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The Veersche Gat and the Zandkreek formed a sea-arm, which since its closure has been known as the Veerse Meer. Changes in the environment and plankton before and after the closure are reviewed. For the quantitative plankton investigations the method ofLohmann (1908) was applied. As parts of the Oosterschelde system the Veersche Gat and the Zandkreek were poly-euhaline biotopes. Partial isolation of the central stations of the Zandkreek was caused by the existence of a tidal divide at Wolfaartsdijk (Fig. 2). The effect of this divide was illustrated by aberrant chlorinity values and by different qualitative and quantitative composition of the plankton communities. The Zandkreek plankton appeared to be a neritic biocoenose, consisting of a phytoplankton community of mainly pelagic diatoms together with a zooplankton community with strongly littoral characteristics. The size of the standing crops generally increased towards the sea. Phytoplankton at the tidal divide sometimes showed an autochthonous development. Sometimes phytoplankton organisms occurring throughout the whole water body were seen to bloom notably within the area of the tidal divide. In summer a strong autochthonous development of littoral zooplankton made the standing crops within the Zandkreek larger than those of the Veersche Gat. In the second and third years after the formation of the Veerse Meer, a decline in salinity transferred this water body from a polyeuhaline into a meso-polyhaline environment. This decrease in salinity was coupled with a strong eutrophication, evidenced, for example, by high phosphorous contents of the water and caused by the discharge of water from the neighbouring polders and by pollution from sewage. The elimination of many polyhaline-marine species was associated with the gradual appearance of many brackish-water species with flagellates and dinoflagellates as the most important groups. At the same time the standing crops increased greatly. The severe winter at the beginning of 1963 resulted in a marked salinity stratification. In the course of the spring when the ice cover disappeared and the temperature of the upper layers rose, the halocline was combined with a thermocline, and this double stratification was observed until August, 1963, when the wind gradually succeeded in mixing all layers of the water.

Plankton investigations in a Dutch sea-arm before and after dike construction