Disturbance and organization of macroalgal assemblages in the Northwest Atlantic

Large seaweeds are often structurally dominant in subtidal and intertidal rocky shore benthic communities of the N.W. Atlantic. The mechanisms by which these algal assemblages are maintained are surprisingly different in the two habitats. In the subtidal community, kelps are dominant space competitors in the absence of strong grazing interactions. In contrast, the large perennial seaweeds of intertidal zones (fucoids and Chondrus crispus) are competitively inferior to both sessile filter feeders and ephemeral, pioneer algal species. Intertidal seaweed beds are maintained by carnivory of whelks, which reduces filter feeder populations, and by herbivorous periwinkles which reduce ephemeral algal populations. Through most of the intertidal zone, disturbance, both biological and physical, dictates which species shall compete and equilibrium conditions obtain subsequently.The roles of subtidal consumers are quite different. Sea urchins are the major algal herbivores and these voracious animals maintain an equilibrium state in which large tracts of subtidal coralline pavement are kept free of kelp forests. Urchins do not seem to play a successional facilitative role for kelps in the way that periwinkles do for fucoids in the intertidal. Control of herbivore populations is thus a key to the maintenance of subtidal foliose algal beds. It is clear that parasitic amoebas can decimate sea urchin populations so that kelp forest dominance is assured. However, the importance of carnivory in limiting urchins in the subtidal community is unclear in the absence of appropriate manipulation experiments. It is possible that carnivorous decapods and fin fish control sea urchin populations and hence foliose algal abundance, but this must remain speculative. The seaweed-dominated state of the subtidal system is an alternative equilibrium condition to the urchin/coralline alga configuration. The structure of the kelp beds is relatively uniform in responding to frequent small-scale, infrequent large-scale, or no, disturbance.     

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     

Spatial distribution patterns of Sargassum horneri in the coastal waters of the Ma'an Archipelago

Seaweed beds are a nearshore marine ecosystem composed of macroalgae and other marine organisms. In recent years, soil erosion, pollution, and other land-based mechanisms have severely reduced nearshore algal populations, particularly in areas influenced by the outflow of the Yangtze and Qiantang rivers. The Ma'an Archipelago is located in the estuary of the Yangtze and Qiantang rivers in the East China Sea. Although the archipelago is relatively distant from the shore, algal populations there have decreased significantly in recent years. The reasons for this decline are unknown, owing to an absence of data on macroalgal spatial distribution in seaweed beds and of long-term monitoring. Numerous environmental factors, operating at different scales, are likely to contribute to the abundance and distribution of macroalgae in seaweed beds. Therefore, it is important to study the beds across spatial scales to understand the ecological factors influencing the beds' presence and decline. During September 2014 and May 2015, we investigated the distribution of Sargassum horneri in germling and adult stages at 12 sites on three islands (Zhongkui, Lvhua, and Gouqi) around the Ma'an Archipelago. Our results showed that: (1) on a whole-archipelago scale, S. horneri density was highest around Zhongkui Island, and lowest around Lvhua Island. Adult plant lengths showed the opposite trend. The ratio of average densities around the three islands was 1 (Lvhua): 3.8 (Gouqi): 13.8 (Zhongkui) at the germling stage, and changed to 1:1.8:4 at the adult stage. (2) At the individual-site scale, wave exposure may explain why S. horneri density is highest in southeastern sites and lowest in northwestern ones. Around Zhongkui Island, S. horneri grew in turbulent conditions, was significantly affected by wave motion, and suffered a high mortality rate between the seedling and adult stages. (3) Within site scale, S. horneri was distributed over a considerably wider depth range around Zhongkui Island than around Lvhua or Gouqi islands. Adult S. horneri was most abundant at 2-m depth. Wave action apparently controlled the upper limit of S. horneri bathymetric distribution, whereas the lower limit depended on sedimentation and light availability.     

基于5.8S rDNA-ITS序列的我国浙江沿海铜藻群体遗传多样性分析

为了调查我国浙江沿海铜藻(Sargassum horneri)群体遗传多样性,对浙江省南麂岛火焜岙、洞头岛、枸杞岛的野生群体和南麂岛马祖岙养殖群体的铜藻44个样品的5.8S rDNA-ITS序列进行了PCR扩增和序列分析,获得DNA片段长度为1485 bp,包括部分ITS1、完整的5.8S rDNA和部分ITS2序列。序列分析表明仅有1个变异位点,说明群体间没有明显的遗传分化,遗传多样性较低。另外,将包含这一变异位点的基因型与GenBank公共数据库中另外2株铜藻的5.8S rDNA-ITS区序列进行比对,共有31个变异位点,其中插入/缺失位点10个。因此,建议加快人工铜藻场的建设以及野生铜藻场的恢复,保持铜藻场的生态平衡。     

Spatial distribution patterns of Sargassum horneri in the coastal waters of the Ma'an Archipelago

Seaweed beds are a nearshore marine ecosystem composed of macroalgae and other marine organisms. In recent years, soil erosion, pollution, and other land-based mechanisms have severely reduced nearshore algal populations, particularly in areas influenced by the outflow of the Yangtze and Qiantang rivers. The Ma'an Archipelago is located in the estuary of the Yangtze and Qiantang rivers in the East China Sea. Although the archipelago is relatively distant from the shore, algal populations there have decreased significantly in recent years. The reasons for this decline are unknown, owing to an absence of data on macroalgal spatial distribution in seaweed beds and of long-term monitoring. Numerous environmental factors, operating at different scales, are likely to contribute to the abundance and distribution of macroalgae in seaweed beds. Therefore, it is important to study the beds across spatial scales to understand the ecological factors influencing the beds' presence and decline. During September 2014 and May 2015, we investigated the distribution of Sargassum horneri in germling and adult stages at 12 sites on three islands (Zhongkui, Lvhua, and Gouqi) around the Ma'an Archipelago. Our results showed that: (1) on a whole-archipelago scale, S. horneri density was highest around Zhongkui Island, and lowest around Lvhua Island. Adult plant lengths showed the opposite trend. The ratio of average densities around the three islands was 1 (Lvhua): 3.8 (Gouqi): 13.8 (Zhongkui) at the germling stage, and changed to 1:1.8:4 at the adult stage. (2) At the individual-site scale, wave exposure may explain why S. horneri density is highest in southeastern sites and lowest in northwestern ones. Around Zhongkui Island, S. horneri grew in turbulent conditions, was significantly affected by wave motion, and suffered a high mortality rate between the seedling and adult stages. (3) Within site scale, S. horneri was distributed over a considerably wider depth range around Zhongkui Island than around Lvhua or Gouqi islands. Adult S. horneri was most abundant at 2-m depth. Wave action apparently controlled the upper limit of S. horneri bathymetric distribution, whereas the lower limit depended on sedimentation and light availability.     

The potential for seaweed resource development in subarctic Canada; Nunavik,Ungava Bay

Ungava Bay is ice covered 6–7 months of the year and evidence of ice scouring of seaweeds is extensive in the intertidal and shallow subtidal. Maximum tidal amplitudes of 16 m, among the highest in Canadian waters, compound this impact. Despite this level of annual perturbation, very extensive and dense beds of fucoids in the intertidal and laminarians in the subtidal are common on the western shores of Ungava Bay. Ground surveys of 24 intertidal stations combined with satellite images delineated 82,000 tons standing crop of Fucus vesiculosus and Fucus evanescens in Payne Bay, of which 36,000 tons were considered harvestable. Subtidally, kelp cover reached peak biomass at 5–10 m consisting of three primary species, Saccharina longicruris, Laminaria digitata, and Laminaria solidungula. In the area of Payne Bay, kelp beds of 100 ha were common, averaging 9–12 kg m⁻² wet weight. The productivity of brown algae at these latitudes has been assumed to be low relative to southern latitudes. Direct measurement of lineal growth indicates productivity is intermediate between arctic and temperate populations. The potential for medium level industrial harvest exists under conservative management strategies within the constraints of subarctic logistics.     

Cultivation of the brown alga Sargassum horneri: sexual reproduction and seedling production in tank culture under reduced solar irradiance in ambient temperature

As a large conspicuous intertidal brown alga, individuals of Sargassum horneri can reach a length of more than 7 m with a fresh weight of 3 kg along the coasts of the Eastern China Sea. The biomass of this alga as a vital component in coastal water ecology has been well documented. In recent years, a steady disappearance of the algal biomass along the once densely populated coastal areas of the Eastern China Sea has drawn attention in China. Efforts have been made to reconstruct the subtidal algal flora or even to grow the alga by use of long-lines. As part of the efforts to establish an efficient technique for producing seedlings of S. horneri, in this investigation a series of culture experiments were carried out in indoor raceway and rectangular tanks under reduced solar irradiance at ambient temperature in 2007–2008. The investigation demonstrated that: (1) sexual reproduction of S. horneri could be accelerated in elevated temperature and light climates, at least 3 months earlier than in the wild; (2) eggs of S. horneri had the potential to be fertilized up to 48 h, much longer than that of known related species; (3) suspension and fixed culture methods were both effective in growing the seedlings to the long-line cultivation stage; and (4) the life cycle of S. horneri in culture could be shortened to 4.5 months, thus establishing this alga as an appropriate model for investigating sexual reproduction in dieocious species of this genus.     

Characterisation of microsatellite loci in the habitat-forming kelp, Ecklonia radiata (Phaeophyceae, Laminariales)

Habitat forming algae play an important role in the ecology of temperate reefs worldwide. Despite this, our understanding of levels of gene flow within and among populations of algae is largely limited to studies on intertidal species; we know comparatively little about important habitat-forming subtidal algae. Here, we develop eight polymorphic microsatellite markers for the characterisation of population genetic diversity and structure in the subtidal kelp, Ecklonia radiata. This large macroalga is the most abundant habitat-forming kelp on the subtidal rocky reefs of temperate Australia and New Zealand where it forms extensive forests that support an astounding diversity of associated taxa.     

Changing Community States in the Gulf of Maine: Synergism Between Invaders, Overfishing and Climate Change

Human activities, including overfishing and species introductions, have had a dramatic impact on benthic communities in the Gulf of Maine within the past two decades. Prior to the 1970s, the climax community in the shallow subtidal was composed of Laminaria spp. kelp beds with an understory of arborescent red algae. In the 1980s, a population explosion of the green sea urchin, Strongylocentrotus droebachiensis, created an alternate community state, urchin barrens. Recently, a new community has been observed in former urchin barrens and kelp beds. This assemblage is principally composed of the introduced species: Codium fragile subsp. tomentosoides (green alga), Membranipora membranacea (bryozoan), Diplosoma listerianum (tunicate), Bonnemaisonia hamifera (red alga) and the opportunistic species Mytilus edulis (mussel) and Desmarestia aculeata (brown alga). In addition to changes in relative abundance, many of these species have greatly expanded their distribution and habitat selection. A model detailing mechanisms for the transition of the traditional kelp bed and urchin barren communities to others is presented and implications for this new community are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

First Record of Undaria Pinnatifida (Laminariales, Phaeophyta) in Southern Patagonia, Argentina

The Japanese kelp Undaria pinnatifida was found on 7th September 2005 in Ría Deseado (Santa Cruz, Argentina). This invasive algae was registered from the lower intertidal to the upper levels of the subtidal zone. The sporophytes were fixed to slabs, gravel and rocks of variable sizes and were also present as epibionts of tunicates. The sporophytes total length ranged between 3 and 88 cm, with a mean of 28.75 cm in the intertidal and 38.3 cm in the subtidal. The density and biomass increased from the intertidal to the subtidal. The kelp population showed a higher number of small immature individuals in the intertidal than␣subtidal where the organisms were bigger and with more individual biomass. This first record of U. pinnatifida in Southern Patagonia extends its southern distribution limit in Argentina, showing progressive expansion in the southwestern Atlantic. More surveys must be conducted to understand the evolution of U. pinnatifida invasion and its impact on the native benthic community.     

Predation on barnacles of intertidal and subtidal mussel beds in the Wadden Sea

Balanids are the numerically dominant epibionts on mussel beds in the Wadden Sea. Near the island of Sylt (German Bight, North Sea), Semibalanus balanoides dominated intertidally and Balanus crenatus subtidally. Field experiments were conducted to test the effects of predation on the density of barnacle recruits. Subtidally, predator exclusion resulted in significantly increased abundances of B. crenatus, while predator exclusion had no significant effects on the density of S. balanoides intertidally. It is suggested that recruitment of B. crenatus to subtidal mussel beds is strongly affected by adult shore crabs (Carcinus maenas) and juvenile starfish (Asterias rubens), whereas recruits of S. balanoides in the intertidal zone are mainly influenced by grazing and bulldozing of the very abundant periwinkle Littorina littorea, which is rare subtidally. Thus, not only do the barnacle species differ between intertidal and subtidal mussel beds, but the biotic control factors do so as well. Electronic Publication     

Influence of subantarctic Macrocystis bed metabolism in diel changes of marine bacterioplankton and CO2 fluxes

The significance of bacterial communities in the fluxes of carbonand energy in giant kelp beds (Macrocystis pyrifera) in KerguelenArchipelago, subantarctica, was estimated by measuring bacterioplanktonbiomass and production over diel cycles in surface seawaterslocated inside and outside of Macrocystis beds. Several physicochemicalparameters [temperature, solar radiation, tide level, dissolvedoxygen, total inorganic carbon (TCO₂), partial pressure of CO₂(pCO₂) and dissolved organic carbon (DOC)] were simultaneouslyrecorded in order to establish possible relationships with observedchanges in bacterial parameters. Bacterial biomass and productionwere significantly higher inside the kelp bed than in the surroundingarea. Furthermore, the results showed large and clear diel variationsof all parameters measured inside the kelp bed. Changes in dissolvedoxygen, TCO₂ and pCO₂ paralleled those of solar radiation, andwere obviously related to the metabolic activity of the kelp.Mean cell volumes and saprophytic bacterial abundance variedover the diel cycles in the same way as photosynthetic activity,while DOC, bacterioplankton production and the frequency ofdividing cells varied in an opposite way with maximal valuesat night-time.     

Possible latitudinal clines in Antarctic intertidal and subtidal zone communities encrusting ephemeral hard substrata

Aim Encrusting faunal communities on rocks were examined from Southern Ocean intertidal and subtidal (6 m) zones to investigate potential change with latitude. Location The site locations were South Georgia (54°S [sub-Antarctic]), Signy Island (60.5°S [maritime Antarctic]) and Adelaide Island (68°S [high Antarctic]). Methods The number of taxa, degree of colonization and bryozoan growth and mortality were measured. Results The communities present were relatively simple in terms of composition and interactions. Spirorbid polychaetes (Annelida) and cheilostomatid bryozoans were the principal components. The community simplicity and dominance of unitary (compared to colonial) fauna increased with latitude. Mean annual mortality of subtidal cheilostomatids also significantly increased with latitude. The number of bryozoan species and potential aspects of community complexity increased with latitude. The growth of the bryozoan Inversiula nutrix increased with latitude reversing the general trend of decreased growth rate. Conclusions These findings suggest that disturbance may significantly increase with latitude within the Southern Ocean. The apparent reversal of typical latitudinal growth trends is probably a result of classical island biogeography, with species decreasing away from a centre of high diversity.     

Habitat linkages: the effect of intertidal saltmarshes and adjacent subtidal habitats on abundance, movement, and growth of an estuarine fish

In this study we used pinfish (Lagodon rhomboides) in field experiments to examine linkages between intertidal saltmarsh and adjacent subtidal habitats. Pinfish are more than twice as abundant in intertidal marshes adjacent to seagrass beds than in those adjacent to the unvegetated subtidal bottom. Movement of pinfish between the marsh edge and the adjacent subtidal habitat was greater for fish captured in areas with both intertidal and subtidal vegetation than in those with intertidal vegetation and adjacent unvegetated mudflats. This movement provides an important link between habitats, allowing transfer of marsh-derived secondary production to subtidal seagrass beds and vice versa. Pinfish held in enclosures with both intertidal and subtidal vegetation were, on average, approximately 90% heavier than fish held in enclosures with intertidal vegetation and unvegetated subtidal bottom. Because saltmarshes and seagrass beds contribute to the production of living marine resources, active measures are being taken to preserve and restore these habitats. The results from this study have direct application to decisions concerning site selection and optimal spatial proximity of saltmarsh and seagrass habitats in the planning of restoration and mitigation projects. To maximize secondary production and utilization of intertidal marshes, managers may opt to restore and/or preserve marshes adjacent to subtidal seagrass beds. Received: 31 May 1996 / Accepted: 23 October 1996     

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     

Intertidal and Subtidal Fouling Assemblages in a Patagonian Harbour (Argentina, Southwest Atlantic)

The composition of the early stages of intertidal and subtidal fouling assemblages in Comodoro Rivadavia harbour (Argentina, 45°52′ S, 67°28′ W) and the influence of shore level and season on their structure were analysed. At the beginning of each season, stones were glued to the substratum with epoxy putty and distributed along 4 vertical transects at intervals of 20 m, at 3 levels: upper intertidal, middle intertidal, and subtidal. Substrata remained in the field for 84–100 days. A total of 48 samples (4 seasons × 3 levels × 4 replicates) were analysed. Species richness increased with depth, with 6 taxa in the upper intertidal, 23 in the middle intertidal and 31 in the subtidal. Seasonal differences in richness were less distinct. Green, red and brown algae were the dominant groups. Invertebrates were mainly represented by filter-feeding, sessile organisms, such as cheilostome bryozoans, spirorbid polychaetes and acorn barnacles. The barnacle Balanus glandula and the bryozoan Cryptosula pallasiana were the only non-indigenous species found in this study. Highly significant differences in structure among shore levels and seasons were evidenced by a two-way ANOSIM test. The upper intertidal is characterized by the filamentous green algae Urospora penicilliformis and Ulothrix flacca. The barnacle Balanus glandula is the most abundant species in the middle intertidal. The subtidal is defined mainly by the presence of the spirorbid polychaetes Paralaeospira levinseni and Romanchella perrieri, and the keyhole limpet Fissurella radiosa. Ordination of samples by season was less clear than by shore level.     

Intertidal community structure differs significantly between substrates dominated by native eelgrass (<Emphasis Type="Italic">Zostera marina</Emphasis> L.) and adjacent to the introduced oyster <Emphasis Type="Italic">Crassostrea gigas</Emphasis> (Thunberg) in British Columbia,Canada

Eelgrass beds represent important habitats for marine organisms, but are in decline in many coastal areas around the world. On Cortes Island, British Columbia, Canada, oysters coexist regionally with native eelgrass (Zostera marina L.), but eelgrass is typically absent directly seaward of oyster beds (the “below-oyster cobble zone”). We compared assemblage structure of nekton (fish and swimming macroinvertebrates) and epibenthos (macroinvertebrates and macroalgae) between eelgrass bed and below-oyster habitats. We sampled the intertidal zone on Cortes Island at low tide using two methods: quadrats to enumerate epibenthic macroinvertebrates and macroalgae, and beach seines to enumerate fish and swimming macroinvertebrates. Using multivariate analysis of similarity (ANOSIM), we found that the structure of nektonic and epibenthic assemblages associated with below-oyster cobble zones were significantly different from those in eelgrass-beds. Univariate measures showed that nektonic species richness and abundance were significantly higher in eelgrass beds than in below-oyster cobble habitat, whereas epibenthic species richness and abundance were significantly higher in below-oyster habitat. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. Handling editor: J. Trexler     

Micropopulation differentiation in phenol content and susceptibility to herbivory in the Chilean kelp Lessonia nigrescenss (Phaeophyta,Laminariales)

Micropopulation differences in phenol content between intertidal and subtidal individuals of the kelp Lessonia nigrescens were found. Subtidal plants showed: (1) significantly higher phenol content than intertidal individuals, in vegetative and reproductive tissues, (2) intra-plant differences, with higher content in apical frond tissues, (3) higher resistance to consumption by herbivorous fishes. The microscopic progeny of subtidal plants showed the same trend as adult plants: (1) haploid spores from subtidal plants had higher phenol content than spores from intertidal individuals, and (2) the microscopic sporophytes derived from subtidal spores and gametophytes were less consumed by herbivorous snails (Tegula tridentata) than those derived from intertidal plant propagules. No increase in phenol content was detected after mechanical injury to experimental fronds, or after transplantation to the subtidal environment.In addition to the absence of inducible responses, the different phenol content between intertidal and subtidal individuals, in adult diploid plants and also in the haploid progeny, suggests that both environments differ someway enough to fix the mentioned features on the plants of Lessonia nigrescens. It is likely that the differences in herbivory between the two distributional extremes contributed to the observed pattern.     

Growth and size distribution of some large brown algae in Ohori,east coast of Korea

About fifty individuals of four dominant large brown algae were collected by random sampling at monthly intervals using SCUBA. Size frequencies of Sargassum confusum and S. horneri were concentrated mainly in the small size classes during the initial growth phase, but were more or less evenly distributed in most size classes during the fast growing phase. Undaria pinnatifida and Costaria costata showed the same tendency as Sargassum spp., that frequency distribution became even when the plants reached their maximum growth. The kelp species recorded their maximum lengths in March, whereas Sargassum spp. showed their peaks in early summer. The relationships between the length and weight in different growth phases appeared to be dependent on the phenology of each species.     

Introduction,establishment and expansion of the Pacific oyster <Emphasis Type="Italic">Crassostrea gigas</Emphasis> in the Oosterschelde (SW Netherlands)

The Pacific oyster Crassostrea gigas was first introduced as an exotic species by oyster farmers in 1964 in the Oosterschelde estuary (SW Netherlands). The initial phase is not well documented but first natural spatfall was recorded in 1975. Excessive spatfall occurred in 1976 and this is considered the start of the expansion phase of the wild oysters. Oyster beds in intertidal and subtidal areas of the Oosterschelde estuary have been growing since. The development in the intertidal area has been reconstructed by using aerial photography, validated by ground truth in 2000–2002. In the subtidal areas extensive oyster beds have been detected by using side scan sonar; on hard substrates along the dikes coverage with oysters up to 90% locally has been recorded by scuba diving surveys. Expansion has also occurred into adjacent water bodies including the Wadden Sea. By forming resistant reefs the oysters induce structural changes in the ecosystem. It is concluded that bed area is still expanding while decrease of the fraction live animals may indicate adjustment of the stock size to the local conditions.