Effecten van antropogene activiteiten op de produktiviteit van het ecosysteem in de westelijke Waddenzee
van der Veer, H.W.; Beukema, J.J.; Cadée, G.C.; Hegeman, J.; Mom, B.; Van Raaphorst, W.; Witte, J. IJ. (1995). Effecten van antropogene activiteiten op de produktiviteit van het ecosysteem in de westelijke Waddenzee. BEON Rapport = BEON-report, 95(9). RIKZ: Den Haag. 56 pp.
Part of: BEON Rapport = BEON-report. Programma Bureau BEON: Den Haag. ISSN 0924-6576
The Wadden Sea plays an important role as a nursery area for fish and crustacean species and as a resting and refuelling area for migrant birds. During the last decades the area has become more and more influenced by anthropogenic activities, such as eutrophication and commercial fisheries. In this report a first attempt is made to study the impact of eutrophication and commercial fisheries on the productivity of the western Wadden Sea. The area of study is the western Dutch part of the Wadden Sea, an area under the influence of fresh water run-offs from the mainland and of intense water exchange with the adjacent coastal zone of the North Sea. In the first contribution, the trends in nutrient loadings of the western Wadden Sea from 1960- 1992 are reviewed. Annual nutrient loads of total-P and total-N in the main river Rhine at the Dutch-German border have decreased after a period of increasing eutrophication up to the late 1970s and early 1980s to values comparable to those in the 19605 and early 19705 again. Part I y due to inputs in the Dutch part of the drainage basin of the Rhine, total nutrient loads from the river Rhine to the coastal zone of the North Sea decreased substantially less during the last decade. The recent decrease in nutrient loads from the river Rhine, however, is not reflected in the total-N and total-P concentrations in the Dutch coastal waters close to the Wadden Sea. The main source of fresh water input in the area are the discharges of lake IJssel, which is fed by a branch of the river Rhine. The annual total-N and total-P loads in the discharges into the Wadden Sea follow a similar trend as in river Rhine, with a decrease during recent years to values as low as before the 19705. The concentrations of total-N and total-P in the western Dutch Wadden Sea followed the steep increase in loads from lake IJssel in the 19705, but showed a different trend hereafter. In contrast to the trends in the discharges of lake IJssel and in the concentrations in the adjacent coastal zone of the North Sea, concentrations of total-N and total-P in the western Wadden Sea dropped since the early 1980-ties and have stabilized at relatively low levels since then. At present concentrations are comparable to those in the 19605 and 19705. In a subsequent contribution the long-term trend in pelagic primary production near the Marsdiep inlet in the western part of the Wadden Sea is discussed and related to the observed trends in nutrient concentrations in the area. Annual primary production amounted to about 150-200 gC.m-2 in the 1960-ties and early 1970-ties. A doubling took place in the early 1980-ties and values have remained high since. Primary production in 1990 was relatively low, but the values for 1991 and 1992 were as high as those in the mid-1980s. A recent lowering is absent. The decline in especially total-P did not yet reduce the productivity in the area, although in the present situation P might have become a limiting factor tor primary production during a short period of the phytoplankton spring bloom. The two other contributions deal with the secundary production of the intertidal flats. Long-term observations of the macrozoobenthic biomass and productivity show biomass values of about 20 g ash-free dry weight (AFDW) per m2 in the beginning of the 1970s, with an increase in the beginning of the 1980s to values of about 30-35 g AFDW per m2. No consistent decline was observed in recent years. Time-series over the period 1974-1993 on settlement and growth of juvenile flatfish (plaice and flounder) and shrimps showed no effect of eutrophication. Year-class strength of both groups appeared to be determined by other factors than the food conditions in the area. At present, there is no indication of a negative impact of decreasing nutrient concentrations in the area on biomass or productivity of the macrozoobenthos and the major juvenile epibenthic predators at the tidal flats. The other anthropogenic activity in the area, commercial fishing activities on the tidal flats for cockles and seed mussels, did have an impact on migrant birds. In the course of 1990 stocks of mussels declined to unprecedentedly low levels in the Dutch Wadden Sea. HardIy a wild mussel bed was left on the tidal flats as a consequence of three years (1988, 1989, 1990) with failing recruitment in combination with intensive fishing for seed mussels. During these three years recruitment of cockles also failed, whereas fishing was continued. Birds taking these bivalves as staple food, the oystercatcher and the eider, experienced food shortage. Significant numbers of eiders left the Dutch Wadden Sea area or died, whereas oystercatchers remained abundant throughout the winter in most of the Dutch Wadden Sea, but suffered from abnormally high mortality.
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