In fall 2018, the Mendocino Complex, the largest wildfire in California’s post-settlement history, burned 459,123 acres including 40% of the Clear Lake watershed. Clear Lake, the largest natural freshwater lake located entirely within California, is hypereutrophic and primarily used for drinking water, tribal use, and supports a significant local fishing economy. The impact of such a large wildfire on a natural, valuable, and vulnerable water resource is unknown and is a main concern for managers. We used long-term state and county lake monitoring data to identify the 1) impacts of wildfire prevalence and burn size on dependent lake water quality variables and 2) water quality changes and trends per and post fire, and over time within the Clear Lake watershed.
Results/Conclusions
We discovered that post-fire epilimnetic total phosphorous (TP), total suspended solids (TSS) and chlorophyll-a (chl-a) did not show strong differences compared to pre-fire concentrations. However, TP epilimnetic concentrations have increased 3 - 5 ug/L/yr since the late 1960s. Anoxic conditions were correlated with higher hypolimnetic TP concentrations, but high-frequency data and consistent monitoring of dissolved oxygen (DO) concentrations will further aid in determining the extent of anoxia in Clear Lake. Long-term TP data were more strongly correlated with maximum air temperatures than precipitation or proportion of burned watershed, suggesting a potential role of warming-induced water column stratification and DO depletion in long-term TP increases. Our results suggest that large, eutrophic or hypereutrophic lakes might be more vulnerable to climate warming rather than episodic wildfires. Nonetheless, our study underscores the value of long-term water quality monitoring and the value in studying fire effects across a wide range of lake types to promote more effective water resource management during changing climates and increasingly frequent large wildfires.