High Mussel Densities Research Articles

Marine transmissible cancer navigates urbanized waters, threatening spillover.

Inter-individual transmission of cancer cells represents a unique form of microparasites increasingly reported in marine bivalves. In this study, we sought to understand the ecology of the propagation of Mytilus trossulus Bivalve Transmissible Neoplasia 2 (MtrBTN2), a transmissible cancer affecting four Mytilus mussel species worldwide. We investigated the prevalence of MtrBTN2 in the mosaic hybrid zone of M. edulis and M. galloprovincialis along the French Atlantic coast, sampling contrasting natural and anthropogenic habitats. We observed a similar prevalence in both species, probably due to the spatial proximity of the two species in this region. Our results showed that ports had higher prevalence of MtrBTN2, with a possible hotspot observed at a shuttle landing dock. No cancer was found in natural beds except for two sites close to the hotspot, suggesting spillover. Ports may provide favourable conditions for the transmission of MtrBTN2, such as high mussel density, stressful conditions, sheltered and confined shores or buffered temperatures. Ships may also spread the disease through biofouling. Our results suggest ports may serve as epidemiological hubs, with maritime routes providing artificial gateways for MtrBTN2 propagation. This highlights the importance of preventing biofouling on docks and ship hulls to limit the spread of marine pathogens hosted by fouling species.

Plankton depletion by mussel grazing negatively impacts the fitness of lobster larvae

Bivalve aquaculture has rapidly expanded in shallow coastal habitats, leading to substantial changes in the natural environment. While the grazing pressure of mussels (Mytilus edulis) on phytoplankton and zooplankton has been well documented, cascading effects of plankton depletion on the development and survival of resident fish and invertebrate larvae remains largely unexplored. This study experimentally assessed the impact of control (no mussel grazing), low (20 mussels·m−3) and high (80 mussels·m−3) mussel densities on a model community composed of various phytoplankton and zooplankton and how changes in plankton characteristics influence the development and survival of lobster (Homarus americanus) larvae. Mussels fed primarily on nanophytoplankton, while the picophytoplankton experienced significantly less grazing. Zooplankton was readily ingested by mussels, showing a sharp decline under low mussel density, whereas it was eliminated from the plankton under high density of mussels. A 65% decrease of polyunsaturated fatty acids (∑ PUFA) was observed in the plankton under mussel filtration compared to control. Lobster larvae reared under control plankton conditions exhibited the fastest development, highest survival rate, carapace length, total lipid and ∑ PUFA contents, and TAG/ST (triacylglycerols/sterols) ratio, indicating the best nutritional condition. Larvae reared under low mussel density showed longer developmental time, lower survival rate and carapace length than that for controls. In addition, these larvae showed a significant reduction in total lipid, ∑ PUFA, and TAG/ST, which collectively suggests poor nutritional status due to limited dietary intake. Under high mussel density, larvae did not molt to stage II and died within 14 days, suggesting that the depleted plankton community was unable to sustain physiological requirements for survival and development of the larvae. Overall, mussel grazing altered the meroplankton community structure and its biochemical composition, ultimately impacting the development, survival, and quality of lobster larvae. These findings provide novel information on complex trophic interactions between bivalves and a meroplankton species and adds a new perspective to fuel discussions on energy and nutrient pathways, particularly long-chain PUFA, in ecosystem carrying capacity studies for shellfish aquaculture in shallow coastal habitats.

Incorporating facilitative interactions into small‐scale eelgrass restoration—challenges and opportunities

Marine ecosystem engineers such as seagrasses and bivalves create important coastal habitats sustaining high biodiversity and ecosystem services. Restoring these habitats is difficult due to the importance of feedback mechanisms that can require large‐scale efforts to ensure success. Incorporating facilitative interactions could increase the feasibility and success of small‐scale restoration efforts, which would limit pressure on donor sites and reduce costs and time associated with restoration. Here, we tested two methods for providing facilitation in small‐scale eelgrass (Zostera marina) restoration plots across northern Europe: (1) co‐restoration with blue mussels (Mytilus edulis, M. trossulus); and (2) the use of biodegradable establishment structures (BESEs). Eelgrass‐mussel co‐restoration showed promise in aquaria, where eelgrass growth was nearly twice as high in treatments with medium and high mussel densities than in treatments without mussels. However, this did not translate to higher shoot length or shoot densities in subsequent field experiments. Rather, hydrodynamic exposure limited both eelgrass and mussel survival, especially in the most exposed sites. The use of BESEs showed more potential in enabling small‐scale restoration success: they effectively enhanced eelgrass survival and reduced mussel loss, and showed potential for enabling mussel recruitment in one site. However, eelgrass planted in BESE plots along with mussels had a lower survival rate than eelgrass planted in BESE plots without mussels. Overall, we show that though co‐restoration did not work at small scales, facilitation by using artificial structures (BESEs) can increase early eelgrass survival and success of small‐scale eelgrass and bivalve restoration.

Amazonian freshwater mussel density: A useful indicator of macroinvertebrate assemblage and habitat quality

Mussels have important ecological functions in freshwater ecosystems, but populations are declining in many rivers worldwide. In the Amazon, which is undergoing widespread human-mediated alterations, information on mussel ecology is lacking. We investigated whether declines in mussel density, due to exploitation, are associated with lower macroinvertebrate assemblage taxon richness and modified habitat functioning in an Amazonian river. Between 2015 and 2017, macroinvertebrate metrics of abundance and number of families, and related habitat variables, were compared between exploited and unexploited mussel beds in the Caeté river, northeastern Pará state, Brazil. Except for Chironomidae abundance, which increased, other macroinvertebrate metrics decreased in exploited mussel beds, especially abundances of shredders and predators. Medium and high mussel densities appear to support abundant and diverse macroinvertebrate assemblages. Macroinvertebrate decreases were mainly correlated with lower organic matter in the sediment and higher dissolved solids in the water column, due to losses in mussel biodeposition and filtration functions. Amazonian freshwater mussel density is a useful indicator of abundant and diverse macroinvertebrate assemblages, and mussel beds may help prioritize locations for habitat conservation in Amazonian rivers, and indirectly for fishes that are essential for mussel dispersion. Aquatic management efforts should explicitly include mussel beds and their diverse associated macroinvertebrate assemblages.

Animal aggregations promote emergent aquatic plant production at the aquatic-terrestrial interface.

The roles mobile animals and abiotic processes play as vectors for resource transfers between ecosystems ("subsidies") are well studied, but the idea that resources from animals with limited mobility may be transported across boundaries through intermediate taxa remains unexplored. Aquatic plants ("macrophytes") are globally distributed and may mediate transfers of aquatic-derived nutrients from aggregations of aquatic animals to terrestrial ecosystems when consumed by terrestrial herbivores. We used mesocosms (94×44cm) to test whether aquatic animal-generated biogeochemical hotspots increase growth and nutrient content in macrophytes using the macrophyte Justicia americana and freshwater mussels. Justicia americana biomass production increased and belowground biomass allocation changed with increasing mussel density. At high mussel density, water-column phosphorus increased and carbon:phosphorus ratios in J. americana tissues decreased. We deployed motion-sensing cameras to explore herbivory on J. americana growing along the margins of the Kiamichi River, Oklahoma, and documented feeding by large mammals (Odocoileus virginianus, Sus scrofa, and Bos taurus). Thus, biogeochemical hotspots generated by aquatic animal aggregations can promote macrophyte production that subsequently is transferred to terrestrial animals. More broadly, this suggests that reductions in aquatic animal biomass may have bottom-up impacts that indirectly affect terrestrial ecosystems via plant-animal interactions bridging ecosystem boundaries.

Effects of mussel and host fish density on reproduction potential of a threatened unionoid mussel: prioritization of conservation locations in management trade-offs

Management decisions in conservation of threatened species require trading off social needs against biodiversity values, including the prioritization of conservation locations, i.e. where conservation efforts should take place. To improve conservation decisions for the thick-shelled river mussel, Unio crassus, a highly threatened temporary parasite on fish, we performed a field study on how mussel and host fish density (European bullhead, Cottus gobio, and common minnow, Phoxinus phoxinus) affect reproduction potential of the mussel at different sites along a river. We assumed that the proportions of gravid mussels would be higher at high mussel density, and result in enhanced glochidia (mussel larvae) encapsulation rates on fish. We also expected the highest ‘glochidia density’—a proxy for the potential number of recruits per stream area, assessed by multiplying glochidia encapsulation rates on fish by fish density, to occur at high mussel density sites. Such river sites, producing many offspring and conveying important conservation values, may thus be prioritized. However, contrary to our assumptions, higher glochidia density and higher proportions of gravid mussels occurred at lower density mussel sites. We also found that P. phoxinus had higher glochidia encapsulation rates than C. gobio, possibly related to species-specific behavioural and life-history traits. Even so, glochidia density was similar for both fish species, reflecting comparable ecological functions in hosts. The results of this study suggest that mussel and host fish densities should be considered along with glochidia density in conservation prioritization and management trade-offs.

Greater macroinvertebrate diversity and freshwater mussel density in meander margins of an Amazon river

AbstractMeanders are complex aquatic environments exhibiting different flow and sediment characteristics that influence macroinvertebrate distribution. Differences in macroinvertebrate composition, freshwater mussel density (mainly Castalia ambigua Lamarck, 1819) and habitat variables were investigated, using uni‐ and multivariate analyses, in two zones, the margin and thalweg, of the meanders of a 7‐km stretch of a morphologically unaltered tropical alluvial lowland river. Clear differences were found between meander zones, with greater taxonomic and functional diversity of macroinvertebrates and greater freshwater mussel densities in the meander margin. A total of 12 families of macroinvertebrates were exclusive to the meander margin, and macroinvertebrate indicators of zones were Philopotamidae and Leptoceridae (meander margin) and Chironomidae and Elmidae (meander thalweg). In the meander margin, there were no differences in macroinvertebrate abundance, and taxonomic and functional group diversity among areas with low, medium and high mussel density. Macroinvertebrate abundance did not vary between zones, but in both, abundance was associated with lower pH and higher electrical conductivity. Low shear velocity, which stabilises the river bed, high organic matter content and percentage silt in the meander margin were associated with higher taxonomic and functional macroinvertebrate diversity, as well as the presence of freshwater mussels. Natural unaltered meanders are spatially heterogeneous in both habitat and biodiversity and, similar to restored meanders, support greater macroinvertebrate taxonomic and functional diversity, as well as maintain freshwater mussel beds, mainly by stabilising the river bed. The meander margin may also be important for attracting mussel host fish that feed on macroinvertebrates and aid mussel dispersal. Thus, the meander margins, and other similar riparian habitats with low shear velocity, for example, side channels and pools, have potentially high conservation value and should be afforded due protection.

A mesocosm investigation of the effects of quagga mussels (Dreissena rostriformis bugensis) on Lake Michigan zooplankton assemblages

Abstract Dreissenid mussels are known to disrupt the base of the food web by filter feeding on phytoplankton; however, they may also directly ingest zooplankton thereby complicating their effects on plankton communities. The objective of this study was to quantify the effects of quagga mussel feeding on the composition and size structure of Lake Michigan zooplankton assemblages. Two mesocosm (six 946 L tanks) experiments were conducted in summer 2013, using quagga mussels and zooplankton collected near Beaver Island, MI, to examine the response of zooplankton communities to the presence and absence of mussels (experiment 1) and varying mussel density (experiment 2). Mesocosms were sampled daily and zooplankton taxa were enumerated and sized using microscopy and FlowCAM® imaging. In experiment 1, the presence of quagga mussels had a rapid negative effect on veliger and copepod nauplii abundance, and a delayed negative effect on rotifer abundance. In experiment 2, mussel density had a negative effect on veliger, nauplii, and copepodite abundance within 24 h. Multivariate analyses revealed a change in zooplankton community composition with increasing mussel density. Ten zooplankton taxa decreased in abundance and frequency as quagga mussel density increased: except for the rotifer Trichocerca sp., treatments with higher mussel densities (i.e., 1327, 3585, and 5389 mussels/m 2 ) had the greatest negative effect on small-bodied zooplankton (≤ 128 μm). This study confirms results from small-scale (≤ 1 L) experiments and demonstrates that quagga mussels can alter zooplankton communities at mesoscales (~ 1000 L), possibly through a combination of direct consumption and resource depletion.

Mussels can both outweigh and interact with the effects of terrestrial to freshwater resource subsidies on littoral benthic communities

Litterfall is an important resource subsidy for lake ecosystems that primarily accumulates in littoral zones. Bivalves are abundant within littoral zones and may modify the effects of terrestrial resource subsidies through trophic interactions and engineering their surrounding habitat. Leaf inputs to lakes and freshwater mussel abundances are changing throughout the boreal ecoregion so we set out to investigate how the co-occurring benthic community might respond.We set up an in situ mesocosm experiment in Ramsey Lake, Sudbury, ON, Canada. Mesocosms contained sediments of either 5% or 35% terrestrial organic matter (tOM), into which we placed mussels (Elliptio complanata) at differing densities (0, 0.4 and 2musselsm−2, with a sham mussel treatment at 0.4musselsm−2). Over one month we recorded the sediment chemistry (dissolved organic carbon, nitrogen and phosphorus), littoral organisms (benthic algae and zooplankton) and mussel growth.At high mussel densities we recorded a 90%, 80%, 45% and 40% reduction in phosphorus, dissolved organic carbon, nitrogen and benthic diatoms, respectively, whereas at low mussel densities we observed a 3-fold increase in zooplankton. We discuss that these results were caused by a combination of bioturbation and trophic interactions. Benthic diatom concentrations were also reduced by 20% in sediments of 35% tOM, likely due to shading and competition with bacteria.Mussel growth increased at high mussel densities but was offset at high tOM, likely due to the organic matter interfering with filter feeding.Our results suggest that mussels can alter the geochemical composition of sediments and abundances of associated littoral organisms, in some cases regardless of tOM quantity. Therefore, the dominant top-down control exerted by freshwater mussels may outweigh bottom-up effects of tOM additions. Generally, our study reveals the importance of considering dominant species when studying the effects of cross-ecosystem resource fluxes.

Depth distribution of the native freshwater mussel (Echyridella menziesii) in warm monomictic lakes: Towards a general model for mussels in lakes

Abstract Freshwater mussels are large, long‐lived and can be important contributors to benthic biomass and processes. They are currently one of the most endangered groups of organisms and it is urgent to develop tools to predict their distribution and the potential effect of their decline or disappearance on these ecosystems. Cyr () showed that the distribution of Elliptio complanata (Unionidae) in Canadian Shield lakes is constrained by physical forces. Here we test Cyr's model in a very different group of mussels (Hyriidae), in different types of lakes (warm monomictic lakes) that cover a wider range of sizes and morphologies. We use data on the depth distribution of Echyridella menziesii along 38 depth transects in 11 warm monomictic lakes located in New Zealand to test three hypotheses: (1) wave‐mixed depth and bottom slope are good predictors of the depth of maximum mussel density, (2) thermocline depth does not limit the distribution of mussels in warm monomictic lakes, and (3) the lower boundary of mussel distribution is determined by the mud deposition boundary. Mussels in New Zealand lakes reach their maximum density within the epilimnion, at increasing depths with increasing lake size and increasing site exposure. The only exceptions were found in large lakes or parts of large lakes with shallow bathymetric slopes, where high mussel densities were found in relatively shallow waters. Mussel density peaks are found much deeper in the volcanic New Zealand lakes than in (glacial) Canadian Shield lakes, a discrepancy we hypothesise could be due to lower sediment stability along the steep slopes of volcanic lakes. Mussels appear to have a very broad range of depth distribution in these warm monomictic lakes. The deepest samples collected in deep oligotrophic lakes (down to 12–30 m) all contained mussels, often in substantial numbers (up to 2–186 mussels/m2), and can only offer a minimal estimate of the lower boundary of their distribution. However, the distribution of mussels in highly productive lakes is limited by hypolimnetic anoxia, and therefore by position of the thermocline. Echyridella menziesii are found in a wide range of substrates. Mussels were found below the mud deposition boundary in many lakes, suggesting that the presence of fine flocculent organic sediments does not prevent them from living in deep areas. The distribution of E. menziesii in warm monomictic lakes appears to be governed by relatively simple physical processes. This supports findings for a very different group of mussels in Canadian Shield lakes and suggests that general models could be developed to predict the distribution of mussels in lakes.

Hydrological and substrate requirements of the thick‐shelled river mussel Unio crassus (Philipsson 1788)

Abstract Conservation and restoration strategies require knowledge of the autecology of target species. The thick‐shelled river mussel Unio crassus is an endangered freshwater bivalve with largely unknown habitat preferences. In this study, hydrological and substrate parameters including shear stress, flow velocity and penetration resistance of substrate were examined in relation to mussel presence/absence data in six streams with recruiting and self‐sustaining populations to derive information on the hydrological and substrate habitat requirements of the species. Mean flow velocities between 0.11 and 0.31 m s−1 and absolute maximum values in the study streams ranging from 0.22 to 0.95 m s−1 indicate that the species is tolerant of a wide range of hydrological conditions. Areas with high mussel densities were characterized by low flow velocities (<0.3 m s−1), low mean penetration resistances (0.36 ± 0.52 kg cm−2) as well as by low near‐bed shear stress (1.06 ± 0.33 x 10−6 N cm−2) compared with non‐colonized sites. Maximum near‐bed flow velocity at mussel beds was 0.33 m s−1 and 0.35 m s−1 at 60% depth. Higher flows with maximum velocities of 0.52 m s−1 and 0.95 m s−1 at 60% depth were measured at non‐colonized sites. Mussel density was significantly negatively correlated with near‐bed shear forces. The results indicate that the hydrological and substrate characteristics of U. crassus habitat are broader than expected. In contrast to the assumption that it depends on moderate to high flows (>0.3 m s−1), streams with low water flow and soft substrate were also identified as suitable habitats. Functional characteristics of the substrate, especially stability and areas with low shear stress (<1.5 N cm−2), seem to be of great importance for the persistence of the species, and monitoring protocols used at present for habitat assessment of U. crassus should be updated. Copyright © 2016 John Wiley & Sons, Ltd.

Effects of near‐bed turbulence on the suspension and settlement of freshwater dreissenid mussel larvae

Summary Larval transport and settlement in benthic invertebrates is theorised to be influenced by bottom roughness and the hydrodynamic forces that roughness generates near the bed. This study of freshwater Dreissena spp. bivalves examined the transport and suspension of pediveliger larvae and larval models in a laboratory flow chamber and larval settlement in Lake Erie. Particle image velocimetry (PIV) measurements in the laboratory, and acoustic Doppler velocimetry (ADV) measurements in the field, were used to estimate small‐scale turbulence in the near‐bed environment over differing roughness. Quadrant analysis was used to determine the frequency of turbulent sweeps and ejections, and the extent of roughness flow regime (skimming versus wake interference flow) was noted to understand the determinants of larval transport and settlement. Skimming flow generated above bottoms with high mussel densities had significantly lower suspended transport (i.e. suspension off the bottom in the flow chamber; 3.31 ± 1.14%) and lower larval settlement in the field (1526 ± 80 larvae m−2 per day) compared to low mussel densities (6.63 ± 1.54% and 1853 ± 47 larvae m−2 per day). Conversely, wake interference flow indicated by high frequencies of turbulent sweeps and ejections generated by the roughness due to mussel patches resulted in high suspended transport in the flow chamber (i.e. via ejections) and the highest larval settlement in the field (i.e. via sweeps; 1943 ± 59 larvae m−2 per day). The spatial configuration of mussel roughness influenced the creation and magnitude of skimming versus wake interference flow, which can inhibit or enhance larval settlement, respectively.

Aggregated filter‐feeding consumers alter nutrient limitation: consequences for ecosystem and community dynamics

Nutrient cycling is a key process linking organisms in ecosystems. This is especially apparent in stream environments in which nutrients are taken up readily and cycled through the system in a downstream trajectory. Ecological stoichiometry predicts that biogeochemical cycles of different elements are interdependent because the organisms that drive these cycles require fixed ratios of nutrients. There is growing recognition that animals play an important role in biogeochemical cycling across ecosystems. In particular, dense aggregations of consumers can create biogeochemical hotspots in aquatic ecosystems via nutrient translocation. We predicted that filter-feeding freshwater mussels, which occur as speciose, high-biomass aggregates, would create biogeochemical hotspots in streams by altering nutrient limitation and algal dynamics. In a field study, we manipulated nitrogen and phosphorus using nutrient-diffusing substrates in areas with high and low mussel abundance, recorded algal growth and community composition, and determined in situ mussel excretion stoichiometry at 18 sites in three rivers (Kiamichi, Little, and Mountain Fork Rivers, south-central United States). Our results indicate that mussels greatly influence ecosystem processes by modifying the nutrients that limit primary productivity. Sites without mussels were N-limited with -26% higher relative abundances of N-fixing blue-green algae, while sites with high mussel densities were co-limited (N and P) and dominated by diatoms. These results corroborated the results of our excretion experiments; our path analysis indicated that mussel excretion has a strong influence on stream water column N:P. Due to the high N:P of mussel excretion, strict N-limitation was alleviated, and the system switched to being co-limited by both N and P. This shows that translocation of nutrients by mussel aggregations is important to nutrient dynamics and algal species composition in these rivers. Our study highlights the importance of consumers and this imperiled faunal group on nutrient cycling and community dynamics in aquatic ecosystems.

Predator size interacts with habitat structure to determine the allometric scaling of the functional response

While both predator body size and prey refuge provided by habitat structure have been established as major factors influencing the functional response (per capita consumption rate as a function of prey density), potential interactions between these factors have rarely been explored. Using a crab predator (Panopeus herbstii) – mussel prey (Brachidontes exustus) system, we examined the allometric scaling of the functional response in oyster (Crassostrea virginica) reef habitat, where crevices within oyster clusters provide mussels refuge from predation. A field survey of mussel distribution showed that mussels attach closer to the cluster periphery at high mussel density, indicating the potential for saturation of the refuge. In functional response experiments, the consumption rate of large crabs was depressed at low prey density relative to small crabs, while at high prey density the reverse was true. Specifically, the attack rate coefficient and handling time both decreased non‐linearly with crab size. An additional manipulation revealed that at low prey densities, the ability of large crabs to maneuver their claws and bodies to extract mussels from crevices was inhibited relative to small crabs by the structured habitat, reducing their attack rate. At high prey densities, crevices were saturated, forcing mussels to the edge of clusters where crabs were only limited by handling time. Our study illuminates a potentially general mechanism where the quality of the prey refuge provided by habitat structure is dependent on the relative size of the predator. Thus anthropogenic influences that alter the natural crab size distribution or degrade reef habitat structure could threaten the long‐term stability of the crab –mussel interaction in reefs.

Recruitment of the threatened mussel Margaritifera margaritifera in relation to mussel population size, mussel density and host density

ABSTRACT Anthropogenic, abiotic factors are considered the main cause of recruitment failure of unionid mussels, including the freshwater pearl mussel (Margaritifera margaritifera). This large‐scale investigation instead examined the relationship between biotic factors and mussel recruitment. Mussel population size and density were positively related to recruitment and to juvenile mussel density. Juvenile mussel density appeared to be the more accurate measure of recruitment of the two. Host fish density of young‐of‐the‐year and older brown trout (Salmo trutta) were positively related to recruitment. Moreover, the mean density of both age classes of fish, when grouped into density classes, was positively related to juvenile mussel density, an effect that decreased at trout densities above 10 trout 100 m‐2. Mussel population size and density were relatively more important to recruitment than trout density. To increase recruitment of juvenile mussels, managers should apply measures that increase mussel density, and trout density up to about 10 trout 100 m‐2 in proximity to mussel beds. Mussel beds may also be managed; one possible measure within small and sparse mussel populations may be to concentrate the remaining mussels in areas where trout density is high. Likewise, young‐of‐the‐year trout may also be moved to areas of high mussel density, as young trout individuals tend not to move far during their first year. This may increase mussel larval infection rates and mussel recruitment. Copyright © 2012 John Wiley & Sons, Ltd.

Alternative mechanisms alter the emergent properties of self-organization in mussel beds

Theoretical models predict that spatial self-organization can have important, unexpected implications by affecting the functioning of ecosystems in terms of resilience and productivity. Whether and how these emergent effects depend on specific formulations of the underlying mechanisms are questions that are often ignored. Here, we compare two alternative models of regular spatial pattern formation in mussel beds that have different mechanistic descriptions of the facilitative interactions between mussels. The first mechanism involves a reduced mussel loss rate at high density owing to mutual protection between the mussels, which is the basis of prior studies on the pattern formation in mussels. The second mechanism assumes, based on novel experimental evidence, that mussels feed more efficiently on top of mussel-generated hummocks. Model simulations point out that the second mechanism produces very similar types of spatial patterns in mussel beds. Yet the mechanisms predict a strikingly contrasting effect of these spatial patterns on ecosystem functioning, in terms of productivity and resilience. In the first model, where high mussel densities reduce mussel loss rates, patterns are predicted to strongly increase productivity and decrease the recovery time of the bed following a disturbance. When pattern formation is generated by increased feeding efficiency on hummocks, only minor emergent effects of pattern formation on ecosystem functioning are predicted. Our results provide a warning against predictions of the implications and emergent properties of spatial self-organization, when the mechanisms that underlie self-organization are incompletely understood and not based on the experimental study.

Turbidity‐mediated interactions between invasive filter‐feeding mussels and native bioturbating mayflies

Summary1. Invasive dreissenid mussels are known to cause large ecosystem changes because of their high filter‐feeding capacity, while native bioturbators may interfere with the mussels filter feeding. In this experiment, we investigated indirect environmental interactions between invasive filter‐feeding dreissenid mussels (zebra and quagga mussels) and native recolonizing bioturbating hexagenid mayflies (Hexagenia) at two mussel densities and two Hexagenia densities in a 2‐month long laboratory experiment.2. Mean turbidity increased with increasing density of Hexagenia and decreased with increasing density of mussels. Turbidity showed the fastest decline at the highest mussel density, and no decline or a lower rate of decline at the low mussel density, dependent on Hexagenia density.3. Mussel growth decreased with increasing Hexagenia density at low but not at high mussel density. Moreover, growth of mussels decreased as a function of increased mean turbidity at low mussel density but not at high mussel density. Filtering activity at the highest mussel density increased after introduction of food at the lower two densities of Hexagenia, but was constantly high at the highest Hexagenia density.4. There was no difference in emergence of Hexagenia among the treatments, but mortality of Hexagenia was higher in the presence of mussels than in their absence.5. Our results indicate that interactions between dreissenids and hexagenids are mediated through the sediment, and depend on density of both dreissenids and hexagenids. As the natural densities of these animals vary considerably within lakes, their growth and survival because of indirect environmental interactions is expected to vary spatially.

Effects of native predators and eelgrass habitat structure on the introduced Asian mussel Musculista senhousia (Benson in Cantor) in southern California

The ability of predators to control the abundance of non-native species has been little explored in marine systems. Native predators may be used to control non-native species or may confer invasion resistance to communities if predation rates on invaders are density-dependent. We studied the response of southern California native predators to the density of Musculista senhousia (Benson in Cantor, 1842), a small, fast growing mussel that has been introduced from Japan to several coastlines worldwide. We performed field experiments to determine if M. senhousia proportional mortality is density-dependent and if eelgrass Zostera marina L. habitat structure influenced mussel density-dependent mortality. We also evaluated the effect of seagrass habitat structure on the aggregative and functional responses of the predatory gastropod Pteropurpura festiva (Hinds, 1844) to Asian mussel density. In the summer of 2002, P. festiva aggregated in plots with high mussel density and was responsible for nearly all predation on M. senhousia. However, M. senhousia proportional mortality was inversely density-dependent at all levels of eelgrass above-ground and below-ground habitat structure. Asian mussel proportional mortality also was inversely density-dependent and was not influenced by eelgrass habitat structure in the spring of 2004 when wading birds were the chief predator of mussels. In contrast to results for mussel proportional mortality, the aggregative and functional responses of P. festiva varied with seagrass habitat structure. P. festiva density increased with Asian mussel density in plots with low simulated habitat structure, but the relationship between P. festiva density and Asian mussel density was parabolic at zero, intermediate and high levels of habitat structure. In field enclosures, P. festiva exhibited a Type I (linear) functional response to Asian mussel density at low levels of eelgrass structure, and a Type II (hyperbolic) functional response to mussel density at high levels of eelgrass structure. Our results and those of others suggest that the degree to which local benthic communities in southern California are resistant to Asian mussel invasion depends on habitat structure, mussel settlement rates, and the density and diversity of predators.

Assessing the potential benthic–pelagic coupling in episodic blue mussel ( Mytilus edulis) settlement events within eelgrass ( Zostera marina) communities

Coastal marine seagrass ecosystems are important nursery grounds for commercially and recreationally important species, and they serve as key settlement and recruitment sites for other species. We investigated several years (2001–2003) where episodic settlement events of blue mussels ( Mytilus edulis) occurred in Barnegat Bay, NJ, USA. Population assessment indicated that blue mussels settled in eelgrass beds ( Zostera marina) in late spring with peak densities exceeding 170,000 m −2. Based on calculated filtration rates of M. edulis, we determined that for at least 53 days in 2001, the density and size distribution of M. edulis were sufficient to filter the water column volume in excess of twice a day, with maximum calculated filtration rates exceeding 8 m 3 water m −2 day −1. While the settlement event in 2001 was very localized, in 2003, the settlement event was considerably more widespread throughout the bay, with maximum settling densities exceeding 175,000 individuals m −2. Associated with these high densities, maximum calculated filtration rates exceeded 15 m 3 water m −2 day −1. This filtration potential may have impeded the localized development of a brown-tide ( Aureococcus anophagefferens) bloom in 2001, which occurred in other regions of the bay, but the widespread settlement event seen in 2003 may have impeded the development of any brown-tide blooms in Barnegat Bay during that summer. The decline in mussel densities throughout the summer may be a result of elevated water temperatures in this back bay, but at one site, the high settlement of M. edulis was followed by a substantial migration (>40 individuals m −2) of small sea stars ( Asterias forbesii). In 2001, A. forbesii was a significant factor in reducing M. edulis density by the end of the summer at the Barnegat Inlet site and a community level assessment showed significant positive correlations between mussel aggregations and sea star densities ( r=0.68–0.73, P<0.001). At this same site in 2003, the sea stars were again present in high densities (26 m −2) and were a potential mechanism for mussel decline. In other regions of the bay, sea star densities are very low, but numerous other predatory species exist, including blue crabs ( Callinectes sapidus), green crabs ( Carcinus maenus), spider crabs ( Libinia spp.), and several Xanthid crabs. Given the high mussel densities seen in this study and the considerable predation by sea stars and other benthic predators, the benthic–pelagic coupling which these mussels provide in this system contributes to the high secondary production in these grass beds.

Modelling the impact of benthic filter‐feeders on the composition and biomass of river plankton

SUMMARY 1. The POTAMON model [Everbecq E. et al. (2001) Water Research, 35, 901] has been used to simulate the effect of benthic bivalves (mainly Dreissena polymorpha) on the phytoplankton and zooplankton in a lowland Western European river (the Moselle). Here we use a modified version of the POTAMON model with five categories of phytoplankton (Stephanodiscus, Cyclotella‐like, large diatoms, Skeletonema and non‐siliceous algae) to model filter‐feeding effects of benthic bivalves in the Moselle. Zooplankton has been represented in the model by two categories, Brachionus‐like and Keratella‐like rotifers.2. According to density estimates from field surveys (Bachmann V. et al. (1995) Hydroécologie Appliquée, 7, 185, Bachmann V. &amp; Usseglio‐Polatera P. (1999) Hydrobiologia, 410, 39), zebra mussel density varied among river stretches, and increased through the year to a maximum in summer. Dreissena filtration rates from the literature were used, and mussels have been assumed to feed on different phytoplankton categories (but less on large and filamentous diatoms) as well as on rotifers.3. The simulations suggest a significant impact of benthic filter‐feeders on potamoplankton and water quality in those stretches where the mussels are abundant, their impact being maximal in summer. Consequently, different plankton groups were not affected to the same extent, depending on their period of development and on indirect effects, such as predation by mussels on herbivorous zooplankton.4. A daily carbon balance for a typical summer shows the effect of benthic filter‐feeders on planktonic and benthic processes: the flux of organic matter to the bottom is greatly enhanced at high mussel density; conversely, production and breakdown of organic carbon in the water column are reduced. Mussel removal would drive the carbon balance of the river toward autotrophy only in the downstream stretches.