Jean-François Guillaud

Satellite and in situ observations of a late winter phytoplankton bloom, in the northern Bay of Biscay

A phytoplankton bloom was observed in late winter 2000, on the continental shelf offshore of southern Brittany, in northwestern Bay of Biscay. This bloom appeared initially along the 120-m isobath, in stratified and clear waters, at the interface between the oceanic water and the plumes of southern Brittany rivers (mainly the Loire and Vilaine). The development of the bloom was triggered by favourable meteorological conditions, characterised by solar irradiance reaching the maximum level expected for that period of the year. Outside of the bloom area, the phytoplankton photosynthesis was irradiance limited: inshore, because of the stronger attenuation of the light; offshore, because of the weak stratification. The hydrological conditions at the onset of the bloom were observed in the field, during the oceanographic cruise MODYCOT. However, without SeaWiFS, the only observations related to this major event in the primary production would have been those of the coastal phytoplankton network (REPHY (REseau PHYtoplankton)).

Biogeochemical modelling in the Bay of Seine (France): an improvement by introducing phosphorus in nutrient cycles

As part of the French National Programme for Coastal Oceanography, this paper focuses on improvement of biogeochemical modelling in the: Bay of Seine (Eastern Channel), by introducing phosphorus in nutrient cycles. The Bay of Seine receives the Seine river, which exhibits very high nutrient concentrations, and this coastal zone constitutes a typical case of eutrophication in a river plume area. In terms of analyses, sequential extraction and analysis of sedimentary phosphorus were used in order, to measure various forms of particulate phosphorus in suspended matter and sediment (calcium-bound phosphate, Fe/Al-bound phosphate, exchangeable phosphate and organic phosphorus). In the modelling approach, the Bay is divided into 42 boxes and a two-layer, vertical thermohaline model is linked with the horizontal circulation scheme to take vertical stratification into account. The previous biological sub-model, with only nitrogen and silicon as nutrients, was improved by taking account of (a) various forms of bioavailable phosphorus (dissolved phosphate, exchangeable phosphate and organic phosphorus) and (b) experimental parameters, which govern the adsorption/desorption of phosphate at the solid-water interface. The ecological model accurately reproduces dissolved nutrient behaviour in the river plume, and concentrations of diatoms, flagellates and zooplankton are consistent with observed values. Simulated exchangeable phosphate and organic phosphorus in suspended matter are in the range of measured concentrations, whereas these particulate variables are simulated with less accuracy in the sediment. After evaluating the respective roles of nitrogen, phosphorus and silicon as limiting factors for phytoplanktonic growth, the model was used to estimate consequences of reduced phosphorus or nitrogen input for phytoplanktonic production in the Bay of Seine.

Modelling the eutrophication process in a River Plume: The Seine Case Study (France)

Abstract As part of a French National Program aiming at better understanding and modelling the coastal eutrophication process, the Bay of Seine (Eastern Channel) has been chosen as a typical case of phytoplanktonic eutrophication in a river plume. Flow rates and nutrient concentrations in the river Seine show a continuous increase of N loadings during the last decade, whereas Si loadings are stationary and P loadings decrease, due to a reduction of industrial discharges. Seasonal cruises made in 1978 and 1992 showed the nutrient enrichment of the eastern part of the Bay and the high chlorophyll content of the plume area, especially in the surface layer. In order to quantify the respective roles of horizontal advection, vertical stratification, meteorologically-induced versus man-induced variations of nutrient loadings in the global eutrophication process, the mathematical modelling approach was applied. Based on a one nautical mile mesh size map of computed lagrangian tidal residual currents, a box-model ...

New instruments to monitor coastal sea water masses according to European Water Framework Directive, Trophimatique project

The automated monitoring of physico-chemical parameters in the coastal zone has been using large buoys and fixed infrastructures. A better understanding of many estuaries is needed in order to fulfil the agenda and requirements of the European Water Framework Directive. In order to address the fluxes of nutrients as well as algal blooms and low oxygen events, the “TROPHIMATIQUE” project has developed a new generation of instruments able to be deployed in sea waters. The smaller size of the new instruments is intended to facilitate the maintenance.