Late fall phytoplankton dynamics in three lakes, Rocky Mountain National Park

We studied phytoplankton population dynamics during the month preceding formation of ice cover in three small subalpine lakes in Rocky Mountain National Park, Colorado, U.S.A. The outflow from Emerald Lake, which is surrounded by talus, flows into Dream Lake, which is surrounded by sub-alpine forest. Nymph Lake is a lower seepage lake with abundant macrophytes in summer. The major ion concentrations in the three lakes were similar during the study, although Emerald and Dream Lakes had higher concentrations of nitrate and silica than Nymph Lake. A principal component analysis (PCA) showed that the phytoplankton in Emerald and Dream Lakes were distinct from the phytoplankton in Nymph Lake. The species composition changed in each lake during the late fall. The patterns of change in Emerald and Dream Lakes were similar on the PCA diagram despite the greater abundance of diatoms in Dream Lake and the decreasing flow from Emerald Lake into Dream Lake during the fall. In Nymph Lake, a progressive shift in species distribution occurred with a decrease in the most abundant chlorophyte, Chlamydomonas sp., and increases in several species, including two chrysophytes and the diatom Eunotia sp. The marked change in species composition in all three lakes suggests that phytoplankton populations are influenced by changes in water temperatures and incident solar radiation that occur during the late fall. We also compared these data with phytoplankton data for two fall periods from two other hydrologically connected Rocky Mountain lakes. PCA analysis showed that the difference between years was greater than the change during the fall and that the fall species composition in these two lakes was distinct from that in Emerald and Dream Lakes or in Nymph Lake. Studying phytoplankton dynamics in alpine and sub-alpine lakes may offer clues as to how these ecosystems may respond to projected climate changes in the Rocky Mountain region, such as warmer temperatures and later formation of ice-cover.