Although often small in area, shallow freshwaters are abundant in many landscapes, and their roles in watershed biogeochemical cycles are increasingly appreciated. However, shallow waterbodies have been far less studied than larger lakes and rivers. To understand the aggregate role of small waterbodies at watershed scales, their distinctive features need more investigation. Biogeochemical processes in waterbodies are strongly influenced by sediment-water interactions, which influence aquatic ecosystem metabolism and productivity, fluxes of nutrients to downstream ecosystems, and atmospheric greenhouse gas exchanges. The influence of the sediment-water interface (SWI) is especially great in smaller waterbodies because of their shallow depths and high biological productivity, and because they often receive high inputs of groundwater, nutrients, and detrital organic matter. Thick accumulations of flocculent sediments, or floc, are common in many shallow freshwaters that have productive aquatic and/or riparian vegetation and lack strong current or wave action. Loosely structured floc layers form a transitional zone that is potentially more connected to overlying water than relatively consolidated sediments, and are subject to dynamic variation in physical structure, thermal stratification, and reduction-oxidation (redox) status. Despite the prevalence of floc, its biogeochemical importance has been little studied. These sediments represent a potentially reactive SWI that can be considered an ecotone between overlying water, sediment porewater, and deeper groundwater. As with other better studied SWIs (e.g., deep lacustrine and marine waters, river hyporheic zones), floc sediments may play an important role in the biogeochemical cycles of freshwater ecosystems, and their distribution in shallow waters may be very extensive.In this dissertation I employ a broad array of approaches to reveal the biogeochemical importance of floc in shallow freshwaters. In Chapter 1, I describe the physicochemical properties of floc and examine which environmental features serve as predictors of floc thickness across a diverse set of shallow waterbodies in southwestern Michigan. In Chapter 2, I investigate organic matter decomposition rates in floc and quantify the temperature-sensitivity of this process. In Chapter 3, I consider the potential for floc to remove nitrate from overlying waters and compare nitrate removal rates for floc to rates reported in the literature for other common sediment types. Finally, in Chapter 4, I evaluate two physical exchange processes, diffusion and buoyancy-induced flow, using heat transport modeling and direct observation of flow to determine the drivers of solute exchanges between overlying waters and floc porewaters.This research confirms that floc is abundant in shallow freshwaters and reveals that floc can be distinguished by high organic matter percentages, low bulk densities, and high volumes of occluded gas bubbles compared with other organic sediments. Floc layers are active sites for decomposition despite persistent anoxia, and results suggest floc accumulations are sustained by particularly high rates of organic matter input rather than slow decomposition. Further, decomposition rates could increase 12-56% with a 1-4°C increase in water temperatures—a likely scenario for this region in the next 100 years. Third, floc has the potential to remove nitrate from overlying waters, but the limitations of our approach hinder our ability to conclude that removal rates are much greater than those measured with other sediments. Lastly, buoyancy-induced mixing of overlying water and floc porewaters is an important mechanism driving the exchange of heat and fluid across the SWI in shallow waterbodies, at least seasonally.
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