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The Sluice-dock at Ostend
Polk, P. (1978). The Sluice-dock at Ostend. Rapp. et Proc.-Verb. Cons. Int. Explor. Mer 173: 43-48
In: Rapports et Procès-Verbaux des Réunions du Conseil Permanent International pour l'Exploration de la Mer. Conseil Permanent International pour l'Exploration de la Mer: Copenhague. ISSN 0074-4336
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| Abstract |
The sluice-dock at Ostend is an artificial lagoon (1.5 m deep and with an area of 86 ha) connected to the harbour of Ostend and the North Sea by sluices (vannes). At low tides, during spring tide, the whole dock can be emptied, washing out the planktonic biomass. As an oyster culture exists in this biotope, applied and fundamental research is done. Planktonic larvae of oyster predators can be eliminated from the biotope by washing out the whole water volume during their bloom and replacing it by new sea-water. The oyster pest Crepidula fornicata (introduced with oyster spatfall from the Netherlands and reproducing heavily in the sluice-dock, otherwise not occurring along the Belgian coast) is controlled in this way (before: 15,746 individuals settled on 828 test plates (14 . 5 x 4 cm) after: 17 individuals on 804 test plates during the bloom). In the absence of tides, oyster spawning is correlated with the periodicity of the state of the moon. At the moment, no successful reproduction of Ostrea occurs; perhaps an unknown pollution or eutrophication occurs in the biotope. Very soon after the sluice-dock plankton has been washed out and coastal fauna and flora introduced, the latter is partly replaced by a typical sluice-dock fauna: e.g. the mud, accumulated by Polydora ciliata is colonized by Jassa falcate along the Belgian coast, by the vicariant Corophium insidiosum in the sluice-dock. As a partly typical sluice-dock fauna readily develops after washing out the water, eggs must remain in the sediment: resting eggs ('dauereier') of Acartia bifilosa and Eurytemora americana are found in the sediment. During windless periods, vertical migrations have been studied during 24 hour periods: the plankton community has specific migration velocities for each species and each developmental stage has its own migration pattern. The synecological study of the biotope includes biomass activities and interactions of phyto-, zoo- and bacteria-plankton. C and N cycles are studied. The primary production, due to nannoplankton, is of the same order of magnitude as that in the North Sea if expressed per m SUP-2 ; very high if expressed as per m SUP-3 . The phytoplankton shows mortality during night (diminution of chlorophyll a, augmentation of phaeopigments). Grazing by zooplankton does not regulate totally the phytoplankton dynamics. A high zooplankton production occurs, but the niche of the tertiary level remains open: no predation on zooplankton exists. Therefore the zooplankton are regulated by endogenous factors. |
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