• Sciences de l'environnement/Biodiversité et Ecologie
  
• Sciences du Vivant
  
• Sciences de l'environnement/Environnement et Société
  
• Sciences de l'environnement/Milieux et Changements globaux
  
• sdv/sdv.ee/sdv.ee.eco
  
• Planète et Univers/Océan, Atmosphère
  

In coastal systems, the multiplicity of sources fueling the pool of particulate organic matter (POM) leads to divergent estimations of POM composition. Eleven systems (two littoral systems, eight embayments and semi-enclosed systems and one estuary) distributed along the three maritime façades of France were studied for two to eight years in order to quantify the relative contribution of organic matter sources to the surface-water POM pool in coastal systems. This study was based on carbon and nitrogen elemental and isotopic ratios, used for running mixing models. The POM of the estuary is dominated by terrestrial material (93% on average), whereas the POM of the other systems is dominated by phytoplankton (84% on average). Nevertheless, for the latter systems, the POM composition varies in space, with 1) systems where POM is highly composed of phytoplankton (≥ 93%), 2) systems characterized by a non-negligible contribution of benthic (8 to 19%) and/or riverine (7 to 19%) sources, and 3) the Mediterranean systems characterized by the contribution of diazotroph organisms (ca. 14%). A continent-to-ocean gradient of riverine and/or benthic POM contribution is observed. Finally, time series reveal 1) seasonal variations of POM composition, 2) differences in seasonality between systems, and 3) an inshore-offshore gradient of seasonality within each system that were sampled at several stations. Spatial and seasonal patterns of POM composition are mainly due to local to regional processes such as hydrodynamics and sedimentary hydrodynamic (e.g. resuspension processes, changes in river flows, wind patterns influencing along-shore currents) but also due to the geomorphology of the systems (depth of the water column, distance to the shore). Future studies investigating the link between these forcings and POM composition would help to better understand the dynamics of POM composition in coastal systems.

Increasing abundance of non-commercial sprats and decreasing biomass and landings of commercial anchovies and sardines justify the need to study the feeding ecology and trophic niche overlap of these planktivorous species in the Gulf of Lions. Their diet has been investigated on the basis of stomach content and stable isotope analyses in 2011 and 2012 according to different depths and regions in the study area. The main prey were Corycaeidae copepods, Clauso/Paracalanus, Euterpina acutifrons and Microsetella, for sprats and small copepods, such as Microsetella, Oncaea and Corycaeidae, for anchovies and sardines. This is the first time that the diet of sprats is described in the Gulf of Lions. Sprats fed on a larger size spectrum of prey and seem to be more generalist feeders compared to anchovies and sardines. Ontogenetic changes as well as spatial and temporal variations of the diet occurred in the three species. Stable isotope analysis revealed mobility of sardines and sprats among feeding areas while anchovies exhibited preferred feeding areas. Sprats showed a higher relative condition assessed by C/N ratios than sardines and anchovies. Our results showed an overlap of the trophic niches for the three species, indicating a potential trophic competition in the Gulf of Lions. (C) 2015 Elsevier B.V. All rights reserved.

This paper presents results on the spatial and temporal distribution patterns of mesozooplankton in the naturally fertilized region to the east of the Kerguelen Islands (Southern Ocean) visited at early bloom stage during the KEOPS2 survey (15 October to 20 November 2011). The aim of this study was to compare the zooplankton response in contrasted environments localized over the Kergue-len Plateau in waters of the east shelf and shelf edge and in productive oceanic deep waters characterized by conditions of complex circulation and rapidly changing phytoplankton biomass. The mesozooplankton community responded to the spring bloom earlier on the plateau than in the oceanic waters, where complex mesoscale circulation stimulated initial more or less ephemeral blooms before a broader bloom extension. Taxonomic compositions showed a high degree of similarity across the whole region, and the populations initially responded to spring bloom with a large production of lar-val forms increasing abundances, without biomass changes. Taxonomic composition and stable isotope ratios of size-fractionated zooplankton indicated the strong domination of herbivores, and the total zooplankton biomass values over the survey presented a significant correlation with the integrated chlorophyll concentrations in the mixed layer. The biomass stocks observed at the beginning of the KEOPS2 cruise were around 1.7 g C m −2 above the plateau and 1.2 g C m −2 in oceanic waters. Zooplankton biomass in oceanic waters remained on average below 2 g C m −2 over the study period, except for one station in the Polar Front zone (F-L), whereas zooplankton biomasses were around 4 g C m −2 on the plateau at the end of the survey. The most remarkable feature during the sampling period was the stronger increase in abundance in the oceanic waters (25 × 10 3 to 160 × 10 3 ind m −2) than on the plateau (25 × 10 3 to 90 × 10 3 ind m −2). The size structure and tax-onomic distribution patterns revealed a cumulative contribution of various larval stages of dominant copepods and euphausiids particularly in the oceanic waters, with clearly identifiable stages of progress during a Lagrangian time series survey. The reproduction and early stage development of dominant species were sustained by mesoscale-related initial ephemeral blooms in oceanic waters, but growth was still food-limited and zooplankton biomass stagnated. In contrast, zooplankton abundance and biomass on the shelf were both in a growing phase, at slightly different rates, due to growth under sub-optimal conditions. Combined with our observations during the KEOPS1 survey (January–February 2005), the present results deliver a consistent understanding of patterns in mesozooplankton abundance and biomass from early spring to summer in the poorly documented oceanic region east of the Kerguelen Islands.

During the KEOPS II survey in spring (Oct-Nov 2011), the northeast and southeast region Kerguelen Island (between 46°59S and 50°37 S and 69°50E and 74°59E) were intensively studied to understand the physical and biogeochemical processes governing the onset of the Kerguelen spring bloom. Mesozooplankton community spatial distribution, taxonomy structure, size structure and biomass were studied from 41 zooplankton net tow samples -(0-250m vertical net tows) analyzed both with a ZOOSCAN and a binocular. 13C and 15N content between distinct size fractions, pigment gut contents and respiration rates of of dominant species were measured as well. Here we report on first results obtained along two transects (East-West and North South) and along a lagrangian sampling in the central part of the region during the bloom development. Biomass, abundance and size structures show temporal and spatial differences linked to the different water masses and trophic conditions. Day-night sampling revealed strong variations in abundance and biomass during the bloom phase due to large organisms, mainly euphausiids. Different spatial patterns of zooplankton size spectra were found with the highest slope values on the shelf area showing a large ratio of small organisms and the lowest within the central part of the region.

Mesozooplankton from the epipelagic layer (0-200 m) was studied in two contrasted ecosystems: the first one is located in the Austral Ocean, around the Southern part of the Kerguelen shelf and over the HNLC (High Nutrient Low Chlorophyll) area, at the end of summer bloom period supported by natural iron enrichment (KEOPS, January-February 2005). The second study focused on the whole Mediterranean Sea, along a 3 000 km transect characterized by an oligotrophy gradient decrease from West to East (BOUM, June-July 2008) with low phosphorus concentration. Mesozooplanktonic community was characterized by both its stock composition (abundance and biomass) and its structure (taxonomic composition and size spectrum). Spatial distribution at the regional scale was studied and linked to different environmental and trophic parameters. The community impact on primary production was also estimated from different physiological process analyses (ingestion, respiration and excretion). Mesozooplanktonic community showed higher abundance and biomass over the Kerguelen shelf than in HNLC area. The community was essentially composed by copepodite stages, large quantity of exuviae and high respiration rates suggesting active growth. However, phytoplankton based ingestion rates were low implying the use of other food source such as microzooplankton. In the Mediterranean, abundance showed an increasing westward gradient with a southward decreasing gradient in the Occidental Basin. This distribution was strongly linked to the chlorophyll concentration. Mesozooplankton grazing on primary producers was important. Biogeochemical flux associated with the metabolic activities supported in excess of 100 % of the primary production needs in term of carbon, nitrogen and phosphorus. Relationships between the specific diversity and the environmental variables (physical, chemical and biological) showed a high regionalization in the Mediterranean Sea whereas in the Kerguelen shelf these relations are weak.