Differences In Assemblage Composition Research Articles

Betapart: an R package for the study of beta diversity

Summary1. Beta diversity, that is, the variation in species composition among sites, can be the result of species replacement between sites (turnover) and species loss from site to site (nestedness).2. We present betapart, an R package for computing total dissimilarity as Sørensen or Jaccard indices, as well as their respective turnover and nestedness components.3. betapart allows the assessment of spatial patterns of beta diversity using multiple‐site dissimilarity measures accounting for compositional heterogeneity across several sites or pairwise measures providing distance matrices accounting for the multivariate structure of dissimilarity.4. betapart also allows computing patterns of temporal difference in assemblage composition, and its turnover and nestedness components.5. Several example analyses are shown, using the data included in the package, to illustrate the relevance of separating the turnover and nestedness components of beta diversity to infer different mechanisms behind biodiversity patterns.

Establishing expectations for pan-European diatom based ecological status assessments

The European Union (EU) Water Framework Directive depends, for effective implementation, on Member States (MSs) agreeing to a concept of the unimpacted “reference” state, which will then provide the “expected” value in Ecological Quality Ratio (EQR) calculations. Reference assemblages of organism groups will, in turn, vary, due to geological, hydrological, climatic, physicochemical and biological factors. Member States tackle this by establishing “types” which share common characteristics. However, for the purposes of ensuring consistent application, broad transboundary types were also established within five Geographical Intercalibration Groups (GIGs, referred to here as “regions”) as part of the EU's intercalibration exercise. In this paper, we evaluate these types using river diatom assemblages and also provide reference threshold values for two common metrics used in pan-European diatom assessments. A database was assembled, representing 14 EU Member States from Ireland and Portugal in the West, to Estonia and Cyprus in the East, in order to explore biogeographical patterns in assemblages unaffected by anthropogenic pressures. Multivariate analyses were used to examine this pattern and its relationship with geographic, typological and abiotic parameters. After taxonomic harmonisation, NMDS ordination of samples indicated weak differences in assemblage composition among regions. ANOSIM analyses, in turn, indicated that MS was the best factor to group similar samples whereas alkalinity, recognised as the primary environmental variable structuring diatom communities, although significant, explained less variability in the dataset. This, we believe, reflects the importance of methodological factors other than taxonomy (e.g. counting protocol, sample season) that may be constant within a MS but which vary between MSs. When two diatom metrics, the TI and IPS, were applied to the data, differences in the distribution of the metric scores between MS were generally not statistically significant even though some differences between regions were apparent. A trend of increasing values of TI (decreasing values of IPS) was observed in the sequence: Nordic<Alpine<Mediterranean<Central-Baltic<Eastern Continental regions. Additionally, some differences were observed among types within the Mediterranean and Nordic regions, though not for other regions. The data used in this exercise provides us with a region and, in some cases, a type specific benchmark dataset against which national reference data can be compared.

Locating faunal breaks in the nearshore fish assemblage of Victoria, Australia

Marine communities are frequently biogeographically structured, despite the potential for dispersal. Previous research on a variety of marine taxa in south-eastern Australia has suggested that a biogeographic break occurs along the coastline of Victoria. However, little of this research has focussed on nearshore ichthyofauna and the location of the break remains debated. Using fish abundance measured by two methods: underwater visual census (UVC); and baited remote underwater video (BRUV) at six locations along the open coast of Victoria, we examined (1) whether there is sufficient concordance among species to indicate the presence of a faunal break; and if present (2) where any such breaks occur. Differences in assemblage composition between locations were tested with analyses of similarity and examination of residuals from regressions of pairwise dissimilarities against coastline distance. Data collected using UVC revealed two large faunal breaks co-located with a habitat discontinuity, the convergence of two currents and a thermal gradient. Data collected by BRUV revealed only a gradation of change across the study region. Greater understanding of the biogeographic structure of these communities will facilitate more effective management, especially in light of anticipated range shifts in response to global climate change.

River drying lowers the diversity and alters the composition of an assemblage of desert riparian arthropods

Summary 1. Many studies have shown negative effects of river drying on in‐stream animals. However, the influence of river drying on riparian animals remains poorly studied. We examined ground‐dwelling riparian arthropod assemblages along a drying section of the semi‐arid San Pedro River in southeastern Arizona, U.S.A.2. We found strong differences in assemblage composition, taxon diversity and the abundance of key taxa between dry and flowing sites, with higher diversity and abundance of most taxa at flowing sites.3. Changes in assemblage composition, taxon diversity and abundance of representative taxa were associated with a combined measure of water availability that included distance to water and type of water. Other environmental variables showed a weaker association with changes in these arthropod assemblages.4. Thus, we found evidence that desert riparian arthropods are sensitive to river drying and to reduction in water resources. Increases in drying along this river may reduce the diversity and the abundance of many groups of ground‐dwelling arthropods, leading to marked shifts in community composition.

Effects of oil drilling activity on the deep water megabenthos of the Orinoco Fan, Venezuela

The response of a deep-water megafaunal assemblage to sedimentation disturbance from hydrocarbon drilling was investigated using remotely operated vehicle video off the Atlantic coast of Venezuela. This was the first assessment of megafauna in bathyal waters in this region. A two-way analysis of variance (ANOVA) design was used to assess patterns in density and assemblage structure both temporally, before and after the drilling event, and spatially, at different distances from the disturbance. High levels of sedimentation occurred within a radius of 20 to 50 m from the drilling site. Megafaunal densities were reduced with high levels of disturbance (from 0.60 m−2 to 0.17 m−2 &lt;20 m from the drilling site). The responses of motile and sessile fauna were different. Sessile fauna were most common (77% total) and reflected trends for total density. Motile megafaunal density was generally higher after drilling (up to double the pre-drill density). Species richness was reduced by disturbance and proximity to the disturbance. Multivariate ANOVA revealed significant differences in assemblage composition with distance and before and after drilling but no interaction. This was most likely a result of variable species-specific responses to disturbance. Megafaunal densities were generally much higher than reported densities from comparable depths in the Gulf of Mexico or from deeper locations in the Caribbean Sea. The responses to sedimentation disturbance were generally less obvious than observed elsewhere and may result from the fauna being adapted to the naturally high levels of sedimentation deriving from the Orinoco River.

Polychaete fauna inhabiting sediments associated with the stinking vase sponge (Ircinia campana Lamarck, 1814), St. John, United States Virgin Islands

As part of a general polychaete survey of St. John, United States Virgin Islands, an analysis was conducted on macrofaunal polychaete assemblages found in sediments trapped within the stinking vase sponge (Iricinia campana Lamarck, 1814) growing in shallow water at less than 4 m depth. Higher species numbers and greater abundances were found within the sponges than in adjacent sand. Polychaetes comprised over 90% of all macrofauna found in the two habitats, with 41 species within 21 families: Acoetidae, Amphinomidae, Capitellidae, Chrysopetalidae, Cirratulidae, Dorvilleidae, Eunicidae, Euphrosinidae, Glyceridae, Hesionidae, Maldanidae, Nereididae, Oenonidae, Orbiniidae, Opheliidae, Sabellidae, Sigalionidae, Spionidae, Syllidae, Terebellidae, and Trichobranchidae. Univariate statistics, community ordination (nonmetric multidimensional scaling) and a multiple response permutation procedure revealed differences in assemblage compositions between vase sponge sediments and open sand. Seven polychaete species found in vase sponges were not found in any other habitats in the survey area. Vase sponges appear to be population refugia for many soft-bottom polychaete species scarcely found or absent in adjacent reef habitats, by supplying protection from predators and currents, finer and more organically enriched sediments within the concavities, and food derived from the sponges.

Terrestrial arthropod fauna of arid areas of SE Spain: Diversity, biogeography, and conservation

The goal of this paper is to provide a general description of the terrestrial arthropod fauna of arid areas of SE Iberian Peninsula. The available data show that invertebrate communities in these environments are dominated by arthropods and that the composition of the assemblages are similar to those described in other arid regions of the world. The number of species of different arthropod groups (Coleoptera, Hymenoptera, Orthoptera, Lepidoptera, Araneae) in these environments show that arid regions of SE Iberian Peninsula are inhabited by a diverse terrestrial arthropod fauna. An ecological feature of arthropod communities in arid zones of SE Spain is the spatial and temporal variability of assemblage composition at different scales, a characteristic of faunal assemblages in arid environments. Similarity in composition of assemblages between sites (Beta diversity) shows a high variability in faunal composition between apparently similar environments. The distribution patterns of species show that 8.4–9.6% of the species in these environments are local and SE Iberian Peninsula endemics. A number of eremic Mediterranean or Palaearctic species are also associated exclusively with these environments, highlighting the singularity of the invertebrate fauna in these arid habitats. Differences in assemblage composition of arid areas with more mesic surrounding environments show that arid zones of SE Spain are inhabited by a distinct fauna, increasing faunal diversity in SE Spain. A recent analysis of endangered and threatened invertebrate species in southern Spain recognizes the arid environments of SE Spain as priority areas for conservation, where habitat loss and fragmentation, changes in agricultural and grazing uses, and inadequate management constitute the main threats. We conclude that the fauna of the SE Iberian arid zones represents an important component of the high ecological diversity of the Mediterranean ecosystem in SE Spain, not only because of their arthropod diversity but also due to the important implications that these areas have for conservation in the entire SE region of the Iberian Peninsula.

Testing a new classification of morphological functional groups of marine macroalgae for the detection of responses to stress

Morpho-functional group classifications have been widely used in studies concerning benthic macroalgal communities, due to their easy application and the reduced cost and workload compared with species-level identifications. However, their effectiveness in representing spatio-temporal patterns and describing the effect of disturbances has been disputed. We propose a new, expanded classification of morphological functional groups based on thallus structure, growth form, branching pattern, and taxonomic affinities. This approach was tested on subtidal macroalgal assemblages of the western Mediterranean. We examined the differences in composition of assemblages subjected to different types of stressors using species, traditional morphological groups, and new morphological groups. Results showed that the new morphological groups allowed to detect patterns highlighted through species analyses generally better than the use of traditional morpho-functional groups. The new morphological groups may represent a valid tool in monitoring programs and environmental impact assessments, where it is important to detect early stages of environmental changes using methods that allow to examine a large number of samples in a limited time.

Agricultural matrix provides modest habitat value for ants on mixed farms in eastern Australia

Agricultural development has contributed to the global erosion of biodiversity. The farmed matrix in agricultural landscapes can and must be important for the conservation of biodiversity and provision of ecosystem services, but this assumes that the matrix has biodiversity value. We investigate the contribution of pastures and crops to ant diversity on mixed farms in eastern Australia. Remnant native woodlands, pastures of native grasses, sown pastures of exotic species, and crops were sampled for epigaeic ants on 3 farms using pitfall trapping. Ants were sorted to species and assigned to functional groups. Ant species richness and functionality followed consistent patterns across the three farms. Significant differences in assemblage composition were found between the major habitat types, and in species richness between woodland and non-woodland habitats (native and sown pastures, and crops), which did not contribute appreciably to farm-level biodiversity: 1–10% of species were found only in the farmed matrix. Insect conservation in agricultural landscapes is important for the provision of ecosystem services, including pest control and the maintenance of soil condition. As the farmed matrix makes only a modest contribution to farm-scale biodiversity, appropriate management of the unfarmed parts of the landscape is critical and habitat restoration may be warranted where the level of native vegetation is low. Maintaining a mix of land uses within the production matrix will also be a necessary bet-hedging strategy in a world with changing climates, commodities, community expectations and farming practices.

Anthropogenic disturbance and landscape patterns affect diversity patterns of aquatic benthic macroinvertebrates

Measures of species diversity are valuable tools for assessing ecosystem health. However, most assessments have addressed individual sites or regional taxon pools, with few comparisons of differences in assemblage composition within or among regions. We examined the effects of anthropogenic disturbance on local richness (a diversity) and species turnover (b diversity) of benthic macroinvertebrates in small streams within and between 2 ecoregions (Northern Piedmont vs Southeastern Plains ecoregions) of the Patuxent River basin (Maryland, USA). Regional species pools did not differ between ecoregions (Piedmont = 166 taxa, Plains = 162 taxa); however, local richness was lower in the Plains (mean = 17.4 taxa/stream) compared to the Piedmont (mean = 22.2 taxa/stream). When streams were categorized into disturbance classes (low, medium, high), local richness did not differ among categories for either region. However, at the entire Patuxent scale, local richness tended to decrease with % impervious cover in a watershed. Variation in species composition, analyzed with nonmetric multidimensional scaling (nMDS), differed significantly between Piedmont and Plains streams, and Plains streams had higher b diversity than Piedmont streams. When partitioned by disturbance category and region, b diversity differed only between the low-disturbance sites (Plains . Piedmont). Relationships between b diversity and environmental variables varied by region. b diversity was weakly negatively related to % row-crop cover in a watershed at the entire Patuxent scale. For the Piedmont region, b diversity tended to decrease with % forest, % pasture, and % row-crop cover in a watershed. Such negative relationships between b diversity and landuse variables indicate a possible homogenization of the assemblage. The incongruence between diversity measures and composition measures, together with differing effects of anthropogenic land use on b diversity in the 2 regions, emphasizes the need to incorporate both a and b diversity and regional environmental factors in conservation/land management studies.

A century later: Long-term change of an inshore temperate marine fish assemblage

There is compelling evidence that European marine fish assemblages have undergone extensive changes in composition over the last century. However, our knowledge of which species have changed in abundance and body size distributions, and the reasons for these changes, is limited due to a paucity of historical data. Here we report a study of long-term change in a marine fish assemblage from the inshore waters of the Western English Channel, near Plymouth. We compiled data from historic trawls undertaken between 1913 and 1922, and resurveyed those sites in 2008 and 2009. Our results revealed highly significant temporal differences in assemblage composition, but the scale of change was not consistent among taxonomic groups. Dramatic changes were recorded within the elasmobranchs, characterised by a reduction in abundance of all skate (Rajiidae) species, apparent extirpation of the angel shark ( Squatina squatina), and large increases in the abundance of lesser-spotted catshark ( Scyliorhinus canicula). By contrast we observed less evidence of change among ‘flatfishes’ (Pleuronectiformes) or ‘roundfishes’ (other teleosts). Changes were also observed in length–frequency distributions, with a significant decline in the size distribution of elasmobranchs (excluding S. canicula), but no significant change in size distributions of either group of teleosts. These data provide further evidence that larger, slow-maturing species have undergone declines in UK waters over the last century, and form useful benchmarks for assessment of future changes in this coastal faunal assemblage.

Saproxylic beetle assemblages on low stumps, high stumps and logs: Implications for environmental effects of stump harvesting

The aim of this study was to evaluate how future harvesting of spruce low stumps as a renewable energy source, might influence the saproxylic (wood living) beetle fauna. Species richness, abundance and assemblages of saproxylic beetles were compared on low stumps, high stumps and logs of spruce on 10 clear-cuts in northern Sweden. The sampling was conducted with emergence traps and a total of 929 individuals and 120 species were collected. No significant differences in beetle abundance or species richness were detected between substrate types. However, there were clear differences in assemblage composition between all substrate types. Our results suggest that low stumps created at final felling support as many species and individuals per volume dead wood as other dead wood substrates and that they are therefore overlooked as substrates for wood living organisms. Consequently, harvesting of low stumps might have negative effects on the diversity of saproxylic beetles, as up to 80% of the dead wood remaining on clear-cuts is in the form of low stumps. Future studies should evaluate if compensatory measures can be performed to minimize negative impacts.

Simulated climate change in dry habitats: do spiders respond to experimental small-scale drought?

Ground invertebrates such as spiders react to changing conditions in their terrestrial environments. Due to climate change, changes of species diversity, community composition and ecological traits (e.g., habitat specialization) can be assumed. Since it is often impossible or impracticable to carry out large-scale investigations concerning the impact of microclimate change on soil arthropods, studies on responses of arthropod communities to simulated climate change at a smaller scale may be a useful alternative. I conducted a field experiment to detect potential changes in species richness, community structure and ecological traits of spiders caused by prolonged drought. In a semi-dry grassland/Juniperus communis heath complex, five 16-m 2 plots were subjected to either a drought (excluding all rain) or non-drought treatment. Activity densities of spiders were measured using pitfall traps from July to September, 2008. Although differences in microclimate between treatments were significant, no significant treatment effect on either species richness or activity densities was found. Ordination analyses (NMDS) and multivariate analysis of variance (MANOVA) revealed no significant difference in assemblage composition between the treatments, nor were any changes in ecological traits detected. Spiders were not a suitable model group for detecting any changes in the present study, but comparable experiments yielded changes for at least some spider families and especially for microarthropods. For future small-scale studies I recommend a multi-species group approach with micro- and macroarthropods, using a broad spectrum of sampling techniques.

Squat lobster assemblages on seamounts differ from some, but not all, deep‐sea habitats of comparable depth

AbstractThis study was carried out to test the hypothesis that benthic communities on seamounts are distinct from those of other deep‐sea habitats at comparable depths. Analysis of the squat lobster fauna of deep‐sea habitats in the Southwestern Pacific revealed that the species composition of assemblages on seamounts was not statistically dissimilar from assemblages on slope and plateau habitat at comparable depths. However, compositional differences were observed between seamount and rise and ridge habitat. Differences in assemblage composition between seamount and ridge habitat were statistically significant for two of the four ridge systems examined. Assemblages on seamounts that were distinct from non‐seamount ridge habitat were typically dominated by small‐bodied species with an abbreviated larval stage. Various environmental variables were correlated with the observed assemblage patterns observed; depth‐related variables may account for differences between seamount and rise assemblages, whilst differences in POC flux likely play a role in determining the assemblage compositional patterns between seamount and non‐seamount ridge habitat. Extensive pre‐analysis data treatment was required to ensure that multivariate analyses of assemblage data from seamount and non‐seamount habitats were robust. Our results confirm the findings of recent studies that found no compositional differences in assemblages from seamount and slope habitats, and support the idea that dissimilarity between seamount assemblages on different ridge systems increases with geographic distance. Further research will be required before the generality of these findings can be confirmed.

Long‐term dynamics of bird diversity in forest and suburb: decay, turnover or homogenization?

AbstractAim Urbanization and deforestation are important drivers of biodiversity change. However, long‐term changes in faunal communities within urbanizing regions are poorly understood. We investigated how well observed community changes in both space and time agree with expectations based on current paradigms in urban ecology.Location Greater Brisbane region, Australia.Methods We compared bird assemblages in two time‐periods 15 years apart, at multiple sites in remnant forest and residential suburbs across an urbanizing landscape. Differences in assemblage composition, species abundances and functional groupings were assessed within and between habitats.Results Compared with forest, suburbs in both time‐periods had over twice the total bird abundance, a different species composition, greater between‐site community similarity, a greater proportion of non‐native species and greater dominance by large‐bodied species. These differences corresponded with changes in sites whose habitat was converted from forest to suburb. Between time‐periods, abundances of 58% of suburban species changed significantly compared with those of 11% in forest. Increaser species outnumbered decreasers in suburbs, with the reverse in forest. Abundance of small‐bodied birds decreased 70% in suburbs and 20% in forest. Broad‐spectrum competitors and nest predators were common among suburban increasers. Among invasive species, the number of increasers was counterbalanced by decreasers. Both site‐scale species richness and between‐site community similarity increased to a small extent in both habitats.Main conclusions Species composition and ecological function of suburban bird communities were very dynamic. Suburban assemblages were neither a subset of forest species nor an increasingly non‐native compilation. Communities in large forest patches were comparatively stable. The notion of habitat‐specific species turnover better characterizes the nature of most changes than either species decline or homogenization, even though both of these were evident. There is considerable scope for careful urban planning, focused on both among‐ and within‐habitat variety, to sustain bird diversity in urbanizing landscapes.

Harpoons and travellers

Two ethnographic collections of whale bone tools from Tierra del Fuego and the Strait of Magallanes are analysed and compared with contemporary specimens recovered from archaeological contexts in the region. The comparison reveals clear differences in assemblage composition, methods of production and tool size. In the ethnographic collections barbed harpoon heads not only constitute the most common tools, but are typically made on a more limited selection of anatomical parts, and typically shaped using metal rather than stone tools as well as being significantly larger than their archaeological counterparts. The history of the ethnographic collections is reviewed as a prelude to discussing the differences in detail. We offer an hypothesis to explain them based on Scheinsohn's (1990–1992) observations that these ethnographic tools were no longer manufactured for their use as hunting weapons, but were produced for exchange with Europeans in the 19th and early 20th century. The differences in design and prod...

Spatial and seasonal variations in benthic algal assemblages in streams in monsoonal Hong Kong

Samples from stone surfaces were collected in pools within four unpolluted hillstreams (two shaded and two unshaded) in monsoonal Hong Kong (lat. 23 degrees N) to elucidate the extent of spatial (within and among streams) and temporal (seasonal) variations in algal biomass and assemblage composition. Sampling continued for over 12 months, incorporating the dry season when streams were at baseflow, and the wet season when spates were frequent. We anticipated that algal biomass would be lower in shaded streams and during the wet season, with associated seasonal differences in assemblage composition or relative abundance of different growth forms (e. g. erect versus prostrate). Benthic chlorophyll a (a proxy for algal biomass) varied among streams from an annual mean of 11.0-22.3 mg m(-2). Dry-season standing stocks were 18% higher than during the wet season when spate-induced disturbance reduced algal standing stocks. Algal biomass varied significantly at the stream scale, but not at the pool scale, and was lower in unshaded streams, where standing stocks may have been limited by high densities of algivorous balitorid loaches (mainly Pseudogastromyzon myersi). An overriding effect of grazers on algal biomass could also have reduced variations resulting from spate-induced disturbance. Significant differences in assemblage composition among streams, which were dominated by diatoms and cyanobacteria (totally 82 taxa) were not systematically related to shading conditions. Seasonal variations in algal assemblages were statistically significant but rather minor, and did not involve major shifts in composition or growth form caused by spate-induced disturbance. The abundance of filamentous cyanobacteria in all the streams may have been due to 'gardening' by balitorid loaches that removed erect or stalked diatoms and favoured cyanobacteria that persist through basal regeneration of filaments. This explanation requires validation through manipulative experiments.

Conservation importance of limestone karst outcrops for Palaeotropical bats in a fragmented landscape

Limestone karst landscapes are important for biodiversity yet are increasingly threatened by development activities such as mining. Furthermore, karsts are often scattered and isolated by agriculture, and are rarely considered in landscape planning because of a paucity of biodiversity data. We determined the conservation significance of an isolated limestone karst outcrop for insectivorous bats by quantifying the influence of this roosting resource on local assemblage structure across a fragmented landscape in peninsular Malaysia. Using a combination of rank abundance, gradient and randomisation analyses, we demonstrate that bat assemblages at nine forest sites are structured following a spatial gradient of increasing distance from a karst roosting resource. The assemblage at our karst site was dominated by a superabundance of three cave-roosting species, two of which were also found to dominate assemblages up to 11 km away. Cave-roosting bats exhibited a significant decay in abundance related to the distance from karst, with sites closest to karst also characterised by a rarity of tree cavity/foliage-roosting species that were otherwise common. Gradient analysis revealed that differences in assemblage composition were largely associated with the distance from the karst and, to a lesser extent, forest isolation and area. Our findings suggest that isolated karst outcrops can serve as important population reservoirs for cave-roosting bats, which subsidise diversity levels in forest fragments that might otherwise be expected to decline over time. While conservation efforts need to focus on maintaining large areas of connected forest, landscape management needs to ensure protection of karsts as point resources for cave-roosting bats.

Species identity, diversity and microbial carbon flow in reassembling macrobenthic communities

Intense disturbance may locally destroy patches of habitat and shape the landscape into a mosaic of reassembling communities. The development of ecosystem properties during such community reassembly is poorly understood. In intertidal bare sediments, trophic relations between microphytobenthos or heterotrophic bacteria and macrofauna invertebrates may guarantee fundamental ecosystem properties such as carbon flow through the food web. We studied the dynamic relation between reassembling macrofauna communities and such microbial carbon flow during recovery after severe disturbance. We deliberately induced prolonged hypoxia in winter and early summer and allowed recolonisation for periods of two and five months. Carbon flow was quantified from basal resources (microphytobenthos and bacteria) to intermediate consumers using13C as a tracer. Within the period of study (5 months), microbial carbon flow fully recovered, although macrofauna diversity was still very low compared to the natural communities (ranging from 6 to 17 species). More than 90% of microbial carbon flow to macrofauna was due to the consumers that recolonised within two months. Two of these species were dominant contributors to microphytobenthos carbon transfer to fauna. Furthermore, at an early stage of reassembly, this ecosystem property was remarkably similar when disturbance took place at different times of the year (winter or early summer), although there were differences in assemblage composition and functional diversity. We conclude that species assemblages and ecosystem function developed relatively independently in this benthic system. We discuss which ecological factors may have caused such non‐parallel development of macrofaunal communities and carbon flow.

The distribution of megabenthic, invertebrate epifauna in the Balearic Basin (western Mediterranean) between 400 and 2300 m: Environmental gradients influencing assemblages composition and biomass trends

The distribution of megabenthic epifauna (invertebrates) in the Balearic Basin (western Mediterranean) has been analyzed at depths between 427 and 2265 m after compiling samplings performed in 1985–1992 and 2007–2008 with an OTSB-14 bottom trawl. 84 epibenthic taxa of invertebrates (excluded decapod crustaceans) were collected. Epibenthic assemblages were organized in five groups (n-MDS analyses) as a function of increasing depth: upper slope assemblage, U, hauls between 427 and 660 m; middle slope assemblages M1 and M2, hauls between 663–876 m and 864–1412 m, respectively; lower slope assemblages L1 and L2, hauls between 1488–1789 m and 1798–2265 m, respectively). We found significant differences in assemblage composition between all depth-adjacent pairs of groups. Trends in the distribution of biomass vs. depth and within assemblages varied when hauls taken over insular were compared to those over mainland slopes. Over insular slopes we found (n-MDS) only four distinct depth assemblages, with significant differences between all depth-adjacent group pairs, except between L1 and L2. Over the mainland slope, two peaks of biomass situated at U (427–660 m) and at L1 (1488–1789 m) were clearly identified, attributable to the echinoid Brissopsis lyrifera and holothurian Molpadia musculus at U and to the synallactid holothurian Mesothuria intestinalis at L1. The distribution of biomass vs. depth on insular slopes did not follow this pattern, showing no significant biomass peak below 1000 m and a total biomass an order of magnitude lower than adjacent to the mainland. After compiling available environmental data over the mainland slope off Barcelona, we found coincidence between the peak biomass of Mesothuria intestinalis and: i) a significant increase of labile OM (%OrgC, C/N, hydrolizable aminoacids–EHAA, and the EHAA/THAA-total hydrolizable aminoacids-ratio) over 1600 m; and ii) an increase of turbidity and T at 1500–1600 m in February 2008. We suggest that such OM inputs must likely be associated to the formation of nepheloid layers close to submarine canyons, probably associated with oceanographic processes in deep water masses in the area. This would explain why aggregations of M. intestinalis were linked to the mainland part of the Balearic basin, with highest densities located south of canyons. If hotspots of biomass as cited here for M. intestinalis are regulated by factors such as river inputs, both natural climatic changes (e.g. changes in rainfall regimes) and human impact (e.g. river damming) may affect deep-Mediterranean communities below 1000 m.