Kelp ecosystems form widespread underwater forests playing a major role in structuring the biodiversity at a regional scale. Some seaweeds such as Laminaria digitata are also economically important, being exploited for their alginate and iodine content. Although some studies have shown that kelp ecosystems are regressing and that multiple causes are likely to be at the origin of the disappearance of certain populations, the extent to which global climate change may play a role remains speculative. Here we show that many populations of L. digitata along European coasts are on the verge of local extinction due to a climate-caused increase in sea temperature. By modeling the spatial distribution of the seaweed, we evaluate the possible implications of global climate change for the geographical patterns of the species using temperature data from the Coupled Model Intercomparison Project phase 5 (CMIP5). Projections of the future range of L. digitata throughout the 21st century show large shifts in the suitable habitat of the kelp and a northward retreat of the southern limit of its current geographic distribution from France to Danish coasts and the southern regions of the United Kingdom. However, these projections depend on the intensity of warming. A medium to high warming is expected to lead to the extirpation of the species as early as the first half of the 21st century and there is high confidence that regional extinction will spread northwards by the end of this century. These changes are likely to cause the decline of species whose life cycle is closely dependent upon L. digitata and lead to the establishment of new ecosystems with lower ecological and economic values.

Summary: The physiological response of the scleractinian coral Turbinaria reniformis to ammonium enrichment (3 µM) was examined at 26°C as well as during a 7d increase in temperature to 31°C (thermal stress). At 26°C, ammonium supplementation slightly changed the coral physiology. It induced a decrease in symbiont density, compensated by an increase in chlorophyll content per symbiont cell. Organic carbon release was reduced, likely due to a better utilization of the photosynthesized carbon (i.e. incorporation into proteins, kept into the coral tissue). The δ15N signature of the ammonium-enriched symbionts and host tissue was also significantly decreased by 4 and 2‰ respectively, compared to the non enriched conditions, suggesting an important uptake of inorganic nitrogen by the holobiont. Under thermal stress, coral colonies that were not nitrogen enriched experienced a drastic decrease in photosynthetic and photoprotective pigments (chlorophyll a, β carotene, diadinoxanthin, diatoxanthin, peridinin), followed by a similar decrease in the rates of photosynthesis and calcification. Organic carbon release was not affected by this thermal stress. Conversely, nitrogen-enriched corals showed an increase in their pigment concentrations, which maintained rates of photosynthesis and calcification to ca. 60% and 100% of those obtained under control conditions respectively. These corals however lost more organic carbon into the environment. Overall, these results indicate that inorganic nitrogen availability can be important to determine the resilience of some scleractinian coral species to thermal stress, and can have an equivalent function as heterotrophic feeding concerning the maintenance of coral metabolism under stress conditions.