Escape from the heat: thermal stratification in a well-mixed estuary and implications for fish species facing a changing climate

Abstract

Climate change may cause organisms to seek thermal refuge from rising temperatures, either by shifting their ranges or seeking microrefugia within their existing ranges. We evaluate the potential for thermal stratification to provide refuge for two fish species in the San Francisco Estuary (Estuary): Chinook Salmon (Oncorhynchus tshawytscha Walbaum, 1792) and Delta Smelt (Hypomesus transpacificus McAllister, 1963). We compiled water temperature data from multiple monitoring programs to evaluate spatial, daily, hourly, intra-annual, and inter-annual trends in stratification using generalized additive models. We used our models to predict the locations and periods of time that the bottom of the water column could function as thermal refuge for salmon and smelt. Periods in which the bottom was cooler than surface primarily occurred during the peak of summer and during the afternoons, with more prominent stratification during warmer years. Although the Estuary is often exceedingly warm for fish species and well-mixed overall, we identified potential thermal refugia in a long and deep terminal channel for Delta Smelt, and in the periods bordering summer for Chinook Salmon. Thermal stratification may increase as the climate warms, and pockets of cooler water at depth, though limited, may become more important for at-risk fishes in the future.

Allochthonous marsh subsidies enhances food web productivity in an estuary and its surrounding ecosystem mosaic

“…Allochthonous contributions to the pelagic food web were lower in 2015 than in 2011, and the river-to-sea habitat gradient was less obvious. This reflects how juvenile salmon move among habitats to avoid elevated temperatures and lower water levels in the river and estuary [ 16 , 91 , 93 , 94 ]. Furthermore, it supports existing literature that shows how drought conditions can reduce inputs of terrestrial organic matter from the surrounding marsh into the estuary by shifting vascular plant biomass availability and prey community structure [ 20 , 95 – 97 ].…”

Disentangling abiotic and biotic controls of age‐0 Pacific herring population stability across the San Francisco Estuary

“…On the other hand, juvenile herring are generally associated with cool, salty conditions in spring, so they may mostly benefit from the combination of seasonally favorable coastal upwelling events and low flows that expand their range upriver. Habitat restoration throughout the estuary may also enhance opportunities for thermal refugia: tidal marshes can export cool water after nighttime evaporative cooling (Enright et al, 2013), eelgrass meadows can provide dense canopy cover in shallow subtidal areas (Boyer et al, 2017), and deep subtidal areas can allow stratification (Mahardja et al, 2022; Vroom et al, 2017). Collectively, a diversity of region‐specific management strategies could help buffer juvenile herring from elevated mortality rates when weak coastal upwelling, marine heat waves, warm droughts, or other climate drivers negatively impact habitat suitability and food supply in the estuary.…”

A bioenergetics‐based index of habitat suitability: Spatial dynamics of foraging constraints and food limitation for a rare estuarine fish

“…Over the summer to fall, as salinity in seaward regions compresses suitable Delta Smelt habitat, food limitation may intensify. In the driest years, Delta Smelt become restricted to upstream habitats limited by turbidity and the episodic effects of high temperatures, though some fish may find refuge in deeper areas of the Shipping Channel region, where thermal stratification and localized turbidity maxima occur (Young et al 2021; Mahardja et al 2022). By early fall, declining temperatures may provide opportunities for greater growth potential, but declining turbidity and prey availability become increasingly limiting.…”