Understanding the effects of recent atmospheric heatwaves on seagrass-inhabited intertidal shellfish beds: a mesocosm...

Shellfish beds are key estuarine habitats coinhabited by commercial species and ecosystem engineers such as the seagrass Zostera noltei. Organisms inhabiting the intertidal areas of these beds are threatened by marine and atmospheric heatwaves. The present study aimed to assess the effects of sediment heating during periods of emersion on the physiology of juveniles of three commercial clams (Ruditapes decussatus, Ruditapes philippinarum and Venerupis corrugata), the seagrass Z. noltei, and their interactions. Three sediment temperature treatments (control, 20-27 and 20-32 °C ramps) were applied in a tidal mesocosm system, to simulate summer conditions in NW Spain. Responses of clams and Z. noltei were measured after four days of heatwave (day 4) and after a recovery period of four days (day 8). Although some effects were observed in all three clam species, the feeding and respiration rates remained within normal ranges. However, the probability of growth of V. corrugata clearly decreased, confirming that this species is particularly sensitive to warming. In Z. noltei, the heatwave conditions negatively affected the morphometric and photosynthetic measurements. Nonetheless, the plant acted as a thermal buffer, as the number of degree hours was ∼20 % lower in boxes containing Z. noltei than in those with bare sand. The presence of clams did not enhance the performance of Z. noltei and no relevant interactions were observed. Shellfish managers should remain alert to future heatwaves because some key species are already living close to their thermal limits and these could modify ecosystem functioning and affect clam fisheries.