GROUNDWATER NUTRIENT SUPPLY TO NEARSHORE PERIPHYTON IN ULTRA-OLIGOTROPHIC LAKE TAHOE (Invited Presentation)

Nearshore environments of large lakes are increasingly experiencing threats from climate change and stresses from anthropogenic inputs of nutrients. In clear, ultra-oligotrophic lakes such as Lake Tahoe, the impacts of climate have affected thermal stratification, lake clarity, timing and delivery of streamflow, and changes in nearshore ecological communities. However, since the 1960s, there have been regions of the Lake Tahoe nearshore zone that have experienced excessive growth of attached algae (periphyton). We present two case studies investigating the role of groundwater in simulating nearshore periphyton. The first study investigated watershed processes such as timing of nutrient discharge by streams and groundwater and the spatial patterns in growth from monitoring locations adjacent and distal to where a stream enters the lake. The second uses a multi-isotopic approach to identify the source of nutrients in periphyton (δ¹⁵N, δ¹³C and δ³⁴S) and groundwater (δ¹⁵N, δ¹⁸O, and Δ¹⁷O) to determine whether sources change seasonally with streamflow and/or atmospheric inputs. While atmospheric deposition is typically assumed the largest influx of nitrogen and phosphorus to Lake Tahoe, the source of nutrients that contribute to nearshore periphyton hotspots is largely unknown. Analysis indicate that nutrients found in groundwater are largely responsible for stimulating nearshore periphyton growth, and the rate of nutrient inputs to the lake are influenced by recharge within the watershed, wave action, and proximity to the lakeshore. The results also indicate attached periphyton respond to the source of nutrients from groundwater when other sources of nutrients such as from in-lake mixing and surface water have very low concentrations. Isotopic data collected from stream inputs show distinct sources of nitrogen related to nitrification of snow ammonia, whereas groundwater samples show a mix of sources related to ammonia fertilizer, soil ammonia, and nitrate fertilizer. Differences in the composition of isotopic values and potential sources within the same transect are likely due to different groundwater flow paths entering the lake and more direct contributions of atmospheric nitrate to landscape and then to subsequent groundwater discharge to the nearshore. This investigation demonstrates the viability of stable isotopic approach to distinguish between the different nitrate sources delivered through streams and groundwater to Lake Tahoe nearshore.