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This report includes descriptions of data collection and findings from in-water and aerial surveys of nearshore algae in Lake Tahoe between April 2023 - March 2025. Eight nearshore sites were surveyed for periphyton (attached algae) on 15 dates, including snorkel and dive sampling, and UAV aerial surveys. Eight sites were surveyed for metaphyton (un-attached algae), including dive sampling and unmanned aerial vehicle (UAV) surveys, on 10 dates. A total of 18 helicopter flights were conducted along the entire Tahoe shoreline within a week of in-water sampling events to capture whole-shoreline imagery. In this report we describe results of data collection from the 2023 - 2025 sampling events and quantify temporal and spatial variation in algal cover, biomass, and nutrient composition using generalized additive mixed models (GAMMs). We provide interpretation of spatial and temporal variation in periphyton and metaphyton growth and identify remaining knowledge gaps. We also provide recommendations for future monitoring and methods development for remote-sensing approaches (UAV, helicopter imagery, satellite imagery).

The two years of our study both featured high lake levels and similar water temperatures, complicating assessment of interannual variation in algal growth. We observed similar seasonal patterns in algal growth in both years and few significant interannual differences in algal biomass, cover, or nutrient composition for either periphyton or metaphyton (e.g., non-significant effects of ‘year’ in GAMMs), with a few exceptions. For periphyton, key findings included: (1) Significant spatial (among-site, across-depth) variation in periphyton biomass (ash-free dry weight-AFDW, periphyton biomass index-PBI), nutrient content (C, N, P), and isotopic composition (δ13C, δ15N). West shore sites with shallow slopes (Tahoe City, Pineland) supported the greatest periphyton biomass and areal cover. Periphyton AFDW was significantly greater at 1.5 m than at 0.5 m depth. (2) Periphyton taxonomy was dominated by diatom species, but cyanobacteria present in samples consistently contained heterocysts, which are sites of nitrogen fixation, suggesting potential nitrogen limitation of periphyton growth. (3) Helicopter surveys identified extensive growth of filamentous green algae (Zygnema) in periphyton at several west shore locations, despite green algae rarely being abundant within fixed sampling areas. (4) The δ15N signature of periphyton was similar to atmospheric values and did not vary seasonally, suggesting nitrogen fixation within periphyton mats may contribute to nearshore algal growth in Lake Tahoe.