Recruitment of shore crabsCarcinus maenas on tidal flats: Mussel clumps as an important refuge for juveniles

During the late summer and early fall, juvenile shore crabs (Carcinus maenas L.) occurred in high abundances in mussel clumps scattered on tidal flats of the Wadden Sea. Abundances were much lower on bare tidal flats without mussel clumps and decreased substantially from July to November, whereas numbers in mussel clumps remained high. Large crabs left the tidal flats in early fall, whereas juveniles undertook tidal migrations only in the late fall. In March very few shore crabs were found in the intertidal area. The size of juvenile shore crabs living between mussels did not increase significantly during fall. On the bare tidal flats surrounding the mussels, a size increase was observed. Mussel beds and mussel clumps serve as a spatial refuge for the early benthic phases of juvenile shore crabs. Between mussels they can hide effectively from their epibenthic predators. Juvenile shore crabs do not leave the intertidal area and the mussel habitats before their major predators have left the area. Mussel clumps scattered over the tidal flats may be a critical refuge for juvenile shore crabs settling on tidal flats. Intensified efforts in mussel culturing in the European Wadden Sea during recent decades may have caused an increased abundance of mussel clumps on tidal flats, thus enhancing habitat availability for some mussel-clump inhabitants.     

An analysis of mussel bed habitats in the Dutch Wadden Sea

A habitat suitability analysis for littoral mussel beds in the Dutch Wadden Sea was carried out. The analysis was based on the presence of mussel beds in the years 1960–1970, and a number of environmental characteristics: wave action, flow velocity, median grain size, emersion times and distance to a gully border. The habitat model describes mussel bed appearance quantitatively. It predicts the distribution of mussel beds quite well, as well as the distribution of spatfall in the years 1994 and 1996. From the analysis we found that wave action (maximum orbital velocity) was the main structuring factor. A low orbital velocity was preferred. Neither very low, nor maximum flow velocities were favourable for mussel beds. Very coarse sands or silty environments were not preferred. Sites close to the low water line showed lower mussel bed appearance; when emersion time was above 50% , hardly any mussel beds could be found. The habitat suitability analysis and the construction of a habitat suitability map was performed in the framework of the discussions on a further or reduced exploitation of the tidal flats in the Dutch Wadden Sea by cockle and mussel fishery activities. Electronic Publication     

High mortality of Pacific oysters in a cold winter in the North-Frisian Wadden Sea

Mortality of introduced Pacific oysters (Crassostrea gigas) was studied in the northern Wadden Sea in response to an ice winter. After a decade of mild winters, in January and February 2010, the first severe winter occurred since the Pacific oysters became dominant on former intertidal blue mussel (Mytilus edulis) beds in the North-Frisian Wadden Sea. After the ice winter, mortality of Pacific oysters on densely populated beds in the List tidal basin reached about 90%, indicating much higher losses in comparison to former mild winters. At lower densities between the islands of Amrum and Föhr, oysters were less or even not affected. Although Pacific oysters are assumed to be very tolerant to frost, the duration of cold water- and air temperatures accompanied by mechanical stress of the ice burden might have caused the high mortality in the winter 2009/2010 in formerly dense beds.     

Subtidal and intertidal mussel beds (Mytilus edulis L.) in the Wadden Sea: diversity differences of associated epifauna

In 1997 and 1998, surveys were performed to compare species composition, abundance and diversity of non-attached epifauna (>1 mm) in low intertidal and adjacent shallow subtidal zones of three mussel beds (Mytilus edulis L.) near the island of Sylt in the North Sea. The community structure was similar when compared within tidal zones: no significant differences in species numbers and abundances were recorded between locations and between years. A comparison between tidal zones, however, revealed higher diversity, species densities and total species numbers in the subtidal (per 1,000 cm²: H ′=2.0±0.16; 12 ±1 species density; 22 species) than the intertidal zone (per 1,000 cm²: H ′=0.7±0.27; 6±2 species density; 19 species). Abundances significantly dropped with increasing submergence from 2,052 (±468) m^–2 to 1,184 (±475) m^–2. This was mainly due to significantly higher densities of both juvenile periwinkles, Littorina littorea, and crabs, Carcinus maenas, in intertidal mussel beds. However, many less dominant species were significantly more abundant in subtidal mussel beds. This study revealed that in the non-attached epifaunal community of mussel beds the tidal level effect within metres was strong, whilst the spatial variability in a much wider (kilometre) range but the same tidal level was negligible. The high epifaunal diversity in the subtidal zone suggests that the protective measures for mussel beds against the effects of mussel fishery should be extended from the intertidal to the subtidal zone, if the integrity of the mussel bed community in the Wadden Sea National Park is to be maintained. Electronic Publication     

Variations in the mussel population of the Dutch Wadden Sea in relation to monitoring of other ecological parameters

The pattern of distribution of intertidal mussel beds is relatively constant over a number of years although their surface area can vary greatly. The abundance of mussels shows much greater fluctuations. In the western part of the Dutch Wadden Sea, west of the Terschelling tidal divide, the amount of mussels on natural beds fluctuated between 1 and 24 million kg fresh weight during the years 1949 to 1988. In the eastern part of the Dutch Wadden Sea the biomass varied between 5.5 and 180 million kg. The influence of the mussels on the ecosystem therefore can be very different between years. When many mussels are present the whole watermass can be filtered every few days. In years with few mussels present the filtering may take one month. It is argued that monitor programmes for a.o. nutrients, chlorophyll and growth rates of benthic organisms are of limited value if there is no indication about the total amount of mussels in the area. Presented at the VI International Wadden Sea Symposium (Biologische Anstalt Helgoland, Wattenmeerstation Sylt, D-2282 List, FRG, 1–4 November 1988)     

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     

Differential effects of the severe winter of 1995/96 on the intertidal bivalves Mytilus edulis, Cerastoderma edule and Mya arenaria in the Northern Wadden Sea

Intertidal mussel beds were severely damaged by scouring ice floes during the winter of 1995/96. Aerial surveys before and after the winter showed that more clusters of mussel beds vanished in a region with a higher areal share of tidal flats and a lower salinity, suggesting that the amount of ice present determined the magnitude of the disturbance on beds of Mytilus edulis. Nehls and Thiel [(1993) Neth J Sea Res 31:181–187] observed a strikingly similar spatial pattern of disturbances caused by severe storms in the Wadden Sea. Areas on mussel beds mechanically undisturbed by ice showed no reduced abundance and biomass of mussels, indicating that temperature alone was of little importance as a lethal factor. Conversely, Cerastoderma edule was strongly affected by low temperature. On average 80% died during the winter with extinctions up to 100% in the high tidal zone. At the lowest tidal level, surviving cockles were larger than those killed by the frost. A reinvestigation of sampled sites in autumn revealed that substantial further mortality had occurred during spring and summer which may constitute a time-lag effect of the preceding winter. There was no increased mortality in juvenile and adult Mya arenaria during the winter of 1995/96, confirming that this clam is a hard-winter species like Macoma balthica. Received in revised form: 7 May 2001 Electronic Publication     

Community dynamics of intertidal soft-bottom mussel beds over two decades

Macrozoobenthos communities in the North Sea showed pronounced changes over the past decade in relation to an increasing number of invasive species and climate change. We analysed data sets spanning 22 years on abundance, biomass and species composition of intertidal soft bottom mussel beds near the island of Sylt (German Bight) in the Northern Wadden Sea, based on surveys from 1983/1984, 1990, 1993 and from 1999 to 2005. Mussel bed area and blue mussel biomass decreased, and a change in the dominance structure in the associated community comparing 1984 to mid-1990s with the period from 1999 to 2005 was observed. Coverage of the mussel beds with the algae Fucus vesiculosus decreased since the end of the 1990s. Within the study period biomass and densities of the associated community increased significantly. Dominance structure changed mainly because of increasing abundances of associated epibenthic taxa. Apart from the Pacific oyster Crassostrea gigas all other alien species were already present in the area during the study period. Community changes already started before Pacific oysters became abundant. An attempt is made to evaluate effects on the observed changes of decreasing mussel biomass, ageing of mussel beds, decreasing fucoid coverage and increasing abundances of invader. All four factors are assumed to contribute to changing community structure of intertidal mussel beds.     

Variability in annual recruitment success as a determinant of long-term and large-scale variation in annual production of intertidal Wadden Sea mussels (<Emphasis Type="Italic">Mytilus edulis</Emphasis>)

To understand the background of the strong variation and recent decline of stocks and production of mussels (Mytilus edulis) on tidal flats of the Wadden Sea, we analysed long-term (twice-annual for 26 years) and multi-station (15 sites) estimates of numbers, mean individual weights, biomass, and annual production on Balgzand, a 50-km² tidal-flat area in the westernmost part of the Wadden Sea (The Netherlands). Somatic production was estimated from summed growth increments of soft tissues per half-year period and expressed in ash-free dry mass (AFDM). In adults, positive values in spring/summer regularly alternated with negative values in autumn/winter, when up to ∼25% (mean: 14%) of individual weight gains in the preceding season were lost. No weight losses were observed during the first winter of the life of mussels. The 26-year mean of net somatic tissue production P amounted to 5.5 g AFDM m⁻² a⁻¹ at a mean biomass B of 3.2 g AFDM m⁻²; the ratio P/B varied strongly with age composition of the mussel population and ranged between 0.5 and 3.0 a⁻¹ (mean: 1.7). Within the restricted areas of mussel beds, mean biomass and annual production values were two orders of magnitude higher. In the Wadden Sea, mussel beds cover a typical 1% of extensive tidal flat areas. Numerical densities of recruits showed straight-line relationships with subsequent life-time year-class production. Once recruits had reached an age of ∼10 months, their numbers predicted subsequent production within narrow limits. Production per recruit averaged 0.21 g AFDM for 10-mo recruits and was not related to recruit density. Local variation in annual production varied strongly, with maximal values between mid-tide and low-tide level, where recruitment was also maximal. Production per recruit was higher at low than at high intertidal levels. Frequently failing recruitment is indicated as the main cause of declining mussel stocks in the Wadden Sea. As in other bivalve species, a declining frequency of the occurrence of cold winters appears to govern declining recruitment success and consequently declining production.     

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.     

Modeling the influence of a young mussel bed on fine sediment dynamics on an intertidal flat in the Wadden Sea

Mussel beds are known to affect fine sediment dynamics and morphology on mudflat scale, a clear example of ecosystem engineering. Current research into possible ecological engineering applications of mussel beds makes quantitative modeling desirable. In this study a process-based model of the interaction between a young mussel bed and fine sediment was set up for use in the hydrodynamic and morphological model Delft3D-FLOW. The model encompasses the hydraulic roughness of the mussel bed, active capture of suspended sediment by filter feeding and changed bed properties due to biodeposited matter. The mussel bed implementation in Delft3D-FLOW was applied in a test case: a Wadden Sea intertidal mudflat model. It was concluded that a combination of active deposition via filtration and slow down of the flow due to increased roughness leads to high net deposition in the mussel bed. In addition, the ability of young mussels to quickly climb on top of deposited material results in rapid trapping of large amounts of fine sediment. In the wake of the mussel bed, deposition is also high because of reduced flow velocities. The effects of different existing mussel bed patterns were also evaluated. Patchiness and specifically striped patterns cause mussel beds to experience less sedimentation than uniformly covered beds of the same size and may therefore be favorable to mussels.     

Introduced Pacific oysters (Crassostrea gigas) in the northern Wadden Sea: invasion accelerated by warm summers?

Among the increasing number of species introduced to coastal regions by man, only a few are able to establish themselves and spread in their new environments. We will show that the Pacific oyster (Crassostrea gigas) took 17 years before a large population of several million oysters became established on natural mussel beds in the vicinity of an oyster farm near the island of Sylt (northern Wadden Sea, eastern North Sea). The first oyster, which had dispersed as a larva and settled on a mussel bed, was discovered 5 years after oyster farming had commenced. Data on abundance and size-frequency distribution of oysters on intertidal mussel beds around the island indicate that recruitment was patchy and occurred only in 6 out of 18 years. Significant proportions of these cohorts survived for at least 5 years. The population slowly expanded its range from intertidal to subtidal locations as well as from Sylt north- and southwards along the coastline. Abundances of more than 300 oysters m^–2 on mussel beds were observed in 2003, only after two consecutive spatfalls in 2001 and 2002. Analyses of mean monthly water temperatures indicate that recruitment coincided with above-average temperatures in July and August when spawning and planktonic dispersal occurs. We conclude that the further invasion of C. gigas in the northern Wadden Sea will depend on high late-summer water temperatures.Communicated by H.D. Franke     

The American slipper limpet <Emphasis Type="Italic">Crepidula fornicata </Emphasis>(L.) in the northern Wadden Sea 70 years after its introduction

In 1934 the American slipper limpet Crepidula fornicata (L.) was first recorded in the northern Wadden Sea in the Sylt-R?m? basin, presumably imported with Dutch oysters in the preceding years. The present account is the first investigation of the Crepidula population since its early spread on the former oyster beds was studied in 1948. A field survey in 2000 revealed the greatest abundance of Crepidula in the intertidal/subtidal transition zone on mussel (Mytilus edulis) beds. Here, average abundance and biomass was 141 m^–2 and 30 g organic dry weight per square metre, respectively. On tidal flats with regular and extended periods of emersion as well as in the subtidal with swift currents in the gullies, Crepidula abundance was low. The main substrate of attachment was live mussels. Compared with the years following their initial introduction, Crepidula is more abundant today and has shifted from the now extinct oyster beds to the epifaunal community of the mussel beds. Their present abundance is considerably lower than at more southern European coasts where the species may dominate the epifauna. Low winter temperatures are suggested to have limited the population expansion in the northern Wadden Sea until now. Electronic Publication     

High survival and growth rates of introduced Pacific oysters may cause restrictions on habitat use by native mussels in the Wadden Sea

Pacific oysters Crassostrea gigas (Thunberg, 1793) were introduced to the northern Wadden Sea (North Sea, Germany) by aquaculture in 1986 and finally became established. Even though at first recruitment success was rare, three consecutive warm summers led to a massive increase in oyster abundances and to the overgrowth of native mussel beds (Mytilus edulis L.). These mussels constitute biogenic reefs on the sand and mud flats in this area. Survival and growth of the invading C. gigas were investigated and compared with the native mussels in order to predict the further development of the oyster population and the scope for coexistence of both species. Field experiments revealed high survival of juvenile C. gigas (approximately 70%) during the first three months after settlement. Survival during the first winter varied between > 90% during a mild and 25% during a cold winter and was independent of substrate (i.e., mussels or oysters) and tide level. Within their first year C. gigas reached a mean length of 35-53 mm, and within two years they grew to 68-82 mm, which is about twice the size native mussels would attain during that time. Growth of juvenile oysters was not affected by substrate (i.e., sand, mussels, and other oysters), barnacle epibionts and tide level, but was facilitated by fucoid algae. By contrast, growth of juvenile mussels was significantly higher on sand flats than on mussel or oyster beds and higher in the subtidal compared to intertidal locations. Cover with fucoid algae increased mussel growth but decreased their condition expressed as dry flesh weight versus shell weight. High survival and growth rates may compensate for years with low recruitment, and may therefore allow a fast population increase. This may lead to restrictions on habitat use by native mussels in the Wadden Sea.     

Historical changes in the benthos of the Wadden Sea around the island of Sylt in the North Sea

The situation regarding the distribution and abundance of seagrass, macroalgae and benthic fauna near the island of Sylt in the south-eastern North Sea during the period 1923 to 1940 is compared with that of the 1980s. Evidence of organic enrichment in recent times is provided by (1) massive growth of green algal mats on sheltered tidal flats, (2) a decline of red algae in the subtidal zone, (3) an expansion of mussel beds along low water line and down to 20 m depth, (4) increased abundance of polychaetes inhabiting intertidal and subtidal sandy bottoms. Seagrass beds have undergone complex changes which remain unexplained. Intensified erosion has contributed to the loss of habitats in the intertidal zone, and probably affected sessile epifauna in the deep channels. Here, direct removal and disturbance by the bottom-trawling fishery may also have contributed to the observed species impoverishment. Presented at the VI International Wadden Sea Symposium (Biologische Anstalt Helgoland, Wattenmeerstation Sylt, D-2282 List, FRG, 1–4 November 1988)     

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.     

Historic mussel beds (Mytilus edulis) in the sedimentary record of a back-barrier tidal flat near Spiekeroog Island, southern North Sea

Mussel beds show irregular cycles of appearance, disappearance and reappearance at preferred distinct locations on intertidal flats. These cycles are documented in the depth profile by the presence of shell-rich sediments intercalated with sandy layers. Such mussel bed layers were regularly found over the past 12 years on the Swinnplate, a back-barrier tidal flat south of Spiekeroog Island, southern North Sea. They are characterised by the occurrence of sub-articulated pairs of Mytilus shells. Based on life span considerations, a period of at least 35–40 years over which mussel beds may have repeatedly established themselves is suggested. A survey in spring 2000 revealed a reduced occurrence of old, embedded beds on the Swinnplate. The present results, based on core profiles from 1992 to 2000 and amino acid age determination, show that alternations of shell layers indicative of former Mytilus beds and sediment layers provide insight into the historic development of tidal flat environments. The frequent occurrence of sub-articulated valve pairs in the shell layers documents that the embedding of the mussel beds took place soon after the death of the organisms without prior physical disturbance. Successions of historic Mytilus beds provide evidence that mussel beds are a regular, but not necessarily permanent, faunal feature of back-barrier tidal flats of the North Sea. Electronic Publication     

Effects ofFucus vesiculosus covering intertidal mussel beds in the Wadden Sea

The brown algaFucus vesiculosus formamytili (Nienburg) Nienhuis covered about 70% of mussel bed (Mytilus edulis) surface area in the lower intertidal zone of Königshafen, a sheltered sandy bay near the island of Sylt in the North Sea. Mean biomass in dense patches was 584 g ash-free dry weight m^?2 in summer. On experimental mussel beds, fucoid cover enhanced mud accumulation and decreased mussel density. The position of mussels underneath algal canopy was mainly endobenthic (87% of mussels with >1/3 of shell sunk into mud). In the absence of fucoids, mussels generated epibenthic garlands (81% of mussels with <1/3 of shell buried in mud). Mussel density underneath fucoid cover was 40 to 73% of mussel density without algae. On natural beds, barnacles (Balanidae), periwinkles (Littorina littorea) and crabs (particularly juveniles ofCarcinus maenas) were significantly less abundant in the presence of fucoids, presumably because most of the mussels were covered with sediment, whereas in the absence of fucoids, epibenthic mussel clumps provided substratum as well as interstitial hiding places. The endobenthic macrofauna showed little difference between covered and uncovered mussel beds. On the other hand, grazing herbivores — the flat periwinkleLittorina mariae, the isopodJaera albifrons and the amphipodsGammarus spp. — were more abundant at equivalent sites with fucoid cover. The patchy growth ofFucus vesiculosus on mussel beds in the intertidal Wadden Sea affects mussels and their epibionts negatively, but supports various herbivores and increases overall benthic diversity.     

Documentation of sites of intertidal blue mussel (Mytilus edulis L.) beds of the Lower Saxonian Wadden Sea, southern North Sea (as of 2003) and the role of their structure for spatfall settlement

Field surveys (dating back to 1950) and aerial photograph series (dating back to 1966) were evaluated to determine sites of intertidal blue mussel (Mytilus edulis) beds at the Wadden Sea coast of Lower Saxony. Maps were prepared indicating sites of blue mussel beds during the last decades. A table gives additional information on the presence (or absence) of blue mussel beds at each site at the time of large-scale surveys. Altogether 187 sites of M. edulis beds were recorded in the investigation area. In spring 1996, there were still only 19 sites where mussel beds still occurred, although at 51 sites residual mussel-bed structures were present, e.g. shell bases of former beds or protruding patches (which had been occupied by M. edulis before the beds vanished) and open spaces. At that time, the majority of the sites contained neither mussel beds nor mussel-bed structures. The analysis of recent data confirmed that mussel larvae have preferred to settle in sites of present mussel beds and sites with bases of former mussel beds. There was no preferential selection of one of these categories (settled beds vs. shell bases). On the other hand, the presence of mussel beds or mussel bed structures is not obligatory for settlement, because sites without those structures were also re-settled by the spatfall in 1996, even though on a smaller scale.     

Mussel beds — amensalism or amelioration for intertidal fauna?

The faunal assemblages of a mussel bed (Mytilus edulis L.) and ambient sandflat were compared to study how a bioherm of suspension feeding organisms affects benthic communities in a tidal flat. During a survey of mussel beds in the Wadden Sea at the island of Sylt (North Sea), a total of 52 macrofaunal species and 44 meiobenthic plathelminth species were detected. They occupied different microhabitats in the mussel bed. 56% of the macrofauna species were dwelling in the sediment beneath the mussels and 42% were epibenthic or epiphytic. The latter were restricted in their occurrence to the mussel bed. Along a transect from the sandflat to the mussel bed the mean species densities of macrofauna did not differ significantly, while abundances were significantly lower in the mussel bed than in the sandflat. The composition of the assemblages shifted from a dominance of Polychaeta in the sandflat to Oligochaeta in the mussel bed. Surface filter-feeding polychaetes of the sandflat (Tharyx marioni) were displaced by deposit feeding polychaetes under the mussel cover (Capitella capitata, Heteromastus filiformis). The total meiobenthic density was lower and single taxa (Ostracoda, Plathelminthes, Nematoda) were significantly less abundant in the mud of the mussel bed. The plathelminth assemblage was dominated by grazing species (Archaphanostoma agile), and differed in community structure from a sandflat aseemblage. An amensalistic relationship was found between the suspension-feeding mussels and suspension-feeding infauna, while deposit-feeders were enhanced. The presence of epibenthic microhabitats results in a variety of trophic groups co-occurring in a mussel bed. This is hypothesized as trophic group amelioration and described as an attribute of heterotrophic reefs.