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.

A new, fully‐automated, closed‐chamber system was developed for measuring photosynthetic activity in aquatic plants, algae, or corals during immersion. The performance of this system, which monitors oxygen exchange, was evaluated both in the laboratory and in situ under natural conditions using the seaweed Laminaria digitata. Intact, large individuals were placed inside the chamber and kept in place by a plastic grid in a transparent Perspex dome. The grid separated the upper incubation chamber containing the alga from the detectors that were situated in the lower chamber. Oxygen was measured using a novel method based on lifetime optical fluorescence sensor technology that provides an extremely stable and precise measurement of dissolved oxygen. The circulation and homogenization of the medium between the samples and the detectors in this closed system were provided by two pumps. The medium could be renewed by another pump that opened to the external ambient seawater and controlled by a solenoid valve. All the mechanics were driven by an electronic card that allowed choice of filling time, time of measurement, and time of medium renewal. This system provides a new tool to study, in detail, the photosynthesis of whole aquatic organisms under natural field conditions during immersion, combining high time‐resolution of oxygen exchange with a long temporal scale of in situ measurements. By allowing automatic and very accurate measurements without any intervention during monitoring, this system will be useful for estimating and comparing production of primary producers or for assessing the influence of environmental factors on production.

International audienceTwo populations of Laminaria digitata (Hudson) Lamouroux (Ann Mus Hist Nat Paris 20:21-47, 1813) were examined for their seasonal photosynthetic acclimation to clear and turbid-light environments along the French coast of the English Channel. Photosynthesis-irradiance curves, pigment concentrations and the daily in situ integrated oxygen production rates were measured in both populations. Despite the great differences in light attenuation between the sites, the two populations achieved similar oxygen production rates in the field, in relation to high maximal photosynthetic rates, total pigment concentrations and antenna (fucoxanthin + chlorophyll c)/chl a pigment ratios in sporophytes from the turbid environment. Environmental conditions (i.e. light, temperature and nitrogen availability) changed throughout the year in both sites. While the seasonal acclimation trends were evident in the clear-light environment, the strategy in the turbid-light environment differed, tending to maximize light capture throughout the year. This study highlights the diversity of the response of a single species to contrasted light environments