Linkages between hydrology and seasonal variations of nutrients and periphyton in a large oligotrophic subalpine lake

Periphyton is important to lake ecosystems, contributing to primary production, nutrient cycling, and benthic metabolism. Increases in periphyton growth in lakes can be indicative of changes in water quality, shifts in ecosystem structure, and increases in nutrient fluxes. In oligotrophic lakes, conservationists are interested in characterizing the influence of hydrological drivers on excessive periphyton growth along nearshore areas. We collected nutrient samples bi-weekly from groundwater and surface water during a 9-month monitoring period to evaluate the timing and availability of nutrients to eulittoral periphyton in Lake Tahoe. Groundwater discharge rates were measured synoptically using seepage meters and estimated indirectly using continuous head gradient measurements and aquifer properties estimated by slug tests. The discharge measurements made from the seepage meter measurements provide information about the spatial variability perpendicular from shore along and the change in groundwater discharge due to wave action. Algal biomass sampled from substrates and observed using underwater photographs were used to correlate seasonal growth and nutrient concentrations in groundwater and lake water. Results indicate that groundwater and nutrient discharge are temporally variable due to seasonal changes in recharge within the watershed, wave action, and lake stage. Groundwater discharge was enhanced by the seasonally-low lake stage and episodic recharge caused by precipitation falling as rain in the watershed. Increases in dissolved phosphorus and nitrate in the lake during winter are attributed to groundwater discharge and correlates to increases in algal biomass in the nearshore area. Results indicate that nutrient-rich groundwater discharge appears to stimulate seasonal periphyton blooms along the eulittoral zone of Lake Tahoe.