Structure Of Macrofaunal Assemblages Research Articles

Enhancing multiple scales of seafloor biodiversity with mussel restoration

Restoration projects are underway internationally in response to global declines in shellfish beds. As diverse biological assemblages underpin a variety of ecosystem services, understanding broader changes in biodiversity associated with mussel restoration becomes increasingly valuable to scientists and restoration practitioners. Studies generally show bivalve beds increase species richness and abundance, but results are scale-dependent and conditional on the mobility of specific communities observed. We examined biodiversity at multiple scales to determine how communities with varying levels of mobility are influenced by subtidal mussel restoration. Significant changes in assemblage structure were observed in both mobile fish and epifaunal communities, with enhanced species richness and total abundance of associated individuals. In contrast, we observed site-dependent effects of bivalve restoration on macrofaunal community structure and composition, with sheltered, harbour mussel bed communities numerically dominated by detritivores accustomed to organically enriched, muddy sediments. Sediment organic matter significantly increased within mussel beds, and distance-based linear models showed that sediment organic matter was an important predictor of macrofaunal assemblage structure on mussel beds, highlighting the significance of benthic-pelagic coupling and biodeposition to soft-sediment communities. This study contributes novel methods and ecological insights on the role of species mobility and site selection in structuring restoration outcomes, better informing future mussel restoration efforts aimed at emphasising functionally-driven ecosystem services.

Can short-term meteorological events alter subtropical estuarine macrobenthic assemblages in seagrass meadows (Patos Lagoon Estuary - Southern Brazil)?

Short-term meteorological events, or occluded fronts, can cause variations in the diversity and density of macrozoobenthic assemblages from a seagrass meadow and sandflat at the subtropical Patos Lagoon Estuary (32 °S; Brazil). The first occluded front (EV1), with an average wind speed of 3.71 ms−1, did not disturb the macrofaunal assemblage because seagrass dampened the effect and increased the belowground biomass, seagrass coverage and canopy height. In contrast, after the second occluded front (EV2), with an average wind speed of 3.08 ms−1, the meteorological disturbance induced a reduction in vegetation coverage and epifaunal diversity in seagrass meadows. However, the EV2 disturbance increased the infaunal density in the sandflat and the diversity in the seagrass meadow. In this way, after EV2, the macrofaunal assemblages were modified in the sandflats. Our study shows the important role of shelter and habitat complexity provided by seagrass beds and highlights that, because of vegetation coverage loss, the occluded fronts, which are becoming more intense and frequent in a global warming context, can cause changes in macrofaunal assemblage structure.

Macrofaunal assemblages from two low-energy sandy beaches within contrasting salinity environments in Northeastern Cuba

Low energy sheltered beaches occur in a wide range of coastal environments. Invertebrate macrofaunal assemblages for this beach type have been less studied compared with those at exposed sandy beaches. An estuarine and a reef-fringed beach in northeastern Cuba were sampled to assess the effects of freshwater input on the intertidal assemblages, and to describe the spatial zonation at both beach types. A total of 13 species were recorded; species richness between beaches was similar with ten species at the estuarine beach and nine species at the reef-fringed beach. Salinity and swash width differed significantly between the two beaches. Macrofaunal assemblages also differed between beaches, and the differences persisted over time. Moreover, salinity and swash width resulted in relevant predictors for differences in macrofaunal composition. Species were clustered in two main cross-shore zones corresponding with the resurgence and the retention zones. These findings contribute to the understanding of factors affecting macrofaunal communities in sandy beaches that have been under-represented in global studies. In addition, this study contributes to the knowledge of the structure and composition of macrofaunal assemblages in the less well studied beaches of Cuba.

Biodiversity, trait composition and ecological functioning: impacts of coastal urbanisation on subtropical mudflats

The world’s coastlines have become heavily modified over the last century, with the adjacent natural habitats declining in biodiversity and health under increasing pressure from urbanisation. In this study we assessed the structure and biological traits of macrofaunal assemblages from 24 south-east Queensland mudflats in order to determine whether ecological functioning (e.g. primary production, nutrient cycling) and the delivery of ecosystem services was affected by urbanisation. This work represents the most comprehensive assessment of mudflat assemblages in the region to date. The mudflats contained 50 macrofaunal taxa and so were comparable to other local intertidal systems. Summer assemblages contained more species, more individuals and had differing taxonomic composition. When indexed as a proportion of subcatchment area, urbanisation did not correspond to a clear impact on macrofaunal composition; rather, the nature of the industry or activity was critical. Mudflats from subcatchments with industries producing organic wastes significantly differed from subcatchments with <1.3% cover of these industries. Functioning was conserved in mudflats experiencing current levels of enrichment, but this may decline with growing pressure from human populations. The results of this study illustrate that large-scale spatial data, such as from satellites, can be used to detect the cumulative effects of urbanisation when the pressures are highly resolved.

Submarine canyons support distinct macrofaunal assemblages on the deep SE Brazil margin

Submarine canyons are common topographic features of continental margins where increased organic matter trapping and complex topography may enhance the abundance of benthic assemblages. This heterogeneity may result in differences in the composition of benthic assemblages and their diversity between canyon and nearby slope sediments. Here we studied and compared benthic macrofaunal assemblages of four submarine canyons and nearby slope transects (150–1300 m depth) on the E-SE Brazil margin (19–23 oS) along the Espírito Santo (ESB) and Campos Basins (CB). We asked if these submarine canyons would increase sediment total organic carbon (TOC) content and thus support higher abundances, lower diversity and dissimilar composition of macrofaunal benthic organisms if compared to open slope stations at similar depths. In general, we found limited support for a higher sediment TOC or macrofaunal abundances in canyons compared to slopes, although a steeper unimodal curve of TOC with depth in canyons was evidenced. Significant differences in sediment grain size and macrofaunal assemblages also occurred at spatial scales of basins, water depth and interactions between depth and environment (slope/canyon); highlighting the importance of intra-canyon heterogeneity processes. However, canyons and slope environments had significant differences in the structure of macrofaunal assemblages that were partially explained by water depth, TOC and mean grain size. The spatial heterogeneity within canyons and a variable proximity to continental runoff lead to a high dissimilarity of upper slope canyon assemblages indicating that these environments should be included in marine spatial conservation planning along Brazil’s EEZ in view of increasing offshore deep-sea exploration.

Effects of Tropical Saltmarsh Patches on the Structure of Benthic Macrofaunal Assemblages

Habitat complexity can affect assemblage abundance, richness and composition by providing resources and mediating ecological interactions for an array of organisms in many ecosystems. Temperate saltmarshes are well studied, but little is known about tropical saltmarshes. Here we tested if the presence of saltmarsh patches would increase macrofaunal richness, abundance and influence the structure of macrofaunal assemblages when compared to non-vegetated tidal flats. Macrofaunal abundance was higher in vegetated than in non-vegetated areas, but richness did not differ among areas. Macrofaunal assemblage structure was significantly different among areas and composition differed mainly due to greater abundances of epifaunal species in vegetated areas. Within saltmarshes, the macrofaunal composition generally had greater variability than in non-vegetated areas, likely due to greater habitat complexity generating different environmental conditions. Additionally, megalopa and juvenile individuals of Leptuca leptodactyla were more abundant in vegetated areas indicating that tropical saltmarshes may have an important role as a nursery ground for fiddler crabs. This study increased knowledge of tropical saltmarsh patterns and suggests that further research should test the ecological roles of tropical saltmarshes, such as a nursery.

Similar Epiphytic Macrofauna Inhabiting the IntroducedSargassum muticumand Native Fucoids on the Atlantic Coast of Morocco

The diversity and structure of epiphytic macrofaunal assemblages on the introduced invasive brown macroalga Sargassum muticum and native fucoids (Bifurcaria bifurcata, Cystoseira humilis, Cystoseira tamariscifolia and Sargassum vulgare) were compared over a 1-year period in intertidal rockpools on the Atlantic coast of Morocco. In comparison with the epiphytic fauna associated to other macroalgae from European shores, we found that the associated epiphytic macrofauna was moderately diverse (H'<3 bit) and was mainly dominated by gastropods and crustaceans. The most abundant taxa were the isopod Dynamene bidentata and the gastropods Steromphala umbilicalis and Steromphala pennanti. The epiphytic macrofauna community structure differed slightly between the invader and the natives along the year. There was significant temporal variation in the total number of individuals of epiphytic macrofauna per thallus dry weight for the associated epifauna with the highest abundance found on C. tamariscifolia in the spring. The composition of the epiphytic macrofauna on each macroalga species was fairly constant over time. Overall, our results demonstrated that the mobile epiphytic macrofauna on this rocky area showed low macroalgal specificity. They also revealed that the invader S. muticum supports similar epiphytic macrofaunal communities as the native brown macroalgae and provides an additional appropriate habitat for a generalist associated epiphytic macrofauna.

Assessing differences in macrofaunal assemblages as a factor of sieve mesh size, distance between samples, and time of sampling

With increasing cascading effects of climate change on the marine environment, as well as pollution and anthropogenic utilization of the seafloor, there is increasing interest in tracking changes to benthic communities. Macrofaunal surveys are traditionally conducted as part of pre-incident environmental assessment studies and post-incident monitoring studies when there is a potential impact to the seafloor. These surveys usually characterize the structure and/or spatiotemporal distribution of macrofaunal assemblages collected with sediment cores; however, many different sampling protocols have been used. An assessment of the comparability of past and current survey methods was in need to facilitate future surveys and comparisons. This was the aim of the present study, conducted off the Oregon coast in waters 25-35m deep. Our results show that the use of a sieve with a 1.0-mm mesh size gives results for community structure comparable to results obtained from a 0.5-mm mesh size, which allows reliable comparisons of recent and past spatiotemporal surveys of macroinfauna. In addition to our primary objective of comparing methods, we also found interacting effects of seasons and depths of collection. Seasonal differences (summer and fall) were seen in infaunal assemblages in the wave-induced sediment motion zone but not deeper. Thus, studies where wave-induced sediment motion can structure the benthic communities, especially during the winter months, should consider this effect when making temporal comparisons. In addition, some macrofauna taxa-like polychaetes and amphipods show high interannual variabilities, so spatiotemporal studies should make sure to cover several years before drawing any conclusions.

Are intertidal soft sediment assemblages affected by repeated oil spill events? A field-based experimental approach

This study investigates the impact of repeated diesel spills on the structure of intertidal macrofaunal assemblages of a subtropical estuary. Three frequencies of exposure events were compared against two dosages of oil in a factorial experiment with asymmetrical controls. Hypotheses were tested to distinguish between (i) the overall effect of oil spills, (ii) the effect of diesel dosage via different exposure regimes, and (iii) the effect of time since last spill. Repeated oil spills dramatically altered the overall structure of assemblages and reduced the total density of macrofauna and densities of dominant taxa. Increasing the frequency of oil spills negatively affected macrofauna. In general, frequent low-dosage oil spills were more deleterious than infrequent high-dosage ones. However, increases in densities of some taxa, mainly the gastropod Heleobia australis, were observed in response to infrequent spills. Our results highlight the importance of repeated exposure events in determining the extent of oil impacts.

Functional uniformity underlies the common spatial structure of macrofaunal assemblages in intertidal seagrass beds

Previous work has shown that the intertidal seagrass macrobenthos at three geographically and ecologically disparate localities (in the north-east Atlantic, south-west Indian and south-west Pacific Oceans) possess similar relative species occurrence distributions and uniform species densities. These common features are here demonstrated to be related to the presence in those assemblages of: (1) similar functional diversities and evennesses, (2) the same set of dominant component functional groups, and (3) similar ranked relative occurrence distributions both of those groups and of the component genera within each of the larger groups. The two lower-latitude systems were particularly similar in all these respects. Although sharing the same subset of individual functional groups, however, the relative importance of members of that subset varied from locality to locality and even within a single locality, whilst still maintaining the same ranked relative functional-group occurrence distribution. Therefore the broad structure of available macrobenthic functional roles and the relative occurrences of the component taxa in intertidal seagrass beds (and hence, granted stochastic assembly, the total numbers of taxa supported by unit area) are likely to be linked causally, although the form of the relationship is unclear.

Effects of proximity to offshore fish farms over soft-bottom macrofauna

Sea-cage fish farms impact the seabed within their immediate vicinity, potentially affecting recipient communities. We assessed whether proximity to three sea-cage fish farms at the Canary Islands altered patterns in the abundance, assemblage structure and richness of soft-bottom macrofauna. We related among-farm variability in dissimilarities in macrofaunal assemblage structure between seabeds beneath cages and controls to differences in hydrodynamics, production and seabed topography. Contrasting patterns of species abundances with varying proximity to fish farms were observed: some species decreased while other species increased their abundances with increasing distance at some farms. Although faunal assemblages at 0 m (i.e. beneath the cages) were different, in terms of assemblage structure, from those found at controls, pairwise differences in assemblage structure among distances away varied among the studied fish farms. Species richness showed inconsistent patterns with proximity to cages among fish farms. In summary, inconsistent patterns of macrofaunal assemblages with varying proximity to aquaculture facilities preclude confident predictions on the way offshore aquaculture alters macrofauna in the study region.

Benthic macrofaunal assemblages in multispecific seagrass meadows of the southern Philippines: Variation among vegetation dominated by different seagrass species

Although the influence of seagrasses on the diversity and abundance of associated fauna is generally well understood, the effects of seagrass specific identity are not. To determine whether benthic epifaunal and infaunal assemblage patterns vary by the identity of dominant seagrass species in shallow water meadows, we compared quantitative measures of faunal species richness, abundance, and assemblage structure among beds dominated by Cymodocea rotundata, Enhalus acoroides, and Thalassia hemprichii; the study site was at Lopez Jaena, Misamis Occidental Province in the southern Philippines. The Cymodocea-dominated vegetation had higher seagrass shoot density and lower seagrass biomass than those dominated by Enhalus and Thalassia. Across vegetation types, we encountered 30 and 15 species of epifauna and infauna at average densities of 1.73 and 0.82 animals/core (0.0314m2), respectively. Neither densities and species richnesses of epifauna and infauna nor species composition varied significantly by vegetation type. Multivariate analyses of macrofaunal assemblage structure and abiotic/biotic environmental factors demonstrated that seagrass aboveground biomass explained a significant proportion of the multivariate variation in epifaunal species composition (39%); none of the potential explanatory variables was related to variation in ecological parameters of the infauna. Thus, seagrass specific identity is not a good predictor of either macrofaunal abundance or diversity patterns. Although the ecological parameters of the benthic macrofauna may be influenced by seagrass biomass and structure, responses differ between epifauna and infauna.

Structure and taxonomic composition of free-living nematode and macrofaunal assemblages in a eutrophic subtropical harbour, Hong Kong

The spatial and seasonal taxonomic composition patterns of macrofauna and nematodes in a eutrophic subtropical harbour, previously suffered from sewage pollution, were studied in relation to a number of sediment parameters. In the polluted, inner-harbour area, levels of organic contents and heavy metals were high, whereas species number, abundance and diversity of nematodes and macrofauna were the lowest in comparison to the cleaner, outer-harbour area. Different taxonomic composition patterns of nematodes and macrofaunal assemblages were found between inner-harbour and outer-harbour area, which was highly correlated with sediment nutrient levels. Different responses of macrofaunal and nematode communities to sewage pollution suggested that macrofauna might be more tolerant than nematodes to eutrophic conditions due to their ability to modify the sediment. The present findings indicated the usefulness of studying both nematode and macrofaunal communities, in order to reveal different aspects of the benthic ecosystems in response to organic enrichment.

Does mollusc shell debris determine patterns of macrofaunal recolonisation on a tidal flat? Experimental evidence from reciprocal transplantations

Mollusc shells introduce complexity and heterogeneity into soft-sediment habitats and might regulate the structure and functioning of benthic assemblages. Two areas of an unvegetated tidal flat located at the mouth of Paranaguá Bay (southern Brazil) largely differ in substrata characteristics and macrofaunal assemblage structure. Sediments of the inner part of the studied site are structurally more complex than those of the peripheral portion, due to high densities of empty shells of the bivalve Anomalocardia brasiliana. The effects of this biogenic matrix on macrofaunal recolonisation during a three-month period were assessed through a reciprocal transplantation experiment of defaunated sediment between shelled and bare habitats. Hypotheses were tested to distinguish between three possible causes of variation: (i) habitat complexity, (ii) intrinsic factors of each area (regardless of the presence/absence of shells), and (iii) their interaction. Habitat structural heterogeneity provided by empty shells does not explain the large differences between macrofauna in shelled and bare habitats. Assemblages that colonised transplanted sediments became similar to those in controls at the area to which they were moved, but differed from those in controls at the area where they originated. The lack of a consistent response of assemblages to the presence of shells can be explained not only by the extremely fast recovery following small-scale experimental disturbances, but also by the dispersal mechanisms related to reproductive traits of the dominant taxa. Changes in assemblages were greatly influenced by the tanaid Monokalliapseudes schubarti, the numerically-dominant species that showed consistent differences among treatments. The reduced dispersal capacity due to the lack of planktonic larval stages associated with high fecundity, continuous reproduction and fast growth were determinant features for the rapid colonisation of defaunated sediments, regardless of habitat complexity. Despite the well-recognised impact of mollusc shell production on soft-sediment habitats, our study showed that its effects are scale-dependent, both spatially and temporally, and might be regulated by the structure of local assemblages and species-specific responses related to dispersal mechanisms.

Confounding response of macrofauna from a confluence of impacts: brine and sewage pollution

Throughout the shores of the world, a confluence of different sources of pollution is common, e.g., through outlets releasing brine, industrial, and domestic sewage. In this study, we assessed whether a combined disposal of domestic sewage and brine altered the patterns of abundance and assemblage structure of subtidal macrofauna inhabiting sandy sea beds on the south coast of Gran Canaria (Canary Islands, NE Atlantic Ocean). Samples were collected in two surveys (May 2008 and January 2009) at three distances (0, 15, and 30 m) away from the discharge point. Macrofaunal abundances showed different patterns with varying proximity from the disposal point in the two surveys. In May 2008, lower abundances were observed at 0 m (53.89±46.82 ind.) than at 15 m (120±91.7 ind.) and 30 m (283.89±189.33 ind.) away from the disposal point. In January 2009, however, higher abundances were observed at 0 m (131.33±58.69 ind.) than at 30 m (100±24.44 ind.) and 15 m (84.78±58.39 ind.) away from the disposal point. Significant differences in macrofaunal assemblage structure were found with varying proximity from the discharge point in both surveys. The effect of proximity from the disposal point was confounded by changes in sedimentary composition between surveys, which contributed to explain differences in macrofaunal abundances and assemblage structure with varying proximity from the discharge point. As a result, confounding drivers of macrofaunal assemblage structure, here, changes in sediment composition, accounted for changes in macrofaunal abundances and assemblage structure with varying proximity from a combined disposal point of brine and sewage.

Sediment community responses to marine vs. terrigenous organic matter in a submarine canyon

Abstract. The Whittard Canyon is a branching submarine canyon on the Celtic continental margin, which may act as a conduit for sediment and organic matter (OM) transport from the European continental slope to the abyssal sea floor. In situ stable-isotope labelling experiments were conducted in the eastern and western branches of the Whittard Canyon, testing short-term (3–7 days) responses of sediment communities to deposition of nitrogen-rich marine (Thalassiosira weissflogii) and nitrogen-poor terrigenous (Triticum aestivum) phytodetritus. 13C and 15N labels were traced into faunal biomass and bulk sediments, and the 13C label traced into bacterial polar lipid fatty acids (PLFAs). Isotopic labels penetrated to 5 cm sediment depth, with no differences between stations or experimental treatments (substrate or time). Macrofaunal assemblage structure differed between the eastern and western canyon branches. Following deposition of marine phytodetritus, no changes in macrofaunal feeding activity were observed between the eastern and western branches, with little change between 3 and 7 days. Macrofaunal C and N uptake was substantially lower following deposition of terrigenous phytodetritus with feeding activity governed by a strong N demand. Bacterial C uptake was greatest in the western branch of the Whittard Canyon, but feeding activity decreased between 3 and 7 days. Bacterial processing of marine and terrigenous OM were similar to the macrofauna in surficial (0–1 cm) sediments. However, in deeper sediments bacteria utilised greater proportions of terrigenous OM. Bacterial biomass decreased following phytodetritus deposition and was negatively correlated to macrofaunal feeding activity. Consequently, this study suggests that macrofaunal–bacterial interactions influence benthic C cycling in the Whittard Canyon, resulting in differential fates for marine and terrigenous OM.

Macrobenthic assemblage structure and organismal stoichiometry control faunal processing of particulate organic carbon and nitrogen in oxygen minimum zone sediments

Abstract. The Arabian Sea oxygen minimum zone (OMZ) impinges on the western Indian continental margin between 150 and 1500 m, causing gradients in oxygen availability and sediment geochemistry at the sea floor. Oxygen availability and sediment geochemistry are important factors structuring macrofaunal assemblages in marine sediments. However, relationships between macrofaunal assemblage structure and sea-floor carbon and nitrogen cycling are poorly understood. We conducted in situ 13C:15N tracer experiments in the OMZ core (540 m [O2] = 0.35 μmol l–1) and lower OMZ boundary (800–1100 m, [O2] = 2.2–15.0 μmol l–1) to investigate how macrofaunal assemblage structure, affected by different oxygen levels, and C:N coupling influence the fate of particulate organic matter. No macrofauna were present in the OMZ core. Within the OMZ boundary, relatively high abundance and biomass resulted in the highest macrofaunal assimilation of particulate organic carbon (POC) and nitrogen (PON) at the lower oxygen 800 m stations ([O2] = 2.2–2.36 μmol l–1). At these stations the numerically dominant cirratulid polychaetes exhibited greatest POC and PON uptake. By contrast, at the higher oxygen 1100 m station ([O2] = 15.0 μmol l–1) macrofaunal C and N assimilation was lower, with POC assimilation dominated by one large solitary ascidian. Macrofaunal POC and PON assimilation were influenced by changes in oxygen availability, and significantly correlated to differences in macrofaunal assemblage structure between stations. However, macrofaunal feeding responses were ultimately characterised by preferential organic nitrogen assimilation, relative to their internal C:N budgets.

Variability of macrofaunal assemblages on the surroundings of a brine disposal

Desalination plants generate large volumes of hypersaline brine, likely affecting recipient communities. We aimed to assess whether proximity to a brine discharge point altered the abundance, assemblage structure and diversity of macrobenthic fauna. Collection of samples took place twice (May 2008 and January 2009) at 0, 15 and 30m away from a brine discharge point. Total macrofaunal abundance increased with increasing distance from the brine discharge point, though the magnitude of differences was inconsistent between successive years, probably as a result of a change in particle size distribution. Proximity to the brine discharge point also altered patterns in macrofaunal assemblage structure. The macrofaunal species density was higher at 15 and 30m than at 0m. In conclusion, proximity to a brine discharge point significantly altered the ecological pattern of macrobenthic fauna, though disentangling the effect of the increase in salinity from particle size distribution remains undetermined.

Macrofaunal communities of threatened subtidal maërl seabeds on Tenerife (Canary Islands, north-east Atlantic Ocean) in summer

This study contributes with the first data on physical and taxonomical structure of macrofaunal assemblages of maerl beds from the Canary Islands. Maerl beds and Cymodocea nodosa meadows of the Canary Islands are considered biodiversity hot-spots in terms of taxonomic and functional biodiversity with a broad geographical and depth ranges. The authors have studied the structure of the macrofaunal assemblages on different habitat types (Cymodocea, Caulerpa, sabellid field, garden eel and maerl beds). Samples were taken at a range of depths between 14 and 46 m. Correlations were performed among abiotic variables (granulometry, organic matter, nitrogen and phosphates) and the most abundant taxa. Similarity analysis was performed to explore the patchiness of seabeds at a local scale. Significant differences were found in macrofaunal assemblages among seabed types, with highest abundances and lowest biodiversity in sabellid fields, where the sabellid Bispira viola dominated. The polychaetes Aponuphis bilineata and Chone filicauda and the mollusc Turritella brochii were the most abundant taxa on maerl beds. The mosaic of granulometric conditions would explain the associated macroinfaunal community structure and contribute to the creation of diversity on these relatively well preserved seabeds at a local scale.

Macrobenthic assemblage structure in a cool-temperate intertidal dwarf eelgrass bed in comparison with those from lower latitudes

The evolution and ecology of latitudinal patterns in marine macrofaunal biodiversity and assemblage structure are contentious. With the aim of investigating the occurrence of such patterns in intertidal dwarf eelgrass beds (Nanozosteraspp.), those at cool-temperate Scolt Head Island, UK (latitude 52°N), were examined and compared with equivalent systems in warm-temperate Knysna, South Africa (34°S), and subtropical Moreton Bay, Australia (27°S): systems that had earlier been examined using an identical methodology. The Scolt Head bed supported the highest faunal density (mean 6568 0.1m -2), lowest observed and estimated species richness (32 and 33 spp., respectively), lowest species diversity (Simpson's index of diversity 0.34), highest mean constancy index (23.0), lowest proportion of total numbers contributed by singletons or doubletons (0.06%), largest Berger-Parker dominance index (0.81), and least degree of variability between component samples. The beds at Knysna were intermediate between those at Scolt Head and at Moreton Bay in all those respects, as well as in nature and composition of the fauna. Similar dwarf eelgrass beds at Arcachon, France, likewise appear intermediate between those of Scolt Head and Knysna. Community variance in all three regions was greatest at the smallest spatial scale of ≤1m, at which scale two of the three sites also displayed randomly organized faunal assemblages, although this changed to non-random at larger spatial scales. Assemblages were deterministically structured at all spatial scales at Knysna. A consistent series of changes in structural characteristics of macrofaunal dwarf-eelgrass assemblages therefore occurred along the latitudinal sequence of Scolt Head (53°), Arcachon (45°), Knysna (34°), and Moreton Bay (27°). Community assembly patterns did not change with latitude, however, but did so with spatial scale in two of the three regions. © 2011 The Linnean Society of London.