Adrianne P Smits, Facundo Scordo, Minmeng Tang, Alicia Cortés, Mary Jade Farruggia, Joshua Culpepper, Sudeep Chandra, Yufang Jin, Sergio Valbuena, Shohei Watanabe, Geoff Schladow, Steven Sadro

Increasingly severe wildfires release smoke plumes that cover entire continents, depositing aerosols and reducing solar radiation fluxes to millions of freshwater ecosystems, yet little is known about their impacts on inland waters. This large scale study 1) quantified annual and seasonal trends in the spatial extent of dense smoke cover in California, USA, over the last 18 years (2006 - 2022), and 2) assessed the impacts of dense smoke cover on daily gross primary production (GPP) and ecosystem respiration (R) in 10 lakes spanning a large gradient in nutrient concentration and water clarity, during the three smokiest years in our dataset (2018, 2020, 2021). We found that the maximum spatial extent of dense smoke cover between June-October has increased to 70% of California’s area since 2006, with the greatest increases in August and September. In the three smokiest years, lakes were exposed to an average of 33 days of dense smoke between July and October, resulting in substantial reductions in shortwave radiation fluxes and 3 to 4-fold increases in atmospheric fine particulate matter concentrations (PM2.5). However, responses of lake GPP to smoke cover were extremely variable among and within lakes, as well as between years. In contrast, the response of rates of ecosystem respiration to smoke was related to lake nutrient concentrations and water temperature –respiration rates decreased during smoke cover in cold, oligotrophic lakes but not in warm, eutrophic lakes. The impacts of dense, prolonged smoke cover on inland waters are likely to be highly variable within and among regions due to mediating effects of lake attributes and seasonal timing of wildfires.