Diversity,abundance and composition in macrofaunal molluscs from the Ross Sea (Antarctica): results of fine-mesh sampling along a latitudinal gradient

The Latitudinal Gradient Program (2002–2011) aimed at understanding the marine and terrestrial ecosystems existing along the Victoria Land coast (Ross Sea), an area characterized by strong latitudinal clines in environmental factors. During the program’s voyage of the Italian RV “Italica” in 2004, a fine-mesh towed gear, the “Rauschert dredge”, was deployed for the first time at 18 stations in four latitudinal distinct shelf areas between ~71°S and ~74°S. The collected samples contained undescribed species and new records for the Ross Sea from a variety of different marine taxa. Here, we describe the molluscan fauna and investigate evidences for latitudinal effects on molluscan diversity, abundance and assemblage composition. No significant latitudinal trends were detected: while diversity did not vary significantly with latitude, species richness showed an apparent but non-significant decrease with increasing latitude. Beta-diversity was found to be high both within and between latitudinally distinct shelf areas. A large fraction (~20 %) of the collected molluscs corresponded to new species records for the Ross Sea or undescribed species. Rarity in Antarctic molluscan occurrences was confirmed, with singletons (i.e. species represented by only a single individual) accounting for a 22 % and uniques (i.e. species occurring in one sample only) for a 43.5 % of the total presence. Our study of the smaller macrofaunal benthic fraction showed that Antarctic marine research still has far to go to have robust reference baselines to measure possible changes in benthic communities, even in the case of the assumed well-known, well-sampled and well-studied group of Ross Sea shelf molluscs. We advocate the use of fine-mesh trawling gears for routine sampling activities in future Antarctic expeditions to assess the full marine biodiversity.     

The role of notothenioid fish in the food web of the Ross Sea shelf waters: a review

The Ross Sea, a large, high-latitude (72–78°S) embayment of the Antarctic continental shelf, averages 500 m deep, with troughs to 1,200 m and the shelf break at 700 m. It is covered by pack ice for 9 months of the year. The fish fauna of about 80 species includes primarily 4 families and 53 species of the endemic perciform suborder Notothenioidei. This review focuses on the diet and role in the food web of notothenioids and top-level bird and mammal predators, and also includes new information on the diets of artedidraconids and bathydraconids. Although principally a benthic group, notothenioids have diversified to form an adaptive radiation that includes pelagic and semipelagic species. In the southern Ross Sea, notothenioids dominate the fish fauna at levels of abundance and biomass >90% and are, therefore, inordinately important in the food web. Antarctic krill (Euphausia superba) and mesopelagic fishes are virtually absent from the shelf waters of the Ross Sea. Of the four notothenioid families, nototheniids show the most ecological and dietary diversification, with pelagic, cryopelagic, epibenthic and benthic species. Neutrally buoyant Pleuragramma antarcticum constitutes >90% of both the abundance and biomass of the midwater fish fauna. Most benthic nototheniids are opportunistic and feed on seasonally or locally abundant zooplanktonic prey. Artedidraconids are benthic sit-and-wait predators. Larger bathydraconids are benthic predators on fish while smaller species feed mainly on benthic crustaceans. Channichthyids are less dependent on the bottom for food than other notothenioids. Some species combine benthic and pelagic life styles; others are predominantly pelagic and all consume euphausiids and/or fish. South polar skuas, Antarctic petrels, Adélie and emperor penguins, Weddell seals and minke and killer whales are the higher vertebrate components of the food web, and all prey on notothenioids to some extent. Based on the frequency of occurrence of prey items in the stomachs of fish, bird and mammal predators, P. antarcticum and ice krill E. crystallorophias are the key species in the food web of the Ross Sea. P. antarcticum is a component of the diet of at least 11 species of nototheniid, bathydraconid and channichthyid fish and, at frequencies of occurrence from 71 to 100%, is especially important for Dissostichus mawsoni, Gvozdarus svetovidovi and some channichthyids. At least 16 species of notothenioids serve as prey for bird and mammal predators, but P. antarcticum is the most important and is a major component of the diet of south polar skua, Adélie and emperor penguins and Weddell seals, at frequencies of occurrence from 26 to 100%. E. crystallorophias is consumed by some nototheniid and channichthyid fish and can be of importance in the diet of emperor and Adélie penguins, although in the latter case, this is dependent on location and time of year.Unlike the linear phytoplanktonE. superbaconsumers of the E. superba food chain hypothesized for much of the Southern Ocean, the food web of the Ross Sea shelf is non-linear, with complex prey-predator interactions. Notothenioid fish play a key role: as predators, they occupy most of the trophic niches available in the ecosystem, relying on benthic, zooplanktonic and nektonic organisms; as prey, they are important food resources for each other and for most top predators living and foraging on the shelf. They also constitute the major link between lower (invertebrates) and higher (birds and mammals) levels of the food web. This is especially true for P. antarcticum. Along with E. crystallorophias, its ecological role in the Ross Sea is equivalent to that of myctophids and E. superba elsewhere in the Southern Ocean.     

Polychaete assemblages of the northwestern Ross Sea shelf: worming out the environmental drivers of Antarctic macrobenthic assemblage composition

We have limited understanding of which environmental factors structure the distribution patterns and composition of Antarctic macrobenthos assemblages, and the spatial scales on which such factors operate. In 2004, the “BioRoss Survey” was conducted on the northwestern Ross Sea shelf between Cape Adare and Cape Hallett in depths of 50–750 m to describe and quantify the assemblage patterns of benthic macroinvertebrates. In order to determine the influence of primary productivity, disturbance and habitat heterogeneity on the distribution and composition of the macrofaunal assemblages, polychaete data derived from 52 grab samples were analysed. Although differences in the composition of polychaete assemblages among different sampling transects and depth strata were not particularly pronounced (yet statistically significant), the results suggested that large-scale differences in both primary productivity and iceberg disturbance influence distribution patterns. The combination of sediment chl a content, sorting coefficient, sponge spicule content and distance to the nearest iceberg scour best explained polychaete assemblage patterns. This finding supports previous contentions that multiple environmental drivers working at varying scales influence Antarctic shelf assemblages. The results do not supply support for a pronounced decoupling of pelagic and benthic systems, as has been suggested by another study of deeper water benthic assemblages on the Ross Sea shelf.     

Composition and distribution of the peracarid crustacean fauna along a latitudinal transect off Victoria Land (Ross Sea,Antarctica) with special emphasis on the Cumacea

The following study was the first to describe composition and structure of the peracarid fauna systematically along a latitudinal transect off Victoria Land (Ross Sea, Antarctica). During the 19th Antarctic expedition of the Italian research vessel “Italica” in February 2004, macrobenthic samples were collected by means of a Rauschert dredge with a mesh size of 500 μm at depths between 85 and 515 m. The composition of peracarid crustaceans, especially Cumacea was investigated. Peracarida contributed 63% to the total abundance of the fauna. The peracarid samples were dominated by amphipods (66%), whereas cumaceans were represented with 7%. Previously, only 13 cumacean species were known, now the number of species recorded from the Ross Sea increased to 34. Thus, the cumacean fauna of the Ross Sea, which was regarded as the poorest in terms of species richness, has to be considered as equivalent to that of other high Antarctic areas. Most important cumacean families concerning abundance and species richness were Leuconidae, Nannastacidae, and Diastylidae. Cumacean diversity was lowest at the northernmost area (Cape Adare). At the area off Coulman Island, which is characterized by muddy sediment, diversity was highest. Diversity and species number were higher at the deeper stations and abundance increased with latitude. A review of the bathymetric distribution of the Cumacea from the Ross Sea reveals that most species distribute across the Antarctic continental shelf and slope. So far, only few deep-sea records justify the assumption of a shallow-water–deep-sea relationship in some species of Ross Sea Cumacea, which is discussed from an evolutionary point of view.     

Comparative feeding ecology of four sympatric skate species off central California, USA

The big (Raja binoculata), California (R. inornata), longnose (R. rhina), and sandpaper (Bathyraja kincaidii), skates are commonly found on soft-bottom regions of the central California continental shelf and upper slope. The feeding ecology of this assemblage was compared to evaluate the degree of trophic separation among species, based on the results of previous species-specific diet studies. Specimens were collected from fishery independent trawl surveys conducted during September 2002–March 2003 at depths of 9–536 m. Using single and compound measures, diet composition of small (≤60 cm TL) and large (>60 cm TL) individuals were compared within continental shelf (≤200 m) and slope (>200 m) regions using traditional, multivariate, and novel techniques. Diet compositions within size classes were similar in both regions. Diet compositions between size classes generally differed, however, with fishes more important and crustaceans (especially shrimps and euphausiids) less important in the diets of large individuals. Crabs contributed substantially to skate diet compositions on the shelf, but were uncommon prey items at deeper depths, probably because of their relative scarcity in slope waters. Conversely, cephalopods were common prey items at slope depths, but were rarely ingested at shelf depths. The studied skate assemblage appears to consist primarily of generalist crustacean and fish predators that exhibit high dietary overlap at similar sizes. It is possible that resource competition among skates and groundfish species has been reduced because of considerable recent declines in the biomass of upper trophic level groundfishes. Skates may therefore play important roles in contemporary benthic food web dynamics off central California.     

Impact of iceberg scouring on macrobenthic communities in the high-Antarctic Weddell Sea

UW-video guided multibox corer sampling in- and outside iceberg scours provided quantitative macrozoobenthos samples for analyses of effects of grounding icebergs on infaunal benthic communities. These studies were performed on the southeastern Weddell Sea shelf off Kapp Norvegia and Austasen. Based on the UW-video sequences, stations were grouped a priori into two different "disturbance categories" and into undisturbed areas.Average biomass of major taxa in the cores of undisturbed areas was significantly higher (14,716.5 g wet weight m^-2) than in old (405.3 g w. wt. m^-2) or in young scour marks (9.2 g w. wt. m^-2). The habitat taxon richness, too, was highest in undisturbed areas (on average, 11.8 taxonomic units occurred per core), decreased in old scour marks (9.0) and was lowest in young scours (6.8). In undisturbed areas, a higher developed community structure was reflected by a greater variety of taxonomic groups, some of which were principally absent in scours.In young scours, the number of taxa was significantly reduced. Motile forms such as echinoderms and crustaceans, mainly amphipods, and juvenile polychaetes, in particular pioneering species of the family Spionidae, started the recolonization of the devastated areas. Burrowing organisms occurred in older scours where the initially overcompacted sediment had softened. In the course of the re-establishment of macrofaunal communities (after some years/decades), the faunal composition is expected to change towards a "normal" dominance of suspension-feeding organisms, mainly sponges and bryozoans, being typical for wide areas on the southeastern Weddell Sea shelf.A more detailed taxonomical approach, using 167 polychaete species as representatives of the macrozoobenthos, also revealed significant differences between undisturbed and affected areas. The mean abundances (784, 389, and 242 ind. m^-2, respectively), as well as habitat species richness (11.6, 5.5, and 3.1, respectively), decreased from undisturbed areas to old to young iceberg scours. Similarly, a large variety of motile and sessile forms was encountered among the polychaetes at undisturbed sites, whereas in scours, the polychaete fauna was impoverished in terms of species richness, abundance and variety of feeding types and lifestyles.     

Composition, abundance and stratification of soft-bottom macrobenthos from selected areas of the Ross Sea shelf (Antarctica)

Composition, abundance and stratification of soft-bottom macrobenthos were studied at three selected sites on the Ross Sea shelf (Antarctica) with different geomorphology and sedimentation regimes. Sites A (southwest Ross Sea, 810 m depth) and B (Joides basin, 580 m depth) were characterized by biogenic mud and clay sediments, whereas site C (Mawson bank, 450 m depth) featured sandy sediments mixed with a conspicuous biogenic component characterized by shells and tests of calcareous invertebrates (mainly barnacles of the genus Bathylasma). The macrofauna of sites A and B was mainly composed of infaunal polychaetes and bivalves. The assemblages comprised both surface and sub-surface deposit feeders, including some conveyor-belt polychaetes (Maldanidae and Capitellidae) that are responsible for high sediment mixing and bioturbation. The macrobenthos of site C was dominated by crustaceans, polychaetes and echinoderms (ophiuroids), and mainly by filter feeders and epifaunal or interstitial forms. Abundances were higher (up to 1040 ind. m⁻²) at site B than at sites A and C (430 and 516 ind. m⁻², respectively). At sites A and B the benthos was mainly concentrated in the upper 5 cm of the sediment, and abundances declined sharply in the deeper sediment layers. These results indicate a high degree of consistency between sediment features and benthic community structure, which are both strongly related to local hydrography and bottom dynamics. Sites A and B represent areas where the organic input to the seafloor by vertical sedimentation from the upper water column is high. Site C, however, is characterized by high sediment instability and food particles advecting mainly horizontally. The community is more physically controlled by unpredictable, and probably frequent, disturbance events (e.g., bottom turbid currents, sediment reworking and displacement). Individuals were relatively small, indicating that probably they are not able to grow up to the adult size and reproduce. The community may be represented by “pseudopopulations” depending on the settlement of larvae invading from neighbouring areas. Accepted: 23 October 1998     

Impact of hard-bottom substrata on the small-scale distribution of fish and decapods in shallow subtidal temperate waters

The micro-scale spatial distribution patterns of a demersal fish and decapod crustacean assemblage were assessed in a hard-bottom kelp environment in the southern North Sea. Using quadrats along line transects, we assessed the in situ fish and crustacean abundance in relation to substratum types (rock, cobbles and large pebbles) and the density of algae. Six fish and four crustacean species were abundant, with Ctenolabrus rupestris clearly dominating the fish community and Galathea squamifera dominating the crustacean community. Differences in the substratum types had an even stronger effect on the micro-scale distribution than the density of the dominating algae species. Kelp had a negative effect on the fish abundances, with significantly lower average densities in kelp beds compared with adjacent open areas. Averaged over all of the substrata, the most attractive substratum for the fish was large pebbles. In contrast, crustaceans did not show a specific substratum affinity. The results clearly indicate that, similar to other complex systems, significant micro-scale species–habitat associations occur in northern hard-bottom environments. However, because of the frequently harsh environmental conditions, these habitats are mainly sampled from ships with sampling gear, and the resulting data cannot be used to resolve small-scale species–habitat associations. A detailed substratum classification and community assessment, often only possible using SCUBA diving, is therefore important to reach a better understanding of the functional relationships between species and their environment in northern temperate waters, knowledge that is very important with respect to the increasing environmental pressure caused by global climate change.     

Temporal and spatial variability of mobile fauna on a submarine cliff and boulder scree complex: a community in flux

The mobile component of a community inhabiting a submarine boulder scree/cliff was investigated at Lough Hyne, Ireland at dawn, midday, dusk and night over a 1-week period. Line transects (50 m) were placed in the infralittoral (6 m) and circumlittoral (18 m) zones and also the interface between these two zones (12 m). The dominant mobile fauna of this cliff consisted of echinoderms (6 species), crustaceans (10 species) and fish (23 species). A different component community was identified at each time/depth interval using Multi-Dimensional Scaling (MDS) even though both species diversity (Shannon–Wiener indices) and richness (number of species) remained constant. These changes in community composition provided indirect evidence for migration by these mobile organisms. However, little evidence was found for migration between different zones with the exception of the several wrasse species. These species were observed to spend the daytime foraging in the deeper zone, but returned to the upper zone at night presumably for protection from predators. For the majority of species, migration was considered to occur to cryptic habitats such as holes and crevices. The number of organisms declined during the night, although crustacean numbers peaked, while fish and echinoderms were most abundant during day, possibly due to predator-prey interactions. This submarine community is in a state of flux, whereby, community characteristics, including trophic and energetic relationships, varied over small temporal (daily) and spatial (m) scales.     

Underwater video observation of the Antarctic toothfish Dissostichus mawsoni (Perciformes: Nototheniidae) in the Ross Sea, Antarctica

A towed benthic camera system captured images of a ca. 66-cm-long Antarctic toothfish partially under a boulder at 76°30'S, 174°59'E (depth 454 m) in the southern Ross Sea. This is noteworthy because, excluding McMurdo Sound, there are only two published records for this species in the Ross Sea and none in an offshore locality. Adult Dissostichus mawsoni are neutrally buoyant and live and feed in the lower reaches of the water column. Benthic perching is unexpected, suggesting that this subadult is not neutrally buoyant.     

Characterization of organic matter in cross-margin sediment transects of an upwelling region in the Campos Basin (SW Atlantic, Brazil) using lipid biomarkers

Lipid biomarkers [fatty acids (FAs), sterols and alcohols] and total organic carbon (TOC) were analyzed in 48 surface (0–2 cm) sediment samples collected twice (winter 2008/2009 and summer/2009) in two transects ranging from 25 to 3,000 m depths. This sampling array encompassed the major upwelling region in the southeastern Brazilian continental margin, where the river influence is probably minimal. The objectives were (1) to evaluate the sources, transport and major areas of organic matter (OM) accumulation in the continental margin and (2) to identify the fraction of OM that is potentially available to secondary benthic producers. As expected from the regional oceanographic characteristics, lipids derived from primary and secondary autochthonous producers (0.073–5.3 mg gTOC^?1) made the major fraction of the sedimentary OM, whereas lipids from allochthonous sources (0.043–0.40 mg gTOC^?1) and from bacteria (<0.01–0.43 mg gTOC^?1) were of relatively less importance. The accumulation of OM in the sediments was highly dependent on the coupling of physical (hydrodynamics) and biological (response to upwelling) factors. It was found that while some restricted areas in the shelf was a sink of labile OM, the export of this material to the upper and middle slope (400–1,000 m depths) can represent an important source of bioavailable OM to the deep sea benthic community.     

Invasion of amphisteginid foraminifera in the Adriatic Sea

Amphisteginid foraminifera are larger symbiont-bearing foraminifera that are abundant in tropical and subtropical reef and shelf regions of the world’s oceans. There is now unequivocal evidence that climate change has led to an expansion of tropical belts. As temperatures rise, surface isotherms are shifting poleward and habitat ranges are moving towards higher latitudes. Fueled by substantial warming, the Mediterranean has been identified as one of the most severely affected areas. This study documents for the first time that amphisteginid foraminifera have now crossed the Strait of Otranto and invaded the eastern Adriatic coast along southern Albania. Sampling on the opposite side along the southern part of the Italian boot shows, however, that amphisteginid foraminifera have not yet colonized the western coast of the Adriatic. The asymmetric invasion of the Adriatic displays spatial heterogeneity, where the progressive colonization follows the major surface currents with a northerly flow along the eastern coast and a southerly return along the western coastline. Previous sampling along Ionian Sea sample sites provides a baseline chronology allowing computations of range expansion rates. The observed recent rate of range expansion in Amphistegina is computed between 4.0 and 10 km/year^?1 and provides strong support for current species model projections. Based on the new data, the range boundary shift is projected to lead to a total northward range expansion of 5.2° latitude in the year 2100. The latitudinal range extension computed for the Mediterranean is thus almost twice as large as those computed for the southern hemisphere. The ongoing range extension into new areas of the Mediterranean Sea is shown to trigger changes in community structures with potential consequences for ecosystem functioning. Numerical abundances and impacts of invasive amphisteginids are greatest at shallow depths <20 m. Evidence is now accumulating that the invasion of amphisteginid foraminifera results in (1) a loss of benthic foraminiferal biodiversity, (2) alterations of foraminiferal community structures and (3) shifts in abundances of functional groups of foraminifera.     

Are mesophotic coral ecosystems distinct communities and can they serve as refugia for shallow reefs?

We analyzed an extensive dataset of over 9000 benthic and suprabenthic species found throughout the Gulf of Mexico (GoMx) to assess whether mesophotic coral ecosystems represent distinct assemblages and evaluate their potential to serve as refugia for shallow reef communities. We assessed community structure of the overall benthic community from 0 to 300 m via non-metric multidimensional scaling (NMDS) of species presence across depth bands. We used the Jaccard index of similarity to calculate the proportion of shared species between adjacent depth bands, measure species turnover with depth, and assess taxonomic overlap between shallow reefs versus progressively deeper depth bands. NMDS ordinations showed that the traditionally defined mesophotic range (30–150 m) as a whole is not a distinct community. In contrast, taxonomically distinct communities, determined by hierarchical clustering, were found at 0–70, 60–120, 110–200, and 190–300 m. Clustering highlighted an important separation in the benthic community at ~60 m, which was especially important for actinopterygian fishes. Species turnover between adjacent depths decreased with depth for all taxa combined and individual taxa, with peaks at ~60, 90–120, and 190–200 m. Fishes showed lower turnover from shallow to upper mesophotic depths (0–50 m) than all taxa combined, a substantial peak at 60 m, followed by a precipitous and continued decline in turnover thereafter. Taxonomic overlap between shallow (0–20 m) and progressively deeper zones declined steadily with depth in all taxa and individual taxa, suggesting that mid- and lower mesophotic habitats have less (but not inconsequential) potential to serve as refugia (60–150 m, 15–25% overlap with shallow habitats) than upper mesophotic zones (30–60 m, 30–45% overlap with shallow habitats) for all taxa combined. We conclude that the traditional mesophotic zone is home to three ecological communities in the GoMx, one that is confluent with shallow reefs, a distinct mesophotic assemblage spanning 60–120 m, and a third that extends onto the outer continental shelf.     

福建洛阳河口潮间带大型底栖甲壳动物群落及其功能群研究

2013年8月、2013年10月、2014年1月和2014年4月分别在洛阳河口潮间带选取5种生境(红树林区Q1,Q2,Q3;光滩Q4,林缘光滩Q5)开展调查,以探讨不同生境间甲壳动物群落结构及功能群差异和影响因素。调查共获得甲壳动物18科35种,优势种和重要种有弧边招潮(Uca arcuata)、薄片蜾蠃蜚(Sinocorophium lamellatum)、宁波泥蟹(Ilyoplax ningpoensis)和秀丽长方蟹(Metaplax elegans),以穴居型植食者为主;不同生境间优势种存在明显变化,游泳型甲壳动物主要出现在生境Q1、Q2和Q3。洛阳江红树林生境Q1、Q2、Q3底栖甲壳动物物种多样性和功能群复杂性高于林缘光滩和光滩生境,而生物量和栖息密度则相反。通过对5种不同生境的物种数、生物量、密度、功能群、物种多样性指数进行生境-季节间双因素方差分析发现,不同生境间与季节间甲壳动物物种数、栖息密度、功能群、均匀度指数与多样性指数存在显著差异;生物量在生境间差异不显著,丰富度指数在季节间差异不显著;生境Q1、Q2和Q3差异不显著。而底栖甲壳动物群落相似性和非度量多维度标序(n MDS)分析表明林缘光滩Q5的甲壳动物群落结构与Q1、Q2、Q3的相似度高,与光滩生境相似度较低,这可能是因为红树林、林缘光滩、光滩三者生境的复杂程度、食物丰富程度和食物来源存在差异有关。综合分析表明生境复杂程度与季节是影响洛阳河口潮间带大型底栖甲壳动物分布的主要因素。     

互花米草入侵对红树林底栖甲壳动物和鱼类功能群及生态位的影响

为了解互花米草入侵对红树林湿地生态系统底栖甲壳动物和鱼类的影响,本研究利用地笼方式于2020年8月、2021年1月和4月对福建漳江口2个原生红树林样地(白骨壤和秋茄)、2个互花米草入侵样地以及1个光滩样地的底栖甲壳动物和鱼类进行采样调查。采用相对重要性指数、Shannon多样性指数、Pielou均匀度指数和Margalef丰富度指数分析底栖甲壳动物和鱼类的优势种及物种多样性,统计其功能群,并计算其生态位宽度和生态位重叠指数。结果表明: 3个季节共采集到底栖甲壳动物和鱼类种类37种,隶属于2门2纲8目17科,其中大多数为暖水性和广盐性物种,且以肉食性和杂食性功能群为主。双因素方差分析和非度量多维尺度分析结果表明,相比光滩,互花米草入侵后,底栖甲壳动物和鱼类的群落多样性指数变化不显著,但功能群发生了显著变化,浮游生物食性功能群以及肉食性和植食性功能群物种均有一定程度的增加。相比红树林样地,互花米草样地内底栖甲壳动物和鱼类物种丰富度更高。优势物种的时空生态位宽度变化范围在0～1.4186,其中棱鮻最高(1.4186),其次是中华乌塘鳢(1.0168)、刀额新对虾(0.9469)、脊尾白虾(0.8922)。     

Distribution and abundance of demersal fishes and of some other marine benthic organisims on the continental shelf (0-60 m) of French Guiana

In order to establish the distribution and abundance of demersal fishes and of some other benthic organisms on the continental shelf of French Guiana (western central Atlantic), ninety-five bottom trawls were effected at depths ranging from 0 to 60 m in October 1993. A total of one hundred and ten different species were identified including eight decapod crustaceans, two cephalopods, one sea turtle and ninety-nine fishes. Despite the high species richness in benthic macrofauna, most species showed a low count, and only twelve of them comprised more than 80% of total captures. This study has mainly highlighted the existence of a well-pronounced bathymetric zonation. This phenomenon would appear to be linked to environmental factors, especially salinity and substrate granulometry, which are greatly conditioned by fresh water and sediment flowing in large rivers such the Amazon River. The results revealed three distinct faunistic assemblages: a littoral (or shallow-water) community (0-30 m) distributed in coastal estuarine waters characterized by muddy bottoms, a middle-shelf community (30-50 m), and a lower-shelf community (depths > 50 m) distributed in deeper marine waters characterized by sandy bottoms.     

The seaward limit of wave effects on coastal macrobenthos

Exposed sandy coasts are predominantly physically controlled environments where benthic communities are structured by the independent response of species to the physical environment, with minimal biological interactions (swash exclusion hypothesis). This prevalence of physical control may be regarded as a typical property of exposed coastal areas. In an offshore direction, the importance of wave effects on the benthos will diminish until a depth is reached where they are no longer significant [wave exclusion hypothesis (WEH)]. This loss of a coastal property may be used to define an offshore depth limit of the coastal zone. We used a large set of benthos data from the SE North Sea to test whether an offshore limit of the coast can be clearly recognised despite strong small-scale variability and how this limit would vary seasonally and from year to year. In accordance with WEH, both species density and total abundance of macrobenthos were low in the surf zone, strongly increased with depth, and averaged over all sampling dates became relatively constant below 30 m depth. Seasonally, these gradients were weaker during summer recruitment than during autumn. Species richness, by contrast, showed no significant difference with depth. In single years, the depth of the turning point from increasing abundances to constant abundances varied between 20 and 31 m (equivalent to 40–80 km off the coastline) depending on wave height. We conclude that this zone can be derived from benthic community gradients.     

Spatial distribution patterns of ascidians (Ascidiacea: Tunicata) on the continental shelves off the northern Antarctic Peninsula

Ascidians (Ascidiacea: Tunicata) are sessile suspension feeders that represent dominant epifaunal components of the Southern Ocean shelf benthos and play a significant role in the pelagic–benthic coupling. Here, we report the results of a first study on the relationship between the distribution patterns of eight common and/or abundant (putative) ascidian species, and environmental drivers in the waters off the northern Antarctic Peninsula. During RV Polarstern cruise XXIX/3 (PS81) in January–March 2013, we used seabed imaging surveys along 28 photographic transects of 2 km length each at water depths from 70 to 770 m in three regions (northwestern Weddell Sea, southern Bransfield Strait and southern Drake Passage), differing in their general environmental setting, primarily oceanographic characteristics and sea-ice dynamics, to comparatively analyze the spatial patterns in the abundance of the selected ascidians, reliably to be identified in the photographs, at three nested spatial scales. At a regional (100-km) scale, the ascidian assemblages of the Weddell Sea differed significantly from those of the other two regions, whereas at an intermediate 10-km scale no such differences were detected among habitat types (bank, upper slope, slope, deep/canyon) on the shelf and at the shelf break within each region. These spatial patterns were superimposed by a marked small-scale (10-m) patchiness of ascidian distribution within the 2-km-long transects. Among the environmental variables considered in our study, a combination of water-mass characteristics, sea-ice dynamics (approximated by 5-year averages in sea-ice cover in the region of or surrounding the photographic stations), as well as the seabed ruggedness, was identified as explaining best the distribution patterns of the ascidians.