Franck Touratier

Simulated carbon and nitrogen flows of the planktonic food web during an upwelling relaxation period in St Helena Bay (southern Benguela ecosystem)

Abstract A vertically resolved ecosystem model is developed to simulate the dynamics of the pelagic food web in St Helena Bay during a representative period of relaxation after an upwelling event. The proposed model aims at coupling three biogeochemical cycles (carbon, nitrogen and silicon), using several recently developed concepts of the stoichiometric approach. A consequence of this approach is that important qualitative aspects are introduced, such as indicators of phytoplankton physiological state or variable food C:N ratios. For instance, the sedimentation and exudation rates for phytoplankton vary according to physiological state. An attempt is made to parameterize and simulate the diel cycles for vertical migration and feeding rhythms of large zooplankton, two important mesoscale processes that are thought to influence the overall dynamics of the huge phytoplankton blooms in the region. Observations of the Anchor Station Experiment 1987 (ASE’87) are used to assess the quality of the model. There is overall agreement between observations and the corresponding simulated results. The timing, the magnitude, and the vertical structure of the phytoplankton bloom are well reproduced. The balances for carbon and nitrogen flows and stocks compare well to the numerous estimates found from the literature for the southern Benguela region. On the basis of the model results, the origin of the new nutrients, the fate of the carbon fixed by phytoplankton, and the importance of the microheterotrophic pathways are discussed. It is concluded that sediments of the St Helena Bay and surrounding areas may play a crucial role in increasing the level of phytoplankton production. The results also suggest that exudation is the main process by which the carbon fixed by phytoplankton would have been lost, and that microheterotrophic pathways would have been intense during the experiment.

Time variability of the north-western Mediterranean Sea pH over 1995-2011.

Factors controlling ocean acidification and its temporal variations were studied over the 1995-2011 period at the Dyfamed site at 10 m depth, in the North Mediterranean Sea. The results indicated a mean annual decrease of 0.003 ± 0.001 pH units on the seawater scale. The seasonal variability was characterized by a pH decrease during springtime and a strong pH increase in late fall. Anthropogenic CO2 (CANT) absorption by the ocean was the key driver of seawater acidification in this region, accounting for about 70% of the observed drop in pH, followed by water temperature (about 30%). The total inorganic carbon (CT) data showed a CT increase of 30.0 ± 1.0 μmol kg(-1) per decade. This decadal increase is mainly due to the CANT penetration (43.2 μmol kg(-1) per decade) in surface waters, which is mitigated for by relatively small opposing changes in CT due to physical and biological processes.

Northeast Water Polynya 1993: construction and modelling of a time series representative of the summer anticyclonic gyre pelagic ecosystem

Abstract A multidisciplinary international oceanographic expedition was conducted in the Northeast Water Polynya (NEW, northeast of Greenland) from May to August 1993, to understand the formation of the polynya and its influence on the pelagic ecosystem. The residual circulation in the polynya is characterized by an anticyclonic gyre, which follows a system of troughs. The numerous data acquired during the expedition and the resulting publications provide the background for an ecological modelling study. Since the sampling scheme during this expedition was not appropriate for implementing ecological models, the first objective of the present study was to build up a multivariate time series. Stations in the time series were selected by taking into account the residual circulation in the anticyclonic gyre. Model outputs were compared to data along the time series. From the time series alone, it was not possible to fully understand the development of a second phytoplankton bloom, so that we formulated three hypotheses on the functioning of the ecosystem during that period: (H1) horizontal supply of nitrate from waters located to the north; (H2) vertical diffusion of nitrate; (H3) local remineralization processes in the surface layer. H2 and H3 are evaluated using two ecological models, in which the same components are simulated, but the first considers only the nitrogen cycle and the second simulates both the carbon and nitrogen cycles. Comparing the chemical and biological variables simulated by the models to the observed time series data by reference to the three hypotheses lead to the conclusion that H2 is the most likely hypothesis. This also means that the summer pelagic ecosystem in the anticyclonic gyre was perhaps dominated by a short food chain that mostly comprised large phytoplankton, copepods and appendicularians. However, because the parameterization of vertical mixing used to test hypothesis H2 may be an oversimplification of field conditions, the hypothesis cannot be fully tested.

Ocean salinity from satellite-derived temperature in the Antarctic Ocean

Abstract The aim of the MINERVE project (Mesures à l’INterface Eau-aiR de la Variabilité des Échanges de CO2) is to observe and understand the seasonal and interannual variability of the partial pressure of CO2 (pCO2) in surface waters using hydrological and biogeochemical data in the Southern Ocean south of Australia. Logistics routes of the RV Astrolabe provide access to scarcely studied areas, thus allowing us to understand the different processes acting in this area of the Antarctic Ocean. The surface area covered by these cruises, however, is tiny compared with the total surface area of the Antarctic Ocean. Correlations between in situ surface temperature and salinity data were applied to satellite images of sea surface temperature to map ocean surface salinity over a much wider area than under the cruise tracks. Comparisons with salinity data from satellites which provide ~100 km resolution and 0.1 accuracy indicate that we are able to map salinity at 4 km resolution and almost the same accuracy of ± 0.1.

Generalized total least squares to characterize biogeochemical processes of the ocean

The chemical composition of the global ocean is governed by biological, chemical, and physical processes. These processes interact with each other so that the concentrations of carbon, oxygen, nitrogen (mainly from nitrate, nitrite, ammonium), and phosphorous (mainly from phosphate), vary in constant proportions, referred to as the Redfield ratios. We construct here the generalized total least squares estimator of these ratios. The significance of our approach is twofold; it respects the hydrological characteristics of the studied areas, and it can be applied identically in any area where enough data are available. The tests applied to Atlantic Ocean data highlight a variability of the Redfield ratios, both with geographical location and with depth. This variability emphasizes the importance of local and accurate estimates of Redfield ratios.