Blue mussels (Mytilus edulis) are ecosystem engineers with strong effects on species diversity and abundances. Mussel beds appear to be declining in the Gulf of Maine, apparently due to climate change and predation by the invasive green crab, Carcinus maenas. As mussels die, they create a legacy of large expanses of shell biogenic structure. In Maine, USA, we used bottom traps to examine effects of four bottom cover types (i.e., live mussels, whole shells, fragmented shells, bare sediment) and wind condition (i.e., days with high, intermediate, and low values) on flow-related ecosystem processes. Significant differences in transport of sediment, meiofauna, and macrofauna were found among cover types and days, with no significant interaction between the two factors. Wind condition had positive effects on transport. Shell hash, especially fragmented shells, had negative effects, possibly because it acted as bed armor to reduce wind-generated erosion and resuspension. Copepods had the greatest mobility and shortest turnover times (0.15 d), followed by nematodes (1.96 d) and the macrofauna dominant, Tubificoides benedeni (2.35 d). Shell legacy effects may play an important role in soft-bottom system responses to wind-generated ecosystem processes, particularly in collapsed mussel beds, with implications for recolonization, connectivity, and the creation and maintenance of spatial pattern.

Mussels are well-known ecosystem engineers in soft-bottom systems. Mytilus edulis beds have myriad effects on sediment, benthic organisms, and ecosystem processes such as hydrodynamic transport of sediment and animals. When mussels die, they may leave behind massive amounts of whole (empty) and fragmented shells. The legacy effects of this long-lasting biogenic material (i.e., shell hash) on benthic systems are poorly understood. We measured percent cover values of 4 bottom cover types, i.e., live mussels, whole shells, fragmented shells, and bare sediment, at the mussel bed in Carrying Place Cove, Harrington, Maine, USA, and examined their effects on sediment characteristics, community structure of macrofauna and meiofauna, and ecosystem processes of sediment flux and dispersal of postlarval macrofauna and meiofauna. We predicted that live mussels are the cover type with the greatest effects compared to bare sediment, followed by fragmented shells and then whole shells. We discovered mostly bare sediment, substantial cover of whole and fragmented shells, and almost no live mussels in what had in past years been a robust bed. We found significant univariate and multivariate differences in sediment and animals across cover types, especially for meiofauna. Fragmented shell material in particular may be an important driver in this system. Our results are the first to quantify the 4 mussel bed cover types and demonstrate their effects. Mussel beds in the Gulf of Maine have experienced severe declines in the past two decades, attributed primarily to climate change and the invasive green crab, Carcinus maenas. Our results may be useful in predicting the responses of soft-bottom systems as intact mussel beds die off, leaving large areas of bare sediment and shell hash.

Mussels have myriad effects on population, community, and ecosystem processes. Their aggregation behavior is an inducible defense that links non-consumptive effects of predators to benthic spatial pattern formation. Aggregation increases intraspecific competition but can be beneficial due to lower perimeter-related predation and other risks. Mytilus edulis aggregation responses to predation threats have not been investigated outside of Europe. We studied the effects of chemical cues from heterospecifics (predators Carcinus maenas, Nucella lapillus; herbivore Littorina littorea) and conspecifics (injured and intact M. edulis) on M. edulis aggregation behavior in Maine, USA. Mussels self-organized into fractal power-law spatial patterns like those in the field. Aggregations had lower perimeter: area (P:A) ratios than singletons, despite having more complex, irregular shapes with higher fractal dimensions (D). However, with one exception, no significant differences in aggregation rate, P: A ratio, and D were observed for any chemical cue treatment when compared to no-cue controls. Our experiment revealed higher aggregation rates than reported from similar experiments, leaving little scope for additional aggregation when exposed to chemical cues. We suggest that increased aggregation in response to predation threat is context-dependent: costs outweigh benefits beyond some optimal aggregation size, and mussels in our experiment were at the upper aggregation limit beyond which more aggregation could have negative consequences. Bet-hedging with a power-law distribution of aggregation shapes and sizes may be the optimal spatial strategy, especially if predation and other risks are variable in space and time.

Mussels ( Mytilus edulis ) build massive, spatially complex, biogenic structures that alter the biotic and abiotic environment and provide a variety of ecosystem services. Unlike rocky shores, where mussels can attach to the primary substrate, soft sediments are unsuitable for mussel attachment. We used a simple lattice model, field sampling, and field and laboratory experiments to examine facilitation of recruitment (i.e., preferential larval, juvenile, and adult attachment to mussel biogenic structure) and its role in the development of power‐law spatial patterns observed in Maine, USA, soft‐bottom mussel beds. The model demonstrated that recruitment facilitation produces power‐law spatial structure similar to that in natural beds. Field results provided strong evidence for facilitation of recruitment to other mussels—they do not simply map onto a hard‐substrate template of gravel and shell hash. Mussels were spatially decoupled from non‐mussel hard substrates to which they can potentially recruit. Recent larval recruits were positively correlated with adult mussels, but not with other hard substrates. Mussels made byssal thread attachments to other mussels in much higher proportions than to other hard substrates. In a field experiment, mussel recruitment was highest to live mussels, followed by mussel shell hash and gravel, with almost no recruitment to muddy sand. In a laboratory experiment, evenly dispersed mussels rapidly self‐organized into power‐law clusters similar to those observed in nature. Collectively, the results indicate that facilitation of recruitment to existing mussels plays a major role in soft‐bottom spatial pattern development. The interaction between large‐scale resource availability (hard substrate) and local‐scale recruitment facilitation may be responsible for creating complex power‐law spatial structure in soft‐bottom mussel beds.

In this edition of Next Page, Professor of Environmental Studies John Commito reveals his love for all things Maine and why his neighbors don’t believe he reads half of what he says he does. This blog post is available at The Cupola: Scholarship at Gettysburg College: http://cupola.gettysburg.edu/nextpage/3 John Commito, Professor of Environmental Studies October 1, 2013 In this edition of Next Page, Professor of Environmental Studies John Commito reveals his love for all things Maine and why his neighbors don’t believe he reads half of what he says he does.

A great diversity of organisms modify the physical structure of estuarine and coastal environments. These physical ecosystem engineers - particularly, dune and marsh plants, mangroves, seagrasses, kelps, reef-forming corals and bivalves, burrowing crustaceans, and infauna - often have substantive functional impacts over large areas and across distinct geographic regions. Here, we use a general framework for physical ecosystem engineering to illustrate how these organisms can exert control on sedimentary processes, coastal protection, and habitat availability to other organisms. We then discuss the management implications of coastal and estuarine engineering, concluding with a brief prospectus on research and management challenges.

Mussels are important ecosystem engineers that have significant impacts on the ecology of the seafloor. In this study, we attempt to characterize and explain infaunal and epifaunal abundance patterns across scales in a spatially complex, intertidal, soft-bottom habitat: beds of the semi-infaunal mussel, Mytilus edulis. We used univariate and multivariate analyses in a nested sampling design at three locations in eastern Maine to show the relationships between environmental and macrofaunal assemblage variables. The results support the hypotheses that: 1) environmental and macrofaunal assemblage variables vary widely across spatial scales and exhibit maximum variance at different spatial scales; 2) abundance and diversity patterns of macrofauna are linked to spatial variation in environmental variables in the form of biogenic structure (live mussels and mussel shell hash), terrestrially-derived gravel, and, to a lesser degree, the proportion of silt-clay in the sediment; and 3) epifauna are generally positively correlated with hard-substrate features such as live mussels and gravel, and negatively correlated with the silt-clay content of the sediment, although the latter relationships were not strong. On the other hand, the results did not support the hypotheses that: 4) most infauna, specifically those with free-swimming larvae, are negatively correlated with live mussels and silt-clay proportion; instead, they were often positively correlated with live mussels and had weak, insignificant correlations with silt-clay; and 5) infauna without free-swimming larvae are positively correlated with live mussels and silt-clay proportion; instead, they showed some positive correlations with live mussels, but a mix of weak, insignificant correlations with silt-clay. Thus, mussel beds had a variety of scale-dependent relationships, both positive and negative, with each of the macrofaunal groups. The results show that biogenic structure in the form of live mussels and mussel shell hash is important in this system, as is terrestrial gravel, and that these variables are more important than silt-clay content for the macrofaunal assemblage. Mussel beds play an important role in regulating macrofaunal diversity in soft-bottom habitats, and their effects vary across spatial scales.

Biogenic structures built by ecosystem engineers such as corals, bivalves, polychaetes, and sea grasses provide habitat for benthic vertebrates and invertebrates. The polychaete Sabellaria alveolata is an important foundation species whose reef structure adds topographic complexity and high levels of biodiversity to the otherwise low-relief, low diversity, soft-bottom environments in the Bay of Mont Saint-Michel, France, where the largest such reef formations in Europe are found. In this bay, reefs are being increasingly colonised by oysters (Crassostrea gigas) from local aquaculture operations and by green algae (Ulva sp.) due to the increasing inputs of nitrates from terrestrial origin. The purpose of this study was to investigate the possible impacts of epibiotic oysters and green algae on the S. alveolata population and reef community structure in the Bay of Mont Saint-Michel, France. Univariate and multivariate comparisons of macrofauna were conducted for five reef types: controls (no epibionts), low oyster density, high oyster density, green algae, and oyster and green algae. Results showed that all the three reef types with oysters had significantly higher species richness and diversity values than control and algae-only reef types. Pairwise ANOSIM and SIMPER comparisons of controls versus the four reef types with epibionts revealed that all three of the reef types with oysters were significantly different from controls, but there was no significant difference between controls and algae-only reef types. A striking feature of the reef comparisons is that no single species in this species-rich system contributed more than 8.86% to the dissimilarity between the reef types. Thus, k-dominance curves for species abundances were not effective in revealing differences among the reef types. Our results demonstrate that recent anthropogenic inputs of oysters affect the reef species assemblage more strongly than algal epibionts. In addition, epibionts, especially green algae, alter S. alveolata population structure, causing a reduction in new recruits that over the long run may cause significant damage to the reef structure itself. These results are a first step towards understanding anthropogenic threats to S. alveolata reefs and may be useful in the development of strategies for their protection and management.

Mussels (Mytilus edulis L.) are unusual because they thrive in both rocky shore and soft-bottom habitats. Despite their ecological and economic importance, little is known about their spatial structure. Mussels do not generally recruit to bare soft substrate because larvae and postlarvae cannot attach to a bottom of small sediment particles. They attach to hard objects on the sediment surface (especially other mussels), so soft-bottom mussel beds may be spatially organized in ways that are fundamentally different from those on rocky shores. The purpose of our study was to characterize the scales of spatial variability for several mussel abundance parameters in soft-bottom, intertidal M. edulis beds in coastal Maine. We used a random factor nested-ANOVA design of 200 cm2 Cores within 1 m2 Quadrats within 6 m Transects within Positions within bed Sites along 70 km (euclidean distance) of the Maine coast. Based on the literature and our field observations, we hypothesized that Sites and Positions account for most of the spatial variance in soft-botttom mussel beds. We rejected this hypothesis. Sites and Positions were not important in explaining variation in total mussel density, density of new recruits, or density of larger mussels. Although most of the variance in surface silt–clay fraction did occur at these levels, most mussel variation occurred at smaller spatial scales, specifically at the Quadrat scale for new recruits and total mussels and at the Transect scale for larger mussels. Variance in mussel parameters was not closely linked to the silt–clay fraction of surface sediment or to Site rankings of wind exposure and tidal flow. Variance in total mussel density was due primarily to variance in recruitment. No single scale explained more than about half the mussel variance, and no single scale was best at explaining all the mussel parameters. Greater knowledge about mussel bed spatial variability would be useful because it can help direct scale-dependent sampling regimes, field experiments, and coastal management practices.

The blue mussel, Mytilus edulis L., forms dense and variable patch mosaics composed of aggregates of mussel individuals. Knowledge of mussel bed spatial pattern at multiple scales is important for understanding the form and function of intertidal systems where mussels are prominent features. This study extends prior work demonstrating fractal patterns of mussel boundaries in soft-bottom systems at the quadrat-scale by investigating fractal structure using GIS methods at both the quadrat- and bed-scales. The study pursues three goals for mussel beds in eastern Maine: (1) to compare quadrat-scale fractal dimensions obtained using manual methods with those obtained using digital imagery and techniques, (2) to determine if fractal patterns identified at the quadrat-scale are also present at the bed-scale, (3) and to evaluate the effectiveness of aerial photography and image analysis techniques. Photographs of randomly located quadrats (2500 cm(2) each) were scan digitized and classified into mussel presence/absence classes. Fractal dimensions of mussel/non-mussel boundaries were calculated using the box-counting method and compared with results obtained using analog photographs and methods. Digital aerial photographs at low tide were acquired for beds at two sites and classified using image processing techniques, and bed-scale fractal dimensions were calculated. At the quadrat-scale, fractal dimensions and their relationship with percent cover differed consistently in absolute value from results using manual methods but agreed in demonstrating fractal patterns for all quadrats and a parabolic trend with percent cover very similar to the one revealed manually. At the bed-scale, both sites were shown to be fractal, with higher dimension value for the bed that subjectively appeared more fragmented and highly dissected. Because mussels are important soft-bottom ecosystem engineers, i.e., foundation species that regulate species composition and abundances, the fractal spatial distribution identified in this study suggests that the species affected by them also exhibit fractal patterns. These results indicate the effectiveness of archive imagery and GIS methods for characterizing intertidal systems and point to the feasibility of future image acquisition.

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.

Gemma gemma is a small ovoviviparous bivalve distributed in shallow sand flats along the North American Atlantic and Gulf of Mexico coasts. Genetic variation in G. gemma was analysed by means of Inter-Simple Sequence Repeats (ISSRs) at the following levels: (i) between localities (Maine and Virginia), (ii) among 10-m-diameter patches within localities, and (iii) within patches. Thirty individuals/patch and three patches/locality were analysed. Individuals were genotyped for 67 ISSR polymorphic loci from five primers. The portion of the genetic variation found between localities (2%) was small compared to that found either among patches within localities (37%) or within patches (61%). ISSRs in G. gemma allowed the detection of significant differentiation at individual and patch levels. By contrast, a low degree of genetic variability was found between localities. The small-scale genetic heterogeneity does not follow a simple, consistent pattern. Our results contrast with the generally accepted rule that aplanic species are locally homogeneous and globally heterogeneous and teleplanic species are the inverse.

Gemma gemma (Totten, 1834) e un bivalve ovoviviparo privo di stadi larvali pelagici, i giovani si sviluppano dall'uovo fecondato all'interno della madre e vengono rilasciati quando raggiungono una determinata dimensione. Essi rimangono vicino alla madre per un breve periodo di tempo, poi sono in grado di disperdersi soltanto passivamente con il sedimento sospeso dall'azione idrodinamica. Per valutare l'efficacia della dispersione passiva sono stati confrontati i bandeggi ISSR tra individui entro patch di 10m di diametro, tra patch entro ciascuna delle localita e tra le due localita (Maine e Virginia, USA). Sono stati analizzati 30 individui/patch e 3 patch/localita per un totale di 180 individui. Sono stati utilizzati 5 primer che hanno amplificato mediante PCR un totale di 67 bande. L'AMOVA ha rilevato una piccola porzione di varianza genetica tra Maine e Virginia (2.3%, P>0.05), mentre proporzioni piu elevate sono state rilevate tra patch entro localita (36.7%, P <0.0001). Questa `patchiness genetica caotica' a piccola scala spaziale puo essere dovuta alla variabilita del numero e dei genotipi delle reclute. La specie risulta geneticamente strutturata come un mosaico di gruppi di individui generati dalla stessa madre. La presenza di alcuni genotipi diversi all'interno di questi gruppi denota, comunque, che la dispersione passiva puo giocare un ruolo importante nel mantenere la diversita genetica a piccola scala.

Meiofaunal nematodes and copepods disperse passively with sediment bedload, and copepods also display active emergence and reentry behavior. Epigrowth-feeders may be the nematode feeding group most susceptible to passive transport because they live closest to the sediment surface. We used bottom traps at a nematode-dominated intertidal mudflat in Maine, USA, to test the hypotheses that (1) meiofauna taxa disperse in relative proportions different from those of the ambient community; (2) copepods have the highest relative dispersal rate (number of individuals trap−1 day−1 ambient individual−1) and are not as tightly linked as other taxa to sediment flux; and (3) epigrowth-feeders have the highest nematode relative dispersal rate. Results supported all three hypotheses. Nematodes accounted for 95.8% of the individuals in cores, but only 38.9% of the individuals in traps. Copepods accounted for 1.5% of the individuals in cores, but 56.7% of the individuals in traps. Less common taxa also had different relative proportions in cores and traps, as did nematode feeding groups and individual species. The relative dispersal rate was far higher for copepods than for any other taxonomic group, and the absolute (number of individuals trap−1 day−1) and bulk (number of individuals g sediment−1 trap−1 day−1) dispersal rates for copepods were equal to those of the 65-fold more abundant nematodes and higher than those for all other taxa. The non-selective deposit-feeders were the most abundant nematode feeding group in the ambient community, but the epigrowth-feeders as a group and as individual species had the highest absolute, relative, and bulk dispersal rates. Non-metric multidimensional scaling (MDS) using analysis of similarity (ANOSIM) and species similarity percentages (SIMPER) reflected these differences between ambient and dispersing nematode assemblages. Significant positive regression relationships between sediment weight and the number of individuals captured in traps for nematodes and some other taxa indicated that they moved passively in the bedload. Lack of a significant regression relationship for copepods suggested an active behavioral component to dispersal. Meiofauna populations in this soft-bottom community were highly dynamic, demonstrating that the role of dispersal must be included in any consideration of the ecology of soft-bottom systems at local and regional spatial scales.

Seafloor topographic complexity is ecologically important because it provides habitat structure and alters boundary-layer flow over the bottom. Despite its importance, there is little agreement on how to define and measure surface complexity. The purpose of this investigation was to utilize fractal geometry of vertical cross-section profiles to characterize the surface topography of the soft-bottom mussel bed (Mytilus edulis L.) at Bob’s Cove, ME, USA. Mussels there have been shown previously to have spatially ordered fractal characteristics in the horizontal plane. Two hypotheses were tested. The first was that the bed surface is fractal over the spatial scale of 1.44–200 mm, with fractal dimension less than or equal to 1.26, the value for the Koch curve, our model for bed profiles. The second was that bed surface topography (i.e., in vertical profile) is less complex than the mussel bed spatial pattern (i.e., aerial view in the horizontal plane). Both hypotheses were supported. Cross-sections of plaster casts of the bed produced 88 surface profiles, all of which were fractal over the entire spatial scale of more two orders of magnitude employed in the analysis. Fractal dimension values (D) for individual profiles ranged from 1.031 to 1.310. Fractal dimensions of entire casts ranged up to mean (1.242±0.046) and median (1.251) values similar to 1.26, the theoretical value of the Koch curve. The bed surface was less complex than the bed spatial pattern because every profile had D<1.36, the smallest value previously obtained from aerial views of the bed. The investigation demonstrated for the first time that surface topography of a soft-bottom mussel bed was fractal at a spatial scale relevant to hydrodynamic processes and habitat structure important for benthic organisms. The technique of using cross-section profiles from casts of the bed surface avoided possible underestimates of fractal dimension that can result from other profiling methods reported in the literature. The results demonstrate that fractal dimension can be useful in the analysis of habitat space and water flow over any irregular seafloor surface because it incorporates the size, shape, and scale of roughness elements into a simple, numerical metric.