Hydrobiologia

doi: 10.1007/BF00024615pmid: N/A

Nienhuis, P. H.;Smaal, A. C.

doi: 10.1007/BF00024616pmid: N/A

Abstract During the period 1980–1990 long-term physical, chemical and ecological studies were carried out, to study the changes induced by the building of a storm-surge barrier in the mouth of the Oosterschelde estuary and two large auxiliary compartment dams in the rear ends of the estuary. The storm-surge barrier was constructed in the mouth of Oosterschelde estuary (SW Netherlands) during the period 1979–1986. The barrier allows the tides to enter the estuary freely, and, on the other hand, the barrier guarantees safety for the human population and their properties when a stormflood threatens the area. Oosterschelde estuary is isolated from the river input, the rear ends of the ecosystem were separated from the estuary by sea-walls and the strongly decreased tidal exchange with the North Sea induced sheltered circumstances. The Oosterschelde changed from a turbid estuary into a tidal bay, and yet primary production responses appear to be robust and resilient, and the biological communities showed only quantitative shifts from the dominance of specific species assemblages to other assemblages. In many cases predicted changes in the structure of the biological communities could not be verified owing to the large natural variability mainly caused by physical factors (e.g. temperature).

doi: 10.1007/BF00024617pmid: N/A

Vroon, Jacques

doi: 10.1007/BF00024618pmid: N/A

Abstract Hydrodynamic conditions in the Oosterschelde have changed as a result of the Delta Project. Engineering works were carried out between 1960 and 1970 in the northern branch of the Oosterschelde. The Grevelingen dam was built in 1964 and the Volkerak dam was completed in the spring of 1969. With the completion of these dams, the northern branch of the Oosterschelde was cut off from other estuaries, and river flow was then regulated by a sluice complex in the Volkerak dam. Between 1980 and 1987 a storm-surge barrier was constructed across the mouth of the Oosterschelde, and two compartmentalisation dams were built in the eastern part of the basin. The storm surge barrier decreased the effective cross-sectional area at the mouth from 80 000 m2 to 17 900 m2. The compartmentalisation dams reduce the surface area of the Oosterschelde basin from 452 km2 to 351 km2. The hydraulic impact of the works is demonstrated with data from field measurements and data from model calculations.

Mulder, Jan P. M.;Louters, Teunis

doi: 10.1007/BF00024619pmid: N/A

Abstract The completion in 1986/87 of an open storm-surge barrier in the inlet and of secondary dams in the landward parts of the Oosterschelde tidal basin (SW Netherlands) has had and will continue to have a significant impact on geomorphological developments. An analysis of historic data, and of recent detailed bathymetric and morphodynamic process data, indicates that former trends have reversed. At present the Oosterschelde is a sedimentation basin with a degrading intertidal area and silting up of channels. The continuing reduction in intertidal area, the decreasing geomorphological gradients, the increasing fine sediment content of channel deposits, combined with a general reduction in hydrodynamics, imply significant ecological effects.

Brinke, Wilfried B. M. ten;Dronkers, Job;Mulder, Jan P. M.

doi: 10.1007/BF00024620pmid: N/A

Abstract Changes in the budget of fine sediments in the Oosterschelde have been measured. These are related to the partial closure of the tidal basin. Before the engineering works, soil texture of most of the basin was sandy. After the works, unconsolidated fine sediments occurred at several locations throughout the Oosterschelde, mainly in the deeper parts of channels and on musselbeds. Fine sediments accumulate due to the reduction in current velocities. Most of the fine sediment comes from the North Sea; internal sources of fine sediments (primary production and erosion of intertidal flats) are of minor importance. Due to the works, the direction of net transport of fine sediments has changed from an export (before the works) into an import. A qualitative discussion of the underlying processes is presented. The changes in the budget of fine sediments have both positive and negative ecological consequences. Muddy deposits on dike slopes have reduced hard bottom macrozoobenthos. The reduced nutrients input due to reduced fresh water input has not resulted in a reduced primary production because it is counter-balanced by a decrease in turbidity.

Bakker, Cees

doi: 10.1007/BF00024621pmid: N/A

Wetsteyn, L. P. M. J.;Kromkamp, J. C.

doi: 10.1007/BF00024622pmid: N/A

Abstract Turbidity, nutrient concentrations and phytoplankton primary production were monitored in the Oosterschelde before, during and after the construction of a storm-surge barrier and two compartment dams. Flow velocities and suspended matter concentrations decreased severely, causing an increased transparency of the watercolumn. In the eastern and northern compartments, the previously pronounced seasonal variation disappeared. Reduction of the freshwater load and decreasing nutrient concentrations in the adjacent North Sea coastal waters resulted in lower nitrite + nitrate and silicate concentrations. Autumn phosphate concentrations remained at the same level as before the nutrient reduction. Silicate was a limiting nutrient during the pre-barrier period and nitrogen and silicate were limiting during the post-barrier period. Annual patterns in chlorophyll-a concentrations in the western and central compartments showed no obvious trend; in the eastern and northern compartments higher values were measured from 1985 onwards. Primary production during the period 1980–1990 varied between 176 and 550 g C m−2 yr−1. The annual primary production in the western compartment had decreased, while in the central and eastern compartments annual primary production did not change: the formerly existing gradient disappeared. In the northern compartment higher chlorophyll-a concentrations and high annual production suggest that the phytoplankton could benefit from the increased transparency while nutrient concentrations were still high enough to support phytoplankton growth. Changes in photosynthetic physiological parameters were observed which suggested shade adaptation. This is in contrast to improved light conditions and reduced nutrient availability. The apparent incoherence with light-shade adaptation theory may be explained by the species shift that occurred. As a result of the opposite effects of a more favourable light climate and a reduced nutrient availability, together with the resulting species shift, the annual primary production showed a large degree of homeostasis.

Bakker, C.;Herman, P. M. J.;Vink, M.

doi: 10.1007/BF00024623pmid: N/A

Abstract During the pre-barrier period (1982–83), the Oosterschelde phytoplankton were a diatom-dominated community, comprising a species-rich assemblage throughout the year. Assemblages of spring, early summer and summer, developed in response to a gradually evolving turbidity-light gradient during the course of the year. During the barrier-construction period (1984–87), characterized by decreasing current velocities, increasing sedimentation of suspended matter, increasing water transparencies and unchanged nutrient conditions, the growth season for the phytoplankton started earlier and lasted longer. Some flagellate species responded by much higher biomass than before. The impact of short-term climatic factors during this period, notably severe winters, could be illustrated with examples of clear responses of some species (e.g. Biddulphia aurita). In the post-barrier years (1987–90) a changed light-nutrient-salinity regime (i.e. much light, limitation of nitrate, high salinity) was demonstrated and an extended summerseason developed, without the original gradual transitions. This was reflected in an a-seasonal trend of the phytoplankton assemblage, where summer species were already observed in spring and spring species decreased in abundance. In summer small flagellates increased and some weakly silicified diatom species made their appearance. In the eastern compartment no colony formation of Phaeocystis occurred in summer and this was thought to be due to nitrate limitation. Changes in abundance of some species (Phaeocystis, Ditylum brightwellii, Skeletonema costatum), occurring during the entire period of investigation (1982–90), could be explained using field observations compared with experimental evidence from the literature. The relationship between species composition and biomass on the one hand and environmental variables on the other hand, was analysed in a Canonical Correspondence Analysis, for both compartments separately.

Bakker, C.;Vink, M.

doi: 10.1007/BF00024624pmid: N/A

Abstract Nutrients The inflow of Rhine water into the Oosterschelde was strongly reduced from 1987 onwards. This caused the winter concentrations of silicate and nitrate to decrease in the Eastern compartment, while those in the deeper Western compartment, more dependent on North Sea concentrations, hardly changed. The result was a levelling of the former East-West gradients for these nutrients. In East, summer concentrations of nitrate reached limiting levels in the post-barrier period and molar nitrate/ammonium ratios became < 1, indicating that any release of nitrogen must be important to stimulate phytoplankton growth in this area. Silicate summer concentrations in East, on the other hand, were higher in the new situation. In West, differences in summer nutrient concentrations between the old and new situation were smaller than in East, due to the still continuing exchange with the North Sea. Phytoplankton diatoms and flagellates In East during summer, N-depletion and longer residence times caused the phytoplankton to become strongly dependent on nutrient regeneration processes and increased zooplankton grazing. Average diatom biomass declined, but flagellate biomass rose during summer. Spring conditions for phytoplankton development in this area improved due to the increased water transparency, nutrients being present in excess, and this resulted in a higher ‘new’ production of diatoms than before. In West, summer biomass of diatoms decreased, probably due to increased consumption by mussels under conditions of longer residence times; nutrients were not limiting, due to important benthic mineralization processes and exchange with the North Sea. The previously existing West-East biomass gradients disappeared, or sometimes reversed. Experimental (mesocosm studies) as well as field data, reported in the literature, give evidence for the given explanations.