UPWELLED PELAGIC WATER EXCHANGE DRIVING LITTORAL LIMNOLOGY (UPWELL) EXPERIMENT

Spring upwelling events are common in late-May through mid-June. Wind events combine with a weakly stratified lake to generate upwelling. This process can transport nutrient rich water to the surface, potentially driving algal growth, and may transport well-oxygenated surface water to depth. Prior to mid-May, the lake is typically vertically mixed. By late-June, stratification strengthens and winds tend to die down; upwelling becomes less common until fall storms start to blow in around October.

In spring 2018, UC Davis Ph.D. student, Derek Roberts, oversaw a large experiment to improve understanding of the physics and associated water quality/ecological impacts of spring upwelling events. Derek, with other TERC staff and AmeriCorps member, Christine Limon, deployed an array of instruments extending a half-mile offshore. Over $0.5 million of instruments were involved, made possible by researchers from TERC, UC Davis Civil and Environmental Engineering, Bodega Marine Lab, and Stanford University pooling their resources. This instrument array measured currents, temperature, oxygen, and clarity, among other variables. In addition to the instrument deployment, Derek and the team sampled water one to two times each week for nutrient and particle analyses.

Three major wind events induced upwelling on Lake Tahoe’s west shore on May 31, June 4, and June 9th 2018. The animation above shows the lake-response to the June 9th event along a transect perpendicular to shore from the Homewood area. Colors on the upper-panel show surface temperature, and arrows show the direction and magnitude of the horizontal currents. White arrows show currents in the upper 15 m of the water column; grey arrows show currents in the 15-30 m depth range; and black arrows show currents at depths below 30 m. In the large middle-panel, colors correspond to temperature along the transect profile, and arrows show the east-west and up-down currents. The bottom-panel shows wind speeds throughout the event. Note how strong winds on June 9th upwelled cold water across the transect. When the winds relax on June 10, warm water comes rushing back across the lake, driving strong southerly currents alongshore.

For more information please contact Geoff Schladow (gschladow@ucdavis.edu).