Snowfall and pond volume drive summer mountain pond dynamics in California's Sierra Nevada

Abstract

Mountain aquatic ecosystems are especially vulnerable to warming and changes in hydroclimate due to climate change. Within mountain ecosystems, ponds make up a majority of the lentic waterbodies in abundance but are severely understudied compared to other freshwater systems. Understanding the drivers of pond biological, chemical, and physical dynamics in mountain ecosystems is critical for determining their responses to future climate change. Here, we performed a field survey of 30 ponds over four summers (2020–2023) in California's Sierra Nevada to understand how pond dynamics (temperature, water chemistry, primary production, and zooplankton community) are driven by environmental variables. Ponds varied in volume (4–3361 m³), elevation (2299–3444 m), and snowfall, where we captured years of relatively low snowfall (361–803 mm) and one of the highest snowfall years on record (1447–3002 mm). We found that snowfall was a dominant driver of pond dynamics during the ice-free season, similar to mountain lakes. Higher snowfall increased pond volume during the ice-free season, which was associated with a decrease in mean temperatures and thermal variability, a decrease in nutrient concentrations, and a decrease in zooplankton abundance. Sierra Nevada ponds mixed almost every night, unlike low-elevation temperate ponds of similar volume. Our results suggest that montane ponds do not fit previously defined pond paradigms. With the Sierra Nevada predicted to lose 70% of their snowpack by 2100, it is increasingly important to understand the impact of hydroclimate on its understudied ponds. For ponds that are able to persist with lower snowfall, they may be warmer, more thermally variable, with higher nutrients, primary productivity, and therefore higher zooplankton concentrations.