Differential effects of warming on carbon budget, photosynthetic yield and biochemical composition of cold-temperate and Arctic isolates of Laminaria digitata (Phaeophyceae).

Cold-temperate and Arctic hard bottom coastal ecosystems are dominated by kelp forests, which have a high biomass production and provide important ecosystem services, but are subject to change due to ocean warming. However, the photophysiological response to increasing temperature of ecologically relevant species, such as Laminaria digitata, might depend on the local thermal environment where the population has developed. Therefore, the effects of temperature on growth rate, biochemical composition, maximum quantum yield, photosynthetic quotient and carbon budget of young cultured sporophytes of Laminaria digitata from the Arctic at Spitsbergen (SPT; cultured at 4, 10 and 16 °C) and from the cold-temperate North Sea island of Helgoland (HLG; cultured at 10, 16 and 22 °C) were comparatively analyzed. Temperature significantly affected growth rates of L. digitata from SPT and HLG, with the highest rates occurring at 10 °C, but growth did not differ between both isolates neither at 10 °C nor at 16 °C. Nevertheless, maximum quantum yield and carbon fixation rate were highest at 4 °C for the Arctic and at 16 °C for the cold-temperate L. digitata. Significantly higher rates of oxygen production and carbon fixation were observed in the cold-temperate relative to the Artic L. digitata at 10 and 16 °C, respectively. Neither temperature nor biogeographic region of origin affected the photosynthetic quotient, and release rates of dissolved or particulate organic carbon. Total carbon and mannitol content were significantly higher in the Arctic compared to the cold-temperate L. digitata at 10 °C, revealing an increased accumulation of storage compounds in the high latitude L. digitata. We conclude that L. digitata from SPT and HLG differ in their sensitivity to increasing temperatures and that the Arctic population from Spitsbergen is likely to benefit from ocean warming, while the temperate population from Helgoland will be negatively affected by further increases in ambient temperature.