Few coupled lake-watershed studies examine long-term effects of climate on the ecosystem function of lakes in a hydrological context. We use 32 years of hydrological and biogeochemical data from a high-elevation site in the Sierra Nevada of California to characterize variation in snowmelt in relation to climate variability and explore the impact on factors affecting phytoplankton biomass. The magnitude of accumulated winter snow, quantified through basinwide estimates of snow water equivalent (SWE), was the most important climate factor controlling variation in the timing and rate of spring snowmelt. Variations in SWE and snowmelt led to significant differences in lake flushing rate, water temperature, and nitrate concentrations across years. On average in dry years, snowmelt started 25 days earlier and proceeded 7 mm/day slower, and the lake began the ice-free season with nitrate concentrations ~2 μM higher and water temperatures 9°C warmer than in wet years. Flushing rates in wet years were 2.5 times larger than those in dry years. Consequently, particulate organic matter concentrations, a proxy for phytoplankton biomass, were 5–6 μM higher in dry years. There was a temporal trend of increase in particulate organic matter across dry years that corresponded to lake warming independent of variation in SWE. These results suggest that phytoplankton biomass is increasing as a result of both interannual variability in precipitation and long-term warming trends. Our study underscores the need to account for local-scale catchment variability that may affect the accumulation of winter snowpack when predicting climate responses in lakes.