CLIMATE AND HYDROLOGIC DRIVERS OF LAKE ECOSYSTEM DYNAMICS IN AN ALPINE CATCHMENT, COLORADO, USA

Alpine regions are vulnerable to climate change, but detailed records are limited. Analysis of data from 7 alpine and sub-alpine lakes in in the Colorado Front Range shows that spring ice-off dates have shifted 7 days earlier over the last 33 years. Further, winter/spring snowfall and spring/summer air temperatures were found to be strong predictors of ice cover duration. In turn, the physical characteristics and phytoplankton community dynamics of the most studied lake varied with ice-off timing. Summers with earlier ice-off had slower flushing rates, stronger stratification and higher conductivity, pH and inorganic nitrogen. A NMDS ordination and RDA analysis of the data revealed a strong response to climate by the phytoplankton community at the surface. A much weaker response was found at 9 m. Using composite climate variables representing “too-much-summer”, we found that these variables explained 29.1% of phytoplankton community composition at the surface, but only 9.7% at 9 m. Observed photobleaching of dissolved organic matter suggests that greater exposure to ultraviolet light at the surface may contribute to differences between the responses at the surface and 9 m. The links among hydrological, chemical and biological responses to climate highlight the integration by lakes of larger-scale ecosystem change.