Algae are important to many of the processes that characterize wetland ecosystems. Despite their importance, we know relatively little about the factors that regulate algal communities in wetlands. This is particularly true for northern boreal regions where wetlands are abundant and are considered to be extremely vulnerable to disturbances associated with climate change. My dissertation research investigates how nutrients, grazing, light, and hydrology regulate algal primary production and taxonomic structure in high latitude wetlands. I documented spatial and temporal variability in algal structure and function in six northern boreal wetlands in interior Alaska to determine the contribution of algal primary production to wetland primary production and evaluate the relationships between environmental parameters characteristic of different wetland types and algal productivity, abundance, and taxonomic composition. There were significant spatial and seasonal differences in algal biomass and taxonomic composition both within and between wetlands. Measures of algal productivity were equivalent, and sometimes higher, than macrophyte production, contributing up to 50% of wetland primary production. Water depth and nutrients were significant predictors of algal taxonomic composition. In Chapter 3 I evaluated the potential for grazers to regulate benthic algal communities in boreal wetlands with future increases in nutrient concentrations using an in situ mesocosm experiment in an Alaskan marsh. I tested the hypothesis that nutrient enrichment stimulates algal accumulation and grazers regulate algal responses to nutrients by suppressing algal accumulation but increasing productivity through nutrient recycling. My results support my hypothesis and provide some of the first evidence that nutrients and grazing are important factors regulating benthic algal biomass and community composition in a northern boreal wetland. My data also suggest the potential importance of consumer-driven nutrient recycling to algal productivity and wetland biogeochemistry. In Chapter 4 I investigated how changes in light availability, in the presence and absence of grazing, limit the ability for algae to respond to nutrient inputs using an in situ mesocosm experiment. I hypothesized that light limitation and herbivory are both responsible for regulating the algal response to nutrient enrichment. My analyses indicate that grazers are responsible for maintaining low algal biomass even with increased nutrient availability, not light. In the final chapter, I evaluated how changes in hydrology predicted with climate change might influence algal community structure in boreal peatlands. I monitored shifts in algal taxonomic composition in response to an ecosystem-scale water table manipulation, including both drought and flooding conditions in a rich fen in interior Alaska. The observed changes in algal taxonomic composition in response to fluctuations in water table suggests that increased frequency of drought and flooding events expected with climate change may significantly alter algal structure and function in boreal wetlands. Furthermore, my results suggest the potential importance of nitrogen-fixation by cyanobacteria to wetland biogeochemistry. The results of this dissertation improve our understanding of the abiotic and biotic factors that regulate benthic algal productivity and taxonomic composition in northern boreal wetlands. My results indicate that algae are an ecologically important component of wetland ecosystems. These results have important implications for energy flow in boreal wetlands and indicate that the current wetland structure and function may be altered by climate change.
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