Environmental controls on algal community structure and function in boreal wetlands.

摘要

Compared to lakes and streams, we know relatively little about algal ecology in freshwater wetlands. This discrepancy is particularly acute in boreal regions, where wetlands are abundant and processes related to climate change are expected to alter the hydrology, pH and nutrient concentrations of aquatic ecosystems.;To evaluate how accelerated nutrient inputs might affect algal structure and function in boreal wetlands, I enriched mesocosms in an Alaskan marsh with all possible combinations of nitrogen (N), phosphorus (P) and silica (Si). Algal growth was not stimulated following enrichment with any nutrient alone or with P and Si together, but increased significantly with the addition of N in any combination with P and Si. Water column concentration of dissolved organic carbon (DOC) increased linearly with algal biomass. The wetland algal community shifted from one dominated by Euglena and Mougeotia to small coccoid taxa (Chroococcus and Gloeocystis) and Nitzschia inside N-combination treatments. This study provides several lines of evidence for co-limitation, and the central importance of N as a co-limiting nutrient for the wetland algal community. Changes in algal dynamics with increased nutrient concentrations could have important implications for wetland food webs, and suggests that algae may provide a link between increasing nutrient inputs and altered wetland C cycling in this region.;In chapter 3, I examined the response of an algal community to a wide range of pH levels, both decreased and increased from ambient conditions in an Alaskan marsh. Alkalinization resulted in an increase in the concentrations of inorganic nutrients and a significant increase in algal growth. There were distinct shifts in euglenoid taxa in the alkaline treatment, including an increase of Trachelomonas and a decrease of Euglena relative to the control. Acidification resulted in an increase of Mougeotia (Chlorophyta, Zygnemataceae) and a decrease in overall taxa richness, which coincided with a significant reduction in concentrations of dissolved inorganic C. Trends observed in this study indicate that alkalization may significantly alter algal community structure and loosen nutrient constraints on algal productivity, while acidification may reduce algal diversity in boreal wetlands.;In the final chapter, I monitored algal responses to a water table manipulation in an Alaskan fen to evaluate how changes in hydrology might affect C cycling and energy flow in boreal peatlands. I measured consistently higher algal productivity at sites exposed to seasonal drying than in ambient or flooded conditions. Approximately 18% of the C fixed by algae during photosynthesis was released into the overlying water column as DOC. At this rate, algae were contributing more than 6 84 mg DOC L-1 h-1 to the water column during peak productivity in the drought treatment. Approximately 25% of the algal exudates were carbohydrates, about 38% of which was glucose. I examined the biodegradability of algal exudates and found they were extremely labile, decreasing by more than 55% within the first 24 hours of incubation. Bacterial growth increased rapidly in the presence of algal exudates. These findings show that algae have the potential to contribute a significant amount of labile C to boreal peatlands, and may become increasingly important for energy flow in these ecosystems if seasonal drying trends become more frequent with future climate change.