphnid reproduction and grazing pressures eventually creates starvation food levels and the daphnid population begins to crash. Despite the apparent function that residual energy reserves have in causing negative daphnid population biomass production rates and possible population extinction, residual energy reserves were observed to possess a redeeming quality by providing a deposit from which necessary energy can be extracted to sustain the most efficient daphnid size classes through periods of food scarcity ensuring the population's continued existence. Daphnid population dynamics during the period when the popula- tion is characterized by oscillating conditions of high and low daph- nid population biomass levels and abundant and scarce food availability, could be considered the daphnid's vacillating steady-state existence. Daphnid population oscillations stimulated by organismal time lags induced by residual energy reserve accumulation or allocation and sub- sequent use delay daphnid physiological state response to the prevail- ing environmental conditions. Lipid deposit formation or allocation 106 and subsequent utilization corresponding to existing daphnid popula- tion food per mass ratios varies around the population maintenance threshold food per mass ratio. Characteristic of growing daphnid population cultures are great latent unrealized grazing, growth, and reproductive potentials that only exhibited when algal biomass availability appreciably exceeds daphnid population biomass. As demonstrated, daphnid population cul- tures, once they attain their characteristic oscillating steady-state existence, are confined to the constraints of a population induced food limited environment as indicated in their sub-optimal population parameters mentioned previously. This latent potential represents a considerable unrealized population response mechanism should food availability increase dramatically. It is these unrealized grazing, growth, and reproductive poten- tials which allows filter-feeding cladocerans to maintain the low phytOplankton abundance critical to biomanipulation of eutrophic lakes. It appears that once these cladocerans reach their oscillating steady- state and the accompanying low phytoplankton abundance, their signifi- cant unrealized grazing, growth, and reproductive potentials inhibits the development of blooms of any phytoplankton which can be ingested and assimilated by the cladocerans. Thus, if piscivorous fish can maintain control of zooplanktivorous fish predation upon filter-feeding cladocerans, the cladocerans will maintain phytoplankton biomass at desirable levels. CONCLUSIONS Feeding rate kinetics of 2, pulex fed 9, reinhardi at 27.C. can be empirically modeled to fit a Michaelis-Menten type hyperbolic function. Filtering rate kinetics of Q, pulex fed 9. reinhardi at 27 C. can be empirically modeled to fit a negative exponential function. There was no distinct experimental evidence supporting a feeding or filtering rate empirical model threshold food concentration correction for 2, pulex fed 9, reinhardi at 27 C. Daphnid filtering rates can be predicted from a multivariate functional relationship of both algal cell concentration and daphnid carapace length. Intermediate daphnid size classes were observed to possess the most efficient and competitive intra-specific mass normalized filter- feeding and respiration rate kinetics at low food concentrations. All measured daphnid feeding and filtering rate kinetic quantities were observed to differ with daphnid carapace length and therefore, body mass. At zero algal cell concentration food levels, daphnid body growth occurred, apparently supported by maternally allocated energy reserves possessed by the neonates prior to experimental inoculation. 107 10. ll. 12. 13. 108 To predict individual daphnid specific growth rates as a function of algal cell concentration, average measured daphnid specific growth rates and corresponding algal cell concentrations were fit to the threshold food concentration corrected hyperbolic empirical model. It was demonstrated, that daphnid individual specific growth rates increase with increasing algal cell concentration approaching an asymptote at 100,000 algal cells/ml beyond which the rate of daphnid specific growth as a function of increasing food concen- tration decreases. Daphnid fecundity (eggs per female per clutch), can be predicted from a multivariate functional relationship of both daphnid carapace length and algal cell concentration. The measured accelerated daphnid growth and prolific fecundity as a function of carapace length, at elevated food concentrations explains the ability of 2, pulex to explicit favorable food condi- tions and to rapidly incorporate net energy gain into biomass production. Residual energy reserves, maternally allocated or accumulated in the hemocoel of daphnids when food is abundant, are later meta- bolized when food is scarce to temporarily sustain daphnid basal metabolism, body growth, and reproduction. Observed high size-specific daphnid mortality rates exhibited by the largest and smallest daphnid size classes at high population biomass levels when food is scarce, is due to their inferior and 14. 15. 16. 17. 18. 109 inefficient mass normalized filter-feeding and respiration rates, exacerbated by deficient maternally allocated and accumulated energy res erves o All daphnid size classes composing a population culture were calculated to obtain a larger quantity of algal biomass per day when daphnid population biomass levels were low than when popula- tion biomass levels were high, with the most efficient intermediate daphnid size classes consuming the most algal biomass at either population biomass level. Daphnid population specific growth rates along with other measured population dynamic parameters all varied as an interactive functional relationship of the calculated food per mass ratio and therefore, the food per mass ratio represents the primary controlling factor regulating daphnid population dynamics. Daphnid populations cultured at the 40,000 and 80,000 algal cells/ml food levels appear to require 29 percent of the total body biomass in food per day just to maintain the population. An empirical threshold corrected hyperbolic model was applied to predict daphnid population specific growth rate as a function of the food per mass ratio. Total daphnid population biomass rather than daphnid population numbers appeared to be a more biologically meaningful measure upon which to express population dynamic phenomena in the unstable experimental daphnid assemblages. 19. 20. 21. 22. 23. 24. 25. 110 Sufficient allocated maternal energy reserves are necessary at limiting food concentrations to sustain the relatively inefficient neonate until it develops into a more energetically efficient and competitively superior intermediate sized daphnid. With the renewed batch culture system, it was demonstrated, that the critical p0pu1ation threshold food concentration for 2, pulex fed g. reinhardi at 27 C. lies between 20,000 and 40,000 algal cells/ ml. To provide an accurate estimate of a daphnid population's threshold food concentration, it is not enough to measure only egg production as a function of food density, for to maintain daphnid population growth, the eggs must be able to survive and in turn, produce viable neonates . In the populations of Q, pulex studied, a constant population bio- mass equilibrium is not realized because there is a time lag be- tween the decrease of food concentration and the delayed response of decreasing birth rates. The mechanism responsible for the time lag provoking population oscillations has been identified as accumulated and maternally allocated lipid deposits. From the presented data it appears that a continuing population bio- mass oscillation is as close as 2. pulex gets to a steady-state equilibrium. The average food consumption value of all daphnid size classes was 38 percent of daphnid body weight per day at low population 26. 27. 28. 29. 30. lll biomass where population biomass was increasing. At high population biomass the average food consumption value of all daphnid size classes was 21 percent of daphnid body weight per day when population biomass production was decreasing. Realized average daphnid population daily food consumption never exceeded 13 percent of the calculated potential daily food consump- tion, even when daphnid population biomass levels were low. By having to persist under less than optimum food conditions, which the daphnid population creates for itself through excess grazing and reproductive pressure supported by residual energy reserves, daphnid growth, reproductive, and filter-feeding potentials are inhibited and a population of stunted daphnids develops. Utilization of stored or conferred lipid deposits desensitizes the daphnid population to rapidly declining food conditions when population biomass is high and provides energy to stimulate excess reproduction and grazing pressures beyond the culture system's limited energy capacity creating starvation food conditions and subsequent negative population biomass production rates. Despite the apparent function that residual energy reserves have in causing negative daphnid population biomass production rates and possible population extinction, residual energy reserves were observed to possess a redeeming quality by providing a deposit from which necessary energy can be extracted to sustain the most effici- ent daphnid size classes through periods of food scarcity ensuring the population's continued existence. 31. 32. 33. 34. 35. 36. 112 Daphnid population dynamics during the period when the population is characterized by oscillating conditions of high and low daphnid population biomass levels and abundant and scarce food availability, could be considered the daphnid's vacillating steady-state existence. Daphnid population oscillations stimulated by organismal time lags induced by residual energy reserve accumulation or allocation and subsequent use delay daphnid physiological state response to the prevailing environmental conditions. Lipid deposit formation or allocation and subsequent utilization corresponding to existing daphnid population food per mass ratios varies around the population maintenance threshold food per mass ratio. Characteristic of viable evolving daphnid population cultures are great latent unrealized grazing, growth, and reproductive potentials that are only exhibited when algal biomass availability appreciably exceeds daphnid population biomass. The unrealized grazing, growth, and reproductive potentials of 2, pulex at dynamic population equilibrium allows these filter- feeding cladocerans to maintain the low phytoplankton abundance critical to biomanipulation of eutrophic lakes. Maternally allocated energy reserves provided to the neonate as a function of food available to the adult was identified as the critical intrinsic physiological mechanism determining the viability of a filter-feeding cladoceran population when zooplanktivorous fish predation is reduced. 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