Relevance

Water clarity refers to the transparency or clearness of the water and is a commonly used indicator for the health of a water body. Federal, state, and regional agencies have adopted regulations to protect Lake Tahoe’s renowned clarity, which includes both the pelagic (deep water) and the littoral (nearshore) zones. The nearshore represents an important socio-economic value because this is where most visitors and residents experience the lake first-hand. Both California and Nevada recognize the unique ecological and aesthetic values of the nearshore environment, and both have adopted standards to protect nearshore water clarity. Secchi disk transparency is measured in the pelagic zone of Lake Tahoe, but this approach does not work in the littoral (nearshore) zone where water depth is insufficient for the method. Instead, instrument measurements of turbidity and light transmissivity are used as indicators of nearshore clarity.

Human and Environmental Drivers

Nearshore turbidity is primarily driven by the concentration and type of fine particulate materials suspended in water. Nutrient loading affects clarity by increasing phytoplankton growth (suspended organic particles). Suspended sediment loading affects clarity by contributing more fine inorganic particles. Both types of particulates cause nearshore clarity loss by scattering light and through light absorption. Heterogeneity of nearshore features leads to considerable variability in effects from environmental and anthropogenic drivers. Main drivers include seasonal runoff and lake water-column mixing, as well as episodic storm runoff and localized upwelling events. Deep-water zones close to the shoreline may mitigate the intensity of these effects by mixing offshore water with nearshore water and diluting clarity-reducing constituents. Extended shallow-water shelves can accentuate impacts by retaining higher concentrations of nutrients and suspended sediment particles, as well as by providing warmer water conditions for increased biological activity and sediment resuspension from waves and boat wakes. Urban stormwater runoff generally contains much higher concentrations of nutrients and fine sediment particles than found in the lake or in runoff from undisturbed areas. Urban stormwater discharges to the lake generally derive from impervious areas that include transportation routes and associated conveyance systems. They cause locally elevated concentrations of phytoplankton and suspended fine sediment particles that contribute to diminished nearshore clarity. Stream water concentrations of nutrients and sediments are naturally higher than pelagic lake water, so even streams from undisturbed watersheds contribute fine sediment particles and nutrients. Streams that pass through disturbed watersheds, however, contribute significantly higher concentrations of nutrients and fine sediment particles than streams from undisturbed watersheds. Inputs from groundwater seepage directly into the lake can increase concentrations of dissolved nutrients (e.g., nitrogen and phosphorus), which increase concentrations of the microscopic suspended algae that decrease nearshore clarity. Upwelling events and seasonal lake mixing deliver deep-lake waters to the nearshore. These waters often can be nutrient-rich relative to nearshore conditions. Accumulated fine sediments that have settled in the nearshore may have an impact on transparency during times of high winds or when spring snowmelt increases lake levels and exposes the newly submerged land surface to wave action. The cumulative effect of boat wakes during peak recreation periods can induce episodic sediment resuspension in the nearshore. Atmospheric deposition of fine sediments and adsorbed nutrients from road dust can have a disproportionately greater effect on the nearshore compared to deep lake sites due to proximity. Long-term climate trends are likely to impact nearshore conditions as more precipitation arrives through rain rather than snow, with higher contributions of nutrients and fine sediments at some locations from increased runoff scouring of the landscape. Higher surface water temperatures from warming climate conditions may also increase nutrient cycling and phytoplankton production.

Example EIP Projects

• Baldwin Beach BMP Retrofit and Pavement Reconditioning
• P048: Lake Tahoe Nearshore Evaluation and Monitoring Framework
• Pilot Implementation of Nearshore Monitoring Framework for Clarity Metrics