Assessing the effects of nitrogen deposition on mountain waters: A study of phytoplankton community dynamics

We assessed the phytoplankton communities and the relevant aquatic chemistr y in three large lakes in the Greater Yellowstone Ecosystem. While N limita tion of phytoplankton is most common, it appears that a recent regional inc rease in atmospheric N deposition is causing both P and Si limitation to oc cur to some degree. N additions in semi-continuous laboratory bioassays of mixed diatom assemblages produced a dramatic increase in biomass in two of the three study lakes. Relative abundances of species were altered relative to control treatments with both added N and Si. Higher levels of N primari ly favored the alga Fragilaria crotonensis. This result is consistent with previous laboratory and field observations which suggest that F. crotonensi s is not a good competitior for N, and only thrives in N rich environments. We hypothesize that continued increases in N loading will alter natural sp ecies assemblages in all the study lakes. (C) 1998 Published by Elsevier Sc ience Ltd. All rights reserved.