The abundance and distribution of intertidal canopy-forming macroalgae are threatened by the increase in sea surface temperature and in the frequency and intensity of heatwaves caused by global warming. This study evaluated the physiological response of predominant intertidal macroalgae in the NW Iberian Peninsula (Bifurcaria bifurcata, Cystoseira tamariscifolia and Codium tomentosum) to increased seawater temperature during immersion and increased air temperatures during consecutive emersion cycles. We combined field mensuration and laboratory experiments in which we measured mortality, growth, maximum quantum yield and C:N content of the macroalgae. Air temperature was a critical factor in determining physiological responses and survivorship of all species, whereas high seawater temperature had sublethal effects. Cystoseira tamariscifolia suffered the greatest decreases in F_v/F_m, growth and the highest mortality under higher air temperatures, whereas C. tomentosum was the most resistant and resilient species. Two consecutive cycles of emersion under atmospheric heatwaves caused cumulative stress in all three macroalgae, affecting the physiological performance and increasing the mortality. The potential expansion of the warm-temperate species B. bifurcata, C. tamariscifolia and C. tomentosum in the NW Iberian Peninsula in response to increasing seawater temperature may be affected by the impact of increased air temperature, especially in a region where the incidence of atmospheric heatwaves is expected to increase.

The increased frequency of heatwaves expected in the context of global warming will affect socio-ecological systems such as shellfish beds at intertidal seagrass meadows. A mesocosm experiment was performed to assess the effects of a simulated atmospheric heatwave during low tide on the bioturbation indicators and growth of the commercial juvenile native Ruditapes decussatus and the introduced clam R. philippinarum, and on their interactions with the seagrass Zostera noltei. Under the heatwave, heat dissipation at 5 cm depth was significantly greater in the sediments below Z. noltei than below bare sand, the photosynthetic efficiency (F_v/F_m) of Z. noltei decreased and the clams tended to grow less. Furthermore, after the heatwave clams below bare sand tended to burrow deeper than those below Z. noltei, indicating that seagrass provided a refuge for clams. Ruditapes philippinarum grew less, and did not burrow as deeply as R. decussatus, which may imply greater vulnerability to desiccation and heat at low tide. The particle displacement coefficient (PDC) of R. philippinarum indicated lower bioturbation values in Z. noltei than in bare sand and was a suitable bioturbation indicator for juvenile Ruditapes spp. clams. In Z. noltei coexisting with R. philippinarum, the F_v/F_m values were higher than without clams after a recovery period, which may be linked to the assimilation of phosphate excreted by the clams and suggests a facilitative interaction. No such interaction was observed with R. deccusatus, probably because of its deeper burrowing depth. The findings suggest reciprocal facilitative interactions between R. philippinarum and Z. noltei and the potential contribution of Z. noltei to the sustainability of clams under global warming scenarios, which may support management actions aimed at enhancing the coexistence between shellfishing activities and seagrass conservation.

Seagrass carbon stocks are vulnerable to physical disturbance. We assessed the effect of clam harvesting on the organic carbon (C_org) stocks in surface sediments in four intertidal Zostera noltei meadows on the Iberian Atlantic coast (Spain and Portugal), by comparing undisturbed and harvested areas. We also monitored the spatial cover of the meadows throughout the growing season. Sedimentary C_org content and C_org stocks were about four times lower in intensively harvested areas than in control areas, but there were not differences between areas with low harvesting pressure and control areas. Reductions of 53-85% in sedimentary C_org stocks of Z. noltei meadows were caused by intensive clam harvesting. The effect of intensive clam harvesting on C_org stocks increased throughout the growing season, but the area covered by the seagrass increased from 21 to 37%, suggesting rapid recovery of seagrass canopies and potential recovery of sedimentary C_org stocks.

Reduced salinity in estuarine areas can negatively affect intertidal seagrass meadows where clams are harvested. However, legacy effects of hyposalinity on seagrasses and infaunal clams and on their interactions have seldom been studied. Legacy effects were examined in the intertidal seagrass Zostera noltei and juveniles of the clams Venerupis corrugata, Ruditapes decussatus and R. philippinarum. Low-salinity stress was applied to mesocosms that included assemblages of seagrass and the 3 clam species that were subsequently transplanted to 2 shellfish beds characterized by different environmental conditions (i.e. seawater temperature, salinity and nutrient concentration). After 2 mo, the morphological and biochemical traits of Z. noltei and the growth and mortality of the clams were measured. Past hyposalinity stress increased clam mortality and decreased Z. noltei leaf length. The native V. corrugata was the most vulnerable to past hyposalinity, whereas the introduced clam R. philippinarum was the most resilient. The presence of clams was associated with greater nitrogen content and biomass of the above-ground parts of Z. noltei. Survival of the clams after past hyposalinity stress was greater below Z. noltei than below bare sediment, indicating persistent positive interactions following the harsh environmental conditions and during low-salinity periods in the field. At the colder and more nutrient-rich site, positive interactions were more frequent and Z. noltei performed better. This supported the influence of abiotic conditions on the recovery process. Nonetheless, enhanced survival of clams below Z. noltei indicates that Z. noltei could favour the sustainability of these shellfisheries after exposure to extreme rainfall events.