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International audience; Microzooplankton play an important role in aquatic food webs through their multiple interactions with other organisms and their impact on carbon export. They are major predators of phytoplankton and bacteria while being preyed on by higher trophic levels. Microzooplankton diversity (Dinoflagellates, DIN and Ciliates, CIL), community structure, interaction with phytoplankton and its potential in channeling carbon to higher trophic levels were studied in contrasting productivity regimes (off- and on-plateau, the latter been naturally fertilized by iron) around the Kerguelen islands in the Southern Ocean (SO). DIN and CIL diversity was sampled in late summer (February-March 2018; project MOBYDICK) and at the onset-of the bloom (KEOPS2 cruise), and assessed by Illumina sequencing of 18S rDNA amplicons and microscopic observations. The diversity obtained by the two approaches could be compared at a relatively high taxonomic level (i.e., often to family level). In particular for DIN, relative abundances and ranking of dominant taxa differed between sequencing and microscopy observations. CIL were always recorded at considerably lower abundances than DIN, the median of their abundances across stations and seasons being 350 and 1370 cells L-1, respectively. During late summer, DIN and CIL biomasses were about 1.5 times higher on- than in off-plateau waters, while community composition was spatially similar. The most abundant DIN at all stations and during both seasons were small Gymnodinium (-1, g = 0.36 ± 0.09 d-1) in late summer. Despite having great potential as a phytoplankton grazer, microzooplankton occurred at low biomass and showed little temporal variability, suggesting that it was controlled by copepod predation. Microzooplankton is a key component of the SO as an intermediate trophic level mediating carbon transfer from primary producers to higher trophic levels.

International audience; The Kerguelen Plateau is a region of natural iron fertilization in the Southern Ocean, within the typically iron limited High Nutrient Low Chlorophyll (HNLC) waters of the eastward flowing Antarctic Circumpolar Current. Between 26 February and 19 March 2018, the MOBYDICK expedition investigated pelagic ecosystem dynamics in the Kerguelen Island region on the northern plateau during the post-phytoplankton bloom period. The survey specifically targeted sampling at two stations in the HNLC waters to the west of the Kerguelen Plateau, and in the iron enriched waters on and to the east of the plateau. A combination of WP2, WP3 and Mesopelagos midwater trawl were used to sample the mesozooplankton (125 μm to ≤ 10 mm), macrozooplankton (10–30 mm) and micronekton (> 30 to 200 mm) communities. Stable carbon (δ13C) and nitrogen (δ15N) isotopes were measured across representative samples of taxa from all stations. Trophic positions was estimated using a Bayesian isotope mixing model (tRophicPosition), that uses both the δ13C and δ15N values of baselines and consumers. Meso/macrozooplankton trophic positions (TPs) were ~ 2.4 on the plateau and > 0.6 TPs higher at the upstream HNLC stations. This provides empirical evidence for shorter food chains on the diatom dominated plateau and longer food chains in the HNLC region dominated by pico and nanophytopklankon. Meso/macrozooplankton TPs were also elevated downstream of the plateau, though to a lesser extent than in the upstream region, likely reflecting the effect of downstream iron transport, supporting elevated phytoplankton production in the downstream region. There was a consistent increase in trophic position between meso/macrozooplankton and micronekton of ~ 0.6 at all stations. Food chain length was therefore determined by the composition of and interactions between the trophic levels below micronekton. Similar trophic structure at the two upstream HNLC stations, on either side of the northern branch of the Polar Front, indicated that food chain length was robust to the differences in community composition, and supports the critical role of size structured trophic interactions in determining food web properties.

The possibility of using the natural biofiltration power of blue mussels (Mytilus edulis, 0.37 ± 0.08 g ind−1 dry weight) to dampen the potential detrimental effect of phytoplankton blooms on juvenile farmed sea bass (Dicentrarchus labrax) was tested in a fish farm during a 35-day mesocosm experiment. Mussel effective clearance rates averaged 41.15 ± 14.19 m3 h−1 and led to a 6.3–13.1-fold reduction of the phytoplankton abundance as well as comparable decreases in chlorophyll a and turbidity. This improvement in seawater quality significantly enhanced fish physiological performances: weight-based growth rates were significantly higher (2.87 ± 0.43% d−1) compared to control exposed to non-filtered (bulk) seawater (2.55 ± 0.44% d−1). The same observation holds for the Fulton condition index and the metabolic activity (RNA:DNA ratio). For fish reared in bulk seawater, diatoms embedded in gills (Rhizosolenia imbricata, Thalassiosira sp.) and mucus overproduction indicated a stress (i.e. mechanical damages) induced by phytoplankton exposure which, in turn, may have affected fish energy balance. The use of mussels as a satisfying mitigation tool reducing phytoplankton bloom impacts is discussed with regard to phytoplankton bloom magnitude and ashore marine fish farming in coastal ecosystems.

International audience; Mercury (Hg) in fish can present a potential risk to wildlife and human consumers. Mercury levels were determined in 12 fish species and 2 invertebrate species from the French coast of the Eastern English Channel collected in June and November 2012. Total mercury concentrations in the muscle tissues ranged between 0.04 and 1.63 µg g-1 (d.w.). Dab and sandeel displayed a significant positive correlation between Hg concentrations and fish length. Seasonal differences in Hg concentrations were observed for dab and shrimp. The lesser weever showed significantly higher concentrations of Hg (0.97 ± 0.34 µg g-1 d. w.) than all other species. Gobies and sprat, which had the smallest size, showed significantly lower concentrations than herring, whiting, pout and sardine. Among the considered species, flatfish such as dab, flounder, plaice and sole displayed intermediate levels of Hg. In the present study, the fish age expressed as specimen length seems to be one of the major factors governing Hg contents in muscle tissues. Finally, mercury concentrations in commercial fish from the Eastern English Channel were below the levels fixed by the European Union for total mercury in edible parts of fish products.

International audience; A 45-day exposure experiment was carried out to assess whether the harmful algal blooming Pseudo-nitzschia delicatissima (i.e. needle shaped diatom and potential domoic acid producer) is involved in recurrent mass mortality events at a coastal fish farm in the southern North Sea. Juvenile sea bass (Dicentrarchus labrax) were exposed to different concentrations of P. delicatissima, ranging from 1 × 106 to 5 × 106 cells L− 1. Despite exposure to high concentrations of P. delicatissima, sea bass weight and length based growth rates (0.05 ± 0.02 g d− 1 and 0.18 ± 0.05 mm d− 1, respectively) and condition (Fulton K = 0.98 ± 0.06 mg mm− 3) were not significantly reduced suggesting that the fish were not affected during the 45-day exposure period. However, signs of gill mucosa irritation (i.e. mucus overproduction) were observed, showing that this diatom induced a mechanical stress, which can affect gill functions or increase susceptibility of fish to secondary infections (e.g. bacterial and viral infections, parasitism). Results are discussed with regard to consequences for wild and farmed fish, and possible synergic noxious effects when P. delicatissima is associated with other potentially harmful taxa (e.g. Phaeocystis globosa) during the phytoplankton spring bloom in the eastern English Channel–North Sea ecosystem.

International audience; The presence of Mnemiopsis leidyi A. Agassiz, 1865 along the French coasts of the Eastern English Channel and the North Sea (EEC-NS) was established via morphological observation and molecular evidence. The earliest records were from surveys carried out in autumn 2005 in Le Havre harbour (Bay of Seine, EEC) and coincided with the historical introduction of the species in other Northern European waters. Since 2009, the species has also been frequently observed along the French coast of the North Sea. Results indicate M. leidyi has established a self-sustaining population in the Bay of Seine, which may act as a source population for northern European harbours via commercial shipping.