Phytoplankton Responses to Lake Nitrogen Enrichment in Mount Rainier, North Cascades, and Olympic National Parks

Williams, Jason; Beutel, Marc

The National Park Service (NPS) is mandated by the Organic Act, Wilderness Act, and 1977 Clean Air Act amendments to protect national parks from adverse effects of air pollution. In the Pacific Northwest, NPS has identified a need for information about the effects of atmospheric nitrogen deposition on park resources. Nitrogen emitted through fossil fuel combustion and agricultural activities can be transported through the atmosphere and deposited on parks as wet or dry nitrogen deposition. Elevated nitrogen deposition can reduce biodiversity, alter species composition, alter biogeochemical cycling, and contribute to acidification in both aquatic and terrestrial ecosystems. In many mountain lakes throughout the northern hemisphere, nitrogen deposition has increased lake nitrate concentrations, thereby stimulating changes in phytoplankton abundance and community composition. It is not yet clear if nitrogen deposition has caused widespread changes in Pacific Northwest mountain lakes. Previous investigations found evidence for deposition-induced phytoplankton changes in only one lake (Hoh Lake at Olympic National Park), and highlighted a need to better understand factors affecting lake sensitivity to nitrogen deposition. At least three conditions must be met for nitrogen deposition to induce phytoplankton changes in mountain lakes. Nitrogen deposition inputs must increase lake nitrogen concentrations, phytoplankton growth must be limited by nitrogen, and lake nitrogen concentration increases must be sufficient to stimulate phytoplankton growth. In this study, we tested these three conditions for mountain lakes at Mount Rainier (MORA), North Cascades (NOCA), and Olympic (OLYM) national parks. We conducted nutrient enrichment experiments in three lakes at each park to determine if phytoplankton growth is nitrogen limited and quantify lake nitrogen concentration thresholds necessary to stimulate phytoplankton changes. Results indicate phytoplankton species and biomass growth was nitrogen-limited or co-limited by nitrogen and phosphorus. Dissolved inorganic nitrogen (DIN=NO3-N + NH4-N) concentrations of 13-25 μg N L-1 were necessary to stimulate an increase in lake phytoplankton biomass, and DIN concentrations 0.02-50 μg N L-1 can stimulate growth of nitrogen-sensitive species, depending on species. We compared minimum concentration necessary to increase phytoplankton biomass (13 μg N L-1) to concentrations observed in previous lake surveys at the parks. This threshold is exceeded in at least 27% of sampled lakes at NOCA, 26% of sampled lakes at MORA, and 0 out of 9 sampled lakes at OLYM. Phytoplankton in mountain lakes appear highly sensitive to future increases in lake nitrogen. It is not clear if nitrogen deposition strongly affects lake nitrogen concentrations. We tested for a correlation between nitrogen deposition rates and lake nitrate across MORA, NOCA, and OLYM using lake chemistry data from previous surveys. We observed a weak positive correlation between deposition and lake nitrate across the three parks. However, many lakes with similar deposition rates had nitrate concentrations that differed by orders of magnitude. This result implies that watershed characteristics such as land cover type, slope, and geology mediate the relationship between nitrogen deposition and lake nitrate. Additional research is needed to establish if future increase or decrease in nitrogen deposition will increase lake nitrogen concentrations, and to identify lakes where deposition-induced concentration increases are most likely.

Phytoplankton Responses to Lake Nitrogen Enrichment in Mount Rainier, North Cascades, and Olympic National Parks

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