The Oosterschelde is a bay at the Dutch North Sea coast with man-made rocky shores.
Originally the Oosterschelde was the eastern arm of the river Schelde. For many ages heavy man laid dikes around the flow to protect the surrounding land from flooding. After a catastrophic flooding in 1953 new plans were carried out. First the Oosterschelde was cut off from the river. The estuarine area with brackish water changed to a salt water bay.
As a final step engineers planned to turn it to a lake by cutting the bay off from the sea. But after fierce protests from ecologists and oyster farmers it was decided to build a flood barrier that only closes at extreme high tides. The flood barrier led to less strong currents and a better visibility.
The dikes around the Oosterschelde continue below sea lever to form steep rocky slopes. The depth of the rocky parts vary, at places where tidal gullies join the dikes they can reach 20 meters depth. On these places currents may free the stones of silt and a rich benthic fauna may be found. Many other places are sandy or silty. Further away from the shore sand may accumulate to form sandbanks.
Marine life in the Oosterschelde has been studied intensively over the last decades. Official studies where carried out in connection with the waterworks. In addition many marine enthusiasts record changes in species diversity. They found that the number of species increases every year. The main reasons for this increase in diversity seem to be the influx of species from the south as a result of the warmer seawater and invasions of exotic species.
The first meters under water are populated by many different species of seaweeds. About 110 species have been recorded, the number of species being quite evenly spread between brown, red and green seaweeds. Kelp forests are absent, probably because the absence of enough light at depth.
Deeper down benthic animals cover the bottom. The diversity is great but on some places one sigle species may be very dominant. For instance Japanese oysters (Crassostrea gigas) thrive so well that they are considered a plague. On places with strong currents plumose anemones may cover most of the stones and at times thick layers of brittle stars cover everything.
On many other places sponges, hydroids and tunicates may be quite numerous attributing to the beauty of the underwater landscape. One of the special attractions in these places are the sea slugs. More than 30 species of these beautiful animals have been recorded.
Also the variety of crustaceans must be mentioned. Especially different species of smaller spider crabs and of swimming crabs catch the eye. Lobsters recovered very well after massive death in the cold winter of 1962/1963; they are quite numerous again.
On the sand or silt bottoms mud Sagartia (Sagartia troglodytes) is very common and the lesser cylinder-anemone (Cerianthus loydii) is not rare.
Fish and other swimming species
About sixty species of fish live in the Oosterschelde area. The bay is a nursery, many species are born or pass their juvenile life here.
Very common species on hard substrates are butterfish (Pholis gunnellus), black goby (Gobius
On the soft bottom many sandgobies (Pomatoschistus minutus), dragonets (Callionymus lyra) and different kinds of flatfish are quite numerous.
Cuttlefish (Sepia officinalis) and squids (Loligo vulgaris) visit the area to mate and deposit their eggs. Since marine enthusiasts placed branches in the bottom to stimulate the cuttlefish to do so their mating behavior can be observed in late spring.
Harbor seals (Phoca vitulina) and harbor porpoises (Phocoena phocoena) returned after many years of absence. Probably these sight dependent hunters benefit from the better visibility that occurred after the flood barrier was build.
Diving is very popular in spring and summer. Expect to meet many other divers at the change of tides. Visibility is rather good during calm weather periods, otherwise the water can be rather murky.
Most dives are made from the dikes. A car and a good map will do. Divesites can be found following the link adjacent to this page. Currents can be strong, so take care and dive only at the change of tides. At low tide visibility will be better. Tide tables can be found at filling stations or following the link.
The Oosterschelde is situated in the Dutch