The boreal kelp Laminaria digitata dominates the low intertidal and upper subtidal zones of moderately exposed rocky shores in north-western Europe. Due to ocean warming, this foundation species is predicted to disappear from French coasts in the near future. Photosynthesis of L. digitata sporophytes was surveyed in situ during spring and summer tidal cycles with emersion periods around midday during hot, sunny weather. The net production (NP) of whole individuals (i.e. the difference between their gross primary production and respiration) was assessed by measuring carbon fluxes inside a closed chamber. Photosynthetic performance of thalli was assessed using pulse-amplitude modulated (PAM) fluorescence parameters, the effective (ΦPSII) and optimal (Fv/Fm) quantum yields of photosystem II. Content in pigments involved in the xanthophyll cycle, a photoprotective mechanism, was measured in thalli to evaluate the de-epoxidation ratio (DR). NP shifted from positive values (reaching 140 μmol C gDW-1 h-1) during morning immersion to negative values uduring emersion (reaching -37 μmol C gDW-1 h-1), and did not return to positive values during the following immersion period when respiration was exacerbated. ΦPSII decreased during emersion (down to 0.01), but recovered during afternoon immersion. Fv/Fm decreased during emersion (down to 0.18) indicating severe photoinhibition. High DR values (up to 0.70) showed the effectiveness of the photoprotective mechanism, which appeared nevertheless insufficient to prevent photodamage during emersion stress. Among the environmental factors contributing to this emersion stress, repeated heat shocks over consecutive tide cycles likely play a leading role. These repeated heat shocks appear to further exacerbate the detrimental effects of warming events on this marginal population of L. digitata.

Given the growing body of evidence on the general decline of kelp beds worldwide, it is crucial to understand the physiological responses of kelp gametophyte stages to environmental parameters. We investigated the physiological responses to light and temperature of gametophytes from two populations of Laminaria digitata in contrasting environments along the French coast of the English Channel. Gametophytes of both populations were highly tolerant of high light through an efficient down-regulation of photosynthesis triggered by the activation of the xanthophyll cycle. Temperature increases promoted photosynthesis and photosystem II showed high resistance to short-term exposure to high temperatures currently encountered in the field. Gametophytes from the two sites displayed some differences in their pigment content and photosynthetic characteristics, but low replication and difference in time of sampling precluded tests of potential local adaptation to the light conditions at each site, as observed in previously published results on adult sporophytes. Gametophytes of L. digitata appeared to be resistant to irradiation and temperature conditions currently experienced in the field, confirming their role in persistence of kelp species under stressful environmental conditions.

Two 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.

The ability of Laminaria digitata (Hudson) J.V. Lamouroux to cope with rapid and drastic changes in light was studied in the field by measuring photosystem II fluorescence, net oxygen production and pigment content. Experiments were conducted during two spring tides of late spring in Northern Brittany where low spring tides occur around noon. Daily patterns of the photosynthetic performance of Laminaria digitata were observed in relation to changes in incident underwater light. Photoinhibitory light exposure induced a sharp decrease of the optimal quantum yield (F(v)/F(m)), a decline of the net oxygen production in the first of the two spring tides investigated and a concomitant increase in the de-epoxidation ratio of the Violaxanthin pool. Photoinhibition persisted at a lower extent at the end of the day and complete recovery was achieved during the night. The implications of the photoinhibition of photosynthesis of Laminaria digitata are discussed in relation to the natural ambient conditions experienced in the field.

From 1999 to 2005, studies carried out in the frame of regional and national French programs aimed to determine whether the Phaeocystis globosa bloom affected the intertidal benthic communities of the French coast of the eastern English Channel in terms of composition and/or functioning. Study sites were chosen to cover most of the typical shore types encountered on this coast (a rocky shore, an exposed sandy beach and a small estuary). Both the presence of active Phaeocystis cells and their degradation product (foam) did have a significant impact on the studied shores. The primary production and growth rates of the kelp Saccharina latissima decreased during the bloom because of a shortage of light and nutrient for the macroalgae. On sandy sediments, the benthic metabolism (community respiration and community primary production), as well as the nitrification rate, were enhanced during foam deposits, in relation with the presence of bacteria and active pelagic cells within the decaying colonies. In estuarine sediments, the most impressive impact was the formation of a crust at the sediment surface due to drying foam. This led to anoxic conditions in the surface sediment and resulted in a high mortality among the benthic community. Some organisms also tended to migrate upward and were then directly accessible to the higher trophic level represented by birds. Phaeocystis then created a shortcut in the estuarine trophic network. Most of these modifications lasted shortly and all the systems considered came back to their regular properties and activities a few weeks after the end of the bloom, except for the most impacted estuarine area.

For the routine determination of organic matter inmarine sediment, a rough estimate is often obtained bymeasuring the loss of weight on ignition(Difference-On-Ignition method: D.O.I.). This D.O.I. is to beused in ecological studies where organic material insediment is used as an environmental variable amongothers. A review of papers using this method showed agreat variability within conditions used. Theseconditions are generally used without accuratejustification. We propose a simple standard procedurebased on grain-size fraction of sediment to determineoptimal temperature and time of ignition, and theinfluence of the sample weight. The method proposed, using the loss of weight on ignition, will give thesame accuracy in measurements of organic matter inmarine sediments.