Climate Change, Not Wildfires, Primary Driver of Clear Lake Water Quality

Angela DePalma-Dow, Lake County Water Resources Department Invasive Species Program Coordinator and “Lady of the Lake” Columnist, Served as Lead Author of Ecosphere-Published Work

Lake County, CA (December 14, 2022) – County of Lake Water Resources staff and researchers from Michigan State University and University of Vermont partnered to investigate current water quality trends in Clear Lake, and their relationship to wildfires and climate change.  The research team evaluated water quality monitoring data from the last three large wildfire years in the Clear Lake Basin, including 2018 (Mendocino Complex), in comparison to long-term water quality nutrient data.

Their findings have now been published in the open-access, Peer-Reviewed Ecology Journal, Ecosphere, in a piece titled, “Turning up the Heat: Long-term water quality responses to wildfires and climate change in a hypereutrophic lake” (DOI: 10.1002/ecs2.4271):

“Right after the Mendocino Complex, our department received many calls from the public concerned about the fire’s impact on Clear Lake water quality,” Notes Angela De Palma-Dow, County Invasive Species Program Coordinator and lead author and primary investigator for the research effort. “And while there is a plethora of wildfire water quality research out there, none includes a lake as large and productive (i.e., biomass- and species-dense, nutrient-rich and eutrophic) as Clear Lake.  This was a great opportunity for our team to closely review the data, and inform public discussion and planning efforts.”

The research effort aimed to identify the influence of wildfires, water and air temperature, and precipitation on water quality in Clear Lake.  Climate change data was collected from a variety of public, open sources.  Water quality data was sourced from a long-term collaborative water monitoring program undertaken by the California Department of Water Resources, County Water Resources staff and local Tribal governments.

For about 60 years, members of this collaborative have collected monthly water quality data from Clear Lake’s three arms. Our Lake has become much more nutrient-rich and productive over time, a process known as “eutrophication.” This trend is pronounced after the mid 1980’s and in times of drought.

Eutrophication is typically fueled by increased nutrient inputs, primarily phosphorus. Nutrients lead to growth of both algae (phytoplankton) and cyanobacteria, turning the water green. When algae and cyanobacteria die and decompose, this can result in foul odors and release of toxins, disrupting recreation in affected areas of the lake. Thick green mats on the surface of the lake can be observable from shorelines.  These conditions increase in frequency and severity when water temperatures are very warm and especially when lake levels are low (e.g., drought).

Researchers analyzed what variables were most associated with increased phosphorus over time, especially during the heavy bloom season (July-October).  Watershed areas burned by wildfire, seasonal air and water temperatures, and seasonal precipitation were assessed.  Phosphorus increases in Clear Lake were primarily driven by lack of rain and warmer air and water temperatures.   

“We found higher phosphorus concentrations, over the long term, were associated with hot and dry weather, not rain, alone, or rain following wildfires,” co-author Dr. Ian McCullough of Michigan State University added.  Climate change may be a greater threat to Clear Lake water quality than wildfires.  

Higher water temperatures, at the top and bottom of the water column, are also associated with observed increases in phosphorus.  These findings are most pronounced in the data after 1985.

“What could be happening here is release of phosphorus from lake sediment,” said Dr. Jennie Brentrup, study co-author and aquatic scientist at the Cary Institute of Ecosystem Studies. “Part of the issue is warmer water holds less oxygen, and decomposing algae and cyanobacteria also consume oxygen at the lake bottom. As the lake heats up and less oxygen is available, more phosphorus can be released from the sediment and mix into surface waters, further fueling the cycle of eutrophication.”

Sedimentary phosphorus release must be considered in future management of Clear Lake.  The Blue Ribbon Committee for the Rehabilitation of Clear Lake (BRC) just approved $2.25 million for a hypolimnetic oxygenation pilot study to be conducted in the Oaks arm of the Lake.  This project will inject oxygen directly into the bottom of the lake to “trap” phosphorus in the sediments, preventing the food source for cyanobacteria; its aim is to stop severe blooms from occurring, in the first place. More information on this project and the BRC is available through the California Natural Resources Agency’s website.

Clear Lake is a critical asset, supporting Lake County fisheries, water-based tourism and local economic activity. Its waters are used for irrigation, recreation, and Tribal cultural practices, and also drinking water for 40,000 people (nearly 60% of the county’s population), the majority of whom are low income and/or members of local Tribes (Big Valley Band of Pomo Indians, Habematolel Pomo of Upper Lake, Robinson Rancheria Pomo Indians of California, Scotts Valley Band of Pomo Indians, and Elem Indian Colony).

County Water Resources staff value this research.

“As stewards of the Lake, we rely on long-term monthly monitoring data,” affirms Scott De Leon, Lake County’s Water Resources Director.  “Wildfires and droughts are expected to grow more frequent, and Angela and the research team’s findings valuably interpret the role multiple climate-informed factors play in the water quality outcomes we are seeing.  Our team continually seeks to augment our understanding and plan future efforts in accordance with the best available data.”

If you have any questions or need more information, please contact the Water Resources Department (WRD), at water.resources@lakecountyca.gov, or by phone, at (707)263-2344.  Also visit WRD’s website, or the “Lake County Water Resources Department” Facebook page, @lakecountywater.

Please direct questions on, “Turning up the Heat: Long-term water quality responses to wildfires and climate change in a hypereutrophic lake,” specifically, to Angela.DePalma-Dow@lakecountyca.gov. 

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