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.     

Faunal structures associated with patches of mussels on East Asian coasts

Aggregations of mussels harbor a variety of associated animals and make it possible for diverse species to coexist at the shore. Species composition and diversity of the associated fauna are controlled by the position of mussel beds or patches, e.g. tidal level, age structure of mussels, quality of ambient water and by mussel species. When patches of mussels were surrounded by algal growth, a difference in the species composition of the associated fauna was recognized between the patches and algal mats. Mechanisms promoting coexistence are discussed. Biodeposit production by mussels may affect the environment both within the bed and the ambient waters. Reducing sediments showing low Eh values caused by the accumulation of biodeposits was observed in calm waters where the polychaete Capitella capitata, an indicator for organic enrichment, occurred both in the intertidal mussel bed and the subtidal sandy bottom communities. In a shallow subtidal sandy bottom of the Gulf of Thailand, where heavy bioturbation by the spatangoid urchin Brissus latecarinatus was occurring, small patches of the mussel Modiolus metcalfi increased species diversity and equitability in this habitat. Species composition was different between mussel patches and pure sandy bottoms. Electronic Publication     

The contribution of marine snow to the particle food supply of the benthic suspension feeder, Mytilus edulis

In order to examine the importance of the settling of large particles to the food supply and feeding behavior of a benthic culture of the blue mussel, Mytilus edulis, we investigated the tidal dynamics of large (>0.5 mm diameter) marine aggregates, commonly known as marine snow, during three tidal cycles in July 1998 at a shallow, subtidal, low current flow regime site along the coast of Maine (Shorey Cove, Roque Island, Englishman's Bay, Maine).In situ, optically measured marine snow showed a distinct tidal signal displaying an increase in size and abundance through high tide with a peak on the early ebb tide as it settled to the bottom. Marine snow volume ranged an order of magnitude through the tidal cycle, from under 8 to over 80 mm³ l⁻¹. An increase in the in situ marine snow volume corresponded with an increase in benthic mussel feeding activity (from 20% to 60% of maximum exhalant siphon area, which is an estimate of pumping rate) and maximum rates of pseudofeces production by the mussels during periods of low tidal current speeds. In contrast, mussels from the same population feeding on surface waters in shipboard chambers produced no pseudofeces and had high pumping rates (80-100% maximum exhalant siphon area) over the whole tidal period. A second peak in benthic mussel pumping rates also occurred during flood tide.Food quality was lower in the bottom waters due to significantly higher particulate inorganic matter (PIM, >2 mg l⁻¹) when compared with the surface waters. PIM accounted for 95% of the total settled mass flux of 3.4 g m⁻² day⁻¹ measured in sediment traps deployed 1 m off the bottom, with organic carbon representing only 2.5% of the mass flux during the mid-summer conditions. At low-current sites such as Shorey Cove, Roque Island, Maine, the settling of marine snow provides an important additional source of food, albeit of low quality, to benthic populations of blue mussels.     

Mechanisms and consequences of competition between an alien mussel, Mytilus galloprovincialis, and an indigenous limpet, Scutellastra argenvillei

The influence of an alien mussel Mytilus galloprovincialis (Lamarck) on an indigenous limpet Scutellastra argenvillei (Krauss) was investigated on the west coast of South Africa by comparing four situations in which limpets occupied rock patches surrounded by mussels: (1) ‘established patches’ of high-density adult limpets; (2) ‘experimentally cleared patches’ from which mussels were manually removed; (3) ‘naturally disturbed patches’ where mussels had been eliminated by wave action; and (4) ‘half-density’ patches formed by thinning established patches. Limpets in established patches were most effective in retarding settlement and lateral invasion by mussels, but all patches shrank due to mussel encroachment. As patches shrank, limpet density fell in established and naturally disturbed patches due to emigration. Limpets living on mussel beds were small, sparse and never achieved sexual maturity; 60% shifted to rock, whereas only 7% moved from rock to mussels. The limpets showed highest fidelity to established patches (79% after 12 months) and lowest fidelity in naturally disturbed patches (20%). Thinning of established patches reduced fidelity to 26%. Mussel beds did not provide a suitable alternative substratum for S. argenvillei after they displaced the limpets from rocks. Moreover, elimination of mussels by waves failed to allow S. argenvillei to re-establish dense, adult populations resembling those prior to arrival of the mussel. Adults of S. argenvillei feed collectively by trapping kelp blades. On the topographically complex mussel beds limpets cannot feed this way. In sum, on exposed shores where M. galloprovincialis achieves maximal recruitment and growth, S. argenvillei is incapable of preventing mussel encroachment and is likely to become completely displaced. Semi-exposed shores do, however, offer a refuge preventing global extinction of the limpet.     

Habitat engineering by the invasive zebra mussel <Emphasis Type="Italic">Dreissena polymorpha</Emphasis> (Pallas) in a boreal coastal lagoon: impact on biodiversity

Habitat engineering role of the invasive zebra mussel Dreissena polymorpha (Pallas) was studied in the Curonian lagoon, a shallow water body in the SE Baltic. Impacts of live zebra mussel clumps and its shell deposits on benthic biodiversity were differentiated and referred to unmodified (bare) sediments. Zebra mussel bed was distinguished from other habitat types by higher benthic invertebrate biomass, abundance, and species richness. The impact of live mussels on biodiversity was more pronounced than the effect of shell deposits. The structure of macrofaunal community in the habitats with >10³ g/m² of shell deposits devoid of live mussels was similar to that found within the zebra mussel bed. There was a continuous shift in species composition and abundance along the gradient ‘bare sediments—shell deposits—zebra mussel bed’. The engineering impact of zebra mussel on the benthic community became apparent both in individual patches and landscape-level analyses.     

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.     

Hard rock versus soft bottom: the fauna associated with intertidal mussel beds on hard bottoms along the coast of Chile,and considerations on the functional role of mussel beds

The fauna associated with hard bottom mussel beds along the exposed Pacific coast of Chile was examined. The abundance of adult (>10 mm body length) purple mussels Perumytilus purpuratus varied between 32 and 75 individuals per 50 cm², and their biomass between 4.8 and 8.6 g AFDW per 50 cm² at eight sampling sites between Arica (18°S) and Chiloé (42°S). At all sampling sites, the associated fauna was dominated by suspension-feeding organisms (cirripeds, spionid and sabellid polychaetes, a small bivalve) followed by grazing peracarids and gastropods. Predators and scavengers also reached high abundances while deposit- and detritus-feeding organisms were of minor importance. The majority of organisms associated with these hard bottom mussel beds feed on resources obtained from the water column or growing on the mussels rather than on materials deposited by the mussels. This is in contrast to the fauna associated with mussel beds on soft bottoms, which comprises many species feeding on material accumulated by mussels (faeces and pseudofaeces) and deposited within the mussel bed. Many of the organisms dwelling between mussels both on hard bottoms and on soft bottoms have direct development, but organisms with pelagic development also occur abundantly within mussel beds. We propose that species with direct development are disproportionately favoured by the structurally complex habitat with diverse interstitial spaces between the mussels, which provides ample shelter for small organisms. We conclude that mussels on hard-bottoms primarily provide substratum for associated fauna while mussels on soft bottoms provide both substratum and food resources. Electronic Publication     

Space as a limiting resource in freshwater systems: competition between zebra mussels (Dreissena polymorpha) and freshwater sponges (Porifera)

Species interactions between two types of sessile benthic invertebrates, the zebra mussel (Dreissena polymorpha) and freshwater sponges (Porifera), were evaluated in Michigan City IN Harbor in southern Lake Michigan during 1996. The study objective was to define whether competition plays a role in structuring benthic communities using experimental techniques commonly employed in marine systems. Sponges were uninhibited by zebra mussel presence and overgrew zebra mussel shells on hard vertical substrata. In contrast, zebra mussels did not overgrow sponge colonies, but did show an ability to re-capture hard substrata if relinquished by the sponge. The negative affect of sponges on zebra mussels through overgrowth and recruitment suggests interactions that could eventually displace zebra mussels from these benthic communities. However, seasonal reduction of sponge biomass from autumn through winter appears to allow the zebra mussel a periodic respite from overgrowth, preventing exclusion of zebra mussels from the community and allowing these two taxa to co-exist.     

Mussels matter: postlarval dispersal dynamics altered by a spatially complex ecosystem engineer

This study investigated postlarval dispersal of soft-bottom macrofauna at a spatially complex intertidal mudflat comprising patches of bare sediment and an ecosystem engineer, the mussel Mytilus edulis. At each of four sites in Guard Point Cove, Maine, USA, we took core samples and deployed bedload traps in bare sediment and mussel bed habitats to estimate ambient densities, rates of sediment flux, and several measures of postlarval dispersal. Univariate and multivariate nonmetric multidimensional scaling (nMDS) results showed few significant site effects and no habitat×site interactions. In contrast, there were numerous significant habitat effects. Compared to the bare sediment, the mussel bed habitat had: fewer species; higher ambient density and proportional abundance of the oligochaete Tubificoides benedeni (the dominant species in both habitats); lower ambient densities and proportional abundances of major taxa and the nonoligochaetes as a group; and higher sediment flux and relative (i.e., per capita) dispersal of nonoligochaetes. Macrofauna species dispersed in relative proportions that were different from those in the ambient assemblage. Per capita T. benedeni transport rates were low in mussel beds compared to those for nonoligochaetes, consistent with the view that beds represent favorable habitat for oligochaetes. The number of total macrofauna individuals trap⁻¹ day⁻¹ was negatively correlated with ambient density and positively correlated with sediment flux in both habitats, but these relationships were significant only in the mussel bed. The results indicate that altered transport rates of sediment and postlarvae are important mechanisms by which mussels act as ecosystem engineers to modify soft-bottom habitats. Differential transport rates caused by aggregations of mussels and other foundation species must be considered in explanations of spatial pattern in soft-bottom communities.     

Lift as a mechanism of patch initiation in mussel beds

The mussel Mytilus californianus is the dominant competitor for space in the mid-intertidal zone of wave-swept rocky shores in the Pacific Northwest where it forms extensive tightly packed beds. The rate at which patches are formed in these beds, can play an important rôle in community ecology by controlling the establishment and persistence of fugitive species. Despite the biological importance of physical disturbance, the mechanism of patch initiation has not been adequately explained. Battering by logs can create patches, but is the predominant mechanism only on shores near active logging sites. In other areas, it has been speculated that the hydrodynamic forces associated with storm waves somehow cause patches to form. However, the forces acting along the direction of flow — drag and the acceleration reaction — are unlikely to initiate patch formation. Here, it is suggested that fluid-dynamic lift forces imposed on mussel beds by breaking waves are sufficient to dislodge individual mussels and trigger patch formation. Arguments are presented suggesting that the likelihood of dislodgment by lift is consistent with the observed rate of patch formation in the absence of log battering.     

Mytilid mussels: global habitat engineers in coastal sediments

Dense beds of mussels of the family Mytilidae occur worldwide on soft-bottoms in cold and warm temperate coastal waters and have usually been considered hot spots of biodiversity. We examined intertidal mussel beds at four distant locations around the globe with the same sampling method, to find out whether this “hot spot” designation holds universally. We studied species assemblages within the matrices of byssally interconnected mussels engineered by Mytilus edulis in the North Sea, by mixed Perumytilus purpuratus and Mytilus chilensis at the southern Chilean coast, by Musculista senhousia in the Yellow Sea and by Xenostrobus inconstans at the coast of southern Australia. In all cases, species assemblages inside mussel beds were significantly different from those outside with many species being restricted to one habitat type. However, species richness and diversity were not generally higher in mussel beds than in ambient sediments without mussels. In the North Sea (M. edulis) and at the Chilean coast (P. purpuratus, M. chilensis), mussel beds have markedly higher species numbers and diversities than surrounding sediments, but this was not the case for mussel beds in Australia (X. inconstans) and the Yellow Sea (M. senhousia) where numbers of associated species were only slightly higher and somewhat lower than in adjacent sediments, respectively. In conclusion, although soft bottom mytilid mussels generally enhance habitat heterogeneity and species diversity at the ecosystem level, mussel beds themselves are not universal centres of biodiversity, but the effects on associated species are site specific.     

Inshore–offshore comparison of parasite infestation in Mytilus edulis: implications for open ocean aquaculture

Offshore production offers a new perspective for mussel aquaculture in the German Bight (North Sea) as no expansion is possible in the intertidal and subtidal zone of the Wadden Sea because of restrictions on the number of licenses. The development of offshore wind farms offers a unique opportunity because of the associated infrastructure. Service platforms, as well as the pylons themselves, offer perfect structures for mooring longlines and other culture units. One of the advantages of offshore culture may be a lower parasite load in offshore mussels compared with mussels produced under traditional inshore bottom culture. By sampling mussel spat from offshore suspended buoys or collectors, we simulated an offshore culture situation and compared parasite infestation rates with those in mussel spat obtained from suspended inshore buoys or collectors, in mussels from inshore benthic subtidal beds and from inshore benthic intertidal mussel beds. Mussels from offshore sites were free of trematodes and shell‐boring polychaetes. Parasitic copepods only occurred at a single offshore site, on a 20‐year‐old research platform, but not on buoys or collectors exposed for shorter time periods. All three monitored parasite taxa were present at all other sites. The highest prevalence was found for trematodes in inshore benthic intertidal mussels (78.7 ± 6.4%) and locally reached 100%. Through a variety of detrimental effects, trematodes, parasitic copepods and shell‐boring polychaetes are known to affect growth performance and product quality. We therefore propose that offshore mussel production could be a promising culture procedure because it seems to result in lower parasite burden than at traditional culture sites. Whether offshore production also results in better survival and growth, compared with inshore mussel culture on a commercial scale, needs to be investigated further.     

Isolated and combined impacts of blue mussels (Mytilus edulis) and barnacles (Balanus improvisus) on structure and diversity of a fouling community

Effects of two presumably dominant competitors, the blue mussel Mytilus edulis and the barnacle Balanus improvisus on recruitment, population dynamics and community structure on hard substrata were experimentally investigated in the subtidal Kiel Fjord, Western Baltic. The hypothesis that blue mussels and/or barnacles are local dominants and strongly influence succession and community structure was tested by monitoring succession in the presence and absence of simulated predation on either or both species. Manipulations included blue mussel removal, barnacle removal, combined blue mussel and barnacle removal, as well as a control treatment for natural (non-manipulated) succession. In the second part of the experiment, recovery from the treatments was monitored over 1 year.During the manipulative phase of the experiment, blue mussels had a negative effect on recruitment of species, whereas barnacles had no significant effect. Even so, a negative synergistic effect of blue mussels and barnacles was detected. Calculation of species richness and diversity H′ (Shannon Index) showed a negative synergistic effect of blue mussels and barnacles on community structure. Additionally, diversity H′ was negatively affected by the dominant competitor M. edulis. These effects were also detectable in the ANOSIM-Analysis. The non-manipulative phase of the experiment brought about a drastic loss of diversity and species richness. Blue mussels dominated all four communities. Barnacles were the only other species still being able to coexist with mussels. Effects of simulated predation disappeared fast.Thus, in the absence of predation on blue mussels, M. edulis within a few months dominates available space, and diversity of the benthic community is low. In contrast, when mussel dominance is controlled by specific predators, more species may persist and diversity remains high.     

Evidence of cannibalism and bentho-pelagic coupling within the life cycle of the mussel, Perna canaliculus

The feeding ecology of the green-lipped mussel, Perna canaliculus, was investigated within three intertidal mussel beds along Ninety Mile Beach, northern New Zealand, between August 2000 and March 2001. Adult mussels of different sizes (45-105 mm in shell length) were collected from the intertidal sites about 30 min after being submerged by the incoming tide for gut content analyses. Results of these analyses indicate that mussels consume a variety of phytoplankton, micro- and mesozooplankton, including mussel larvae and post-larvae. Cannibalism of juveniles of up to 620 μm was recorded for intertidal mussels, and conspecifics of up to 2.4 mm were found within the stomachs of additional mussels collected in August 2000 from a nearby subtidal site. For all three intertidal populations, mussel larvae and juveniles contribute about 70% of the food particle consumption during the spawning peak in August, while phytoplankton and other zooplankton constitute the majority of the food source (about 99%) in March, during gametogenesis. Larger intertidal mussels tended to have more food particles in their stomachs than smaller mussels within all three populations. Distinctive differences in food consumption among intertidal populations directly coincide with variations in total particulate matter (TPM), particulate organic matter (POM) and percent organic matter (OM) in the adjacent seawater.Separate experiments designed to test the feeding behavior of mussels feeding at different times during the incoming tide were conducted at one of the intertidal sites during August 2000 and March 2001. Results from these experiments indicate a marked shift in food consumption from bivalves to other mesozooplankton in August, and from phytoplankton to mesozooplankton in March. The observed combination of mussel predatory and grazing behavior over the incoming tide and through the year provides evidence for a strong food-web link between the benthic and pelagic life stages of this species. Furthermore, the high rate of cannibalism during some months of the year suggests that this source of food may significantly contribute to the energy budget of wild populations, with potential implications for evolutionary adaptive success.     

Effects of the exotic invader Musculista senhousia on benthic communities of two Mediterranean lagoons

The impact that the alien mussel Musculista senhousia has on benthic biodiversity and community structure was investigated in two Italian transitional environments, where the species was particularly abundant: the Sacca di Goro (Adriatic Sea), and the Rio Padrongiano Deltaic area (Sardinia, Tyrrhenian Sea). Sampling campaigns were conducted by sampling mussel beds from each of two different patch size classes (small: <1.0 m², and large: >10.0 m²), and the surrounding bare sediments. At both study sites, mussel density varied greatly with time within and between patches. Despite the huge difference in benthic species complement at Goro and Padrongiano (33 and 115 taxa, respectively), at both sites the presence of mussel patches had a gross positive effect on the benthic community. At Goro, many trophic guilds showed a positive relationship with mussel abundance. At Padrongiano, a continuous and directional process of patches colonization (both large and small) through time seemed evident, while at Goro the process was interrupted by high-summer mortalities. At both study sites, M. senhousia played a double role: (1) as secondary substratum, enhancing the environmental structural complexity, and (2) as densely aggregated, living organisms, thus giving rise to a number of possible interactions with other invertebrates. The outcome, however, was strictly dependent by the level of anthropogenic stress, in this case water oxygen deficiency, which was different at the two study sites. Guest editors: A. Razinkovas, Z. R. Gasiūnaitė, J. M. Zaldivar & P. Viaroli European Lagoons and their Watersheds: Function and Biodiversity     

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.     

Distribution, abundance, size and recruitment of the mussel, Mytilus californianus, across a major oceanographic and biogeographic boundary at Point Conception, California, USA

The Point Conception, California, USA region (hereafter PC) is one of the most important biogeographic and oceanographic discontinuities on the US west coast. Here we address how mesoscale oceanographic and environmental variability in the region around PC, CA may influence the distribution, abundance and size of the mussel Mytilus californianus, a competitively dominant species in rocky intertidal assemblages along the northeast Pacific. Strong upwelling and high wave exposure dominate the California coast north of PC, and weak, seasonal upwelling and warmer water temperatures are characteristic of the region south/east of PC. We hypothesized that the gradient in temperature, upwelling, and wave exposure around PC would greatly influence patterns of recruitment and abundance of mussels, potentially underlying large-scale differences in community structure. We evaluated these predictions by surveying intertidal community structure, mussel distribution, size, abundance and recruitment at a range of intertidal sites around PC. We found that intertidal communities north of PC were dominated mainly by macrophytes, while mussels and barnacles were relatively scarce. Intertidal communities south of PC were dominated by mussels and barnacles, with a low abundance of macrophytes. Mussels were larger and mussel beds were more expansive and extended lower in elevation at sites ranging from north to south around PC. At northern sites, high abundances of sea star predators and elevated wave exposure effectively displaced the entire mussel zone upwards. We found no differences in the numbers of mussel recruits to sites around PC, suggesting that spatial patterns of mussel abundance were not driven by differential recruitment. These results suggest that unlike other well-studied systems, supply of benthic larvae does not underly the large-scale gradient in community structure around PC. We suggest that environmental conditions favorable to macroalgal growth north of PC, and conditions favorable to filter-feeder growth south of PC may underly mesoscale patterns of intertidal community structure in this region.     

Complementary influences of co-occurring physical ecosystem engineers on species richness: insights from a Patagonian rocky shore

Structural modification of the environment by physical ecosystem engineers often allows for the occurrence of species that are not able to establish in unengineered habitats, thus leading to increased species richness at the landscape-level (i.e., areas encompassing engineered and unengineered habitats). Unlike previous studies that focused on the contribution of a single engineering species to landscape-level species richness, this study evaluates whether co-occurring engineers—i.e., intertidal mussels (primarily Perumytilus purpuratus) and rock boring bivalves (Lithophaga patagonica)—contribute to landscape-level species richness in a similar or complementary way. Our results show that both mussel and L. patagonica patches harbor a substantial number of invertebrate species in addition to those occurring in the unenegineered rock substrate. However, the distinctive habitat patches created by each engineer add exclusive subsets of species to the study area, which implies that mussel and L. patagonica patches contribute complementarily to overall species richness in our intertidal landscape. Here we postulate that complementary engineering effects on landscape-level species richness will occur when the engineered patches structurally differ from each other and, thus, vary in their relative ability to modulate two or more abiotic conditions and/or resources that prevent species establishment in the unengineered state. In spite of its inherently small spatial scale (500 m), our study highlights the potential for complementary engineering impacts at the larger scales that are usually implied in biodiversity conservation and management (tens to hundreds of kilometers) and outlines a simple conceptual basis and approach to address them.     

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)     

Impacts of bottom and suspended cultures of mussels <Emphasis Type="Italic">Mytilus</Emphasis> spp. on the surrounding sedimentary environment and macrobenthic biodiversity

The aim of this study was to quantify the effect of bottom and suspended mussel cultures, cultured in different physical environments, on the sedimentary environmental conditions and thereby the biodiversity structure of the associated macrofaunal community. We compared two bottom cultures (Limfjorden: microtidal, wind-driven; Oosterschelde: macrotidal) and one suspended culture (Ria de Vigo in an upwelling coastal region). The sedimentary environmental conditions (mud fraction, POC, PON, phosphorus content, chl a breakdown products) were significantly elevated underneath and surrounding bottom and suspended cultures compared to culture-free sediments that were nearby and hydrodynamically similar. The relative change in environmental conditions was more pronounced in the Oosterschelde compared to Limfjorden, most likely due to differences in hydrodynamic forcing and characteristics of the mussel bed. The effect of the suspended cultures in Ria de Vigo on the surrounding sediments was influenced by local topographic and hydrodynamic conditions. The impact of mussels on the benthic community due to biodeposition was clearly seen in the community structure. The species composition changed from species which are typically present in sandy environments to more small opportunistic species, which are typically present in organically enriched sediments. The impact of bottom cultures on the benthic community due to changes in the habitat under the presence of mussels was positive, especially in the Oosterschelde where an increase in the number of epibenthic species was seen. The influence of bottom cultures on the sedimentary environment and on the macrobenthic community seems to be very local. Within the mussel site in Limfjorden, differences were detected between sites where none or almost no mussels were present with sites where mussels were very abundant.