ALBERT

Notes: Abstract Based on data, collected in 1980–1990, the intertidal benthic macrofauna of the Schelde and Ems estuaries was compared. The spatial occurrence of the benthic macrofauna along the salinity gradient, including the freshwater tidal area was emphasized. Both estuaries appeared to have a very similar species composition, especially at genus level. The higher number of species observed in the Schelde estuary was probably due to a greater habitat diversity. In both estuaries species diversity decreased with distance upstream. The total density did not vary along the estuarine gradient, whereas biomass is highest in the polyhaline zone. In both estuaries distinct intertidal benthic communities were observed along the salinity gradient: a marine community in the polyhaline zone, a brackish community in the mesohaline zone, and a third community in the oligohaline and freshwater tidal zones of the estuary. These three communities were very similar between both estuaries. Their main characteristics were discussed together with the occurrence and distribution of the dominant species. For the Schelde estuary and to a lesser extent also for the Ems estuary, there was evidence that anthropogenic stress had a negative effect on the intertidal macrobenthic communities of the oligohaline/freshwater tidal zone. Only Oligochaeta were dominating, whereas the very euryhaline and/or true limnetic species were missing. In the mesohaline zone, the Schelde estuary was dominated by large numbers of short-living, opportunistic species, whereas in the Ems estuary relatively more stable macrobenthic communities were observed. A comparison with some other European estuaries showed in general similar trends as those observed for the Schelde and Ems estuaries.

Notes: Abstract During 17 successive years (1969–1985) the macrozoobenthos has been sampled quantitatively in the latewinter/early-spring period at 15 stations scattered over Balgzand (a 50 km2 tidal flat area in the western-most part of the Wadden Sea) and at 5 stations located in a small (3 km2) area 150 km away from Balgzand in the eastern part of the Dutch Wadden Sea. In 25 species, numbers per m2 were, in most years, sufficiently high at 2 or more of the Balgzand stations to allow between-station comparisons of fluctuation patterns. Comparisons were made by rank correlation. Out of a total of 1003 of such comparisons that could be made with the Balgzand data, 47% yielded significantly positive correlations and less than 1% significantly negative ones. Thus, nearly half of the fluctuation patterns of the populations living at the 15 stations within the Balgzand area showed a high similarity. Synchronization of population fluctuations was augmented particularly by the incidence of severe winters (causing low spring numbers in about a quarter of the species that were sensitive to low temperatures and high reproductive success in several species during the subsequent summer) and further by the increasing trends in numbers in about half of the species, probably as a consequence of eutrophication. Similar results as on Balgzand were obtained within the restricted area Groninger Wad in the eastern part of the Dutch Wadden Sea (Essink & Beukema, this issue). Comparison of the fluctuation patterns between the 2 distant areas also yielded high numbers of significantly positive correlations, though the proportion of the patterns that were similar was lower than these proportions were within the 2 areas. It is concluded that common patterns of fluctuation in numerical densities of macrobenthic species can be assessed over vast areas. Such common patterns will represent the ‘normal’ or ‘base-line’ fluctuations that may be used to distinguish (as departures from such patterns) the effects of local disturbing influences.

Notes: Abstract Since 1600 the surface area of the Dutch Wadden Sea decreased considerably by successive reclamations of saltmarshes. In 1932 the Zuiderzee (3200 km2) was closed off from the Wadden Sea causing in the remaining part an increase in tidal range and current velocities. In 1969 the Lauwerszee (91 km2) was closed off and turned into a freshwater lake as well. Man's use of the Wadden Sea changed simultaneously. Dredging in harbours and shipping routes, and extraction of sand and shells became common practice. Extraction of sand increased manifold between 1960 and 1985. These activities did contribute to the turbidity of the Wadden Sea water. Discharge of nitrogen and phosphorus compounds into the western Wadden Sea increased also manifold since 1950, causing an increase in phytoplankton production, duration of phytoplankton blooms, and intertidal macrozoobenthic biomass. Loads of metals and organochlorine contaminants entering the Wadden Sea were hard to estimate. Fisheries changed drastically since the 1930's. Fishing for ‘Zuiderzee’ herring came to an end shortly after closing off the Zuiderzee. The anchovy fishery ceased in the 1960’, that for flounder in 1983. Undersized brown shrimps were fished until 1971. Selective shrimptrawls and sorting devices with flushing seawater were introduced to reduce mortality among young flatfish and shrimps. Oysters became extinct in the 1960's due to over-exploitation of the natural beds. Production of mussels increased more than ten times between 1950 and 1961 due to ‘culturing’, catches of cockles increased slowly between 1955 and 1984. Whelks were fished until 1970. The most important changes in the biotic system of the Wadden Sea, increased production of microalgae and intertidal macrozoobenthos, can be attributed to increased nutrient loads. Eutrophication provided ample food supply for mussels which are harvested mainly by man and eider duck, and may have caused also increased growth in juvenile plaice. Increased turbidity may have impaired life conditions for adult dab, and have presented also recovery of sublittoral eelgrass beds after their disappearance in the 1930's due to the ‘wasting disease’. Increased turbidity in the Wadden Sea is probably caused by closing off the Zuiderzee (1932), a significant increase of dredge spoil disposal near Hoek van Holland between 1970 and 1983, and a more than 10-fold increase of mussel culturing in the Wadden Sea since 1950. Stocks of several bird species breeding in the Wadden Sea area suffered great losses in the early 1960's due to a pesticide accident. Most of the breeding populations have recovered. PCB's caused a reproduction failure among harbour seals in the 1970's. Since 1980 official Dutch policy aims at multiple use of the Wadden Sea, with emphasis on protection and restoration of the natural environment. The 3rd Water Management Plan (1989) aims at a development of the Wadden Sea ecosystem towards the situation of ca. 1930, i.e. without undoing present sea dikes and reclaimed polders. Management of the Dutch Wadden Sea will therefore have to focus mainly on reduction of eutrophication, pollution and turbidity. Some management options are discussed.

Notes: Abstract Organic waste-water from beetsugar factories is being discharged by pipeline into the eastern part of the Dutch Wadden Sea since 1969 mainly in September–December. The organic waste load was high in the first four years and diminished considerably from 1974 onwards. From 1968 macrobenthos has been monitored at five permanent stations. Two of these stations were situated relatively close to the outfall of the pipe-line. Data on numerical density of seven macrobenthic species were analysed for synchrony of yearly fluctuations in order to discriminate between pollution-induced changes in population densities and changes due to variation of natural environmental factors. A great deal of concordance in density fluctuations was found between the five stations. Comparison with data from the western part of the Dutch Wadden Sea (150 km apart) revealed a relative good concordance of the density fluctuations in eastern (only 3 stations) and western Wadden Sea. The two stations close to the outfall showed an aberrant pattern of density variations due to the waste discharge.

Notes: Abstract From approximately 1960 to 1975 the Ems estuary received several tons of mercury per year from a chlor-alkali plant, a pesticide factory and some minor sources. The discharge has been reduced drastically from 1976 onwards. In 1975 and 1976 measurements were made on the distribution of mercury in the sediment. The horizontal distribution revealed a strong local enrichment of the sediment near the point of discharge. The vertical distribution was found to be in accordance with the local deposition rates. In the water phase no significant change in mercury content from 1975 to 1978/79 could be demonstrated. In 1978/79 a difference between Ems estuary and Dutch Wadden Sea was not significant. In 1978 mercury contents of eelpoutZoarces viviparus in the Ems estuary were about twice as high as in the Wadden Sea. In the Ems estuary a decrease of these contents was found between 1974/75 and 1978. A similar decline in the Wadden Sea may be related to a decreased mercury discharge by the River Rhine.