Cumulative effects of nutrients and pH on the plankton of two mountain lakes

We conducted enclosure experiments to examine the cumulative effects of nutrient enrichment and acidification on the plankton of two mountain lakes with differing nutrient conditions. The low-nitrate lake responded to N, N plus acid, and N plus acid plus P additions, showing four- to seven-fold increases in chlorophyll a, increased photosynthetic rate, compositional shifts toward large chlorophytes, and decreased zooplankton biomass. The high-nitrate lake responded minimally to either N or P alone but responded strongly to combined additions of N plus acid plus P, showing eightfold increases in chlorophyll a, increased cell density and photosynthetic rates, and compositional shifts toward chlorophytes and the dinoflagellate Gymnodinium. In both study lakes, changes in chlorophyll a were linked to addition of limiting nutrients regardless of pH, whereas shifts in phytoplankton species composition were apparently affected by both nutrient conditions and acidity. The most striking changes in species composition and biomass occurred in combined N plus acid plus P treatments, indicating that continued nutrient enrichment may interact with acidification to produce marked changes in the plankton of mountain lakes.