Effects of Introduced Trout on Benthic Algae Across a Climate Gradient

Oligotrophic lakes in the Sierra Nevada mountain range are essential centers of endemic biodiversity and carbon cycling, and sources of fresh water for human populations throughout the western United States. Like many freshwater systems, they are also highly sensitive to interacting impacts from our changing climate and from human interventions such as the introduction of predatory trout species. Despite continued oligotrophic status and a lack of point nutrient sources in the watersheds, filamentous algal blooms (FABs) have been noted in the littoral zones of many of these lakes in recent years. These FABs are thought to be a relatively novel phenomenon. However, scarcity of historical data on algal presence makes it difficult to draw definite conclusions. In this work we use geographic variation among lakes to establish a space-for-time substitution and untangle potential drivers of these FABs on both lake and landscape scales. We hypothesize that an interaction between trout stocking and changing climate may be driving an increase in FABs. We studied benthic algae in ten lakes both with and without trout across a climatic gradient, in 2019, 2020, and 2021. We surveyed the timing, extent, and associated substrates of FABs in each lake and quantified benthic chlorophyll-a concentrations as a proxy for algal productivity on both soft and hard substrates. Finally, we explored remote sensing methods to extend the spatial and temporal coverage of our dataset. In this presentation I share preliminary results on the differences between lakes in our study, discuss potential explanations and drivers of FABs in these oligotrophic alpine lakes, and seek feedback on next steps.