Nutrient Dynamics and Productivity in Three European Estuaries

The Scheldt, Gironde and Sado estuaries exhibit quite different characteristics regarding the annual river discharges, nutrient loads and oxygen conditions. Distribution patterns of nutrients, dissolved oxygen, suspended matter and chlorophyll a as well as the nutrient ratios and half saturation constants, allow the identification and comparison of specific processes. The Scheldt emerges as the most eutrophic estuary being, in general, not nutrient limited and functions as a source of phosphorus and nitrogen. Intense biogeochemical processes take place in this system. The Sado is shown to be a moderately productive system, the upper part being enriched in nutrients and chlorophyll a and showing a tendency for oxygen depletion in summer. Different factors seem to control phytoplankton production along the salinity gradient: light regime landwards and nitrogen seawards. By contrast, the Gironde does not show clearly the presence of sinks or sources regarding nutrients and exhibits a reduced productivity essentially controlled by turbidity. The high amounts of particles seem also to be responsible for localized processes such as phosphate sorption/desorption and silicate regeneration.