|De betekenis van het zout- en silicaatgehalte in Nederlandse kustwateren voor het zeegrasareaal: faserapport 1996|
Kamermans, P.; Hemminga, M.A.; de Jong, D.J.; Dijkema, K.S. (1997). De betekenis van het zout- en silicaatgehalte in Nederlandse kustwateren voor het zeegrasareaal: faserapport 1996. BEON Rapport = BEON-report, 97(4). RIKZ: Den Haag. 22 pp.
Part of: BEON Rapport = BEON-report. Programma Bureau BEON: Den Haag. ISSN 0924-6576, more
The earthly decline of Poseidonian beds is generally blamed on the increased eutrofication and turbidity of the water. Recently, however, through analysis of long term data, gathered over seventy years, until now unnoticed relations have been made between two environmental factors and the existence of Poseidonian beds. The now available data indicate an increased salinity (above circa 17 g Cl. I-1 = 31 PSU or pro mille), leading to a decline of several Poseidonian bed populations in the Netherlands: stable populations of Great Poseidonian Beds(Zostera marina in the Dutch coastal waters are almost exclusively to be found where freshwater influx takes place. Additionally a strong correlation has been found between the amount of silicate and the Poseidonian bed areale in Grevelingen. These research data suggest that both temperate salinity and availability of silicate in the water phase are conditions for a vital Poseidonian bed population and an increase in the Poseidonian bed areale. Based on this hypothesis a research project has been formulated that include about three subresearches. (1) Through experimental research the meaning of salt and salinity content in the water for germination and growth of Great Poseidonian Beds is determined. (2) Analysis of the decline of the Poseidonian beds in the Dutch Wadden Sea considering the previously formulated hypothesis of the water quality. We, hereby, also consider the German and the Danish Wadden Sea. (3) Check on the possibilities available to allow more fluvial water in the Grevelingen and the Easter Scheldt, how this causes a change in salinity and silicate, and what the potential increase of the areale of the Poseidonian beds could be. If the hypothesis is correct, there are possibilities to transform the results of the research into policy guidelines, because via the present locks and pumping engines an obvious influence can be exerted on the relevant water quality parameters. The results of growth experiments of different salt and silicate contents indicate that plants are bred at a lower salinity content than the one present in Grevelingen, follow a different strategy, than plants grown under high salinity conditions (32PSU). More shoots and leaves were found on plants formed in low salinity (22PSU) areas. The surface production, however, was not influenced by the salinity content. This can be explained by looking at the length of the produced leaves. More leaves were found at a low salinity content, but the leaves were smaller than those found at higher salinity contents. More shoots, grown like by Z. marina, at a low salinity content, can be favourable for an increase of the Poseidonian Bed areale. The results of the silicate-experiment have not been able to show positive effects of the increase in the silicate content (from circa 6 µM to 70 µM) on the growth or development of Poseidonian Beds. The possible positive effect of an increased silicate content can be undone by the high salinity level of the water during the experiment. Temporary results indicate that the silicate content of the water could indirectly be influenced via the type epiphyte ( silicate needy or not) which grows on the Z. marina leaves. The other two subresearches (analysis of the development of Poseidonian beds in the Wadden Sea in relation to the water quality and the hydraulically regime and evaluation of increased fresh water supply for the Grevelingen and the Easter Scheldt) are still in an initial phase.