Immigration and settlement pattern of Paralichthys dentatus and P. lethostigma in an estuarine nursery ground,North Carolina,U.S.A.

Immigration and settlement pattern of 0-group summer flounder (Paralichthys dentatus) and southern flounder (P. lethostigma) in an estuarine nursery are described. Flounders entering the estuary were transforming larvae. Despite their sympatric occurrence during larval immigration, the two species were found concentrated in different habitats. Sampling along transects within the estuary indicated that newly settled southern flounder were concentrated on tidal flats towards the head of the estuary while greatest numbers of summer flounder were captured on tidal flats in the middle reach of the system. Logistic regression analysis of presence/absence data indicated that salinity affected the distribution of southern flounder while substrate type was an important factor affecting the distribution of summer flounder. Numbers of both species on tidal flats declined in the spring, southern flounder appeared to move upstream to oligohaline riverine habitat and summer flounder appeared to move to high salinity salt marsh habitat. The segregated distribution apparent in juveniles is largely established during the immigration period. Differences in settling behaviour and subsequent movement and/or differential mortality occurring during settlement, appear important in determining the observed distribution of juvenile summer and southern flounder.     

Distribution and abundance of the zooplankton of the Ems estuary (North Sea)

From 1974 to 1977 a study was made of the abundance and the distribution of the zooplankton species of the Ems estuary (The Netherlands and the Federal Republic of Germany), an area of about 500 km² with extensive tidal flats.The most important component of the zooplankton consisted of holoplanktonic calanoid copepods with, during the summer, a significant contribution of meroplankton, mainly consisting of polychaete and cirripede larvae. Zooplankton abundance showed a marked seasonality, with a pronounced spring peak and a smaller late summer/autumn peak. In the low salinity area the spring bloom was dominated by Eurytemora affinis, which persisted in the salinities below S=5 upriver during the whole year. In the polyhaline area Acartia bifilosa was the main component of the spring bloom. The late summer maximum from August to October was dominated by Acartia tonsa in the meso- and polyhaline area and by A. discaudata and Centropages hamatus in the euhaline part of the estuary bordering on the Wadden Sea proper. The number of abundant (> 1000 ind·m⁻³) species increased from the inner, fresher part of the estuary towards the Wadden Sea but, except for April, average zooplankton density in salinities <18 was similar (2768 ind·m⁻³) to the density in salinities >18 (2817 ind·m⁻³).The distribution patterns show that there are only a few autochthonous species and that the majority of species is allochthonous and penetrates more or less deeply into the estuary from the Wadden Sea and North Sea, varying with species and season.     

Dredging activities in the Dutch Wadden Sea: effects on macrobenthic infauna

Effects of dredging on macrobenthic infauna were studied at several sites and water depths in the estuarine Dutch Wadden Sea. Dredging in tidal flat areas was found to have long-lasting effects: filling-in rates in such areas were extremey slow, sediment composition altered dramatically and recovery of benthos was virtually absent during the long period of filling-in. Such pits on tidal flats persisted for more than 15 years. In subtidal areas recovery of bottom structure as well as the inhabiting infaunal benthos proceeded rapidly, while the original situation may be expected to return. In tidal channels with strong currentts both filling in and recolonization took only about 1 to 3 years. In some subtidal areas with low tidal stream velocities, such as tidal watersheds, recovery proceeded less rapidly: it lasted 5 to 10 years.     

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?

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.     

Comparison of the fish communities of the reef flat,the reef edge and the pelagic system in Flores sea reef environments

During the Snellius-II Expedition beach seines and gill nets were used to sample the fish fauna of reef flats and reef edges of Flores Sea reefs. Beach seining r sulted in the largest samples: in total about 14 700 specimens representing 104 species. These catches from sand flats and seagrass beds show a remarkable diversity. Slightly over 1000 specimens were collcted with gill nets along the reef edges. These much smaller catches show a still wider variety: 143 species. Only six out of the 107 dominant species occur in both habitats. Of the total number of 204 species 43 species were sampled in both habitats. Fishes occurring in both habitats are mainly sand-dwelling species and active predatory species. Pelagic species mainly occur along the reef edge; only epipelagic species are commonly sampled over the reef flat.     

Animal traces on a tidal flat in Hangzhou Bay,China

Despite prevailing low salinity (S ranging from 1 to 10) of the Hangzhou Bay, its extensive (up to 2 km wide) flats along the northern coast have a species-rich macrobenthic fauna of crustaceans, bivalves, gastropods and polychaetes. These produce a variety of crawling, feeding, dwelling and resting traces, some comparable to those observed on the flats of Korea and Taiwan. The characteristic starlike feeding traces of the bivalve Tellina (Moerella) iridescens are described here for the first time. Sediment reworking appears to be largely confined to the surface layer. The burrow-inhabiting crabs are also mainly surface sediment feeders. These Chinese flats lack organisms reworking deeper sediment layers such as lugworms or callianassid crustaceans. As compared to the Wadden Sea, sedimentation rate is higher and subsurface sediment reworking less intense, resulting in relatively few bioturbation structures in vertical sections of the Hangzhou tidal flat sediments.     

Changes in suspended-matter floc size during the tidal cycle in the dollard estuary

Measurements of in situ particle size of suspended matter in a tidal channel in the Dollard (Dutch Wadden Sea) indicated systematic variations in floc size during the tidal cycle that can be explained by assuming settling during slack tide, resuspension during the early ebb and early flood, flocculation of fine particles into large ones during most of the tide, and deflocculation of large flocs into smaller particles during or after settling to the bottom. There was a characteristic difference in floc-size variation during ebb and during flood. During the ebb maximum floc size coincided with maximum suspended-matter concentration (maximum collision frequency); during the flood maximum floc size continued to increase towards high-water slack tide. The adjacent tidal flats had a marked influence on floc size: where the flats were small, the highest percentages of large flocs (>F128 μm) occurred around slack tide when current velocities are low. Where the adjacent flats were broad and extensive, high percentages also occured during intermediate periods. Flocs of maximum size are probably not in equilibrium with the bulk of the suspended matter: they are mainly formed during periods of high suspended-matter concentrations of short duration.     

Variations in settlement and growth of 0-group plaice (Pleuronectes platessa L.) in the dutch wadden sea as determined by otolith microstructure analysis

Larval growth, date of settlement, and subsequent growth of 0-group plaice were compared in the eastern (Dollard) and western (Texel) parts of the Dutch Wadden Sea in autumn 1988. Daily increment analysis in otoliths with SEM showed that specimens sampled in the Texel region, and at 2 Dollard stations, hatched early in the season, had a shorter larval period (50 to 61 d) with high growth rates, and early settlement. All specimens caught at the other 6 stations hatched later, had a long larval phase (62 to 82 d) and settled later in the season.Estimates of maximum growth based on optimal feeding conditions and the prevailing water temperatures in both tidal basins were compared to the growth rates observed in the field. According to previous studies, plaice in the Texel region have optimal growth. All late settlers in the Dollard and part of the early settlers also had optimal growth. However, some early settlers in the Dollard had significantly lower growth rates than the batch of synchronously settled specimens from the Texel region.     

Increased mortality in alternative bivalve prey during a period when the tidal flats of the Dutch Wadden Sea were devoid of mussels

In the course of 1990, stocks of mussels (Mytilus edulis) declined to unprecedentedly low levels in the Dutch Wadden Sea. Hardly a wild mussel bed was left on the tidal flats as a consequence of three years (1988, 1989, and 1990) with failing recruitment and intensive fishing for seed mussels. During these three years, recruitment of cockles (Cerastoderma edule) also failed, whereas fishing was continued. Bird species taking these bivalves as staple food, the oystercatcher (Haematopus ostralegus) and the eider (Somateria mollissima), experienced food shortage. Significant numbers of eiders left the Dutch Wadden Sea area or died, whereas oystercatchers remained abundant trhoughout the winter in most of the Dutch Wadden Sea.Alternative prey species of oystercatchers experienced unsually high mortality rates in the appropriate size classes. This was so in all other common species of bivalves, viz. first-year and older cockles, adult Macoma balthica, and juvenile Mya arenaria. This led to minimal stocks of food for oystercatchers in the late winter of 1991. In March 1991, cockles were depleted and the combined stocks Mya and Macoma would soon have run out of food supply to the overwintering oystercatcher population. Apparently, oystercatchers are able to reduce the stocks of their various bivalve prey species to very low levels.     

The heat balance of a tidal flat area

The results of a heat balance investigation of tidal flats in the western part of the Dutch Wadden Sea are presented. It is shown that heat balance studies of tidal flat areas, which are very complicated due to the tidal cycle, can be carried out with success.It turned out, that bottom properties could be derived from the temperature measurements at different depths; the thermal diffusivity α was established at 1.06 x 10⁻⁶·m²·s⁻¹. The albedo of the dry tidal flat was found as a = 0.14, while for water a = 0.06 can be used. For mean daily heat balance calculations an average value of α = 0.10 should be taken. The net radiation can be estimated with reasonable accuracy from simple weather data from nearby coastal station. Then it is shown that the daily heat balance interacts with the tidal cycle, resulting in a 15-day periodicity in the water temperature as well in the bottom temperature. Further it is demonstrated that with a simple model and some measured “bulk”-parameters, it is possible to predict daily mean water temperatures during the whole measuring period from simple weather data measured at a nearby coastal station.     

Scale and intensity of intertidal habitat use by knots Calidris canutus in the Western Wadden Sea in relation to food,friends and foes

In August–October 1988–1992 we studied the distribution and abundance of knots Calidris canutus around Griend in the western Wadden Sea, and the extent to which these can be explained by benthic prey availability and presence of avian predators. Numbers in the nonbreeding season showed monthly averages of 10 000 to 25 000 birds. Over 100 000 knots were recorded on three occassions. Knots feed in large flocks, individual birds usually experiencing 4 000 to 15 000 flock-mates. The Siberian-breeding/west-African wintering canutus subspecies passed through in late July and early August. Otherwise the Greenlandic/Canadian breeding islandica subspecies was present. Over the period 1964–1992 there were no clear trends in the number of knots, but canutus-knots were particularly abundant in July–August 1991, whereas in 1992 both subspecies were absent.Macoma balthica was the preferred prey of both subspecies. Hydrobia ulvae, Mytilus edulis and Cerastoderma edule were eaten when Macoma was absent close to the surface of the sediment. As Macoma buried deeper from July onwards, canutus faced better average feeding conditions than islandica later in the year. The spatial distributon of knots feeding on the intertidal flats around Griend was best explained by the harvestable biomass of the prevalent prey species in a particular year and season, i.e. Macoma (main prey when their harvestable biomass densities were greater than ca 0.8 g AFDM per m²) and Cerastoderma, and by the avoidance of situations where they run the risk of attack by bird-eating birds. Flocks of knots covered most of the intertidal flats in the Western Dutch Wadden Sea in a couple of tidal cycles. This is about 800 km², much larger than the equivalent area used by knots on their wintering grounds in Mauritania (10–15 km²), a difference that is correlated with prey spectrum, prey availability and predictability.     

Large-scale distribution patterns of the mussel Mytilus edulis in the Wadden Sea of Schleswig-Holstein: Do storms structure the ecosystem?

The distribution of mussel beds in the Wadden Sea of Schleswig-Holstein was mapped by aerial surveys from 1989 to 1991. The number of mussel beds decreased from 94 in 1989 to 49 in 1990, as a result of severe storms in early 1990. Thereafter only small changes were observed. The mussel beds that remained in 1990 were found only in the shelter of islands; all beds in exposed areas had disappeared between the surveys of 1989 and 1990, leaving large areas without mussel beds. Storms are thus identified as a major factor limiting the distribution of mussel beds to the sheltered parts of the Wadden Sea. Beds in the exposed parts of the Wadden Sea are highly dynamic, whereas beds in sheltered areas may persist over long times. A comparison with distribution patterns of older surveys (from 1937, 1968 and 1978) revealed great similarities with the results of recent investigations, indicating a constant distribution pattern over a long period.The results are discussed in relation to eutrophication and the structure of the benthic communities of the Wadden Sea. It is concluded that any eutrophication-induced increase of the mussel population would be restricted to the sheltered parts of the Wadden Sea. Storms will largely determine whether the communities of a given area have to compete with mussels, which are most important filter feeders of the ecosystem. As competition for food is a major factor structuring the benthic communities of the Wadden Sea, it is assumed that storms indirectly affect all other communities, giving deeper-burying, storm-tolerant species a competitive advantage in exposed areas where epibenthic mussels are excluded. The impact of mussel fisheries will be different for persisting and dynamic beds: fishing on persisting beds in sheltered areas may remove the crucial reserve which mussel-feeding birds such as eiders or oystercatchers need in times of low mussel populations.     

Increased phytoplankton primary production in the Marsdiep area (Western Dutch Wadden Sea)

Annual phytoplankton primary production in the Marsdiep tidal inlet increased from ca 150 gC·m⁻² in the period 1964 to 1976, to ca 300 gC·m⁻² in 1981/1982 and 1985. This increase is considerable, but comparable to that observed in the outer Ems estuary, eastern Wadden Sea, from 240 gC·m⁻²a⁻¹ in 1972/1973 to 400–500 gC.m⁻².a⁻¹ in 1976/1980. Although the increase is most probably due to eutrophication, as illustrated by the regular increase in phosphate in the Marsdiep area since 1950, it is difficult to explain why this affected primary production not earlier than the late seventies. Primary production has probably not increased in the more turbid inner parts of the Wadden Sea, where light is the limiting factor, and P values were already higher than in the inlet areas.     

Inshore and offshore diversity of epibenthos dredged in the North Sea

Large epibenthos (>1 cm.) was dredged in the German Wadden Sea and in adjacent offshore areas at 10 to 30 m depth in Helgoland Bight. There were apparent differences in species composition and relative abundances between the 2 regions. The hypothesis of increasing species richness in offshore direction and of increased individual numbers inshore was tested. Neither proposition was found to be generally true.Regional species number was higher offshore (85) than inshore (73). However, a direct comparison of 11 + 11 localities and of 66 + 66 dredge hauls revealed no significant differences. Ratios of species richness for localities versus regions and for hauls versus localities differed significantly, indicating a higher regional heterogeneity offshore and a higher habitat heterogeneity inshore. Thus, causes for an overall similar diversity were found on different spatial dimensions.Few species were encountered on bottoms subject to the riverine waters of Elbe and Weser. Many decapod crustaceans attained stronger populations inshore, while ophiuroids completely dominated the epibenthos offshore and caused higher individual numbers there. Historical sources suggest that the epibenthos was richer in the past than it is now.     

Adaptation to anaerobic metabolism in two mussel species,Mytilus edulis and Mytilus galloprovincialis,from the tidal zone at Arcachon Bay,France

Aspects of anaerobic metabolism were investigated in two sympatric mussel species, viz. Mytilus edulis and Mytilus galloprovincialis, living in the tidal zone in Arcachon Bay, France. Specific activities of pyruvate kinase (PK) and phosphoenolpyruvate kinase (PEP-CK) were remarkably similar in the two sympatric species and generally corresponded more closely to those observed in M. galloprovincialis in the Mediterranean than with M. edulis in the Dutch Wadden Sea. However, the values for the radio PK: PEP-CK for the two species in Arcachon Bay agreed with those of intertidal M. edulis from the Dutch Wadden Sea.Succinate accumulation during the first 24 h of anaerobicsis was about the same as in M. galloprovincialis in the Mediterranean, but decreased during the second 24 h, particularly in M. edulis, obviously due to propionate formation. Decrease in ATP concentrations in the tissues during anaerobiosis corresponded to that of intertidal M. edulis from the Dutch Wadden Sea. With the exception of specific activities of PK and PEP-CK, all properties investigated in both species were as expected in intertidal mussels.     

An evaluation of intertidal feeding habitats from a shorebird perspective: Towards relevant comparisons between temperate and tropical mudflats

Macrozoobenthic communities of intertidal soft sediments are reviewed worldwide from the perspective of a mollusc-eating shorebird species. Based on 19 sites, total biomass figures varied between 5 and 80 g AFDM per m² (average 24 g AFDM per m²); no latitudinal trends are apparent. The contribution made by bivalves and gastropods varies between 1% and 99%, north-temperate intertidal flats having relatively more molluscs than tropical flats. Intertidal flats in the tropics contain a greater variety of taxa, with brachiopods in Indonesia and echinoderms in northwest Australia contributing significantly to biomass only there. Limits to the occurrence of avian predators of intertidal benthos are set by the harvestable fraction of the biomass on offer and the costs of living at a particular site. No systematic differences in the harvestable fraction of the total mollusc-biomass for a worldwide occurring shorebird species specializing on molluscs (knots Calidris canutus) were apparent between temperate and tropical intertidal areas, in spite of large differences in maintenance metabolism incurred by these birds. The harvestable fractions of bivalves in the two West African areas (Banc d'Arguin, Mauritania and Guinea-Bissau) tended to be high (23–84% of total biomass in six species), they were relatively low (2–52% in five species) in the temperate Wadden Sea and the tropical northwest Australian site. Harvestable biomass determines the intake rate of shorebirds, as illustrated by functional-response curves of knots feeding on two bivalves species. We argue that the collection of information on size-depth relationships along with faunal and biomass surveys at a range of sites is bound to greatly increase our understanding of both the biology of tidal-flat invertebrates and the resource base underpinning the spectacular seasonal migrations of shorebirds.     

What determines the densities of feeding birds on tidal flats? A case study on dunlin,Calidris alpina,in the Wadden Sea

The tidal and seasonal pattern of habitat utilization by dunlin was studied in a tidal-flat area in a shallow bay called Königshafen, island of Sylt, by counting the number of dunlins on marked plots at 10-min intervals over whole tidal cycles. Sediment type, tidal elevation, and seasonal changes in food choice were found to influence the feeding densities of dunlin irrespective of total numbers present in the area. Densities of dunlin were generally highest on low muddy substrates. A preference for high sandy substrates was found in late summer. The tidal pattern of habitat utilization differed between areas. In preferred areas dunlin stayed during the whole emersion period. Other areas were only used by following the moving tide line. Seasonal changes in habitat utilization were apparently triggered by changes in food choice. In spring dunlin fed almost exclusively on polychaetes. In late summer a relatively high proportion of shrimps, Crangon crangon, was found in the diet of dunlins. The preference for shrimps may force the birds to stay away from the tide line, where the shrimps can escape into deeper waters. It is recommended that studies on habitat utilization on tidal flats should consider the tidal behaviour of the target species, as single low-tide counts may not give valid results.     

The macrozoobenthos of the subtidal western dutch wadden sea. I. Biomass and species richness

During a one-year period in 1981–1982, a survey was conducted on the macrozoobenthos of the subtidal areas of the western half of the Dutch Wadden Sea. In total 80 species were found, half of them polychaetes. In terms of biomass, Mytilus edulis dominated the macrozoobenthos, with Hydrobia ulvae, Heteromastus filiformis, Carcinus maenas and Macoma balthica as other important species. Numerically important were also the polychaetes Pygospio elegans and Scoloplos armiger. Average macrozoobenthic biomass amounted to 43.7 g·m⁻² ash-free dryweight. This value is in the same range as values from intertidal areas in the Wadden Sea. The relatively high value in comparison with data from similar subtidal areas is attributed to the important mussel culture in the area.     

Food intake and growth of Sarsia tubulosa (SARS, 1835), with quantitative estimates of predation on copepod populations

In laboratory tests food intake by the hydromedusa Sarsia tubulosa, which feeds on copepods, was quantified. Estimates of maximum predation are presented for 10 size classes of Sarsia. Growth rates, too, were determined in the laboratory, at 12°C under ad libitum food conditions. Mean gross food conversion for all size classes averaged 12%.From the results of a frequent sampling programme, carried out in the Texelstroom (a tidal inlet of the Dutch Wadden Sea) in 1983, growth rates of Sarsia in the field equalled maximum growth under experimental conditions, which suggests that Sarsia in situ can feed at an optimum level. Two estimates of predation pressure in the field matched very closely and lead to the conclusion that the impact of Sarsia predation on copepod standing stocks in the Dutch coastal area, including the Wadden Sea, is generally negligible.