Seasonal upwelling patterns drive water quality heterogeneity in the nearshore of a deep lake

Abstract:

Wind-driven upwelling events can rapidly transport deep water into lake littoral zones, creating substantial variability in temperature and nutrient concentration along lake perimeters. The associated internal waves can drive additional heat and nutrient fluxes, both from the hypolimnion and through the benthos. Understanding the importance of wind-driven upwelling events to littoral ecosystems requires quantification of their frequency and magnitude across space and time.

We use a distributed set of temperature measurements from the perimeter of seasonally stratified Lake Tahoe (four years of 30-second data at 8 sites; 2015-2018) to validate upwelling estimates derived from wind data and bi-weekly profiles from the pelagic zone of the lake. Cross-lake temperature differences are well predicted in space and time by a non-dimensional upwelling parameterization. This parameterization is applied to 16 years (2003-2018) of stratified-season lake profile and wind data to characterize the seasonality and magnitude of upwelling in each quadrant of the lake. Prevailing southwesterly winds combine with comparatively weak late-spring and early-summer stratification to drive regular west/southwest shore upwelling, with the median May/June season seeing over 150 hours of full upwelling conditions. This consistent seasonal pattern explains average June temperatures that are more than 2°C colder on the west shore than on the east shore. Late-spring/early-summer upwelling events are rarely large enough to increase littoral nitrate concentrations; only two of the sixteen study years saw events that would be expected to double the extremely low baseline nitrate concentrations (about 2 μg/L). Toward the end of the stratified season, upwelling is less common due to the stabilizing effect of a deep surface-mixed layer. However, strong winds associated with the occasional late-fall/early-winter storm can drive large magnitude upwelling events that may increase littoral nitrate concentrations.