After the closure of the Brouwers dam, the water of the Grevelingen stood still. There were no longer any tidal movements. A large part of the ecosystem around the Grevelingen lake depended on the influence of the sea water. A good example of this is oystercatchers. They lived on the higher shores of the Grevelingen, but looked for their food on the mud flats around the shore, during ebb tide. From the moment the dam was closed, no new food was supplied. Little shell fish died after only a few days. The plants which depended on the supply of salt water also died.
Cemetery in the sea
Two weeks after the closure of the Grevelingen, the shore had become a large cemetery. Rotting plants and animals were lying everywhere. Many types depended on the water of the North Sea for their oxygen and/ or their food. A vicious circle was created. Oxygen was needed for the rotting processes, as a result of which more and more oxygen was taken from the water. The mass death primarily took place under water. Yet the consequences of the closure were also noticeable above the water level. The mud flats, which were kept wet normally, dried up. A mud flat is clay soil
outside the dikes which normally was uncovered during ebb time and which is greenery. Mud bottoms form a large part of the lower bottoms outside the dikes of the provinces of Zeeland, Groningen and Friesland. The bottoms of the IJssel lake polders, which had only recently become uncovered, were also mud flats. Much of the dried-out clay blew away, because there were no plants remaining to hold it together. To fight the dehydration, two measures were taken. Firstly, grasses and grains were sowed to keep the earth together, and secondly, screens were made of branches, and placed on the sand. Dunes formed against these bunches, which prevented further dispersion. Several new types of bird settled down on the original mud flats, such as: avocets, Kentish plovers, ringed plovers and little terns. They used the flats, which were rich of shells, as a breeding place. When more plants started growing, more and more lapwings, redshanks, black-tailed godwits and larks replaced them.
The ‘Hompelvoet’, an island in the Grevelingen, is the largest breeding place for large sterns in the Delta area nowadays, with about 3,000 breeding couples. Some birds come to the Grevelingen especially for the sea grass. Some fish also like this delicacy. In the Netherlands, one can find two sorts of sea grass along the coasts of the Wadden Sea and in the inlets of Zeeland and the province of South-Holland. The first sort, large sea grass, spread over 4,500 hectare (45 square kilometres) after the closure of the Grevelingen in 1971. From 1989, the amount of large sea grass has been cut by 95 per cent (possibly because of slime mould Labyrinthula). The plant grows on places which are not reached during ebb tide. Small sea grass has smaller leaves and grows in places which are uncovered during ebb tide. Sea grass occurs even in the deeper parts of the Grevelingen lake, because the sunlight can reach far through the clear seawater.
The rock goose eats a lot of sea grass, as do coots, widgeons and mute swans. Many jelly fish and black gobies live inside the sea grass. The black goby was a new fish in the Netherlands, discovered for the first time in the Lake of Veere in 1964. ‘Fuikhorens’ did not occur in the Grevelingen before either. While this slug was earlier only spotted in the canals in Walcheren and South-Beveland, it is now one of the most occurring slugs in the lake.
The balance recovered
Two examples will illustrate the fact that the future of the flora and fauna did not look good, right after the closure. After some time, most types survived. Some types have disappeared, while others have appeared. The first example is about the young plaice that lived in the Grevelingen before the closure of the dam. After a while, they encountered the dam during their journey towards the sea. They were disoriented and kept swimming in the local area of the dam. When this news got out, many anglers came to the dam to land the plaice. The plaice would almost have been extinct if measures were not taken in time. New plaices were introduced. Since the completion of the locks in the Brouwers dam, the plaice can swim to the North Sea without being hindered. A second example is the oysters. Everyone was afraid that the typical oysters from Zeeland would disappear from the inlets. During the severe winter of 1962-1963, almost all oysters had died out. It was a great joy when new oysters were discovered. Even after the closure of the Lake of Brouwershaven (‘Brouwershavense Gat’), the oysters did not disappear. The oysters continue to enjoy living there, and millions of young oysters are born every year.