Short-term heat shock exposure affects the productivity of two habitat-forming NE Atlantic kelp species differently

Understanding the mechanisms by which key macroalgal species respond to temperature variation is critical to understanding changes in ecosystem functioning in rapidly warming oceans. This study tests experimentally for effects of short-term sublethal heat shock (20 °C vs. ambient 10 °C) on the health (growth and productivity), physiological performance (photosynthetic variables) and potential for compensatory mechanisms (phenolic content) of two kelp species, Saccharina latissima and Laminaria digitata, in the Northeast Atlantic. We assessed the effect of heat shock duration over five time points (0, 6, 24, 48, 72 h). Growth of S. latissima increased by ∼56 % under heat shock, while L. digitata growth was unaffected by temperature. Gross primary productivity (GPP) of S. latissima was initially greater in heat shock treatments (after 6 h) but declined after 48 h, while L. digitata GPP remained unaffected by heat shock regardless of exposure time. The relative maximum electron transport rate (rETRmax) of S. latissima differed depending on exposure duration, whereas the rETRmax of L. digitata increased with heat shock and over exposure time. Minimum saturating irradiance (E_k) did not differ for S. latissima, but varied for L. digitata over exposure time. Neither species showed changes in phenolic content after 72 h. Our results suggest that both kelp species exhibit the potential for short-term acclimation to sublethal heat shock but that responses are species-specific. These findings highlight the ability of these species to cope with short-term temperature stress, which may provide resistance to extreme temperature events, such as marine heat waves, in the future.