Climate change can alter the physical and biological components of aquatic ecosystems. While changes to physical parameters, such as increasing surface water temperatures, are well documented, the ramifications for biological communities are less clear. In instances where climate change has differential effects across species, temporal mismatches can develop where predator and prey interactions are decoupled. In lakes, this can occur between phytoplankton and zooplankton communities, impairing links in the food web. For example, zooplankton taxa may decline when emergence from resting stages no longer coincides with peak algal abundance. Studies documenting changing phenology in aquatic communities have largely been focused on temperate ecosystems with seasonal ice-cover, but less is known about shifting phenologies in warmer aquatic systems. Lake Powell, one of the largest reservoirs in the United States, lies in the warm, semi-arid climate of the American Southwest and impounds the Colorado River near the upper boundary of Grand Canyon National Park. The reservoir is a critical resource, providing ~70% of the water storage for the Upper Colorado River Basin and setting the downstream conditions for the Colorado River. This study investigates phenological changes in water temperature, stratification, and phytoplankton and zooplankton communities in Lake Powell’s forebay in recent decades.
Results/Conclusions
Winter, spring, and early summer surface water temperatures exhibited significant warming over a 40-year record (warming at 0.26, 0.59, and 0.24 ºC decade-1 respectively based on Thiel-Sen slopes). As surface waters warmed from 1994 to 2014, total phytoplankton increased. The greatest increases were observed in March, when biovolume was 100x higher in 2014 as compared to 1994. At the same time, temporal ranges expanded for many algal species. Notably, Cyanobacteria increased in Lake Powell from early summer to late fall. In December, Cyanobacteria biovolume increased by more than 50x from 1994 to 2014. Trends in zooplankton biomass were less clear from 1992 to 2014, suggesting some disconnection between the two trophic levels. While Cladocera and Rotifera populations also increased in abundance throughout much of the year, Copepods decreased in abundance in the spring and fall. Collectively, the biological and physical changes we document in Lake Powell occurred over a relatively short period of time, with potential implications for both water quality in the reservoir and downstream conditions in the Colorado River.