Karel Essink

Year-to-year variability in the biomass of macrobenthic animals on tidal flats of the Wadden Sea: how predictable is this food source for birds?

Abstract The Wadden Sea is important as a stop-over and wintering area for several species of waders, foraging for shorter or longer times on its tidal flats. The size of the food stocks these birds encounter varies from place to place and from year to year. We studied characteristics of the variability in time of such prey stocks, using long-term data series of annual estimates of biomass of macrobenthic animals collected on tidal flats in various parts of the Dutch and German Wadden Sea. Year-to-year fluctuations were stronger in nearly all individual benhos species than in total macrozoobenthic biomass. The various species differed significantly in their year-to-year variability. Everywhere the tellinid bivalve Macoma balthica was relatively stable, whereas species such as the cockle Cerastoderma edule and the polychaetes Lanice conchilega, Nephtys hombergii and Anaitides mucosa fluctuated heavily and rapidly in all parts of the Wadden Sea where they were found. Within species, fluctuations in biomass of individual age or size classes were stronger than those in total biomass of the species. Several species showed minimal biomass values in the same years over vast areas. This synchronization of scarcity was caused particularly by similar responses to winter character, which was each year similar over the entire Wadden Sea. Such similar responses included low over-winter survival during severe winters and recruitment failure after exceptionally mild winters. Immediately after severe winters, such cold-sensitivee species as C. edule, L. conchilega and N. hombegii were scarce all over the Waden Sea, thus liimiting the possibilities for birds to switch to other parts of the Wadden Sea to find their preferred prey in sufficient quantitu. Simultaneous recruitment failure in several important bivalve species ( C. edule, Mytilus edulis , and Mya arenaria limited the possibility for specialized bivalve consumers to switch to alternative prey types in certain years.

Distribution and life cycle of the North American Spionid polychaeteMarenzelleria viridis (Verrill, 1873) in the Ems estuary

In 1983 the first specimens of the North American spionid polychaeteMarenzelleria viridis were found along the European mainland shore in the Ems estuary. Since then, this polychaete has spread over several estuaries around the North Sea and the Baltic. In the inner part of the Ems estuary juveniles were predominantly present in muddy sediments high in the intertidal zone; in more sandy sediments at higher salinities juveniles and adults co-occured. Detailed information was obtained at a muddy and at a sandy station.Gametes were present in the coelomic fluid from November through March. In May new recruits were found in the sediment samples, reaching densities of over 105 M−2 at the muddy station, andc. 2000 m−2 at the sandy station. During summer, densities decreased at the muddy station, coinciding with a density increase at the sandy station, suggesting migration of juveniles from a nursery to the adult habitat. Cage experiments showed that the decrease of juveniles at the muddy station could be attributed to migration and not to mortality due to predation.At the sandy habitat palps and anterior parts ofM. viridis made up 4–11% of the stomach content of juvenile plaice (Pleuronectes platessa). In juvenile flounder (Platichthys flesus) only in Aprilc. 10% of the stomach content consisted ofM. viridis.During 1983–1990 increasing densities ofM. viridis at the sandy habitat coincided with a reduced abundance ofNereis diversicolor, however, this inverse relationship was not found to be statistically significant. Density fluctuations ofM. viridis andCorophium volutator showed a significant positive relationship, the cause of which is not yet understood.

How predictable is high bivalve recruitment in the Wadden Sea after a severe winter

Higher than average recruitment among bivalves on the intertidal flats of the Wadden Sea was observed many times after severe winters in the period 1940 - 1995. The occurrence of another severe winter in 1995/96 prompted us to test the hypothesis of severe winters leading to universally high bivalve recruitment on a large geographic scale (500 km coastline) in temperate shallow waters. We analysed data sets on bivalve abundance from seven areas in the Dutch, German and Danish Wadden Sea. The longer data sets showed generally higher bivalve recruitment in the 1970Žs and 1980Žs than in the 1990Žs which may be related to the near absence of severe winters since 1987. Considering the period 1988 onwards (the longest possible period in which 1995/96 was the only severe winter), recruitment of Cerastoderma edule was in 1996 - in agreement with our hypothesis - above average at all seven investigated areas. In contrast, recruitment of Macoma balthica and Mya arenaria was for the same period above average only in the southern Wadden Sea (south-west of Jade Bay) but not in the northern Wadden Sea (north of Eiderstedt peninsula). These regional differences may be related to (i) the different topography of the northern Wadden Sea (with barrier islands westwards to the mainland) compared to the southern Wadden Sea (with barrier islands northwards to the mainland) and subsequent differential effects of wind induced currents on bivalve recruitment, (ii) differences in biotic factors such as standing stocks, larval supply or epibenthic predation or (iii) changes in environmental conditions. Our results demonstrate that large-scale comparisons along coasts are an indispensable addition to insights derived from local studies alone.

On the pelagic occurrence and dispersal of the benthic amphipod Corophium volutator

Field data on the pelagic occurrence of Corophium volutator were obtained in 1981–82 in the Dollard (Ems estuary). The pelagic population density averaged over a tidal half cycle was in the range 0–15 m -3 . The pelagic population represents about 0.06% of the benthic population. On most sampling days pelagic occurrence resulted in a net flood surplus transport.

Response of an estuarine ecosystem to reduced organic waste discharge

Since the mid-19th century large amounts of organic wastewere sluiced out into the Ems Estuary on the border between Germany and TheNetherlands. This waste originated from the regional potato flour and cardboardindustries making the inland waterways completely anoxic. In the estuaryseriousoxygen depletion occurred, especially during autumn. Most of the organic wasteentered the estuary in the Dollard, a brackish embayment of the Ems Estuary. Anintensive sanitation scheme was started in the 1970s, leading to a stepwisereduction of the organic waste load on the estuary. In this paper, a review isgiven of the response of the benthos living at intertidal mudflats,viz. microphytobenthos (diatoms), meiofauna (nematodes)andmacrozoobenthos. The benthos response is described mainly on the basis of dataobtained under conditions of high (ca. 1980), intermediate (1987) and largelyreduced (1993) organic waste loading.Reduction of organic loading caused significant changes in abundance, speciescomposition and standing stock of diatoms and nematodes. Macrobenthicpopulations recovered from being severely reduced regularly during the autumnalwaste discharges towards a more stable situation. Altogether, the intertidalmudflat benthos changed from organic waste loading stress towards a normal,estuarine environmental stress. This was especially the case at high and muddyflats in the southeast of the Dollard. At mixed sand/mud flats in the centre ofthe Dollard, a response of macrozoobenthos may have been obscured by theinvasion of the polychaete Marenzelleria cf.wireni.

First Record of the American Jack Knife Clam Ensis Directus on the French Coast of the North Sea

Ensis directus (Bivalvia: Solenidae) was noted for the first time along the French coast of the North Sea in June 1991. High numbers of post larvae after a period of northerly winds indicate that settled larvae originated from adult populations in Belgian or Dutch coastal waters. Future dispersal of Ensis directus could be used as a biological tracer of coastal water movements between the Southern Bight of the North Sea and the English Channel. The American jack knife clam, Ensis directus (Conrad, 1843), synonym Ensis americanus (Binney, 1870) sensu van Urk (1964,1972), was discovered in Europe in the German Bight for the first time in June 1979 (Von Cosel et al. , 1982). This species is assumed to have been transported in its larval stage by a ship containing ballast water. Considering the dimensions of the specimens, this probably happened in the first half of 1978 (Von Cosel et al. , 1982; Muhlenhardt-Siegel et al. , 1983). Since then this species has spread rapidly in the North Sea in subtidal and intertidal areas. Dense populations were found along the German coast within a few years (Von Cosel et al. , 1982; Muhlenhardt-Siegel et al. , 1983; Swennen et al. , 1985). In 1986 Ensis directus was reported from the north and east Danish coasts as far as the Belgian coast (Kerkhof & Dumoulin, 1987) (Figure 1). Since then no new records have been reported.

The effect of organic pollution on local distribution of Nereis diversicolor and Corophium volutator

Abstract Distribution patterns of Nereis diversicolor and Corophium volutator in the SE-Dollard, a 92 km 2 tidal flat area in the NE Netherlands are described. Body condition as well as the maximal size of juvenile worms correlated positively with elevation of the mudflat, probably caused by a better food supply on the higher mudflats. No spatial variation in size was found for C. volutator . N. diversicolor disappeared from ∼1 km 2 of the intertidal mudflats in the autumn of 1985 as a consequence of the seasonal waste discharges, whereas C. volutator disappeared from an area of ∼4 km 2 . Though the discharges decreased by almost 50% from 1982 to 1985, N. diversicolor disappeared from about the same area in the 2 years. N. diversicolor is the main prey species of the avocet Recurvirostra avosetta , large flocks of which stay in the Dollard during autumn migration. Though the avocets lost ∼30% of their potential feeding area in the Dollard in the 1970s because of the high waste discharges, against only 5 to 10% in 1982 and thereafter (at significant lower discharges), the numbers of the avocets did not respond to the decrease of the discharges.

Long-term changes (1977–1994) in intertidal macrozoobenthos of the Dollard (Ems estuary) and effects of introduction of the North American spionid polychaeteMarenzelleria cf.wireni

Intertidal macrozoobenthos was monitored twice a year at three permanent transects in the Dollard (Ems estuary) from 1977 till 1994. In this period organic waste discharges in August-December into the Dollard decreased from 25×l06 kg BOD5 (1970s) to 10×106 kg BOD5 (early 1980s), and finally to 0.5×l06 kg BOD5 (early 1990s). Few long-term trends in numerical density or biomass of separate benthos species could be demonstrated. Crustaceans, mainly the amphipodCorophium volutator, increased in abundance and biomass. In the bivalveMacoma balthica and the polychaeteNereis diversicolor decreasing trends were observed especially in biomass. No relationships with changes in environmental conditions as caused by the decrease in organic waste load could be demonstrated.In 1983 the North American spionid polychaeteMarenzelleria cf.wireni appeared in the Dollard, to become abundant in three years time almost doubling the mean annual macrozoobenthos biomass. This development made the share of polychaetes in the mean annual biomass to increase from 24 to 58%, whereas bivalves fell from 64 to 25%. Because of absence of demonstrable interactions between the original macrobenthic species and the newcomerM. cf.wireni it is concluded that this highly productive species has occupied an empty niche. The dramatic increase of this invader species may have obscured effects of decreased organic waste input on the intertidal benthic community.

Long-term development of biomass of intertidal macrozoobenthos in different parts of the Wadden Sea. Governed by nutrient loads?

Long-term data sets on intertidal macrozoobenthos from Danish, German and Dutch parts of the international Wadden Sea were analysed for trends in biomass of molluscs, polychaetes and crustaceans. Patterns in long-term development were compared with data on nutrient loads from freshwater sources and with regional data on chlorophyll a and nutrient concentrations in the water column. Fluctuation patterns of total biomass were strongly determined by fluctuations of bivalve biomass. Especially in bivalves these fluctuation patterns show synchronisation due to the different impact of severe winters (1979, 1985–87) and mild winters (1988–90). Indications were found for a consistent increasing trend in polychaetes. Most of the data sets do not allow a proper analysis of a possible causal relationship between nutrient input and biomass development of macrozoobenthos. Moreover, for such an analysis better information on nutrient load related food supply for macrozoobenthos is necessary.

Mercury pollution in the Ems estuary

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.