Blue Mussel Beds Research Articles

Drone-based monitoring of intertidal blue mussel beds in the Wadden Sea – comparison of a threshold and two machine learning approaches

In the Wadden Sea, intertidal mussel beds of the blue mussel (Mytilus edulis) and the Pacific oyster (Magallana gigas) form distinct epibenthic communities and local hotspots of high biomass and biodiversity. To detect and evaluate natural and anthropogenic processes, a ground-based monitoring program started over 25 years ago in the German Wadden Sea. In this study, we describe the potential of drones and machine learning approaches for a remote sensing-based integration into an existing monitoring program of intertidal mussel beds. A fixed wing drone was used to cover an area of up to 39ha in a single flight, with an overall time saving potential of 50%. Applying machine learning approaches, a detailed extraction of the intertidal blue mussel bed coverage with an overall accuracy (OA) up to 95.6% was reached, applying a Support Vector Machine (SVM). The application of a multispectral sensor improved the classification performance. Compared to ground-based monitoring, the drone-based method provided significantly more information on the area extension, coverage, and associated algae of the mussel beds. The results show that drones can provide a non-invasive way to survey large and difficult to access areas providing detailed maps of mussel beds and their internal structures.

Soft-bottom tidepools within mixed reefs of native mussels and introduced oysters – refuge for associated species and parasites?

AbstractThe introduction of Pacific oysters to the sedimentary south-eastern North Sea coast and their establishment on intertidal native blue mussel beds has caused the development of mixed reefs of mussels and oysters with extensive tidepools. Tidepools have been intensively studied at rocky shores where they show community structures, which usually differ from that of the surrounding emerging substrates. Tidepools at sedimentary coasts, however, have received less attention. We compared the community structure and species interactions inside and outside tidepools in oyster reefs by determining densities of snails, barnacles and amphipods. Snail densities were similar in and outside tidepools. Barnacle coverage on bivalve shells, however, was lower inside tidepools, which may be caused by higher predation pressure and increased snail grazing under permanently submerged conditions, as was revealed by field and laboratory experiments. Additionally, we studied the occurrence of copepod and trematode parasites in blue mussels inside and outside tidepools. Prevalence and intensity of parasitic copepods was higher in mussels inside tidepools. Trematode parasites, by contrast, showed a lower intensity in mussels inside tidepools. This can be explained by high amphipod densities found inside tidepools because trematode larvae represent a food source of amphipods. Our study suggests that the community structure of oyster reefs within tidepools is not a submerged equivalent to that of intertidal reefs. As their counterparts at rocky shores, they show their own species distribution patterns with particular species interactions and only provide refuge for specific species such as parasitic copepods.

Introduced marine ecosystem engineers change native biotic habitats but not necessarily associated species interactions

Introduced bioengineering organisms may fundamentally change native coastal ecosystems by modifying existing benthic habitat structures and thereby habitat-specific species interactions. The introduction of the Pacific oyster Magallana gigas into the sedimentary coastal area of the south-eastern North Sea and its preferred settlement on native blue mussel shells caused a large-scale shift from monospecific Mytilus edulis beds to current mixed reefs of mussels and oysters. To investigate whether the newly developed biotic habitat affects the occurrence of associated native key organisms and their ecological functions, we studied the long-term density trajectory of the gastropod Littorina littorea and its grazing activity on barnacles attached to Pacific oyster reefs in the northern Wadden Sea. We found no significant correlation between oyster and snail densities on blue mussel beds in the last two decades, which spans a time-period from the beginning of Pacific oyster establishment to today's oyster dominance. A manipulative field experiment revealed that snail density significantly affects the recruitment success of barnacles Semibalanus balanoides on oyster shells with the highest number of barnacle recruits at snail exclusion. Thus, density and grazing activity of the snail L. littorea may control barnacle population dynamics on epibenthic bivalve beds in the Wadden Sea. This interspecific interaction was already known for blue mussel beds before the oyster invasion and, therefore, we conclude that despite the strong modifications that non-native ecosystem engineers cause in native biotic habitats, the ecological functions of associated key species can remain unchanged.

Occurrence, stability, and associated species of subtidal mussel beds in the North Frisian Wadden Sea (German North Sea Coast)

According to historical literature, subtidal blue mussel (Mytilus edulis) beds were relatively widespread in the North Frisian Wadden Sea in the first decades of the 20th century. However, natural subtidal mussel beds subsequently suffered severe losses, partly due to natural causes but also as a consequence of uncontrolled fishing. Especially after the Second World War blue mussels could only be reliably found at a few natural locations by the 2000s. Notably, past sampling techniques using grabs and dredges mean that our knowledge of the areal dimensions and characters of these blue mussel habitats remained limited. Since 2009 broad side scan sonar surveys have been applied to map sediments and to detect subtidal mussel beds or their potential habitats. Apart from several locations with hints of mussels, only two mussel beds were recognized as biogenic reefs according to the EU Habitats Directive. These beds have been monitored annually since 2012. Although the beds have remained relatively stable in terms of area, both beds showed an increasing degradation of their accuracy of topographical discrimination from the surrounding seabed over the years. In line with this degradation, we observed a decline in blue mussel abundance and a strong increase in the dominance of the Pacific oyster (Magallana gigas). However, the final year of the investigation (2018) was characterised by a strong decline in both Mytilus and Magallana populations at both sites.

Towards a sampling design for characterizing habitat-specific benthic biodiversity related to oxygen flux dynamics using Aquatic Eddy Covariance.

The Aquatic Eddy Covariance (AEC) technique has emerged as an important method to quantify in situ seafloor metabolism over large areas of heterogeneous benthic communities, enabling cross-habitat comparisons of seafloor productivity. However, the lack of a corresponding sampling protocol to perform biodiversity comparisons across habitats is impeding a full assessment of marine ecosystem metabolism. Here, we study a range of coastal benthic habitats, from rocky-bed communities defined by either perennial macroalgae or blue mussel beds to soft-sediment communities comprised of either seagrass, patches of different macrophyte species or bare sand. We estimated that the maximum contribution to the AEC metabolic flux can be found for a seafloor area of approximately 80 m2 with a 5 meter upstream distance of the instrument across all the habitats. We conducted a sampling approach to characterize and quantify the dominant features of biodiversity (i.e., community biomass) within the main seafloor area of maximum metabolic contribution (i.e., gross primary production and community respiration) measured by the AEC. We documented a high biomass contribution of the macroalgal Fucus vesiculosus, the seagrass Zostera marina and the macroinvertebrate Mytilus edulis to the net ecosystem metabolism of the habitats. We also documented a significant role of the bare sediments for primary productivity compared to vegetated canopies of the soft sediments. The AEC also provided insight into dynamic short-term drivers of productivity such as PAR availability and water flow velocity for the productivity estimate. We regard this study as an important step forward, setting a framework for upcoming research focusing on linking biodiversity metrics and AEC flux measurements across habitats.

What Is Invasion Biology?

This research presents the invasion history as well as an up-to-date distribution of the Pacific oyster Magallana gigas in European coastal waters, concluding that this invasive species has spread to large parts of all coastal biotopes. Moreover, the measures and management of the Pacific oyster invasions of coastal and marine ecosystems is discussed including restoration and resilience of invaded ecosystems. When Magallana gigas is well established and has reached adjustment phase characterized by firm biogenic reefs, eradication or even decimation is difficult and costly. During the establishment phase and maybe also the expansion phase, where the oyster appear as single individuals spread out on the sediment and yet not forming larger clumps and reefs, it is more realistic to implement mitigation tools. These initiatives cannot be implemented everywhere. They should be reserved especially for protected areas under national or international legislation e.g., Natura2000 areas, where smaller areas can be restored. The present contribution discusses and evaluates known mitigation tools like physical damaging of individual oysters, dredging, small-scale hand picking as well as various fishery concepts. Additionally, this research proposes some novel strategies where triploidity is suggested for implementation to avoid or decimate further spread of the species. As well as some initiatives based on choking and/or starving out the oysters by deploying a thick layer of sand/gravel or food for the competing blue mussels over oyster reefs. The efficiency of the different strategies, as well as the required frequency of follow up initiatives, are evaluated. The major conclusion from this research suggests that decisions to combat the alien oyster are mainly based on political/socioeconomic arguments and less on ecological arguments. This is because reefs of Pacific oysters most likely lead to equal or higher biodiversity as compared to native (e.g., blue mussel) beds and the oysters serve a redundant role in a trophic sense living from same resources as native bivalves. Moreover, oyster reefs serve ecosystem services as protecting shores against erosion and flooding. The most negative impact reported is on some mussel feeding birds that may have a reduced access to food. If eradication/decimation is decided, several mitigation instruments should simultaneously come into play and often repeating treatment strategy is necessary.

Impact on bird fauna of a non-native oyster expanding into blue mussel beds in the Dutch Wadden Sea

Intertidal mussel beds are important for intertidal ecosystems, because they feature a high taxonomic diversity and abundance of benthic organisms and are important foraging grounds for many avian species. After the introduction of the Pacific oyster (Crassostrea gigas) into the European Wadden Sea, many mussel beds developed into oyster dominated bivalve beds. Despite the fact that oysters have been colonizing many European intertidal areas for about two decades, their impact on the ecosystem is still poorly understood. Here, we investigated the impact of oysters on the condition of mussels and on the spatial distribution of birds on 18 bivalve beds with different grades of oyster occurrence throughout the Dutch Wadden Sea. Moreover, in comparing bird densities on bivalve beds with densities expected on the total intertidal area, we could detect which species exhibit a preference for the structured habitat. Overall, 50 different bird species were observed on the beds, of which about half regularly frequent intertidal flats. Most of these species showed a preference for bivalve beds. The condition of mussels decreased with the oyster dominance, whereas the majority of bird species was not affected by the oyster occurrence. However, three of the four species that were negatively affected depend on intertidal mussels as food source. Even though the Pacific oyster is a nonnative species, attempts to fight it may do more harm to avian biodiversity than good.

Joint use of multiple Synthetic Aperture Radar imagery for the detection of bivalve beds and morphological changes on intertidal flats

We analyzed a large amount of high-resolution Synthetic Aperture Radar (SAR) data of dry-fallen intertidal flats on the German North Sea coast with respect to the imaging of sediments, macrophytes, and mussels. TerraSAR-X and Radarsat-2 images of four test areas acquired from 2008 to 2013 form the basis for the present investigation and are used to demonstrate that pairs of SAR images, if combined through basic algebraic operations, can already provide indicators for morphological changes and for bivalve (oyster and mussel) beds. Multi-temporal analyses of series of SAR images allow detecting bivalve beds, since the radar backscattering from those beds is generally high, whereas that from sediments may vary with imaging geometry and environmental conditions. Our results further show evidence that also single-acquisition, dual-polarization SAR imagery can be used in this respect. The polarization coefficient (i.e., the ratio of the difference and the sum of both co-polarizations) can be used to infer indicators for oyster and blue-mussel beds.

Depth-related spatial patterns of sublittoral blue mussel beds and their associated macrofaunal diversity revealed by geostatistical analyses

The blue mussel Mytilus edulis is a foundation species with ecosystem engineering functions in the brackish, non-tidal Baltic Sea. In this study from the western Gulf of Finland, the relationship between the spatial patchiness of blue mussels and the diversity of associated macrofauna was examined across small scales (centimeters to meters) for the first time in subtidal habitats. It was demonstrated using geostatistical tools that blue mussel abundance and the diversity of associated macrofauna varied and interacted at 2 depths. Classic analyses (ANOVAs, correlations and multivariate techniques) detected no relationships between the abundance of blue mussels and their associated macrofaunal diversity, or differences in the abundance of mussels or the diversity of associated macrofauna between depths. Using semivariograms, differences in spatial heterogeneity between depths emerged: i.e. patchiness at 5 m and random patterns at 8 m depth. Cross-semivariograms detected negative spatial co-variation between blue mussel abundance and diversity of macrofauna at 5 m, but positive and neutral spatial relationships at 8 m depth. Combining the approaches suggested that high dislodgment of mussels in shallow environments causes this pattern. Dislodgement effects may be compensated for by increased turnover of small mussels in patches within mussel beds, which would result in reduced habitat space for associated macrofauna. On the basis of our results, it is suggested that patchiness of a foundation species is an ecological response, or result of a disturbance, that reduces the diversity of the associated macrofaunal community.

Multi-frequency SAR data help improving the monitoring of intertidal flats on the German North Sea coast

We demonstrate that Synthetic Apertur Radar (SAR) data have great potential to improve an existing monitoring system based on optical data for intertidal flats and to complement the classification of sediments, macrophytes, and mussels in the German Wadden Sea. Multi-satellite SAR data acquired at different radar bands (L, C, and X band, from ALOS PALSAR, from ERS SAR, Radarsat-2 and ENVISAT ASAR, and from TerraSAR-X, respectively) were used to investigate whether they can be jointly used for crude sediment classification on dry-fallen intertidal flats and for detecting benthic fauna such as blue mussel or oyster beds. In this respect, we show that both multi-satellite and multi-temporal analyses provide valuable input for the routine monitoring of exposed intertidal flats on the German North Sea coast, the latter already improving the identification of the spatial extent of mussel (oyster) beds. In addition, we demonstrate that high-resolution SAR is capable of detecting residuals of historical land use in areas that were lost to the sea during major storm surges in the 14th and 17th centuries.

Large-Scale Spatial Dynamics of Intertidal Mussel (Mytilus edulis L.) Bed Coverage in the German and Dutch Wadden Sea

Intertidal blue mussel beds are important for the functioning and community composition of coastal ecosystems. Modeling spatial dynamics of intertidal mussel beds is complicated because suitable habitat is spatially heterogeneously distributed and recruitment and loss are hard to predict. To get insight into the main determinants of dispersion, growth and loss of intertidal mussel beds, we analyzed spatial distributions and growth patterns in the German and Dutch Wadden Sea. We considered yearly distributions of adult intertidal mussel beds from 36 connected tidal basins between 1999 and 2010 and for the period 1968–1976. We found that in both periods the highest coverage of tidal flats by mussel beds occurs in the sheltered basins in the southern Wadden Sea. We used a stochastic growth model to investigate the effects of density dependence, winter temperature and storminess on changes in mussel bed coverage between 1999 and 2010. In contrast to expectation, we found no evidence that cold winters consistently induced events of synchronous population growth, nor did we find strong evidence for increased removal of adult mussel beds after stormy winter seasons. However, we did find synchronic growth within groups of proximate tidal basins and that synchrony between distant groups is mainly low or negative. Because the boundaries between synchronic groups are located near river mouths and in areas lacking suitable mussel bed habitat, we suggest that the metapopulation is under the control of larval dispersal conditions. Our study demonstrates the importance of moving from simple habitat suitability models to models that incorporate metapopulation processes to understand spatial dynamics of mussel beds. The spatio-dynamic structure revealed in this paper will be instrumental for that purpose.

Predicting the occurrence of rocky reefs in a heterogeneous archipelago area with limited data

The lack of spatial distribution data on marine habitats often presents an obstacle to their protection. The Annex I of the Habitats Directive (European Council Directive 92/43/EEC) lists habitats that are important in biodiversity protection and should be maintained (or restored) to a favourable conservation status. The habitats listed should be protected within an ecological network of protected areas, the Natura 2000 network. However, in the past the establishment of the marine Natura 2000 network has been largely based on insufficient knowledge on the distribution of the habitats. Annex I habitat type reefs are defined as formations of hard compact biogenic or geogenic substrata, which arise from the seafloor in the sublittoral and littoral zone. As obtaining marine data is time-consuming and costly, the bathymetric and substratum data needed for their identification on a larger scale are often scarce. Furthermore, the use of data may be limited due to e.g. national security reasons. This study identifies reefs in a complex archipelago area in the northern Baltic Sea using the best, although limited, data currently available. In the area reefs are elevated rocky outcrops and the associated algal communities and blue mussel beds are vital in maintaining biodiversity in the relatively species poor Baltic Sea. In addition to identifying the physical reef structures, an estimate of their ecological value is obtained by modelling the distribution of four key species occurring on reefs. The results are encouraging, as 55 out of 68 of the potential reefs ground-truthed were confirmed to be reefs. Furthermore the number of predicted species occurring on the reefs, correlated significantly with the number of species observed. The presented maps serve as a valuable background for more detailed mapping of the species diversity occurring on reefs as well as for monitoring their ecological status. Map-based information on important habitats is essential in conservation and marine spatial planning to minimize human impact on marine ecosystems.

The history of intertidal blue mussel beds in the North Frisian Wadden Sea in the 20th century: Can we define reference conditions for conservation targets by analysing aerial photographs?

Conservation decisions often rely on defining a reference status for habitats and species to enable targets to be set and progress measured. Long-lasting and continual anthropogenic impacts on habitats and species make the setting of undisturbed reference values such as diversity, distribution, population size or other ecological characteristics, difficult. In turn this hampers assessment of ecological status.Within the Wadden Sea, intertidal blue mussel beds are important biogenic structures which can be clearly defined from the surrounding flats. As mussel beds are highly productive habitats, they are considered as biological quality indicators for coastal waters. Nonetheless the reference status provokes controversy in discussions between policymakers, stakeholders and researchers. In order to build on existing knowledge of intertidal blue mussel beds in the North Frisian Wadden Sea, we analysed aerial photographs from the 1930s, 1958, 1989, 1998 and 2010. We supplemented this remote sensing data with annual monitoring data from 1999 to 2009 obtained from analysis of aerial photographs and field surveys.Results show a generally high persistency of blue mussel beds likely over eight decades, although sites were probably not permanent throughout the time period and their areal extent had changed. Mussel beds occur mainly on the east side of the islands which provide shelter against storms from the west. Studies of aerial photographs for the 1930s and 1958 demonstrate the importance of historical data to an assessment of the current status of the beds. In particular they help assess the distribution and extent of mussel beds over time.

Habitat change by the formation of alien Crassostrea-reefs in the Wadden Sea and its role as feeding sites for waterbirds

Non-indigenous Pacific oysters (Crassostrea gigas) have been invading the central Wadden Sea since 1998, predominantly settling on intertidal blue mussel (Mytilus edulis) beds which are increasingly transformed into Crassostrea-reefs. Pacific oysters are strong ecosystem engineers and the habitat change is considered to be a threat for waterbirds losing important feeding sites in the intertidal of the Wadden Sea. This study has increased our understanding of the use of foraging habitats by birds according to changing food resources.During the spring and autumn migration period in 2007, we recorded bird densities at two reef types varying in Pacific oyster density and at the adjacent sand flat as a reference site. We also recorded feeding behaviour, choice of prey and assessed peck and intake rate of three target species: Eurasian oystercatcher Haematopus ostralegus, Eurasian curlew Numenius arquata and European herring gull Larus argentatus. To evaluate the use of the Crassostrea-reef in the central Wadden Sea, we compared bird densities of the target species at different intertidal feeding habitats in various regions and compared the biomass intake of Eurasian oystercatcher feeding on different prey species. We show that Eurasian oystercatcher and Eurasian curlew have adapted to the new situation and learned to exploit the food supply offered by Crassostrea-reefs. While foraging mainly on Pacific oysters, Eurasian oystercatchers attained sustainable intake rates even though food resource at dense reef during autumn was very poor due to a lack in harvestable oysters. Consolidation of reefs limits the accessibility of prey for Eurasian oystercatchers whereas a successful recruitment of Pacific oysters enhances the suitability of the habitat. Eurasian curlew was promoted by the engineering effects of the Pacific oyster while feeding extensively on shore crabs at the reefs. In contrast, European herring gulls appear hampered in foraging during low tide and hereby negatively affected by the habitat change from Mytilus-beds to Crassostrea-reefs.

Invertebrate communities associated with blue mussel beds in a patchy environment: a landscape ecology approach

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 471:101-110 (2012) - DOI: https://doi.org/10.3354/meps10010 Invertebrate communities associated with blue mussel beds in a patchy environment: a landscape ecology approach Maria Koivisto1,2,*, Mats Westerbom1,3 1Tvärminne Zoological Station, Helsinki University, J.A. Palménsväg 260, 10900 Hangö, Finland 2Department of Environmental Sciences, Aquatic Sciences, Helsinki University, PO Box 65, 00014 Helsinki University, Finland 3Metsähallitus, Natural Heritage Services, PO Box 94, 01301 Vantaa, Finland *Email: maria.koivisto@eduvantaa.fi ABSTRACT: Landscape ecology has evolved rapidly as a terrestrial discipline over the past centuries due to an increased concern over habitat loss and fragmentation. Proceedings in the marine realm have been comparatively scarce, even though many key habitats in marine ecosystems are becoming equally fragmented. We sampled the macrofauna associated with blue mussel beds in naturally patchy landscapes of islands and reefs in the non-tidal Baltic Sea and tested for the effect of island area, isolation and reef depth on local species richness and composition. The fauna was also compared among islands and reefs. Contrary to expectations, species richness did not increase with island size, and species diversity was higher on deep reefs compared to shallow reefs and islands protruding from the water. Region and depth also had an effect on species assemblage. Geographical distance to nearest island or reef showed no correlation with species richness but correlated positively with faunal density, i.e. a higher degree of isolation increased density. In accordance with other marine studies, our data, therefore, supports the nearest refuge hypothesis, postulating that mobile invertebrates disperse over areas of non-habitat and then concentrate in isolated habitats because habitats in isolation are the nearest available refuge. We show that terrestrial landscape approaches cannot be directly applied to the study system. This has important implications for the planning of marine protected areas and their management. KEY WORDS: Habitat area · Isolation · Seascape · Benthic community · Diversity · Baltic Sea Full text in pdf format PreviousNextCite this article as: Koivisto M, Westerbom M (2012) Invertebrate communities associated with blue mussel beds in a patchy environment: a landscape ecology approach. Mar Ecol Prog Ser 471:101-110. https://doi.org/10.3354/meps10010 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 471. Online publication date: December 19, 2012 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2012 Inter-Research.

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 Fohr, 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.

Succession-driven facilitation of macrofaunal communities in sublittoral blue mussel habitats

Whereas it is well known that ecosystem engineers can have positive effects on biodiversity, fundamental gaps still remain in our understanding of the underlying mechanisms in many marine systems. Individual- and population-level traits of engineering species are likely to determine their ability to modify the environment. Blue mussels function as important ecosystem engineer species in the Baltic Sea where they facilitate the existence of distinct associated communities. The population structure of these blue mussel beds fluctuates widely, but ecosystem effects of these fluctuations are poorly known. In this study, we experimentally and descriptively measured the effect of mussel succession stage with differing biomass, density and size structure on macrofaunal diversity and body size at 5 m depth. We found that mussel succession stage is an important determinant of faunal abundance, species richness and species assemblage. Our results demonstrate that mussel succession stage (size structure, density and biomass) highly affect the associated macrofaunal communities and the strength of facilitation in these systems. We also found that the body size of crustaceans and clams was positively correlated with mussel body size, increasing with a growing mussel body size. We suggest that it is of high importance to consider individual traits of ecosystem engineers and the types of environments in which they play critical roles in order to increase the value of the ecosystem engineering concept in ecology and conservation biology.

Habitat structure and complexity as determinants of biodiversity in blue mussel beds on sublittoral rocky shores

Habitat-forming, ecosystem engineer species are common in most marine systems. Still, much uncertainty exists about how individual and population-level traits of these species contribute to ecosystem processes and how engineering species jointly affect biodiversity. In this manipulative field experiment, we examined how biodiversity in marginal blue mussel beds is affected by blue mussel (1) body size, density and patch context and (2) presence of fucoid and algal structures. In the study area, bladder-wrack (Fucus vesiculosus), filamentous algae and blue mussels (Mytilus edulis) coexist at shallow depths in a variety of patch configurations and offer complex habitats with a high variability of resources. We hypothesized that complexity in terms of mussel bed structure and algal presence determines species composition and abundance. Results from the experiment were compared with macrofaunal communities found in natural populations of both engineering species. Results show that the physical structure and blue mussel patch context are important determinants for species composition and abundance. Results further show that the presence of algal structures positively affects diversity in blue mussel habitats due to increased surface availability and complexity that these algae offer. This study shows that blue mussel beds at the very margin of their distribution have an indisputable function for promoting and maintaining biodiversity and suggest that facilitative effects of habitat-modifying species are important on Baltic Sea rocky shores with fundamental importance to community structure.

Facilitative effects of introduced Pacific oysters on native macroalgae are limited by a secondary invader, the seaweed Sargassum muticum

Introduced habitat-providing organisms such as epibenthic bivalves may facilitate the invasion and expansion of further non-native species which may modify the effects of the primary invader on the native system. In the sedimentary intertidal Wadden Sea (south-eastern North Sea) introduced Pacific oysters ( Crassostrea gigas) have overgrown native blue mussel beds ( Mytilus edulis). These oyster beds are now providing the major attachment substratum for macroalgae. Recently, oysters have expanded their distribution into the shallow subtidal zone of the Wadden Sea, and there support a rich associated species community including the Japanese seaweed Sargassum muticum, which has been presumably introduced together with the oysters. With a block designed field experiment, we explored the effects of S. muticum on the associated community of soft-bottom C. gigas beds in the shallow subtidal. Replicated oyster plots of 1 m 2 were arranged with a density of 0, 7, 15 or 45 S. muticum m − 2 , respectively. We found no effects of different S. muticum densities on associated epi- and endobenthic community compositions associated to the oyster plots. However, the overall coverage of sessile organisms settling on the oyster shells was significantly reduced at high S. muticum densities. The occurrence of abundant native macro-algal species such as Polysiphonia nigrescens, Antithamnion plumula and Elachista fucicola decreased with increasing S. muticum densities. Sessile invertebrates, by contrast, were only marginally affected and we found no effects of S. muticum canopy on diversity and abundance of endofauna organisms. We conclude that increasing densities of S. muticum on C. gigas beds in the shallow subtidal zone of the Wadden Sea limit the occurrence of native macroalgae which otherwise would benefit from the additional hard substratum provided by the oysters. Thus, a secondary invader may abolish the effects of the primary invader for native species by occupying the new formed niche.

Hunting from motorboats displaces Wadden Sea eiders Somateria mollissima from their favoured feeding distribution

AbstractHunting of eiders Somateria mollissima from motorboats is common in Danish marine waters, and to reduce hunting pressure on eiders and other diving duck species in Denmark, motorboat hunting was banned within 42 marine sites covering an area of 2,934 km2, although the effects of this regulation have to date not been examined. Our case study analyses the effects of excluding motorboat hunting from an area of 682 km2 of the Danish Wadden Sea (hereafter the ‘Study Area’) which also supported a large area of blue mussel beds, the preferred food for eiders. Our study covered the entire Danish Wadden Sea (total area 1,225 km2) during the hunting seasons (October‐February) of 1980–2003 using 85 aerial surveys of eiders and motorboats used by hunters. Eider numbers increased by 56% in the Study Area following the ban on motorboat hunting despite a 50% reduction in the eider flyway population over the same period. There was a significant negative relationship between the density of hunter motorboats and that of eiders on a small geographical scale (1.8–2.5 km). Motorboat hunting in the Study Area also affected eider distribution at larger geographical scales (4–12 km), displacing eiders from the Study Area offshore from the Wadden Sea into the North Sea. Following the ban on motorboat hunting, most eiders occurred in the Study Area. Winter (21 December‐31 January) body condition of eiders was greater in the Study Area than the body condition of eiders in the Offshore Area during autumn (20 October‐20 December). Eider abundance relative to blue mussel biomass significantly increased after motorboat hunting was banned in the Study Area, but there was no such change during winter after the hunting ban. Since the ban on motorboat hunting in the Study Area, eider numbers throughout the entire Danish Wadden Sea seem to be regulated by total blue mussel biomass.