COMPARATIVE SPRING DISTRIBUTION OF ZOOPLANKTON IN 3 MACROTIDAL EUROPEAN ESTUARIES

The zooplankton of three european estuaries (Ems, Gironde and Westersc helde) was investigated during spring 1992 by means of samples taken a long the salinity gradient. The three estuaries are comparable in term s of total area, flushing time and salinity gradient but differ by the ir level of eutrophication (highest in the Westerschelde), suspended m atter concentration (highest in the Gironde) and potential phytoplankt on production (highest in the Ems). Copepods and meroplankton dominate d the zooplankton in the three estuaries. The dominant copepod species were Eurytemora affinis and Acartia bifilosa. The distribution of E. affinis along the salinity gradient differed between the estuaries. Pe aks of abundance were observed at O PSU in the Gironde, 6 PSU in the E ms and 9 PSU in the Westerschelde. The downstream shift of the populat ion in the Westerschelde was likely due to anoxic conditions occurring in the oligohaline zone. In the Gironde the downstream distribution o f E. affinis was limited by the very high suspended matter concentrati on found in the maximum turbidity zone. Whatever the estuary, the para meters of the population of E. affinis and maximum abundance values we re similar. However, the influence of the better quality of the availa ble food was suggested in the Ems where individual dry weights and egg production were higher than in the two other estuaries. The influence of a good quality of food in the Ems was confirmed by the development of a large population of Acartia bifilosa (as abundant as E. affinis) and highest values of adult individual weights. The meroplankton (ess entially Polychaete and cirripede larvae) was much more developed in t he Ems than in the Westerschelde and Gironde. This was likely due to t he large extent of mudflats and hard substrates in the Ems favouring a dult settlement and hence the number of larvae locally produced.