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

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 are 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 to 3361 m3), elevation (2299 to 3444 m), and snowfall, where
we captured years of relatively low snowfall (361 to 803 mm) and one of
the highest snowfall years on record (1447 to 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.