UC Santa Barbara

Reservoirs are abundant worldwide, and have profound effects on downstream flow, water chemistry, and downstream biotic communities. However, studies focused on reservoir effects rarely contrast them with lakes, which provide a comparison of natural climatic conditions without the influence of reservoir management. I compared five high-elevation lakes and five reservoirs in the Sierra Nevada, over three years which encompassed a wide range of snowpacks and flow regimes. I sampled lake, reservoir, and outlet stream water chemistry year-round across the three years to quantify seasonal effects of reservoir management. In addition to outlet water chemistry, I collected benthic macroinvertebrates from lake and reservoir outlets during the ice-free season in conjunction with discharge to determine the effects of reservoir management on downstream invertebrate communities. In 2017, I measured aquatic carbon dioxide and diffusive flux from lakes and reservoirs, beginning under ice and until the end of the ice free season, to determine potential sources of high-elevation aquatic CO2 supersaturation and characterize ice-free season CO2 temporal dynamics.

Lake and reservoir nutrient concentrations did not differ in any season or year across the study period. Linear mixed models developed surface and bottom water nutrient concentrations showed that the primary controls were related to basin characteristics and snowpack, but reservoir management in the form of seasonal drawdown was a significant predictor of surface nitrate and both hypolimnetic ammonium and SRP, and indicated that reservoir water deep-release export diminished hypolimnetic nutrient accumulation. Reservoir mean annual discharge was elevated relative to lakes, which in summer and fall of 2016 and 2017 caused significantly higher export of nutrients from reservoirs. However, elevated ammonium export did not cause divergence of lake and reservoir invertebrate assemblages in those seasons, nor did they differ in any season. Other flow metrics, such as peak annual flow and the recession period, were similar between lake and reservoir outlets across years despite reservoir management. Instead, non-metric multidimensional scaling showed that invertebrate communities were related to elevated flow, but not related to low flow metrics such as baseflow and minimum flows, which were greater below reservoirs. Reservoir management altered flow regimes and nutrient flux, but interannual climactic variability was more important for determining invertebrate community structure.

Carbon dioxide was supersaturated in lake and reservoir surface waters for most of the ice-free season of 2017 despite low rates of ecosystem metabolism. Diffusive flux highest for the first 40 days after ice-off, and did not differ significantly between lakes and reservoirs, but was low relative to other water bodies. Linear mixed modeling indicated that the summer CO2 concentrations were primarily related to the duration of ice cover, allowing CO2 to accumulate under ice, which indicates that annual snowpack is a major determinant of summer CO2 evasion.