Methane is the second most important greenhouse gas contributing to climate warming. The open ocean is a minor source of methane to the atmosphere. We report intense methane emissions from the near-shore southern region of the North Sea characterized by the presence of extensive areas with gassy sediments. The average flux intensities (~130 μmol m−2 d−1) are one order of magnitude higher than values characteristic of continental shelves (~30 μmol m−2 d−1) and three orders of magnitude higher than values characteristic of the open ocean (~0.4 μmol m−2 d−1). The high methane concentrations (up to 1,128 nmol L−1) that sustain these fluxes are related to the shallow and well-mixed water column that allows an efficient transfer of methane from the seafloor to surface waters. This differs from deeper and stratified seep areas where there is a large decrease of methane between bottom and surface by microbial oxidation or physical transport. Shallow well-mixed continental shelves represent about 33% of the total continental shelf area, so that marine coastal methane emissions are probably under-estimated. Near-shore and shallow seep areas are hot spots of methane emission, and our data also suggest that emissions could increase in response to warming of surface waters.

We report gross primary production (GPP), community respiration (CR), and net community production (NCP) over a Posidonia oceanica meadow at 10 m in Corsica (Bay of Revellata) based on the open water O2 mass balance from a data set of hourly measurements with an array of three O2 optodes deployed from August 2006 to October 2009. The method was checked by comparison with discrete measurements of metabolic rates derived from benthic chamber incubations also based on the diel change of O2. This comparison was satisfactory and actually highlights the potential caveats of benthic incubation measurements related to O2 accumulation in small chambers leading to photorespiration and an underestimation of GPP. Our data confirmed previous P. oceanica meadows GPP and CR values, strong seasonal variations, and net autotrophy. High‐resolution data revealed strong interannual variability, with a decrease of GPP by 35% and NCP by 87% during 2006–2007 characterized by a mild and less stormy winter compared with 2007–2008 and 2008–2009. P. oceanica meadows are then expected to decrease export of organic carbon to adjacent communities (decrease of NCP), since a decrease in frequency and intensity of marine storms is expected in the future in the Mediterranean Sea as a result of a northward shift of the Atlantic storm track.

Dissolved CH 4 concentrations in the Belgian coastal zone (North Sea) ranged between 670 nmol l-1 nearshore and 4 nmol l-1 offshore. Spatial variations of CH 4 were related to sediment organic matter (OM) content and gassy sediments. In nearshore stations with fine sand or muddy sediments, the CH 4 seasonal cycle followed water temperature, suggesting methanogenesis control by temperature in these OM-rich sediments. In offshore stations with permeable sediments, the CH 4 seasonal cycle showed a yearly peak following the chlorophyll-a spring peak, suggesting that in these OMpoor sediments, methanogenesis depended on freshly produced OM delivery. This does not exclude the possibility that some CH 4 might originate from dimethylsulfide (DMS) or dimethylsulfoniopropionate (DMSP) or methylphosphonate transformations in the most offshore stations. Yet, the average seasonal CH 4 cycle was unrelated to those of DMS(P), very abundant during the Phaeocystis bloom. The annual average CH 4 emission was 126 mmol m-2 y-1 in the most nearshore stations (4 km from the coast) and 28 mmol m-2 y-1 in the most offshore stations (23 km from the coast), 1260-280 times higher than the open ocean average value (0.1 mmol m-2 y-1). The strong control of CH 4 by sediment OM content and by temperature suggests that marine coastal CH 4 emissions, in particular in shallow areas, should respond to future eutrophication and warming of climate. This is supported by the comparison of CH 4 concentrations at five stations obtained in March 1990 and 2016, showing a decreasing trend consistent with alleviation of eutrophication in the area.

The influence of abiotic and biotic variables on the concentration of dimethyl sulfide (DMS), dimethylsulfoniopropionate (DMSP), and dimethylsulfoxide (DMSO), were investigated during an annual cycle in 2016 in the Belgian Coastal Zone (BCZ, North Sea). We reported strong seasonal variations in the concentration of these compounds linked to the phytoplankton succession with high DMS(P,O) producers (mainly Phaeocystis globosa) occurring in spring and low DMS(P,O) producers (various diatoms species) occurring in early spring and autumn. Spatial gradients of DMS and DMSP were related to those of phytoplankton biomass itself related to the inputs of nutrients from the Scheldt estuary. However, the use of a relationship with Chlorophyll-a (Chl-a) concentration is not sufficient to predict DMSP. Accounting for the phytoplankton composition, two different DMSP versus Chl-a correlations could be established, one for diatoms and another one for Phaeocystis colonies. We also reported high nearshore DMSO concentrations uncoupled to Chl-a and DMSP concentrations but linked to high suspended particulate matter (SPM) presumably coming from the Scheldt estuary as indicated by the positive relationship between annual average SPM and salinity.

A complete understanding of the mechanistic basis of marine ecosystem functioning is only possible through integrative and interdisciplinary research. This enables the prediction of change and possibly the mitigation of the consequences of anthropogenic impacts. One major aim of the European Cooperation in Science and Technology (COST) Action ES0609 “Seagrasses productivity. From genes to ecosystem management,” is the calibration and synthesis of various methods and the development of innovative techniques and protocols for studying seagrass ecosystems. During 10 days, 20 researchers representing a range of disciplines (molecular biology, physiology, botany, ecology, oceanography, and underwater acoustics) gathered at The Station de Recherches Sous-marines et Océanographiques (STARESO, Corsica) to study together the nearby Posidonia oceanica meadow. STARESO is located in an oligotrophic area classified as “pristine site” where environmental disturbances caused by anthropogenic pressure are exceptionally low. The healthy P. oceanica meadow, which grows in front of the research station, colonizes the sea bottom from the surface to 37 m depth. During the study, genomic and proteomic approaches were integrated with ecophysiological and physical approaches with the aim of understanding changes in seagrass productivity and metabolism at different depths and along daily cycles. In this paper we report details on the approaches utilized and we forecast the potential of the data that will come from this synergistic approach not only for P. oceanica but for seagrasses in general.

a b s t r a c tWe investigated for the first time the occurrence of dimethylsulfoniopropionate (DMSP) in the leaves of Posidonia oceanica and we report its variability during 4 sampling periods covering the seasonal cycle (February, June, August and November) and along a gradient from 10 m to 30 m depth. The P. oceanica leaf DMSP content expressed per mass of dry weight (dw) ranged from 0.1 to 33.9 mol g dw −1 and averaged 5.0 mol g dw −1 . It was higher than the DMSP content of roots and rhizomes that averaged ∼0.5 mol g dw −1 . The leaf DMSP content showed seasonal variations, being highest in summer when primary production and biomass of P. oceanica were also highest. In August, the leaf DMSP content showed variations with depth, increasing from 30 m to 10 m depth. In summer, the leaf DMSP content was highest in the youngest sections of leaves (closest to base) than in the older ones (closest to apex). The seasonal and depth distribution suggest that the DMSP leaf content is positively related to irradiance, hence, we hypothesize that DMSP in P. oceanica plays a role as an antioxidant against reactive oxygen species, although we cannot unambiguously exclude other potential roles such as grazer deterrent. The average leaf DMSP content of P. oceanica is modest compared to high DMSP producing macroalgae and phytoplankton. Yet, the integrated DMSP stock associated to the meadows of P. oceanica is very large due to its enormous biomass, and at the community level it is 2 orders of magnitude higher than the potential integrated DMSP stock related to phytoplankton in the same area.

We acquired quasi‐continuous measurements of community gross primary production (GPP) by mass balance of O2 measured on a mooring, from August 2006 to October 2016 over a Posidonia oceanica meadow (10 m depth) in the Bay of Revellata (Corsica). Over the 2006–2016 period, annual GPP averaged 88 molO2 m−2 yr−1 and ranged from 61 to 108 molO2 m−2 yr−1. The 2 yr with the lowest annual GPP (2007 and 2015) were characterized by a low occurrence of fall–winter storms, probably leading to the accumulation of leaf litter in fall and early winter; we hypothesize this might have led to occultation of benthic macro‐algae. Among the other years, the inter‐annual variability of GPP was related to changes during the February–August period, as GPP was repeatable among years during the September–January period. For the February–August period, inter‐annual variations of GPP were correlated to chlorophyll a (Chl a), solar radiation (SR) and water temperature. Computed phytoplankton GPP corresponded to a small fraction of community GPP, so the relation between GPP and Chl a probably reflected inter‐annual variations of a common driver that we hypothesize to be nutrient inputs. The correlation of GPP with SR shows that light availability contributed to inter‐annual variations of the development of P. oceanica. The positive relation between GPP and temperature was consistent with the fact that the observed temperatures in the Bay of Revellata were during the study period within the comfort range for the growth of P. oceanica, despite an increase of water temperature of 0.7°C.

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