Response of the Phytoplankton Community in an Alpine Lake to Drought Conditions: Colorado Rocky Mountain Front Range, U.S.A

Abstract

Lakes may serve as sentinels for the impacts of changing climate in alpine areas. In the Rocky Mountain region, 2002 was a year with extremely low snowpack. We examined the summer phytoplankton community in Green Lake 4 for a 6-year period that included the summer of 2002. The phytoplankton community variation was examined in the context of the changes in physical and chemical properties of Green Lake 4. The physical changes associated with the 2002 drought included warmer surface water temperatures and greater hydraulic residence times; whereas the chemical changes included higher concentrations of acid neutralizing capacity (ANC) and major ions. During the summer of 2002 the phytoplankton community was dominated by Synedra sp. and Ankyra sp.; two previously rare species. The growth of Synedra sp. was sufficient to cause a decrease in silica concentrations, which has not been observed in other summers in the water quality monitoring record. The results of a redundancy analysis (RDA) indicated that concentrations of major ions and ANC were aligned with Synedra sp. and Ankyra sp. during the 2002 drought year. Following the 2002 drought year, Chrysococcus sp. and Chlorococcum sp., which became abundant, were aligned with nitrate in the RDA. These results indicate that the response of the phytoplankton community to the extreme drought was most strongly correlated with water quality changes that occurred, rather than temperature and hydraulic residence time. The dominant species in the post-drought phytoplankton community were found to be associated with nitrate, which is brought to the watershed by atmospheric deposition and may represent an anthropogenic driver of phytoplankton community composition.