Thermal buffering by intertidal oyster reefs on mudflats in the European Wadden Sea.

The increase in heatwave frequency and severity due to climate change can threaten intertidal organisms that already experience large temperature fluctuations. Habitat-forming species can mitigate temperature extremes by creating protective microhabitats. While Pacific oysters (Magallana gigas) are known to provide thermal refuge on tropical rocky shores, their influence on temperate intertidal mudflat systems has not been investigated before. We examined the thermal environment within oyster reefs in the European Wadden Sea by recording temperatures in two microhabitat types (crevices between oysters and oyster-formed tidal pools) and comparing them to adjacent mudflats. Temperatures were measured over one year using biomimetic loggers that replicate the thermal properties of the blue mussel (Mytilus edulis), a key co-inhabitant of oyster reefs. Crevices between oysters provided cooler microhabitats particularly in summer when mean temperatures were 0.2-0.4 ± 0.02-0.05 °C lower and daily minimum temperatures were 1.0 ± 0.43 °C lower than on surrounding mudflats, while cumulative heat exposure was only moderately lower and day maximum temperatures were lower than mudflat values only at one of two sites. In contrast, tidal pools had higher day maximum temperatures than the surrounding mudflat across the year. These results demonstrate that oysters can provide localized thermal buffering in temperate mudflat systems, especially through reduced mean temperatures in crevices. However, the effect is less pronounced than in tropical rocky environments. Our findings highlight the context-dependent role of ecosystem engineers in modifying thermal landscapes, with potential implications for species persistence under climate change.