Land-fast ice off Adélie Land (Antarctica): short-term variations in nutrients and chlorophyll just before ice break-up

This study focused on the short-term variations of sea ice microalgal biomass and nutrients, in Adélie Land coastal area (Antarctica). The annual land-fast ice, platelet ice-like layer (PLI) and under-lying seawater were sampled during the 1999 austral spring. The study was conducted during 33 days preceding the ice break-up, with a daily sampling when the meteorological conditions were favourable. Time-series for salinity, chlorophyll a (Chl a), NH4+, NO2−, NO3−, PO4− and Si(OH)4 are shown. We also provide satellite data and meteorological variables for the same period. During the study period, the solid ice thickness varied from 1.47 to 1.05 m. Pigments were concentrated in bottom ice, but progressively invaded the PLI. Phaeo a increased in the PLI during the last weeks, in parallel with increasing NO2− concentration. Si(OH)4, in lowest concentrations in bottom ice, increased progressively in the under-ice water (UIW) and PLI, this was because of offshore water inputs. NH4+ concentrations were high in every sea ice component (particularly in the bottom ice) and were inversely correlated to pigments in the bottom ice. NH4+ concentrations progressively increased in the PLI and underlying seawater just before the break-up. In the bottom ice, PO4− concentrations were related to high pigment concentrations, but with a short lag close to the break up period, indicating in situ regeneration. Neither NO3− nor NH4+ concentrations were exhausted in the bottom ice. NO2− and NH4+ concentrations increased in PLI and under-ice water just before the break-up period, which might indicate strong inorganic nitrogen recycling in land-fast ice. Approximately 0.4 kmol km−1 linear coast of NH4+ (1 kmol NO3− and 2 kmol PO4−) were released to the under-ice seawater during break-up, along with 0.12 tons km−1 Chl a (9.6 tons POC km−1 equivalent). These nutrient sea ice inputs to the coastal zone waters, during the ice recession and break-up, may vary annually depending upon the local meteorological conditions that control ice formation, duration of ice cover and melting

Temporal changes in the biochemical composition of particulate organic matter sedimentation in the Bay of Banyuls-sur-Mer

Gross sedimentation rates (GSRs) were monitored together with meteorological data and the main biochemical characteristics of the collected material (i.e. organic content, C, N, total proteins, carbohydrates, lipids and available proteins) over a three year cycle at a shallow station (18 m) of the Bay of Banyuls, France. GSRs were between 0.26 (11/04/92) and 317.75 gDW m(-2) d(-1) (18/02/94). They followed an annual cycle with low and relatively constant values during spring and summer and high, variable Values during fall and winter. During the study GSRs correlated poorly (state of the sea, precipitation) or even non-significantly (wind speed) with the main meteorological parameters. GSRs correlated negatively with the organic content of the material collected within the sediment traps suggesting the importance of resuspension in controlling GSRs. This hypothesis is consistent with the fact that, among all the measured biochemical parameters, available proteins (i.e. enzymatically hydrolizable proteins) are the most negatively correlated with GSRs. Available proteins showed important temporal changes that are only partly described by changes in carbon and nitrogen contents. These results are discussed in relation to the conceptualisation and the modelling of benthic trophic networks. The major objectives of our research group within the framework of the second phase of the French "Programme National d'Oceanographie Cotiere" are briefly discussed.Les taux de sédimentation brute (GSR) ont été mesurés avec les principaux paramètres météorologiques et les principales caractéristiques biochimiques (i.e. matière organique, C, N, protéines totales, hydrates de carbone, lipides et protéines disponibles) sur une période de trois ans à une station (18 m de profondeur) située en baie de Banyuls, France. Les GSR sont compris entre 0,26 (11/04/92) and 317,75 g m−2 j−1 (18/02/94). Ils suivent un cycle annuel avec des valeurs faibles et relativement constantes durant la période printemps-été et des valeurs plus fortes et beaucoup plus variables durant la période automne-hiver. Les taux de sédimentation brute ne sont corrélés avec les paramètres météorologiques mesurés que faiblement (état de la mer, précipitation) ou même pas significativement (vitesse du vent). Les taux de sédimentation brute sont corrélés négativement avec les contenus organiques du matériel recueilli dans les pièges à particules, ce qui suggère l'importance du processus de resuspension dans le contrôle des taux de sédimentation brute. Cette hypothèse est en accord avec le fait que, parmi tous les paramètres biochimiques mesurés, les protéines disponibles (i.e. dégradables enzymatiquement) sont les plus négativement corrélées avec les taux de sédimentation brute. Les protéines disponibles montrent des variations temporelles importantes qui ne sont qu'imparfaitement décrites par les contenus en carbone et azote. Ces résultats sont discutés en relation avec la conceptualisation et la modélisation des réseaux trophiques benthiques. Les principaux objectifs de notre groupe lors de la seconde phase du Programme national d'océanographie côtière sont brièvement évoqués

Relationship between sedimentary organic matter and benthic fauna within the Gulf of Lion: synthesis on the identification of new biochemical descriptors of sedimentary organic nutritional value

The abilities of several biochemical characteristics of sedimentary organic matter to describe quantitative changes in benthic fauna were assessed based on several surveys carried out within the Gulf of Lion during the last decade. Two sets of high frequency samplings were carried out both inshore and offshore, together with an assessment of seasonal variability along a depth gradient, and a synoptic assessment of mesoscale spatial variability. The considered biochemical parameters were: total organic matter, organic carbon, nitrogen, total proteins, available proteins, carbohydrates, lipids, total and available amino acids. The considered faunal parameters were the abundance and the biomass of both meio- and macrofauna. Our results contribute to unravel the relative importance of food digestibility and composition in controlling particulate organic matter nutritional value. Spectra of both total and available amino acids were almost constant irrespective of the seasons and/or the environments, which were studied during these surveys. This underlines the role of particulate organic matter digestibility relative to its ability to meet specific nutritional requirements in controlling benthic fauna. Multivariate analysis used to relate biochemical and faunal parameters suggest that available amino acids and lipids are the best descriptors of food nutritional value. In contrast with carbohydrates, these two parameters are both associated with the most labile fraction of particulate organic matter. This result suggests that the use of the sum of proteins, carbohydrates and lipids as a proxy for labile organic matter is precluded at least in areas similar to the Gulf of Lion.Les aptitudes de plusieurs des caractéristiques biochimiques de la matière organique particulaire sédimentée à décrire les changements quantitatifs de la faune benthique ont été comparées à partir des résultats de plusieurs campagnes de prélèvements ayant eu lieu dans le golfe du Lion au cours de ces dix dernières années. Ces travaux ont consisté en deux suivis haute fréquence conduits à une station littorale et à une station du large, en une évaluation de la variabilité saisonnière le long d’un gradient côte large, et, enfin, en une étude synoptique de la variabilité spatiale à mesoéchelle. Les paramètres biochimiques considérés sont la matière organique totale, le carbone organique, l’azote, les protéines totales, les protéines disponibles, les sucres, les lipides, les acides aminés totaux et les acides aminés disponibles. Les paramètres faunistiques considérés sont l’abondance et la biomasse de la meio- et de la macrofaune. Les résultats obtenus contribuent à délimiter l’importance relative de la digestibilité et de l’aptitude à satisfaire les besoins nutritionnels dans la définition de la valeur nutritive de la matière organique particulaire sédimentée. Ainsi, la constance des spectres en acides aminés totaux et disponibles quels que soient les environnements et les saisons étudiés, suggère-t-elle un plus grand rôle de la digestibilité de la matière organique particulaire que de son aptitude à satisfaire des besoins nutritionnels spécifiques dans le contrôle de la faune benthique. Par opposition aux sucres, les lipides et les acides aminés disponibles sont associés à la fraction la plus labile de la matière organique particulaire. Ceci rend inopérant l’usage de la somme des concentrations en protéines, sucres et lipides en tant qu’indice de la fraction labile de la matière organique, au moins dans les zones directement comparables au golfe du Lion

Energy transfer in the Congo deep-sea fan: from terrestrially-derived organic matter to chemosynthetic food webs

Large amounts of recent terrestrial organic matter (OM) from the African continent are delivered to the abyssal plain by turbidity currents and accumulate in the Congo deep-sea fan. In the recent lobe complex, large clusters of vesicomyid bivalves are found all along the active channel in areas of reduced sediment. These soft-sediment communities resemble those fuelled by chemoautotrophy in cold-seep settings. The aim of this study was to elucidate feeding strategies in these macrofaunal assemblages as part of a greater effort to understand the link between the inputs of terrestrially-derived OM and the chemosynthetic habitats. The biochemical composition of the sedimentary OM was first analysed in order to evaluate how nutritious the available particulate OM is for the benthic macrofauna. The terrestrial OM is already degraded when it reaches the final depositional area. However, high biopolymeric carbon contents (proteins, carbohydrates and lipids) are found in the channel of the recent lobe complex. In addition, about one to two thirds of the nitrogen can be assigned to peptide-like material. Even if this soil-derived OM is poorly digestible, turbiditic deposits contain such high amounts of organic carbon that there is enough biopolymeric carbon and proteacinous nitrogen to support dense benthic communities that contrast with the usual depauperate abyssal plains. Stable carbon and nitrogen isotopes and fatty acid biomarkers were then used to shed light on the feeding strategies allowing the energy transfer from the terrestrial OM brought by the turbidity currents to the abyssal food web. In the non-reduced sediment, surface detritivorous holothurians and suspension-feeding poriferans rely on detritic OM, thereby depending directly on the turbiditic deposits. The sulphur-oxidising symbiont bearing vesicomyids closely depend on the reprocessing of OM with methane and sulphide as final products. Their carbon and nitrogen isotopic signatures vary greatly among sites and could reflect the intensity of the anaerobic oxidation of methane (AOM) in the sediments. Within the vesicomyid habitats, the heterotrophic fauna exhibits a distinctively light carbon isotopic signature in comparison to the background sediments, clearly indicating the utilisation of chemosynthetically-derived OM. Fatty acid biomarkers further confirm that dorvilleid polychaetes consume aggregates that perform AOM. Terrestrial OM reprocessing by microbial consortium thus ensures its transfer to the benthic food web in the Congo deep-sea fan

The Congolobe project, a multidisciplinary study of Congo deep-sea fan lobe complex: Overview of methods, strategies, observations and sampling

The presently active region of the Congo deep-sea fan (around 330 000 km2), called the terminal lobes or lobe complex, covers an area of 2500 km2 at 4700–5100 m water depth and 750–800 km offshore. It is a unique sedimentary area in the world ocean fed by a submarine canyon and a channel-levee system which presently deliver large amounts of organic carbon originating from the Congo River by turbidity currents. This particularity is due to the deep incision of the shelf by the Congo canyon, up to 30 km into the estuary, which funnels the Congo River sediments into the deep-sea. The connection between the river and the canyon is unique for major world rivers. In 2011, two cruises (WACS leg 2 and Congolobe) were conducted to simultaneously investigate the geology, organic and inorganic geochemistry, and micro- and macro-biology of the terminal lobes of the Congo deep-sea fan. Using this multidisciplinary approach, the morpho-sedimentary features of the lobes were characterized along with the origin and reactivity of organic matter, the recycling and burial of biogenic compounds, the diversity and function of bacterial and archaeal communities within the sediment, and the biodiversity and functioning of the faunal assemblages on the seafloor. Six different sites were selected for this study: Four distributed along the active channel from the lobe complex entrance to the outer rim of the sediment deposition zone, and two positioned cross-axis and at increasing distance from the active channel, thus providing a gradient in turbidite particle delivery and sediment age. This paper aims to provide the general context of this multidisciplinary study. It describes the general features of the site and the overall sampling strategy and provides the initial habitat observations to guide the other in-depth investigations presented in this special issue. Detailed bathymetry of each sampling site using 0.1 m to 1 m resolution multibeam obtained with a remotely operated vehicle (ROV) shows progressive widening and smoothing of the channel-levees with increasing depth and reveals a complex morphology with channel bifurcations, erosional features and massive deposits. Dense ecosystems surveyed in the study area gather high density clusters of two large-sized species of symbiotic Vesicomyidae bivalves and microbial mats. These assemblages, which are rarely observed in sedimentary zones, resemble those based on chemosynthesis at cold-seep sites, such as the active pockmarks encountered along the Congo margin, and share with these sites the dominant vesicomyid species Christineconcha regab. Sedimentation rates estimated in the lobe complex range between 0.5 and 10 cm yr−1, which is 2-3 orders of magnitude higher than values generally encountered at abyssal depths. The bathymetry, faunal assemblages and sedimentation rates make the Congo lobe complex a highly peculiar deep-sea habitat driven by high inputs of terrigenous material delivered by the Congo channel-levee system