Nutrient dynamics and productivity in three European estuaries

The Scheldt, Gironde and Sado estuaries exhibit quite different characteris tics regarding the annual river discharges, nutrient loads and oxygen condi tions. Distribution patterns of nutrients, dissolved oxygen, suspended matt er and chlorophyll a as well as the nutrient ratios and half saturation con stants, allow the identification and comparison of specific processes. The Scheldt emerges as the most eutrophic estuary being, in general, not nutrie nt limited and functions as a source of phosphorus and nitrogen. Intense bi ogeochemical processes take place in this system. The Sado is shown to be a moderately productive system, the upper part being enriched in nutrients a nd chlorophyll a and showing a tendency for oxygen depletion in summer. Dif ferent factors seem to control phytoplankton production along the salinity gradient: light regime landwards and nitrogen seawards. By contrast, the Gi ronde does not show clearly the presence of sinks or sources regarding nutr ients and exhibits a reduced productivity essentially controlled by turbidi ty, The high amounts of particles seem also to be responsible for localized processes such as phosphate sorption/desorption and silicate regeneration. (C) 1999 Elsevier Science Ltd. All rights reserved.