Spatial and temporal patterns of nearshore clarity in Lake Tahoe from fine resolution turbidity measurements

The nearshore areas of lakes respond quickly to watershed runoff, increases in tributary inflows from annual snowmelt, and increased anthropogenic activity in the basin. Therefore, this area of the lake serves both as an early warning system for water quality degradation and as an indicator of the effectiveness of land management practices or sediment control projects. In this study we evaluated the usefulness of combining fine-scale water quality measurements and discrete particle sample analysis to gain a better understanding of seasonal and spatial trends in the nearshore area of Lake Tahoe. Turbidity and mineral composition at 0.5 m depth were measured in nearshore waters near the City of South Lake Tahoe at a spatial resolution of 5-30 m in 2002 and 2003. Particles filtered from discrete samples collected 200 m from shore were analyzed by scanning electron microscopy and chemical analysis using quantum electron dispersive spectrometry. Baseline turbidity levels were extremely low (0.15 NTU) during calm periods in the fall but rose to levels above 4.0 NTU in response to winter and spring precipitation events and spring snowmelt runoff. Discrete samples collected 200 m from shore contained over 80% organic material during the dry part of the year and at least 50% mineral particles during the winter and spring. The effectiveness of this method for detecting variability in nearshore conditions at Lake Tahoe is promising for monitoring the littoral areas of other pristine lakes facing increased anthropogenic pressure and other watershed disturbances.