ZOOPLANKTON BIOMASS IN THE OOSTERSCHELDE (SW NETHERLANDS) BEFORE, DURING AND AFTER THE CONSTRUCTION OF A STORM-SURGE BARRIER

The hydrodynamic consequences of large coastal engineering (barrier-co nstruction) works in the Oosterschelde were: prolonged residence times of the water, increased sinking of particulate material, and higher w ater transparencies. This strongly influenced the phytoplankton (Bakke r et al., 1990; 1994) and phytoplankton biomass increased in the shall ow Eastern compartment of the Oosterschelde (Bakker & Vink, 1994) whil e phytoplankton concentration of the seston rose. Zooplankton biomass, especially of copepods (Temora) and meroplankton (barnacle larvae) in creased during the post-barrier period in the Eastern compartment. It is hypothesized that this is caused by the improved feeding conditions and the increased retention times in this area. The barrier years 198 5 and 1986 were characterized by low current velocities. In the Easter n compartment, this may have favoured the development of the rotifer S ynchaeta (Bakker, 1994) and of the important copepod predator Pleurobr achia (Ctenophora). In the Western compartment, zooplankton developmen ts in the post-barrier years were rather similar to those in the pre-b arrier period. This led to the disappearance of the previously existin g biomass gradients West-East (maxima in West). At present a trend in the opposite direction (maxima in East) is observed.