Experimental effects of elevated temperature and nitrogen deposition on high-elevation aquatic communities

Two widespread drivers of change in high-elevation lakes are climate warming and atmospheric nitrogen deposition, which may have interactive effects on aquatic ecosystems. Using an outdoor mesocosm experiment at 2900 m above-sea level along the Colorado Front Range, we investigated the individual and combined effects of realistic increases in temperature (ambient versus 2.4 °C increase) and nitrogen concentrations (three levels) on lake plankton and hydrochemistry. Relative to the low temperature treatment, enhanced temperatures decreased the overall density of Daphnia pulicaria by ~ 40% and of gravid females specifically by ~ 20%. Increased nitrogen also reduced Daphnia density, especially in the low-temperature treatments, leading to a significant nitrogen-by-temperature interaction. The calanoid copepod Hesperodiaptomus shoshone, in contrast, was unaffected by experimental manipulations of temperature and nitrogen, and declined in abundance over time regardless of treatment. Chlorophyll-a increased to a maximum in week 4 and was unaffected by the temperature manipulation, suggesting that observed effects on Daphnia were likely direct physiological responses to warming rather than bottom-up effects. Nitrate additions caused transient increases in chlorophyll-a, which converged across treatments by the end of the study as nutrients were assimilated. Nitrogen additions also led to progressive increases in dissolved organic carbon concentrations throughout the experiment. Our results suggest that warming has the potential to reduce zooplankton production, consistent with observed decreases in large-bodied Daphnia density with decreasing elevation in the Colorado Rockies. Future work should evaluate how the observed effects on plankton communities scale-up to natural lakes, particularly the relative importance of species-specific stress responses versus indirect food web effects.