Thermal Stress and Suitability for Aquatic Species in the San Francisco Estuary

Increases in water temperature caused by climate change will challenge the management of water and species in the San Francisco Estuary. Our goals were to describe spatial and seasonal patterns in water temperature across the upper estuary, and evaluate how temperature stress and suitability vary across the aquatic ecosystem. We synthesized 10 years of continuous water-temperature data at 75 stations across six regions of the estuary between 2010 and 2019. We identified stressful temperature thresholds for species of interest using published physiological limits and observed distribution, including Endangered Species Act-listed native fish (e.g., osmerids, salmonids), native fish (e.g., cyprinids), non-native fish (e.g., centrarchids, bivalves), and nuisance species such as invasive aquatic vegetation, and harmful cyanobacteria. We then quantified thermal stress across varying spatial and temporal scales and metrics. Analyses indicated there were detectable regional temperature differences, and that Suisun Bay was the only region to provide regular thermal refuge during the warmer parts of the year, though portions of the confluence and Suisun Marsh also provided potential thermal refuge during summer/fall and during cooler parts of the day. Meanwhile, the Central and South Delta experienced the warmest temperatures, exceeding thresholds for listed species throughout summer and early fall. We found that listed species such as juvenile salmonids and osmerids experience more thermal stress across a number of metrics. Fishes with higher heat tolerance (including non-natives and select natives), aquatic vegetation, and Microcystis (a harmful cyanobacteria) had the lowest average number of thermal stress days. This study demonstrates that the estuary is already exhibiting stressful conditions for species of concern, and thermal stress will only increase with climate change. We identify at what times of year and where the estuary may provide refuge from thermal stress conditions across a variety of species, which has important implications for restoration prioritization, design, and species management.