BIOLOGICAL COMPONENTS FROM THE SEINE ESTUARY - FIRST RESULTS

The Seine estuary plays an important role in the dynamics of the easte rn English Channel ecosystem. Nevertheless, its biological compartment is poorly known. This constitutes an important handicap to establish the precise state of the health of this major European estuary. The ob jectives of this study were to identify the life resources of the estu ary: macrobenthos, mesozooplankton, suprabenthos, and fish populations and to define the main trophic links in two parts of the estuary (i.e . the polyhaline and oligohaline zones). There is an impoverishment of the biological diversity from the polyhaline zone to the oligohaline zone. The benthic and pelagic fauna of the Seine estuary is similar to other North-Eastern Atlantic estuaries. But the pelagic fauna, especi ally the copepod Eurytemora affinis and the shrimp Palaemon longirostr is seemed to be more abundant in the Seine estuary than in other estua ries. Two macrobenthic communities occurred in the estuary: a diversif ied and abundant Abm alba-Pectinaria koreni community in the outer par t of the estuary and a Macoma balthica community in the inner part. Th is latter was especially poor in specific richness, density and biomas s, in all areas, except on tidal mud flats. Two trophic chains were id entified. In the oligohaline zone corresponding to the maximum turbidi ty zone (high turbidity, low concentration of oxygen), the trophic cha in was exclusively planktonic due to the dredging of the estuary which prevented permanent benthic fauna formation. In this zone, the number of fishes was relatively low in spite of high biomass of mesozooplank ton and suprabenthos. In the outer part of the estuary, low turbidity and high concentration of oxygen are more favourable to fish populatio ns which feed especially on benthic fauna.