Spatiotemporal dynamics of physicochemical and photosynthetic parameters in the central English Channel
Résumé
Hydrological and photosynthetic parameters were investigated monthly from November 2009 to December 2010 using an opportunity ship, along a transect in the central part of the English Channel. A powerful statistical method, a Partial Triadic Analysis (PTA), was applied on the physicochemical data set and highlighted the functioning of four different ecosystems: (i) the French coast area (FRcoast) subjected to large freshwater inputs, (ii) the North of the Seine Bay (NSBay) influenced by nutrient inputs from the river Seine and offshore, (iii) the centre of the English Channel (CentreEC) characterized by nutrient offshore inputs and (iv) the English coast area (UKcoast) which exhibits low nutrient concentrations despite the proximity of the English coast. In the FRcoast and the NSBay the phytoplankton biomass peaked from mid-January to June. In contrast, in the UKcoast the phytoplankton biomass peak occurred from mid-November to March. In addition, we noticed that these coastal ecosystems showed two types of photosynthetic regulations. Photoacclimation mechanisms were observed in the FRcoast and the NSBay and a control of the photosynthetic parameters by nutrients in the UKcoast. This study highlights the importance of a "top down" control in winter/spring and underlines a complex situation in autumn with an uncoupling between rETRmax and the Chlorophyll a biomass. In order to understand this uncoupling it is necessary to estimate the loss due to zoo-plankton grazing and viral lysis as well as carbon excretion