Predation on 0-group and older year classes of the bivalve Macoma balthica: interaction of size selection and intertidal distribution of epibenthic predators

The bivalve Macoma balthica is a common species in the Wadden Sea and North Sea. Juveniles temporarily use nurseries in the high intertidal. To explain this nursery use, predation pressure was examined for both juvenile and adult Macoma at low and high tidal flats. The study was carried out in the eastern Dutch Wadden Sea.Shrimps Crangon crangon, adult crabs Carcinus maenas, gobies Pomatoschistus and juvenile flatfish were more abundant and larger on low than on high tidal flats, but 0-group Carcinus was more abundant on the high tidal flats. Crangon and 0-group Carcinus stomachs frequently contained Macoma remains. These predators selectively preyed on small 0-group Macoma, both in the field and in laboratory experiments. The effect of predation by epibenthic animals and birds, on the low and high tidal flats, was examined in exclosure experiments (2 mm mesh). There was no effect of epibenthos exclosure on adult Macoma. For 0-group Macoma, densities were higher in exclosures than in the controls where predators had normal access. The density reduction by epibenthic predators was much larger in the low than in the high intertidal. We found no effect of bird predation on densities of 0- and 1+group Macoma.Thus, 0-group Macoma is under high predation pressure by epibenthos in the low intertidal, especially by shrimps, while they are relatively safe in the high intertidal. However, most of the shellfish outgrow their epibenthic predators during their first summer. Therefore, it becomes safe for the bivalves to redistribute to locations where epibenthic predators are abundant, during their first winter. On the other hand, it did not become clear from this study why many of the larger Macoma leave the high intertidal. Concluding, the nursery use of Macoma-spat in the high intertidal is probably, at least partly, an adaptation to avoid epibenthic predation.     

Differential recruitment of bivalve species in the northern Wadden Sea after the severe winter of 1995/96 and of subsequent milder winters

High recruitment of the bivalves Cerastoderma edule, Mytilus edulis, Macoma balthica and Mya arenaria in summer after severe winters is an often reported phenomenon in the Wadden Sea. After the severe winter of 1995/96 however, only Cerastoderma and Mytilus followed this pattern in the Sylt-R?m? Bight. Repeated sampling of Cerastoderma, Macoma and Mya following a severe (1995/96), a moderate (1996/97), and a mild winter (1997/98) revealed that early recruitment was highest after the mild winter. In Cerastoderma the eventual high recruitment at the end of summer 1996 was caused by reduced benthic mortality. Low recruitment of Macoma and Mya after the severe winter may have been caused by a higher susceptibility to epibenthic predation and/or a higher susceptibility to passive re-suspension than in Cerastoderma and Mytilus. In all cases, post-settlement processes were decisive for reproductive success. Received in revised form: 7 May 2001 Electronic Publication     

Large spatial variability in lifetime egg production in an intertidal Baltic tellin (Macoma balthica) population

The extent to which it pays settling larvae of marine benthic organisms to actively select the habitat where they will spend the rest of their life can only be fully appreciated if the fitness consequences of such habitat selection processes are known. We estimated the lifetime egg production of the tellinid bivalve Macoma balthica at 11 sites over a tidal gradient in the western Wadden Sea, using a 30-year data series. The difference in individual lifetime egg production between the best sites in the lower tidal zone and the poorest sites on the high tidal flats was about a factor 10. The differences in lifetime egg production were related to differences in growth and, more importantly, survival. We argue that the large observed differences in reproductive output do not necessarily imply a lack of active habitat selection. As most animals start their last migration before final settlement from the high tidal flats, the choice may be one between a long and risky migration with a low chance of reaching the good habitat versus a more certain but poor existence on the high tidal flats. Electronic Publication     

Long-term studies of macrozoobenthos in intertidal and shallow subtidal habitats near the island of Norderney (East Frisian coast,Germany)

Near the East Frisian island of Norderney two sites are investigated permanently in order to study long-term fluctuations of macrozoobenthos: one transect (since 1977) at the northern side in shallow subtidal waters and another one (since 1976) at the sheltered southern side in the intertidal area of the Wadden Sea. Since 1980 the investigations have been continued in the frame of COST-47, sedimentary intertidal programme (including the shallow subtidal habitats colonized by the Macoma balthica community).The results up to 1984, respectively to 1985, are presented and the changes of abundance of the dominant species are discussed. Discussed are also the influences of water temperatures and sediment disturbances caused by wave action. There is evidence that the intertidal variety of the Macoma balthica community shows a greater stability than the subtidal variety.     

Zur Morphologie vonProstomatella arenicola (Hoplonemertini,Monostilifera)

From observations carried out in the intertidal sands of the Island of Sylt (North Sea),Prostomatella arenicola Friedrich is a typical inhabitant of this Wadden Sea area. Its morphological characters are redescribed. The life cycle ofP. arenicola appears to be univoltine.     

Age-dependent zonation of the periwinkle Littorina littorea (L.) in the Wadden Sea

On sedimentary tidal flats near the island of Sylt (German Bight, North Sea) abundance and size distribution of periwinkles, Littorina littorea L., were studied in low intertidal and in shallow and deep subtidal mussel beds (Mytilus edulis L.). In low intertidal mussel beds, surveys revealed that high densities (1,369±571 m^–2) of juvenile snails (≤13 mm) were positively correlated with strong barnacle epigrowth (Semibalanus balanoides L. and Balanus crenatus Bruguière) on mussels. A subsequent field experiment showed that recruitment of L. littorea was restricted to the intertidal zone. Abundances of periwinkles (213±114 m^–2) and barnacles abruptly decreased in the adjacent shallow subtidal zone, which served as a habitat for older snails (>13 mm). L. littorea was completely absent from disjunct deep (5 m) subtidal mussel beds. Snail abundance varied seasonally with maxima of >4,000 m^–2 in low intertidal mussel beds in October and minima in July, just before the onset of new recruitment. I suggest that the presence of cracks and crevices among the dense barnacle overgrowth in intertidal mussel beds favoured recruitment and survival of juvenile snails. Larger (older) specimens are assumed to actively migrate to the less favourable adjacent subtidal. Therefore, intertidal mussel beds are considered as nurseries for the population of L. littorea in the Wadden Sea. Received in revised form: 25 September 2000 Electronic Publication     

Recruitment and year-to-year variability in a population of Macoma balthica (L.)

Because of methodological problems, macrobenthic studies usually neglect the juvenile stages of invertebrate communities, due to the fact that appearance of recruits in samples is only detected some weeks or even months after their true recruitment. During this period, the temporary meiobenthos undergoes high rates of mortality. From year to year, juvenile survival rate is thus responsible for temporal patterns observed in adult population densities.The results presented here relate to the population dynamics of the tellinid bivalve Macoma balthica (L.). A study of temporary meiobenthos was conducted over two consecutive years in an intertidal Macoma-community located at the mouth of the Gironde Estuary in southwest France. Sampling of juvenile stages required short intervals (2 weeks) between successive samplings and a fine sieving mesh size (63 µm). Other population parameters, such as temporal patterns in density, reproductive cycle, and individual growth, were recorded.Recruitment processes showed a year-to-year variability, with regard to settlement density, settlement period, and survival rate. In 1983, recruitment was moderate and protracted over several months. Only one main recruitment period was detected in 1984, resulting in a high juvenile density. In a previous study (1977), by contrast, recruitment was almost non-existent.This variability is discussed as a function of climatic and sedimentological conditions which prevailed in the estuary throughout the study period. However, none of these physical factors appeared to underlie the recruitment fluctuation in Macoma balthica. It is suggested that biological interactions are of prime importance in regulating population densities in this community.     

Empty habitat in coastal sediments for populations of macrozoobenthos

Species with wide dispersal are expected to have little empty habitat. This was tested by analysing habitat use in the Wadden Sea near the island of Sylt (North Sea). Sampling covered 222 intertidal sites spread across the K?nigshafen tidal flats (4.5 km², mapping approach conducted once) and 270 subtidal sites along 12 km of a tidal channel system (stratified random sampling in spring and autumn). For any single species, 'suitable habitat' was extrapolated from the ranges of water depth and sediment composition present at the ten sites with the highest frequency or abundance of the species. On average, macrobenthic species actually used less than half of the suitable sites within the scale of local populations; this was far less than expected from a lognormal distribution. In the subtidal, abundance of most species changed between the two sampling seasons. A 50% increase in overall abundance was accompanied by a decrease in empty habitat of only about 25%. Thus, a doubling of abundances would not fill the empty space but just of half of it. The polychaete Scoloplos armiger was an exception in occupying almost all of its suitable habitat in the intertidal and subtidal sediments. A distinct patch of high species richness occurred where flood waters persistently form a large gyre which may enhance larval settlement. We suggest that limitations to larval settlement and/or juvenile survival primarily cause the extent of the observed habitat emptiness. Electronic Publication     

Rapid colonization of new habitats in the Wadden Sea by the ovoviviparousLittorina saxatilis (Olivi)

The intertidal periwinkleLittorina saxatilis completely lacks larval dispersal and adult vagility is low. Although this suggests a low dispersal rate,L. saxatilis is frequently found in recently established habitats “exotic” to the Wadden Sea. Populations occur on man-made structures like dikes, breakwater and groynes, some of which are not older than several years. Furthermore,L. saxatilis is found on marsh grassSpartina anglica, introduced to the Wadden Sea in the 1920s and 1930s, as well as on mats of green macroalgae, which have become an abundant feature on the tidal flats since the late 1970s. Seagrass beds are likely to be the original habitat ofL. saxatilis in the Wadden Sea. Since seagrass populations have dramatically declined over the last decades, colonization of new habitat types enabledL. saxatilis to maintain its Wadden Sea populations despite a changing environment. Colonizers can reach new habitats by means of passive transport, especially by rafting on macrophytes and by aerial dispersal attached to birds. In thew Wadden Sea, the ovoviviparously reproducingL. saxatilis has demonstrated its ability to successfully found new populations with only a few individuals. No reduction of genetic variablility (founder effect) was observed in recently established populations.     

Consumer versus resource control and the importance of habitat heterogeneity for estuarine bivalves

The relative influence of consumers (top down) and resources (bottom up) on the distribution and abundance of organisms remains a key question in ecology. We examined the relationships between consumer and resource variables along a productivity gradient for a dominant predator–prey interaction in a marine soft‐sediment system. We 1) quantified density and size of the clam Macoma balthica (prey species) in six replicate sites at each of four habitat types (shallow mud, deep mud, muddy sand and detrital mud) in the Rhode River, Chesapeake Bay. We selected one habitat type of high food availability and clam density (shallow mud) and another of low food availability and clam density (muddy sand) for manipulative experiments. Then, we 2) measured M. balthica survival and growth through transplants, 3) measured food availability as sedimentary organic carbon content, 4) quantified predator density, and 5) calculated predator foraging efficiency in the two habitat types. Clam density in the four habitat types differed and was related to sedimentary carbon availability and predator density. One of the habitats, detrital mud, appeared to be a population sink because it only held juvenile Macoma that never survived to reproductive age. Macoma size and growth, and predator (mainly blue crab Callinectes sapidus) densities were positively correlated with productivity and were higher in shallow mud than muddy sand. In contrast, Macoma mortality, local ‘interaction strength’, and predator foraging efficiency were lower in the productive habitat (shallow mud). Thus, predation intensity was inversely correlated with productivity (food availability); consumer and resource effects differed by habitat type; and, at a relatively small spatial scale, consumer and resource forces jointly determined population dynamics in this soft‐sediment marine system.     

Dynamics of a <Emphasis Type="Italic">Limecola</Emphasis> (<Emphasis Type="Italic">Macoma</Emphasis>) <Emphasis Type="Italic">balthica</Emphasis> population in a tidal flat area in the western Wadden Sea: effects of declining survival and recruitment

We followed the dynamics of the population of the bivalve Limecola (Macoma) balthica in the westernmost part of the Wadden Sea by monitoring for 44 years (1973–2016) its numbers and age composition at 15 sites in a 50-km² tidal-flat area. During the first half of this period, the annual recruitment and adult survival were at a relatively constant level, resulting in rather constant numbers. During the second half of the observation period, annual recruitment and adult survival showed declining trends, resulting in seriously reduced adult abundance. Sudden substantial reductions in adult survival started around 1996 at a few sites to spread over the entire area within 5 years, like an infectious disease. The resulting small adult stocks produced small numbers of recruits. The stock–recruitment curve showed an increasing part up to about 30 adults m^?2, followed by invariably successful recruitments at > 40 adults m^?2. Both recruitment and adult survival were negatively related to water temperatures. However, elevated temperatures after 1997 could not explain the very low survival rates observed after 1996 and the very low recruitment success after 2003. So far, recovery of the population has hardly taken place, with both recruitment and adult survival at lower levels than observed for the initial stable period.     

Estimation of Density-Dependent Mortality of Juvenile Bivalves in the Wadden Sea

We investigated density-dependent mortality within the early months of life of the bivalves Macoma balthica (Baltic tellin) and Cerastoderma edule (common cockle) in the Wadden Sea. Mortality is thought to be density-dependent in juvenile bivalves, because there is no proportional relationship between the size of the reproductive adult stocks and the numbers of recruits for both species. It is not known however, when exactly density dependence in the pre-recruitment phase occurs and how prevalent it is. The magnitude of recruitment determines year class strength in bivalves. Thus, understanding pre-recruit mortality will improve the understanding of population dynamics. We analyzed count data from three years of temporal sampling during the first months after bivalve settlement at ten transects in the Sylt-Rømø-Bay in the northern German Wadden Sea. Analyses of density dependence are sensitive to bias through measurement error. Measurement error was estimated by bootstrapping, and residual deviances were adjusted by adding process error. With simulations the effect of these two types of error on the estimate of the density-dependent mortality coefficient was investigated. In three out of eight time intervals density dependence was detected for M. balthica, and in zero out of six time intervals for C. edule. Biological or environmental stochastic processes dominated over density dependence at the investigated scale.     

Introductions and developments of oysters in the North Sea area: a review

To replenish the exploited native stocks ofOstrea edulis, imports from almost all European coasts have arrived in the North Sea, particularly in the Dutch Oosterschelde estuary. The American oysterCrassostrea virginica and the Portuguese oysterC. angulata have also been imported into the North Sea several times. However, only the introductions of various genetic strains of the Pacific oysterC. gigas have been of lasting success. Spat from British Columbia (Canada) was first imported to the Oosterschelde in 1964. First spatfalls in this area took place in the warm summers of 1975 and 1976. Further larval outbursts occurred in 1982 and 1989, and good settlements took place on culture plots as well as along the dikes of the Oosterschelde. Since 1977 no more cupped oysters have been imported from overseas. The population maintained itself and was able to spread in a northern direction along the Dutch North Sea coast. In Britain, hatchery-producedC. gigas were transferred to several sites, in the 1970s including the British North Sea coast. Here, occurrence in the wild seems to be rather limited up to now. Into the German Wadden Sea,C. gigas larvae and spat from a Scottish hatchery have been introduced since 1971, as were medium-sized oysters from a variety of European sources in the 1980s. Strong spatfalls on intertidal mussel beds in the northern German Wadden Sea occured in 1991 and 1994. For the introductions ofC. gigas along the west European coasts, precautionary measures to minimize unintentional transfers of associated organisms, as recommended by the ICES Code of Practice in 1994, came too late.     

Larval settlement of the bivalvesMya arenaria andM. uzenensis on scallop collectors in Vostok Bay, Sea of Japan

This paper discusses data on the horizontal and vertical distribution of the spat ofMya arenaria andM. uzenensis on collectors for the cultivation of the scallopMizuhopecten yessoensis in Vostok Bay. The maximum settlement of the mollusks on scallop collectors occurred in the innermost part of the bay. The spat density ofM. arenaria on collectors was 2.6 times that ofM. uzenensis. M. arenaria larvae preferred settling on the upper portions of collectors, while the distribution ofM. uzenensis was random. The time of collector placement in the sea influenced spat numbers.     

Associations between macrobenthos and invasive cordgrass, Spartina anglica, in the Danish Wadden Sea

We conducted a survey to provide knowledge on mechanisms controlling spatial and temporal variability of macrobenthos in an intertidal Wadden Sea area partly covered by invasive Spartina anglica. Benthic macrofauna was collected seasonally at seven stations along a transect covering non-vegetated mudflat and vegetated marsh areas. Shannon diversity index was consistently higher for macrobenthos in the open mudflat compared to the marsh area. Infaunal species, like Arenicola marina, Tubificoides benedeni and Macoma balthica, were more abundant in the mudflat than the marsh, while the opposite was evident for epifaunal species, like Hydrobia ulvae and Littorina littorea. The infaunal crustacean, Corophium volutator, on the other hand, appeared particularly attracted to the mudflat–marsh boundary. The biomass of below-ground plant materials and macrodetritus was positively correlated with the total macrofaunal abundance, while the biomass of dead below-ground plant materials alone showed a significant negative correlation with infaunal abundance. Total macrofaunal abundance was highest during summer in the mudflat, while no seasonal pattern was evident in the marsh. The negative influence of S. anglica invasions on infaunal abundance may have serious implications for higher trophic levels, such as waterbirds, that forage in Wadden Sea areas and thus for the overall biodiversity and ecosystem functioning.     

Direct and indirect effects of Littorina littorea (L.) on barnacles growing on mussel beds in the Wadden Sea

On the extensive sedimentary tidal flats of the Wadden Sea, beds of the blue mussel Mytilus edulis represent the only major hard substratum and attachment surface for sessile organisms. On this substratum, the barnacle Semibalanus balanoides is the most frequent epibiont. In summer 1998, it occurred on over 90% of the large mussels (>45 mm shell length) and the dry weight of barnacles reached 65% of mussel dry weight. However, the extent of barnacle overgrowth is not constant and differs widely between years. Periwinkles (Littorina littorea) may reach densities >2000 m^–2 on intertidal mussel beds. Field experiments were conducted to test the effect of periwinkle grazing on barnacle densities. An experimental reduction of grazing and bulldozing pressure by periwinkles resulted in increased recruitment of barnacles, while barnacle numbers decreased with increasing snail density. The highest numbers of barnacles survived in the absence of L. littorea. However, a lack of periwinkle grazing activity also facilitated settlement of ephemeral algae which settled later in the year. Field experiments showed that the growth rate of barnacles decreased in the presence of these ephemeral algae. Thus, L. littorea may reduce initial barnacle settlement, but later may indirectly increase barnacle growth rate by reducing ephemeral algae. It is suggested that periwinkle density may be a key factor in the population dynamics of S. balanoides on intertidal mussel beds in the Wadden Sea.     

The intertidal zoning of cockles (<Emphasis Type="Italic">Cerastoderma edule</Emphasis>) in the Wadden Sea,or why cockle fishery disturbed areas of relatively high biodiversity

Recently, Kraan et al. (ICES J Mar Sci 64:1735–1742, 2007) reported that areas in the Wadden Sea selected by mechanical cockle dredgers were of greater zoobenthic diversity than areas that remained undredged. The present paper attempts to explain this observation, focussing on the elevation-related zoning of the macrobenthic fauna on tidal flats. Dense beds of cockles Cerastoderma edule occur in a specific intermediate intertidal-height zone where more species are present (and reach their maximal abundance) than in either lower or higher zones. This coincidence of preferred intertidal zones in many species, including cockles, may cause the higher species richness (and higher total macrozoobenthic biomass) of the fisher-chosen dredging areas as compared to other nearby non-dredged tidal-flat areas.     

Spatial variability in structural and functional aspects of macrofauna communities and their environmental parameters in the Jade Bay (Wadden Sea Lower Saxony, southern North Sea)

Spatial distribution and functional structure of intertidal benthic macrofauna in relation to environmental variables in the Jade Bay (southern North Sea) were studied and compared with other intertidal areas of the Wadden Sea. A total of 128 stations covering the whole Jade Bay were sampled in summer 2009. A total of 114 taxa were found. Highest species numbers occurred in the subtidal areas, whereas highest mean abundances were found in the upper intertidal areas. Based on species abundance data, six significantly distinct macrofauna communities in the Jade Bay were identified and evaluated with multivariate statistics, univariate correlations and canonical correspondence analysis. Differences in these community patterns were caused by the response of the dominant species (Hydrobia ulvae, Tubificoides benedii, Pygospio elegans, Caulleriella killariensis, Scoloplos armiger, Urothoe poseidonis, Microprotopus maculatus) to prevailing environmental conditions along the gradient from the lower and exposed sandy intertidal areas via intermediate mixed sediments to the upper mudflat areas. Distribution patterns in relation to tidal zonation were best explained by variability in submergence time, Chlorophyll a (chl a) content and sediment composition (mud content), which are proxies for hydrodynamic conditions and food availability. Species inventory and species richness were comparable with other intertidal areas of the Wadden Sea, but the Jade Bay differs from these areas regarding dominant species. Differences in sediment composition and morphological characteristics (macrotidal versus mesotidal Wadden Sea areas) are discussed for comparison of regional differences.     

Notes on growth and behaviour of the American razor clamEnsis directus in the Wadden Sea and the predation on it by birds

Ensis directus (Conrad), a bivalve found in Europe only in the past few years, has filled an empty niche and spread rapidly over the intertidal flats in the Wadden Sea. The growth of the animals was investigated using a few samples taken near Langeness in February 1984. The age of the animals could be determined by reading the clearly visible year marks on the shells. Notwithstanding the low density ofE. directus, some oystercatchers appear to have already specialized in feeding on this new food source.     

The Dutch Wadden Sea: a changed ecosystem

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.