Macrofaunal Community Structure Research Articles

Influence of invasive Spartina growth stages on associated macrofaunal communities

In coastal wetlands, invasive plants often act as ecosystem engineers altering flow, light and sediments which, in turn, can affect benthic animal communities. However, the degree of influence of the engineer will vary significantly as it grows, matures and senesces, and surprisingly little is known about how the influence of an ecosystem engineer varies with ontogeny. We address this issue on the tidal flats of San Francisco Bay where hybrid Spartina (foliosa × alterniflora) invaded 30 years ago. The invasion has altered the physico-chemical properties of the sediment habitat, which we predicted should cause changes in macrofaunal community structure and function. Through mensurative and manipulative approaches we investigated the influence of different growth stages of hybrid Spartina on macrobenthos and the underlying mechanisms. Cross-elevation sampling transects were established covering 5 zones (or stages) of the invasion, running from the tidal flat (pre-invasion) to an unvegetated dieback zone. Additionally, we experimentally removed aboveground plant structure in the mature (inner) marsh to mimic the ’unvegetated areas’. Our results revealed four distinct faunal assemblages, which reflected Spartina-induced changes in the corresponding habitat properties along an elevation gradient: a pre-invaded tidal flat, a leading edge of immature invasion, a center of mature invasion, and a senescing dieback area. These stages of hybrid Spartina invasion were accompanied by a substantial reduction in macrofaunal species richness and an increase in dominance, as well as a strong shift in feeding modes, from surface microalgal feeders to subsurface detritus/Spartina feeders (mainly tubificid oligochaetes and capitellid polychaetes). Knowledge of the varying influence of plant invaders on the sediment ecosystem during different phases of invasion is critical for management of coastal wetlands.

Community structure and nutrition of deep methane‐seep macrobenthos from the North Pacific (Aleutian) Margin and the Gulf of Mexico (Florida Escarpment)

AbstractMethane seeps occur at depths extending to over 7000 m along the world's continental margins, but there is little information about the infaunal communities inhabiting sediments of seeps deeper than 3000 m. Biological sampling was carried out off Unimak Island (3200–3300 m) and Kodiak Island (4500 m) on the Aleutian margin, Pacific Ocean and along the Florida Escarpment (3300 m) in the Gulf of Mexico to investigate the community structure and nutrition of macrofauna at these sites. We addressed whether there are characteristic infaunal communities common to the deep‐water seeps or to the specific habitats (clam beds, pogonophoran fields, and microbial mats) studied here, and ask how these differ from background communities or from shallow‐seep settings sampled previously. We also investigated, using stable isotopic signatures, the utilization of chemosynthetically fixed and methane‐derived organic matter by macrofauna from different regions and habitats. Within seep sites, macrofaunal densities were the greatest in the Florida microbial mats (20,961 ± 11,618 ind·m−2), the lowest in the Florida pogonophoran fields (926 ± 132 ind·m−2), and intermediate in the Unimak and Kodiak seep habitats. Seep macrofaunal densities differed from those in nearby non‐seep sediments only in Florida mat habitats, where a single, abundant species of hesionid polychaete comprised 70% of the macrofauna. Annelids were the dominant taxon (>60%) at all sites and habitats except in Florida background sediments (33%) and Unimak pogonophoran fields (27%). Macrofaunal diversity (H′) was lower at the Florida than the Alaska seeps, with a trend toward reduced richness in clam bed relative to pogonophoran field or non‐seep sediments. Community composition differences between seep and non‐seep sediments were evident in each region except for the Unimak margin, but pogonophoran and clam bed macrofaunal communities did not differ from one another in Alaska. Seep δ13C and δ15N signatures were lighter for seep than non‐seep macrofauna in all regions, indicating use of chemosynthetically derived carbon. The lightest δ13C values (average of species’ means) were observed at the Florida escarpment (−42.8‰). We estimated that on average animal tissues had up to 55% methane‐derived carbon in Florida mats, 31–44% in Florida clam beds and Kodiak clam beds and pogonophoran fields, and 9–23% in Unimak seep habitats. However, some taxa such as hesionid and capitellid polychaetes exhibited tremendous intraspecific δ13C variation (>30‰) between patch types. Overall we found few characteristic communities or features common to the three deep‐water seeps (>3000 m), but common properties across habitats (mat, clam bed, pogonophorans), independent of location or water depth. In general, macrofaunal densities were lower (except at Florida microbial mats), community structure was similar, and reliance on chemosynthesis was greater than observed in shallower seeps off California and Oregon.

Changes of macrobenthos composition under different ENSO cycle conditions on the continental shelf off central Chile

The course of environmental conditions and shelf macrobenthic communities off Central Chile (∼36°S) during the strong 1997–98 El Niño (EN) event is compared with a subsequent and basically “normal” period (2002–2003). Changes in macrofaunal community, feeding mode structure, and biomass size spectra are contrasted over time with changes in oceanographic and sediment settings, in order to assess intra- and inter-annual changes in faunal composition during both ENSO periods. During EN, there was a decrease in biomass and abundance of species known to be well adapted to organic-rich, oxygen-deficient environments, such as the interface-feeding polychaete Paraprionospio pinnata. On the other hand the abundance of highly mobile, burrowing polychaetes remained unaffected, or even increased in biomass. The decline of P. pinnata lasted several years after the demise of warm conditions, possibly due to negative interactions with those more mobile burrowing polychaetes. The percent contribution of subsurface-deposit feeders to total biomass increased during EN (49.3±12.4% during summer) and declined only in the summer-fall period of 2002–03 (11.1±4.1%). An opposite trend was observed for interface and surface-deposit feeders. From EN to summer-fall 2002–03 (i.e., normal to low oxygen conditions) the size-structure of the macrobenthos switched from a larger to a smaller-sized assemblage. However, biomass was maintained due to successful recruitment and high abundance of both P. pinnata and the squat lobster, Pleuroncodes monodon. Our results suggest that the shelf macrofaunal community structure exhibit fluctuations at various time scales, and that these changes are more pronounced during and after a strong EN event. In such cases, the effects of such an event may be recorded at latitudes as far south as 36°S, with consequences in the biota lasting for many years after the demise of warm conditions.

Structure and function of macrofaunal communities influenced by hydrodynamically controlled food availability in the Wadden Sea, the open North Sea, and the Deep-sea. A synopsis

The study areas were situated in the East Frisian Wadden Sea, in the south-eastern North Sea, the Skagerrak and the Kattegat, and in the deep-sea of the Arctic Ocean (Eurasian Basin), the eastern Mediterranean and the Angola Basin.

Nematode and macrofaunal diversity in central Arctic Ocean benthos

Deep-sea diversity studies have revealed intriguing patterns on both local and regional scales, but there is insufficient evidence with which to evaluate these trends in the Arctic Ocean basin. We collected data on the diversity of benthic macrofauna and meiofaunal nematodes along two transects from the shelf margin to the North Pole. Contrary to prevailing paradigms, there was no change in diversity with depth between 1000 and 4273 m. There was a trend, however, toward reduced taxonomic richness for both macrofauna and nematodes with increasing latitude. Regional ( β-) diversity differences were not observed for nematodes, but significant contrasts in Bray–Curtis similarity-based community structure of macrofauna were seen between the Eurasian and Amerasian Basins, as well as between the Lomonosov and Mendeleev Ridges. Since fauna within the deep Arctic Ocean appear to represent a single species pool, we suggest that both local ( α-) and β-diversity may be determined by ecological processes in the Arctic, and are not the consequence of historical or evolutionary processes. Furthermore, insights gained from studies of benthic–pelagic coupling, known to play a significant role in determining benthic community structure and function at high latitudes, may also be useful in investigations of Arctic biodiversity. This model may be valuable in designing future studies of biodiversity, and for predicting potential impacts of climate change on diversity patterns.

Effects of habitat fragmentation per se on decapods and fishes inhabiting seagrass meadows in the northern Gulf of Mexico

The role of habitat fragmentation per se on ecological processes is incompletely under- stood in marine environments. Previous work in fragmented habitats has concentrated on growth of bivalve species, faunal abundances and predation of tethered crabs. Unlike other studies, in the pre- sent study we measured abundances of decapods and fishes and estimated secondary production from in situ sampling of multiple sized, naturally occurring seagrass beds and artificial seagrass units (ASUs). In 2 locations (Thalassia testudinum meadows in Big Lagoon, Florida, and Halodule wrightii meadows in Grand Bay, Alabama), we sampled 7 to 10 seagrass patches ranging in size from 100 m 2 at 2 mo intervals between June and October 2001. We also sampled 10 replicate ASUs of 2 shapes (stellate and circular) and 2 sizes (0.2036 and 0.0487 m 2 ) that were colonized for 4 wk at both locations in June, August and October. Our results indicated that patch size, patch shape, intra-patch location, and degree of isolation can each influence macrofaunal community structure and secondary production estimates; however, their overall impact was inconsistent. Our data suggest that the effects of habitat fragmentation are location-, time- and species-specific, and that fragmentation at this scale may have little impact on faunal assemblages of seagrass meadows.

An investigation of the cumulative impacts of shrimp trawling on mud-bottom fishing grounds in the Gulf of Maine: effects on habitat and macrofaunal community structure

Abstract The cumulative impacts (i.e. collective, multi-year effects) of seasonal commercial shrimp trawling on habitat and macrofaunal community structure were investigated for two mud-bottom fishing grounds and adjacent untrawled areas in the Gulf of Maine. Habitat structure on mud-bottom fishing grounds did not differ significantly from that in similar untrawled areas. Moreover, sediment resuspension associated with shrimp trawling did not appear to result in net loss of deposited material on fishing grounds, but there is evidence that trawling may alter sediment mixing regimes. Visual inspection of the sediment surface in trawled areas revealed minimal evidence of fishing gear disturbance (such as door, bobbin, or net marks), but biological disturbance features, including numerous large burrows, pits, and dense aggregations of the brittle star Ophiura sarsi, were present in both trawled and untrawled areas. Macrofaunal communities on the two fishing grounds exhibited different responses to shrimp trawling, which were attributed to disparities in levels of fishing activity during the 2000–2001 shrimp season. The results suggest that seasonal shrimp trawling produced at least short-term changes (<3 months) in macrofaunal community structure, but did not appear to result in long-term cumulative changes. Resilience to trawling disturbance may be due in part to high levels of biological disturbance generated by benthic megafauna, such as lobsters and fish. By burrowing, pit-digging, and possibly foraging, these animals rework sediments to a depth of 16–17 cm, creating a natural level of disturbance that appears to maintain macrofaunal communities in a perpetually low successional state, so potentially minimizing trawling impacts.

Detecting Anthropogenic Stress in an Ecosystem: 3. Mesoscale Variability and Biotic Indices

Previous work in the Ria Formosa Lagoon has revealed that it is possible to use macrofauna community structure as an indicator of contamination effects. Communities at sites adjacent to sewage outfalls indicated contamination but no pollution gradients were observed. In this study, macrofauna within a sub-section of the earlier work were investigated to determine the extent of mesoscale (10–100 m) variability and new indices of biological community structure. The results were evaluated using univariate (Marine Biotic Index; BI) and multivariate statistical methods (PCA). The computer program Barrier (usually used with genetic data) was also applied to detect “barriers” between spatially referenced samples. PCA identified the most significant factor effecting the distribution of the organisms in the study area to be the mud content of the sediment (% < 63 μm). Communities at sandy sites consisted of suspension feeders and detrital browsers, whilst communities at muddy sites were composed mainly of deposit feeders. The second principal component produced a vector characterized by a contamination gradient. The effects of the sewage discharge appeared to be concentrated at the outfall as it had the highest TOC (10.4%) content and was the only site to be characterized by the BI as slightly polluted. With the use of the BI and Barrier two gradients were observed in the transition of sites from an unpolluted to a polluted state. These gradients appeared not to follow the flow of the channel as expected (high % TOC of sites along the channel), and instead extended west and south from the sewage source. In conclusion, weak gradients were observeable; the multiple approaches of BI, PCA and Barrier analyses in concert were able to confirm the sewage dispersion route through the area.

Relationships Between Chemical Characteristics of Sediments and Macrofaunal Communities in the Cabras Lagoon (Western Mediterranean, Italy)

The Cabras lagoon is a large and shallow transitional system which periodically suffers from dystrophic events leading to massive losses of its biological resources. Excessive organic matter (OM) and sulphur compounds in sediments are often claimed to be major environmental co-factors triggering these events. However, still limited information is available on their relationships with benthic macrofaunal communities in coastal lagoons. In order to assess these relationships, we conducted monthly field investigations between 2001 and 2002, at three regularly-spaced stations set between the inlet and the mouth of the Cabras lagoon. Results showed reduced conditions of sediments, worsening during the warm season. This was consistent with very high OM contents, up to >‰20%, with annual means of 14–16%. Organic pollution-tolerant taxa, such as Tubificidae nc, Neanthes succinea and Polydora ciliata, dominated poor macrofaunal communities. Results of multivariate analysis indicated significant differences in both environmental variables and macrofaunal community structure and species composition between stations and through different periods. This study documented for the first time in a coastal lagoon that inorganic reduced sulphur pools (acid volatile sulphide and chromium reduced sulphur) of sediments are leading (‚best-matching’) environmental factors influencing the spatial and temporal distribution of macrofaunal abundance and biomass. We conclude that impoverished macrofaunal communities in the Cabras lagoon may show a slight recovery during winter–spring, but tend to regress to an early stage of faunal succession in late summer. We suggest that the combination of excessive amounts of sedimentary organic matter and high temperatures tends to lead to anoxic conditions especially in relatively deeper and more saline waters. These events may concur to periodic increase of production rates of toxic hydrogen sulphide in sediments, which rapidly diffuses to the water column with deleterious effects both on the benthic and pelagic components of the lagoon ecosystem.

The relative importance of habitat complexity and surface area in assessing biodiversity: Fractal application on rocky shores

Theoretical work predicts that complex habitats allow more species to co-exist in a given area. However, more field studies are still needed to clarify this relationship, especially in intertidal habitats. Furthermore, the potential separate effects of surface complexity and area on species richness and abundance have rarely been addressed. We tested the hypotheses that a more complex substratum or larger surface area will support a greater number of individuals and species of mobile macrofauna on three rocky shores in Hong Kong. Surface complexity, assessed by using fractals, was an important factor in species–area relationships. The number of species increased proportionally to habitat complexity and this relationship was homogeneous among different shores. Total abundance of animals, however, was more dependent on the available surface area. The slope of the size–frequency distribution of animals in samples taken on surfaces with different fractal dimensions ( D) was significantly steeper with an increase in fractal dimension, showing that the relative abundance of small animals increased with surface complexity. Thus, surface complexity and area may be important in determining different aspects of the macrofaunal community structure on rocky shores. The resulting increase in surface area on more rough surfaces may introduce bias in density and species number assessments when two-dimensional sampling units (i.e., quadrats) are employed. It is necessary, therefore, to account for the surface complexity in the design and interpretation of the results of benthic studies. Using D as an index of surface complexity is very useful, but also involves some practical problems, e.g., surfaces may be anisotropic and different methods may give different estimates of D. Therefore, these different methods need to be calibrated before comparisons of D values between them are meaningful.

Differences in macrofaunal and seagrass assemblages in seagrass beds with and without seaweed farms

Since it was introduced to Zanzibar (Tanzania), seaweed farming has significantly contributed to local, socio-economic development. However, several investigations have shown impacts on the coastal environment near where the farms are located. As many seaweed farms are located on seagrass beds, there is a risk that seaweed farming could affect seagrass beds, and thereby disturb important ecosystem functions and the flow of ecological goods and services. This study compares characteristics of macrophytes (focusing on seagrasses), benthic macrofauna and sediment in seagrass beds, with and without seaweed farms, and a sand bank without vegetation in Chwaka Bay, Zanzibar. The results showed that seagrass beds underneath seaweed farms generally had less seagrass and macroalgae, finer sediment, lower sediment organic matter content and a reduced abundance and biomass of macrofauna, than seagrass beds without seaweed farms. Further, the macrofaunal community structure in seaweed farms showed more similarities to that on the sand bank than in the unfarmed seagrass beds. Most of the dissimilarity was attributable to Lucinidae (suspension-feeding bivalves), which were almost absent in the seaweed farms, resulting in the large difference in biomass between the seaweed farms and the unfarmed seagrass beds. When interpreted together with information from farmers, the observed pattern is believed to be caused by the seaweed farming activities. This indicates that more research is needed to establish the effects of seaweed farming on seagrass beds, and that more attention should be given to the location of farms and the choice of farming methods.

Effect of Scirpus mariqueter Vegetation on Salt Marsh Benthic Macrofaunal Community of the Changjiang Estuary

Benthic macrofaunal community structure was related to vegetation and sediment properties along a low to high marsh elevational gradient in Scirpus mariqueter marsh on the eastern coast of Chongming Island in the Changjiang estuary, China. Multivariate analysis indicated that there were significant differences in macrofaunal community structure between the marsh zones sampled. There were positive relationships between attributes of salt marsh vegetation and macrofaunal community. The main factors explaining macrofaunal community structure along the habitat gradient were shoot height and below-ground plant biomass. In the Scirpus mariqueter marsh studied, above-ground and below-ground plant structure formed a complex habitat, adding structural heterogeneity to the surface sedimentary environment. Habitat heterogeneity is important for supporting diverse macrofaunal assemblages.

Community structure and diversity of soil macrofauna from different urban greenbelts in Jinhua City, Zhejiang Province

Community structure and diversity of soil macrofauna from different urban greenbelts in Jinhua City, Zhejiang Province

Development of macrofaunal community structure in mussel beds on the northern East Pacific Rise

Comparisons of macrofaunal community structure (species composition, abundance, biovolume, rank order, species richness, diversity) in hydrothermal vent mussel beds at 9° 50' N (9N) on the East Pacific Rise were made from analysis of multiple quantitative samples collected in 1999 and 2001 from each of 3 mussel beds of known age (4 to 8 yr in 1999). In addition to this time series approach, a chronosequence based on 2001 samples from 4 mussel beds at 9N and 1 mussel bed at 11° 25' N (11N) allowed comparison of macrofaunal community structure for mussel beds ranging in age from 6 to >14 yr. Many aspects of community structure (rank abundance patterns, dominant species, and diversity) were similar in 1999 and 2001 samples. Young (6 to 7 yr) and old (10 to 14 yr) mussel beds sampled in 2001 could be distinguished using multivariate analyses, based on species abundance and species biovolume matrices, but the differences were in the relative abundance of the dominant species rather than in any substantial changes in the identity of the dominant species. The limpet Lepetodrilus elevatus was the dominant species at all sites in terms of abundance and bio- volume; it and other dominant species may have slightly greater fitness compared to other species that allows them to persist as dominants regardless of the age or location of mussel beds. Decreasing biovolume of macrofaunal communities sampled at 9N and 11N in 2001 was correlated with in- creasing age of the mussel beds and is postulated to be largely a response to biological processes; including competition for inorganic substrates by free living and symbiotic chemoautotrophs and mussel induced mortality of larvae of macrofaunal species, rather than the result of waning hydro- thermal activity.

Benthic Macrofaunal Community Structure along a Well-Defined Salinity Gradient in the Mullica River–Great Bay Estuary**

The benthic macrofaunal community of the Mullica River–Great Bay estuary in the Jacques Cousteau National Estuarine Research Reserve is characterized by both small-scale and large-scale distribution patterns that can be correlated to gradients in physicochemical parameters. Sediment composition is a major factor regulating the local distribution of benthic assemblages in the estuary, with the amount of silt-clay being particularly important. For example, one suite of benthic fauna (i.e., Ampelisca verrilli, Ensis directus, Haustorius arenarius, Pygospio elegans, and Oxyurostylis smithi) reported in the estuary has been found only in sediments with less than 20% silt-clay, whereas another (i.e., Acteocina canaliculata, Lumbrineris tenuis, Maldinopsis elongata, Tellina agilis, Turbonilla sp., and Unciola irrorata) has been observed only in sediments with more than 38% silt-clay. Other physicochemical factors (e.g., organic carbon content of the sediments, dissolved oxygen levels, bottom currents, and turbidity) might also influence the local distribution patterns of the fauna. On the estuary scale, environmental gradients from upriver to down-bay regions strongly affect the distribution of the benthos over broader spatial areas. Hence, four distinct regional assemblages of benthic invertebrates have been identified in the Mullica River–Great Bay estuary, including river-, bay-, lower bay–, and estuary-wide forms. The dominant assemblage—the estuary-wide forms distributed along the length of the estuary—consists of the most abundant populations with the widest salinity tolerances. Examples are Ampelisca abdita, Corophium cylindricum, Cyathura polita, Notomastus latericeus, Polydora ligni, Scoloplos robustus, and Turbonilla sp. Because Great Bay exhibits marked changes in sediment type from predominantly sands in the eastern perimeter to generally finer deposits along the western margin, the abundance, species richness, and species composition of benthic invertebrates vary considerably over relatively small spatial scales.

Deep macrobenthic communities from Nazaré Submarine Canyon (NW Portugal)

Macrofauna community structure within Nazare Submarine Canyon is analysed and used to assess the potential effects of natural enrichment in this area subjected to accumulation of coastal sediments. A transect including three stations (2894, 3514 and 4141 m) was carried out in the Nazare Submarine Canyon (NW Portugal) during a cruise of OMEX II programme (Ocean Margin Exchange), in the winter season of 1999. Although data was not collected in order to calculate sedimentation rates, sampling station at 2894 m is located in an area characterised by high levels of sedimentation, thus a high amount of organic matter is expected to be available for the local communities. Faunistic data are discussed in the context of the different features of the stations sampled. Multivariate analysis clearly separates the shallowest station from the other ones, which otherwise appear to be very similar. It also revealed a perceptible gradient along sediment depth at all stations, from shallow to deeper layers. Exceptionally depressed species richness and low evenness values were observed at the 2894 m station. The high number of individuals of a single species, Cossura sp. A, and the atypical diversity, dominance and evenness values obtained for this station support the hypothesis of community disturbance due to organic enrichment.

Broad-scale environmental gradients among estuarine benthic macrofaunal assemblages of south-eastern Australia: implications for monitoring estuaries

The importance of abiotic factors in explaining patterns of estuarine benthic macrofaunal community structure was examined on a broad spatial scale across south-eastern Australia. Macrofaunal communities were surveyed using an Ekman grab and a modified epibenthic sled (dredge) at each sampling site: data for 24 environmental variables were also collected. Twenty-eight estuaries were sampled on a single occasion during late summer at three stratified locations within each estuary (upper, mid and lower). Macrofaunal community composition was best explained by a common environmental gradient summarising variation in both salinity and longitude. Hence, although the distribution of macrofaunal taxa can be clearly linked to changes in salinity, the geographical position of the sites along an east–west axis, rather than a generalised down-stream gradient, appears to best explain the data. This association was primarily linked to broad-scale changes in estuarine morphology across the geographical range of this survey. A sediment-based environmental gradient among grab samples, but not dredge samples, reflected the largely infaunal nature of the grab samples. In general, the present survey did not support the classification of estuarine assemblages on the basis of a range of physical parameters but, instead, emphasised the continuity of estuarine benthic macrofaunal community structure on a broad spatial scale.

The response of macrofauna communities and shorebirds to macrophyte wrack subsidies on exposed sandy beaches of southern California

To investigate the influence of marine macrophyte wrack subsidies on community structure, relationships between community attributes, including species richness, abundance, and biomass of macrofauna and abundance of shorebirds, and a variety of factors, including the standing crop of wrack and beach morphodynamics, were examined on 15 exposed sandy beaches on the southern California coast. The beaches sampled were primarily modally intermediate morphodynamic types, and three were groomed regularly. Species richness, abundance, and biomass of the macrofauna were high compared to values reported for similar beach types in other regions and were not predicted by morphodynamics or other physical factors. Overall species richness and abundance, and the species richness, abundance, and biomass of wrack-associated fauna and selected taxa were significantly correlated with the standing crop of macrophyte wrack. Wrack-associated macrofauna, such as amphipods, isopods, and insects, made up an average of >37% of the species on ungroomed beaches and comprised 25% or more of the total abundance on half of those beaches. The abundance of two shorebird species, plovers that forage using visual cues, was positively correlated with the standing crop of wrack and with the abundance of wrack-associated invertebrates. Significant differences in community structure, including depressed species richness, abundance, and biomass of macrofauna, especially for wrack-associated taxa, were associated with beach grooming and provided strong evidence for the bottom-up effects of wrack subsidies. Grooming also reduced the prey available to vertebrate predators, such as shorebirds. Substantial ecological effects of the large-scale disturbance and removal of organic material, food resources, and habitat are associated with beach grooming. These results suggest that macrophyte wrack subsidies strongly influence macrofaunal community structure, higher trophic levels, and ecological processes on exposed sandy beaches. The supply of macrophyte wrack should be considered as a factor in ecological studies of exposed sandy beaches, particularly where macrophyte production is high.

Changes in benthic community structure due to clam dredging on the Algarve coast and the importance of seasonal analysis

Patterns in community structure of meiofauna and macrofauna in relation to Portuguese clam dredging were compared during a 2-y-period off Lagos and Vilamoura, south Portugal. SCUBA divers randomly sampled corer and quadrat samples before and immediately after simulating commercial dredge fishery. Univariate measures (abundance, number of taxa, evenness, diversity and biomass) and multivariate analyses (Cluster, MDS and SIMPER) revealed changes in the meio- and macro-benthic community structure caused by dredge disturbance (short-term effects), with a general decrease in all measures. Macrofauna were found to be more sensitive to dredge disturbance, and aggressive predatory behaviour was observed after disturbance in the continuously dredged area. Nevertheless, higher significant natural changes were found between seasonal periods, indicating that dredging may only cause a small-scale and short-term impact.

Preliminary observations of the effects of dredging intensity on the re-colonisation of dredged sediments off the southeast coast of England (Area 222)

The re-colonisation of a site used for the extraction of sand and gravel for ca. 25 years off the southeast coast of the UK was examined 4 years after the cessation of dredging. Effects of different levels of dredging intensity on the rate of macrofaunal re-colonisation were investigated. Values of abundance and total numbers of species were significantly lower ( p<0.05) in an area most recently exposed to the highest level of dredging intensity compared with samples taken from an area of low intensity, and those from a reference site. Differences between previously dredged sediments and the reference location were due to the reduced abundance of a range of macrofaunal species characterising nearby sediments. Multivariate measures of community structure also indicated that there were significant differences ( p<0.01) between the macrofaunal assemblages in the areas exposed to different dredging intensities. Sediment from the area exposed to the highest dredging intensity contained proportionally more sand than other sampled sediments. The extent to which dredging intensity contributed to these differences was difficult to determine owing to the absence of any baseline data. Despite this, univariate and multivariate analyses indicated a strong relationship between macrofaunal community structure and dredging intensity at this site. Correlation analyses also demonstrated that the predominant influence on the macrofaunal community was that of the level of dredging that took place in 1995, the last year that the licensed site was dredged heavily. Preliminary observations indicated that the fauna remained in a perturbed state some 4 years after cessation of dredging. Therefore, relatively rapid ‘recovery’ rates, commonly cited as 2–3 years for European coastal gravelly areas, should not be assumed to be universally applicable. Implications for the future management and scientific study of marine aggregate extraction arising from preliminary observations on the physical and biological status of this site are discussed together with the options for selecting a reference site in the absence of baseline information.