Setting the Delta thermostat: Temperature trends of yesterday, today, and tomorrow

Catarina Pien, Rosemary Hartman, Peggy Lehman, Michelle Nelson and Brittany E. Davis, Office of Water Quality and Estuarine Ecology, California Department of Water Resources, West Sacramento, CA, Eva Bush and J. Louise Conrad, Delta Science Program, Delta Stewardship Council, Sacramento, CA

Background/Question/Methods
Climate change is expected to have localized impacts in the San Francisco Estuary (SFE), which will lead to increased challenges in water management and species conservation. Rising temperatures are likely to shift locations and quantities of suitable habitat for some species. While numerous organizations collect continuous water temperature in the estuary, a landscape-scale synthesis has not been conducted. For this study, we compiled hourly data from >50 continuous water temperature stations between 2009-2019 . We gathered sensor metadata from each station to assess data limitations, and applied consistent quality checks to data from each station. We then analyzed the dataset for spatial and temporal patterns in water temperature based on year, time of year, and region. We also compiled literature on temperature thresholds for species of interest, including native and non-native fishes, aquatic vegetation and algae, and used these thresholds to assess how water temperature could impact habitat suitability for species of interest at different life stages based on year, time of year, and region in the estuary.
Results/Conclusions
Preliminary results indicate that while there are differences in water temperature between years, there is no consistent trend in water temperature between 2009 and 2019. Analyses do indicate that drought years are often warmer than wet years. In these warm years, which are projected to increase, there are often a greater number of days that are thermally stressful to native fishes (temperature > 21°C), and favorable to non-native fishes, aquatic vegetation, and harmful algal blooms. Spatial trends indicate that some freshwater reaches of the upper estuary, primarily those in the South Delta and near the San Joaquin River, regularly reach stressful temperatures for threatened and endangered fish species, and contrast with the Sacramento River and Suisun Bay where water temperatures remain cool. As a next step, we plan to use existing models to project water temperature changes across the Delta, and analyze how these changes will alter the suitable spatial distribution for species of interest. This study aims to inform restoration and management in the SFE by providing information about regions that may provide thermo-refugia as water temperature increases in the estuary. Additionally, this study serves as a model for the synthesis of often disparate and disjointed long-term monitoring datasets.