Coastal fog events buffer stream temperature change in salmonid habitat in California

In coastal California, fog events mitigate heat loading by reducing incoming solar radiation, which has great potential to impact stream temperatures. To date, little is known about how coastal fog events impact stream temperature and salmonid survival. Our objective was to better understand thermal refugia for salmonids in coastal California streams by documenting and comparing stream temperature variation influenced by three sources: coastal fog, riparian shading, and physical stream habitat differences. In tributaries of the Russian River (RR) and within Butano Creek (BC) that are inhabited by juvenile salmonids, we installed temperature sensors in five pools at a foggy site and five pools at a less foggy site within each watershed in early summer 2019. To quantify local climate variability and coastal fog events, we installed micrometeorological stations within and outside the riparian area. We conducted surveys to monitor physical stream habitat, including water depth, velocity, and canopy cover. At the more coastal, fog-influenced sites, we found that stream temperature increased to suboptimal limits for juvenile salmonids during a heat-wave in early June when no fog cover was present (BC T_max =19.8 ºC; RR T_max= 16.5 ºC). This increase of 2.5-3 ºC above monthly average temperature was also observed in deeper pools suggesting their depth (1 - 2 m) was insufficient for thermal stratification to occur. During subsequent fog events, stream temperature returned to a suitable thermal regime for salmonids. These stream temperature fluctuations were independent of riparian shading or physical stream habitat, indicating that microclimate is an important control on salmonid survival by moderating the thermal regime in streams where juvenile salmonids are present. The results of our study a dvance our understanding of the interaction between local climate variability and stream temperature and increase our ability to identify thermal refugia for juvenile salmonids in coastal California streams, which underpins the resilience of a sustainable salmonid population in a changing climate.