Phosphorus (P) is generally considered to be the main limiting nutrient to freshwater phytoplankton productivity. However, recent research is drawing attention to the importance of nitrogen (N) in freshwater eutrophic...Phosphorus (P) is generally considered to be the main limiting nutrient to freshwater phytoplankton productivity. However, recent research is drawing attention to the importance of nitrogen (N) in freshwater eutrophication and N often constrains growth of cyanobacteria in small lakes. In this study we determined phytoplankton nutrient limitation in a large lake, Lake Erie during two growing seasons. During 2010 and 2011, nutrient enrichment bioassays (+P, +N and, +P and N) were conducted monthly from June through September with water collected in Maumee Bay (site MB18) and in the center of the western basin (site WBC). Nutrient concentrations were monitored every other week. At MB18, total P concentration was often >3 mmol/L and nitrate concentration decreased from >250 mmol/L in early summer to mmol/L in late summer. Nitrogen and P levels were about five-fold less at WBC. Bioassays indicated that phytoplankton nutrient limitation varied in summer, spatially, and even among phytoplankton groups. For site MB18, +P increased chlorophyll concentration in one of the eight bioassays, indicating that P did not typically limit production. For site WBC, +P increased chlorophyll concentration in six of the eight bioassays. As a result of very low ambient nitrate concentration (mmol/L) in late summer, +N (without P) increased chlorophyll concentration, suggesting symptoms of N-limitation. The N-fixing cyanobacterium Anabaena became dominant following N-limitation. This study highlights the need to reduce P loading to restore water quality. Furthermore, due to low nitrate concentration, the severity of the cyanobacterial blooms could be worse if not for N-limitation in western Lake Erie.展开更多
Increased water clarity associated with zebra mussel （Dreissena polymorpha） populations may favor benthic algal primary production in freshwater systems previously dominated by pelagic phytoplankton production. Whil...Increased water clarity associated with zebra mussel （Dreissena polymorpha） populations may favor benthic algal primary production in freshwater systems previously dominated by pelagic phytoplankton production. While zebra mussel-mediated water clarity effects on benthic primary production have been implicated in published reports, few production estimates are available. This study estimates benthic primary production in Oneida Lake, NY before and after zebra mussel invasion （1992）, using measured photosynthetic parameters （P^Bmax, α^β and β） from sampled benthic algal communities. In the summers of 2003 and 2004, primary production was measured as O2 evolution from algal communities on hard （cobble） and soft （sediment） substrate from several depths. We also backcast estimates of benthic primary production from measurements of light penetration since 1975. Estimates of whole-lake epipelic and epilithic algal primary production showed a significant （4%） increase and exhibited significantly less interannual variability subsequent to the establishment of zebra mussels. We applied our model to two lakes of differing trophic status; the model significantly overestimated benthic primary production in a hypereutrophic lake, but there was no significant difference between the actual and predicted primary production values in the oligotrophic lake. The hypereutrophic lake had higher zebra mussel densities than Oneida （224 vs. 41 per sample respectively）. Though total community respiration （measured in total darkness） was factored into our model predictions of production, our model may need modification when heterotrophic respiration is a large portion of total community metabolism.展开更多
文摘Phosphorus (P) is generally considered to be the main limiting nutrient to freshwater phytoplankton productivity. However, recent research is drawing attention to the importance of nitrogen (N) in freshwater eutrophication and N often constrains growth of cyanobacteria in small lakes. In this study we determined phytoplankton nutrient limitation in a large lake, Lake Erie during two growing seasons. During 2010 and 2011, nutrient enrichment bioassays (+P, +N and, +P and N) were conducted monthly from June through September with water collected in Maumee Bay (site MB18) and in the center of the western basin (site WBC). Nutrient concentrations were monitored every other week. At MB18, total P concentration was often >3 mmol/L and nitrate concentration decreased from >250 mmol/L in early summer to mmol/L in late summer. Nitrogen and P levels were about five-fold less at WBC. Bioassays indicated that phytoplankton nutrient limitation varied in summer, spatially, and even among phytoplankton groups. For site MB18, +P increased chlorophyll concentration in one of the eight bioassays, indicating that P did not typically limit production. For site WBC, +P increased chlorophyll concentration in six of the eight bioassays. As a result of very low ambient nitrate concentration (mmol/L) in late summer, +N (without P) increased chlorophyll concentration, suggesting symptoms of N-limitation. The N-fixing cyanobacterium Anabaena became dominant following N-limitation. This study highlights the need to reduce P loading to restore water quality. Furthermore, due to low nitrate concentration, the severity of the cyanobacterial blooms could be worse if not for N-limitation in western Lake Erie.
文摘Increased water clarity associated with zebra mussel （Dreissena polymorpha） populations may favor benthic algal primary production in freshwater systems previously dominated by pelagic phytoplankton production. While zebra mussel-mediated water clarity effects on benthic primary production have been implicated in published reports, few production estimates are available. This study estimates benthic primary production in Oneida Lake, NY before and after zebra mussel invasion （1992）, using measured photosynthetic parameters （P^Bmax, α^β and β） from sampled benthic algal communities. In the summers of 2003 and 2004, primary production was measured as O2 evolution from algal communities on hard （cobble） and soft （sediment） substrate from several depths. We also backcast estimates of benthic primary production from measurements of light penetration since 1975. Estimates of whole-lake epipelic and epilithic algal primary production showed a significant （4%） increase and exhibited significantly less interannual variability subsequent to the establishment of zebra mussels. We applied our model to two lakes of differing trophic status; the model significantly overestimated benthic primary production in a hypereutrophic lake, but there was no significant difference between the actual and predicted primary production values in the oligotrophic lake. The hypereutrophic lake had higher zebra mussel densities than Oneida （224 vs. 41 per sample respectively）. Though total community respiration （measured in total darkness） was factored into our model predictions of production, our model may need modification when heterotrophic respiration is a large portion of total community metabolism.
