Eagle ray feeding pits buffer intertidal sediment temperature but do not provide functional refugia under heat stress

Atmospheric heatwaves are increasing in frequency and intensity. Understanding coastal ecosystem responses and the potential for thermal refugia from these events is critical for evaluating the ecological consequences of climate change. In intertidal marine sediments, eagle ray feeding creates pits in the sediment that can retain water at low tide. These pits could buffer against atmospheric heat stress, providing functional refugia for organisms and maintaining ecosystem function. We investigated the role of ray pits as refugia by simulating a 4-day heat stress event on an intertidal flat in Aotearoa New Zealand. We compared ecological responses to heat stress between ambient sediments and ray pits by measuring benthic oxygen fluxes as well as explanatory covariates of cockle density and burrowing behaviour, and sediment characteristics. We found that heat stress caused smaller temperature increases in pits compared with ambient sediments, but this did not influence oxygen fluxes. Instead, we found that reduced cockle densities in pits corresponded with a shift in the relationship between cockle density and net oxygen production (photosynthesis minus respiration). Heat stress did not alter the direction of this relationship, but increased variability around it for both ambient and pit sediments. Measurements of cockle burrowing behaviour indicated that heat-stressed cockles burrow faster across all sediments, which could drive this increased variability in their influence on net oxygen production. Overall, our experimental results show that while ray pits buffer sediment temperature extremes slightly, they do not provide functional refugia from heat stress.