Stable oxygen and carbon isotopes of live benthic foraminifera from the Bay of Biscay: Microhabitat impact and seasonal variability

We determined the stable oxygen and carbon isotopic composition of live (Rose Bengal stained) benthic foraminifera (> 150 μm size fraction) of seven taxa sampled along a downslope transect between 140 to 2000 m water depth in the Bay of Biscay. At the five stations, Hoeglundina elegans, Cibicidoides pachydermus, Uvigerina peregrina, Uvigerina mediterranea preferentially occupy shallow infaunal niches, whereas Melonis barleeanus and Uvigerina elongatastriata occupy an intermediate infaunal microhabitat, and Globobulimina spp. live in a deep infaunal niche close to the zero oxygen boundary.When compared with δ¹⁸O values of calcite formed in equilibrium with bottom waters, U. peregrina forms its test in close equilibrium with bottom water δ¹⁸O. All other foraminiferal taxa calcify with a constant offset to calculated equilibrium calcite. There is no systematic relationship between the foraminiferal microhabitat depth and the Δδ¹⁸O between foraminiferal and equilibrium calcite. We calculated correcting factors for the various taxa, which are needed for constructing multispecies-based oxygen isotope records in paleoceanographic studies of the study area.The δ¹³C values of foraminiferal taxa investigated in this study do neither record bottom water δ¹³C_DIC in a 1 : 1 relationship nor with a constant offset, but appear to be mainly controlled by microhabitat effects. The increase of δ¹³C values of shallow infaunal taxa with increasing water depth reflects the decrease of the exported flux of organic carbon along the bathymetric transect and early diagenetic processes in the surface sediment. This is particularly the case for the shallow infaunal U. peregrina. The δ¹³C values of deep infaunal Globobulimina spp. are much less dependent on the exported organic matter flux. We suggest that the Δδ¹³C between U. peregrina and Globobulimina spp. can shed light on the various pathways of past degradation of organic detritus in the benthic environments.At a station in 550 m water depth, where periodic eutrophication of sediment surface niches was demonstrated previously, we performed a two-year seasonal survey of the isotopic composition of foraminiferal faunas. No marked seasonal changes of the stable carbon isotopic composition of shallow, intermediate and deep infaunal foraminiferal taxa were observed. Thus, the δ¹³C values of foraminiferal individuals belonging to the > 150 μm fraction may result from rather long-term calcification processes lasting for several weeks or months, which limit the impact of ephemeral ¹²C enrichment of shallow infaunal niches on the isotope chemistry of adult individuals during eutrophic periods. Only highly opportunistic taxa reproducing or calcifying during phytoplankton bloom periods and the subsequent deposits of phytoplankton remains in the benthic environment may exhibit a particularly low δ¹³C, indicative of such short productive periods.     

Live (Rose Bengal stained) and dead benthic foraminifera from the oxygen minimum zone of the Pakistan continental margin (Arabian Sea)

Live (Rose Bengal stained) and dead benthic foraminiferal communities (hard-shelled species only) from the Pakistan continental margin oxygen minimum zone (OMZ) have been studied in order to determine the relation between faunal composition and the oxygenation of bottom waters. Samples were taken from 136 m to 1870 m water depth during the intermonsoon season of 2003 (March–April). Live foraminiferal densities show a clear maximum in the first half centimetre of the sediment only few specimens are found down to 4 cm depth. The faunas exhibit a clear zonation across the Pakistan margin OMZ. Down to 500 m water depth, Uvigerina ex gr. U. semiornata and Bolivina aff. B. dilatata dominate the assemblages. These taxa are largely restricted to the upper cm of the sediment. They are adapted to the very low bottom-water oxygen values (≈ 0.1 ml/l in the OMZ core) and the extremely high input of organic carbon on the upper continental slope. The lower part of the OMZ is characterised by cosmopolitan faunas, containing also some taxa that in other areas have been described in deep infaunal microhabitats. The contrast between faunas typical for the upper part of the OMZ, and cosmopolitan faunas in the lower part of the OMZ, may be explained by a difference in the stability of dysoxic conditions over geological time periods. The core of the OMZ has been characterised by prolonged periods of stable, strongly dysoxic conditions. The lower part of the OMZ, on the contrary, has been much more variable over time-scales of 1000s and 10,000 years because of changes in surface productivity and a fluctuating intensity of NADW circulation. We suggest that, as a consequence, well-adapted, shallow infaunal taxa occupy the upper part of the OMZ, whereas in the lower part of the OMZ, cosmopolitan deep infaunal taxa have repeatedly colonised these more intermittent low oxygen environments.     

Sponge assemblages of the deep Weddell Sea: ecological and zoogeographic results of ANDEEP I–III and SYSTCO I expeditions

The aim of this study is to analyze the community structure, ecology and distribution of deep Weddell Sea sponge faunas. Analysis was performed on the basis of sponges sampled during ANDEEP I–III and SYSTCO I expeditions (2002–2008) by RV Polarstern. The material obtained comprises about 800 sponge specimens, representing 129 species, within these are 95 species of demosponges (including 15 new to science), 25 hexactinellid species (7 new) and 9 calcarean species (5 new). Sponges were sampled at 51 stations in depths between 500 and 5,500 m. At most stations, sponge densities were very low, and many species are represented by one or two specimens only. Community structure by Bray–Curtis similarity was analyzed as well as depth range and spatial distribution of the most common species. Zoogeographic affinities of sampled faunas are analyzed. Three associations of sponges are found in the deep Weddell Sea: (1) The Polymastia/Tentorium community, (including Rossella associations) distributed on the lower shelf and continental slope. (2) The Bathydorus community, distributed on the continental slope and upper abyssal. (3) The Caulophacus community, associated with Cladorhizidae, is characteristic for the abyssal plains. The associations follow each other successively both bathymetrically and geographically, from shallow to deep, from shelf and ridge structures into the open abyssal. A distinct faunistic boundary between shelf and deep sea is not present. In general, the sponge fauna of the deep Weddell Sea is regionally restricted and shows stronger affinities only to the sponge fauna of the subantarctic islands.     

Seed bank,biomass, and productivity of Halophila decipiens, a deep water seagrass on the west Florida continental shelf

One of the largest contiguous seagrass ecosystems in the world is located on the shallow continental shelf adjacent to the west coast of Florida, USA and is comprised of seasonally ephemeral Halophila decipiens meadows. Little is known about the demography of the west Florida shelf H. decipiens, which may produce 4.56 × 10⁸ g C day⁻¹ or more during the peak growing season. We documented seagrass distribution, biomass, and productivity, and density of sediment seed reserves, seedlings, flowers and fruits on the southeastern portion of the west Florida shelf by sampling along a transect at three stations in 10, 15, and 20 m water depth. Biomass, flower, fruit, seedling, and seed bank densities tended to be highest at stations in 10–15 m water depth and lowest at 20 m. Flowers and fruit were most prevalent during summer cruises (June and August 1999, July 2000). Seedling germination occurred during summer, fall (October 1999), and winter (January 2000) sampling events, with the highest seedling densities present during the winter. Seed bank density remained consistent through time. A Category I hurricane with sustained winds of 120 km h⁻¹ passed over the stations, but only limited impact on H. decipiens biomass was observed. The presence of a persistent seed bank provides for recovery after storm disturbance, annual reestablishment of populations, and continual maintenance of the 20,000 km² of deep water seagrass habitat present on the west Florida shelf.     

Factors influencing the distribution of Subantarctic deep-sea benthic foraminifera,Campbell and Bounty Plateaux,New Zealand

We investigate the combination of environmental factors that influence the distribution patterns of benthic foraminiferal tests (> 63 μm) in a topographically varied region crossed by both the Subtropical and Subantarctic Fronts, south-east of New Zealand. Seafloor sample sites, extending from outer shelf (50 m) to abyssal (5000 m) depths, are bathed by five different water masses, and receive phytodetritus from Subtropical, Subantarctic and Circumpolar surface water masses. Eight mappable associations are recognised by Q-mode cluster analysis of the benthic foraminiferal census data. Similar associations are identified using cluster analysis based solely on the presence or absence of species. Canonical correspondence analysis and a correlation coefficient matrix were used to relate the faunal data to a set of environmental proxies. These show that factors related to water depth (especially decreasing food supply with increasing depth) are the most significant in determining the overall foraminiferal distribution. Other contributing factors include surface water productivity and its seasonality; bottom water ventilation; energetic state of the benthic boundary layer and resulting substrate texture; and bottom water carbonate corrosiveness. Three shallow-water associations (50–700 m), dominated by Cassidulina carinata, Trifarina angulosa, Globocassidulina canalisuturata, Gavelinopsis praegeri, and Bolivina robusta, occur in coarse substrates on the continental shelf, and on the crests and upper slopes of four seamounts under well-oxygenated, high energy regimes, and high food input. Three mid bathyal to upper abyssal associations (500–3300 m), dominated by Alabaminella weddellensis, C. carinata, and Epistominella exigua, occur in biopelagic sandy mud, beneath a region of strongly seasonal food supply, with their composition influenced by total food flux, ventilation (Oxygen Minimum Zone), and bottom current strength. An unusual lower bathyal association (1200–2100 m), dominated by T. angulosa and Ehrenbergina glabra, occurs in a belt of coarser sandy substrate that runs along the crest of the submarine plateaux slopes beneath the strongly-flowing Subantarctic Front-related currents. A deep abyssal association (3500–5000 m), dominated by Nuttallides umbonifer and Globocassidulina subglobosa, occurs on the abyssal plain beneath oligotrophic lower Circumpolar Water south-east of the Subantarctic Front and is strongly influenced by the cold, carbonate-corrosive conditions.     

The distribution of living benthic foraminifera on the continental slope and rise off southwest Norway

Surface sediment samples taken by ? corer from 45 stations on the Norwegian continental margin and in the Norway Basin have been investigated for their benthic foraminiferal content. Unlike previous studies, the living benthic foraminiferal fauna was differentiated from empty tests comprising the foraminiferal death assemblage. Factor analysis of both the living and dead faunal data reveals six living species assemblages and five corresponding dead assemblages. The additional living assemblage is characterized by the arenaceous speciesCribrostomoides subglobosum that dominates between 1400 and 2000 m water depth, but is rare in the dead faunal data.Trifarina angulosa and, to a lesser extent,Cibicides lobatulus characterize the shallowest foraminiferal assemblage from 200 to 600 m water depth, where it is associated with strong bottom currents and warm, saline Atlantic water of the North Atlantic Drift. On the slope between 600 and 1200 m water depth, theMelonis zaandami Species Assemblage dominates, particularly in areas characterized today by rapid sedimentation of terrigeneous material. Between 1000 and 1400 m depth, where the slope is covered by fine grained, organic-rich, terrigeneous mud, the living foraminiferal assemblage is characterized byCassidulina teretis andPullenia bulloides. Below 1400 m, three foraminiferal assemblages are found:C. subglobosum is found from 1400 to 2000 m,Cibicidoides wuellerstorfi andEpistominella exigua predominantly live from 2000 to 3000 m water depth, and below 3000 m,Oridorsalis umbonatus andTriloculina frigida dominate the fauna.All of theElphidium excavatum tests found in this study and theCassidulina reniforme tests found above 500 m water depth were found to be reworked.Analysis of the sediment grain-size distribution and the organic carbon content in surface samples from the deepest stations suggest that the abundance ofC. wuellerstorfi andE. exigua is positively correlated to relatively coarse (caused by planktic foraminifera) and organic-rich sediments, whereas high frequencies ofO. umbonatus andT. frigida coincide with low organic carbon content. We suggest thatC. wuellerstorfi is adapted to deep-sea environments with relatively high food supply, tolerating relatively low interstitial water oxygen content, whereasO. umbonatus may tolerate lower food supply prefering well-oxygenated interstitial waters.     

Carbon and oxygen isotope geochemistry of live (stained) benthic foraminifera from the Aleutian Margin and the Southern Australian Margin

Comparisons of ambient bottom-water geochemistry and stable isotopic values of the tests of living (stained) calcareous benthic foraminifera from the North Pacific (on the Aleutian Margin, water depth 1988 m) and Murray Canyons group in the Southern Indian Ocean (Australian Margin, water depths 2476 m and 1634 m) provide modern environmental analogs to calibrate paleoenvironmental assessments. Consistent with the hypothesis that microhabitat preferences influence foraminiferal isotopic values, benthic foraminifera from both margins were depleted in ¹³C with respect to bottom-water dissolved inorganic carbon (DIC). The carbon isotope values of deep infaunal foraminifera (Chilostomella oolina, Globobulimina pacifica) showed greater differences from estimates of those of DIC than shallow benthic foraminifera (Bulimina mexicana, Bolivinita quadrilatera, Pullenia bulloides). This study provides new isotopic and ecological information for B. quadrilatera. The mean Δδ¹³C value, defined as foraminiferal δ¹³C values minus estimated ambient δ¹³C values from the Aleutian Margin, is 0.97‰ higher for G. pacifica than the mean from the Murray Canyon. This difference may result either from genetic or biological differences between the populations or from differences in environmental isotopic influences (such as pore water differences) that were not accounted for in the equilibrium calculations. These analyses provide calibration information for the evaluation of bottom water conditions and circulation patterns of ancient oceans based on fossil foraminiferal geochemistry.     

The surface ocean productivity response of deeper water benthic foraminifera in the Atlantic Ocean

We test the relationship of deep sea benthic foraminiferal assemblage composition to the surface ocean productivity gradient in the low latitude Atlantic Ocean using 81 surface sediment samples from a water depth range between 2800 and 3500 m. The samples are selected so that the surface ocean productivity gradient, controlling the flux of organic carbon to the seabed, will be the most important environmental variable. The first two principal components of the assemblage data account for 73% of data variance and are clearly linked to the productivity gradient across the Atlantic. These components show that under higher productivity the assemblages contain a higher abundance of Uvigerina peregrina, Melonis barleeanum, Globobulimina spp. and other taxa with probable infaunal microhabitats. Alabaminella weddellensis, a species linked to episodic phytoplankton debris falls, is also important in these assemblages. As productivity decreases there is a regular shift in assemblage composition so that low productivity assemblages are dominated by Globocassidulina subglobosa and several Cassidulina species along with Epistominella exigua. We hypothesize that these taxa are epifaunal to very shallow infaunal since nearly all organic carbon oxidation occurs near the sediment-water interface in low productivity settings. Discriminant function analysis of the foraminiferal assemblages, with groups selected on the basis of surface ocean productivity, shows clear separation among five productivity levels we used. This analysis demonstrates that productivity variations have a strong influence on assemblage composition. Finally, we used two groups of samples from the Rio-Grande Rise representing water depths from 2007 to 2340 m and 2739 to 3454 m to test for effects produced by changing water depth. All these samples are from a low productivity region and represent nearly identical environmental conditions. Although the low productivity nature of all the Rio-Grande Rise samples is obvious, there are assemblage differences between our depth groups. We cannot account for the assemblage differences with changes in organic carbon flux, dissolution effects or other physical/chemical properties of the ocean. Thus there are as yet unidentified factors related to water depth which cause some assemblage variation in the low productivity setting we investigated.     

Distribution of recent foraminiferal assemblages near the Ombrone River mouth (Northern Tyrrhenian Sea, Italy)

Benthic foraminiferal assemblages in 127 samples, collected at water depth, ranging between 15 and 184 m on the Southern Tuscany continental shelf off the Ombrone River were analysed. Statistical analyses (Cluster and Principal Component Analysis) performed on the 48 most abundant species (>5%) and sedimentological data led to the identification of three clusters related to the size of grain sediment (sand, silt or clay). Q-mode cluster analysis singled out six groups, corresponding to six distinct foraminiferal assemblages: a typical infralittoral assemblage (15-39 m) on sandy silt, clayey silt or silty clay, dominated by Ammonia and Elphidium species, with Eggerelloides scabrus, Rectuvigerina phlegeri and Valvulineria bradyana; a second assemblage (24-78.5 m) associated with vegetated environments or sandy bottoms (Elphidium crispum, Rosalina bradyi, Asterigerinata mamilla, Neoconorbina terquemi, and Tretomphalus concinnus); a third assemblage recognised between 30 and 90 m water depth and characterised by the dominance of the opportunistic species V. bradyana (related to silty bottoms), with Bulimina marginata, R. phlegeri, Ammonia inflata and Ammonia beccarii as other common taxa; an upper circalittoral assemblage (70-100 m) on silty clays, containing B. marginata, Cassidulina carinata and V. bradyana; a lower circalittoral assemblage (95-177 m) on clayey bottoms, with B. marginata, Textularia bocki and Uvigerina mediterranea; and finally, a second lower circalittoral assemblage (104-184 m) on clayey sediments, dominated by two Uvigerina species (U. mediterranea and Uvigerina peregrina), with Sphaeroidina bulloides and B. marginata. The typical V. bradyana assemblage, characterised by relatively low diversity and high dominance, marks the most eutrophicated area running parallel to the coast. The spatial distribution of assemblages is closely associated with sea-bottom sedimentary environments and bathymetry but it is also probably influenced by the outflow of the Ombrone River. The composition, structure and distribution of V. bradyana assemblage suggest an environmental model, useful for paleogeographic reconstruction in areas characterised by a river mouth and a closed morphological setting typical of a nutrient-trap.     

Benthic Distribution of Sewage Sludge Indicated by Clostridium perfringens at a Deep-Ocean Dump Site

Clostridium perfringens in sediment samples collected at the Deep Water Municipal Sewage Sludge Disposal Site (also called the 106-Mile Site), off the coast of New Jersey, was enumerated. The counts of C. perfringens found in sediment samples collected within and to the southwest of the 106-Mile Site were significantly elevated (P < 0.01) compared with counts of samples from reference stations of similar depth (2,400 to 2,700 m), topography, and distance from the continental shelf, indicating that the benthic environment was contaminated by sewage dumping at this site. Low counts of C. perfringens in sediment samples collected at stations between the base of the continental shelf and the 106-Mile Site indicated that coastal runoff was not a significant source of contamination. Elevated counts were observed for samples up to 92 km to the southwest, whereas low counts were obtained for samples from stations to the east of the 106-Mile Site. This distribution is consistent with previous model predictions of sludge deposition. In areas heavily impacted by sludge dumping, C. perfringens counts were generally highest in the top 1 cm of sediment and exceeded 9,000 CFU g (dry weight) of sediment^-1. The patterns of C. perfringens dispersal observed in this study have proved useful for selection of heavily impacted areas and control stations for further ecological evaluation by a multidisciplinary research team.     

Sedimentological history of Bryant Canyon area, northwest Gulf of Mexico, during the last 135 kyr (Marine Isotope Stages 1-6): A proxy record of Mississippi River discharge

Bryant and Eastern Canyons are located in northwest Gulf of Mexico, and are characterized by a complex sedimentological history related to glacioeustatic cycles, river discharges, and interactions between depositional and halokinetic processes. This study is based on detailed sedimentological analysis from forty-eight long cores from these two canyons. This paper determines the evolutionary history of the canyons and assesses the response of sedimentary processes to morphological, climatic, hydrological, and sea-level changes.During the last glaciation, the upper and middle continental slope was supplied with sediments by low density turbidity currents derived from the depositional segregation (deposition of the coarsest material in the most proximal locations) of large turbidity currents initiated on the outer shelf. The lower continental slope was supplied with sediment by westward flowing bottom currents, originated from the entrainment of the most diluted wash-load and tails of turbidity currents from the Mississippi Fan.Bryant and Eastern Canyon systems were active during the penultimate glaciation, Marine Isotope Stage (MIS) 6, and were supplied with sediments by an ancestral shelf-margin Mississippi River delta. Gravity flows transported enormous amounts of sediment to the continental slope and abyssal plain of the northwest Gulf of Mexico. The sea-level rise at MIS 5 led to confinement of river-sourced sediments to the widespread continental shelf of the northwest Gulf of Mexico, and consequently to the cessation of gravity flows. During the first 40 kyr of MIS 5, salt diapirs transformed the canyons into a network of intraslope basins.The sea level dropped to the mid-shelf during MIS 3 and 4, but never reached the shelf-break, and therefore, river-sourced sediments remained largely confined to the shelf. However, seaward sediment transportation was achieved occasionally through turbidity currents related to sediment failures, storms, and high-river discharges. Four high river discharge events have been identified during this period. The first three were centred at 37, 45, and 53 cal ka BP. The last high river discharge occurred at the end of MIS 3 (29.4-33.2 cal ka BP), and resulted in the deposition of closely-spaced, mud turbidites over the entire continental slope. The Laurentide Ice Sheet (LIS) was restricted north of the upper Mississippi River valley during 60 to ∼ 30 cal ka BP and therefore, the high river discharge events are interpreted as melt-water events, related to brief southward advancements of the LIS, which resulted in the flooding of Mississippi River. The extensive lowering of sea level during the last glacial maximum (MIS 2) resulted in the almost direct discharge of Mississippi River sediments to the upper continental slope leading to the development of abundant turbidity currents. Eleven wet-dry cycles during this period are defined; they probably originated from episodic subglacial melt-water floods, released from southern parts of the LIS.The last deglaciation event is characterized by the development of a major melt water event at 16.5-13 cal ka BP that resulted in the deposition of distinct, organic-rich sediments. At about 13 cal ka BP, the melt water discharges of the LIS in North America switched from the Mississippi River to either the St. Lawrence or Mackenzie River valleys, causing the domination of hemipelagic sedimentation on the continental slope of the northwest Gulf of Mexico. Isotopic data indicate that melt-water discharges returned to the Mississippi River Valley at ∼ 11.4 cal ka BP. The absence of any sedimentological indication on the continental slope of the northwest Gulf of Mexico of the return of the melt-water discharges to the Mississippi River is attributed to the confinement of river-sourced sediments on the continental shelf due to the rise of the sea level.     

Recruitment of the infaunal bivalve Soletellina alba (Lamarck, 1818) (Bivalvia: Psammobiidae) in response to different sediment types and water depths within the intermittently open Hopkins River estuary

Studies examining recruitment processes for soft-sediment macroinvertebrate fauna in intermittent estuaries are rare and most studies of active habitat selection have been tested in the laboratory rather than the field. The present field study examined whether recruitment of the infaunal bivalve Soletellina alba was influenced by water depth and sediment particle size in the intermittent Hopkins River estuary, southern Australia. The number of recruits in sediment trays differed between water depths, but active habitat selection was not evident across treatments of varying sediment particle size. The use of sediments with varying particle sizes also provided an opportunity to identify potential discontinuities in body-size distributions of recruits associated with varying habitat architecture. The length (mm) of recruits was converted to the same scale used to express sediment particle size (i.e. phi units: ? = − log₂ of sediment particle size). The size of recruits differed across water depths, but did not differ across treatments with fine (? = 3) versus coarse (? = 1) sediment, and no relationships were apparent between bivalve size and sediments consisting of varying particle size. These patterns of recruitment do not correspond with the distribution of adult S. alba within the Hopkins River estuary. Previous sampling has shown that abundances of juvenile and adult S. alba are variable across time, site and water depth, but are often greater at the deeper water depth (1.05 m below the Australian Height Datum). However, recruitment during the present study was greatest at the shallower water depth (0.05 m below AHD), and the apparent absence of active habitat selection suggests that the distribution of adults is unlikely to be attributable to differences in recruitment associated with sediments of varying particle size.     

Distribution patterns of diatom surface sediment assemblages in the Laptev Sea (Arctic Ocean)

The modern diatom distribution in the Laptev Sea, Arctic Ocean, was investigated in 89 surface sediment samples. Diatom concentrations are relatively low showing values between 0.01×10⁶ and 6.7×10⁶ valves per gram dry sediment. Based on a factor analysis using seventeen taxa or taxa groups five diatom surface sediment assemblages can be defined: the ice diatom assemblage of the central region of the Laptev Sea, the Chaetoceros assemblage of the eastern and southeastern shelf, the Thalassiosira antarctica assemblage of the continental slope and deep sea, the freshwater diatom assemblage in the vicinity of river mouths and deltas, and the Thalassiosira nordenskioeldii assemblage which shows a patchy occurrence on the central Laptev Sea shelf. The distribution pattern of diatom assemblages in surface sediment is significantly related to oceanographic conditions of surface water masses. The main factors controlling the distribution of diatoms in the Laptev Sea are the riverine freshwater input during the summer which strongly affects the salinity conditions, and the sea-ice extent. Furthermore, the composition of the Thalassiosira antarctica assemblage of the continental slope is largely influenced by dissolution and lateral transport processes.     

Spatial distribution of live benthic foraminifera in the Rhône prodelta: Faunal response to a continental–marine organic matter gradient

Benthic foraminifera were collected in the Rhône prodelta (Gulf of Lions, Mediterranean Sea), an enriched zone with high organic matter content. In June 2005, sediment cores were sampled at depths ranging from 20 to 100 m. Four distinct foraminiferal assemblages were determined in the study area, reflecting the geographical distribution of the impact of river supply. The living foraminiferal faunas present a typical picture, with strongly impoverished faunas composed exclusively of stress-tolerant taxa (Fursenkoina fusiformis, Bulimina aculeata, Leptohyalis scottii, and Adelosina longirostra) in the immediate vicinity of the river mouth. This assemblage is well adapted to a high input of continental organic matter and a minimum oxygen penetration depth into the sediment. To the southwest, under the main corridor followed by the river plume, high organic input with a dominantly terrestrial signature (more refractory) may be stressful for many taxa which need organic matter of a more labile quality. In this area, Nonion scaphum, Nonionella turgida and Rectuvigerina phlegeri are present in low densities. On the edge of this area, these taxa show much higher densities. A greater proportion of marine organic carbon could explain their increasing abundances in this area. Towards the east and towards the deepest stations, in the outer part of the enriched zone, biodiversity increases. Faunas at these stations have intermediate densities and contain a number of taxa (Cassidulina carinata, Epistominella vitrea, Valvulineria bradyana, Nonionella iridea/bradyi) at the deepest stations; Bolivina dilatata/spathulata and Textularia porrecta at the eastern stations) that seem to benefit from more marine organic matter. The comparison of geochemical measurements and foraminiferal data strongly suggests that the spatial distribution of foraminifera in the Rhône prodelta is mainly governed by the quality and the quantity of organic matter reaching the sediment–water interface. Since bottom waters are well oxygenated (215–260 µmol/L), and oxygen penetration into the sediment is less than 1 cm at all stations, benthic ecosystem oxygenation appears to have only a minor impact on regional differences in faunal distribution.     

Environmental control on the structure of echinoid assemblages in the Bellingshausen Sea (Antarctica)

The Bellingshausen Sea is one of the most remote and least surveyed seas of the Southern Ocean, so that little was known about benthic communities and those factors that determine community structuring until recently. The present work aims at characterizing the structure and spatial distribution of echinoid assemblages in the Bellingshausen Sea, as well as identifying the environmental factors that determine assemblage structuring. Echinoids were collected at 32 stations using an Agassiz trawl, at depths of 86–3,304?m, during BENTART oceanographic expeditions led in 2003 and 2006. Sediment and bottom water properties were analysed using an USNEL-type box corer and a Neil Brown Instrument System Mark III CTD, respectively. Echinoids were found at all stations, except Peter I Island. Seventeen species were identified, representing 22?% of the echinoid species present in the Southern Ocean and increasing twofold the number of species recorded in the Bellingshausen Sea so far. The echinoid fauna is dominated by the very abundant species Sterechinus antarcticus. Depth is the key factor that determines the nature of echinoid assemblages, which are mainly divided into the continental shelf, the slope and the deep-sea basin. In addition, sediment properties, namely redox values, organic matter and mud content, best match species dispersion on the shelf. Sediment properties affect echinoid distribution depending on species food range and feeding strategy. As it might be expected, sediment properties more strongly influence specialist feeders (Schizasteridae and Cidaridae) than generalists (Echinidae).     

The benthos off King George Island (South Shetland Islands,Antarctica): further evidence for a lack of a latitudinal biomass cline in the Southern Ocean

The benthic fauna off King George Island (South Shetland Islands, Antarctica) was investigated during "Polarstern" expedition ANT XV/3 in March 1998. Samples were taken along two cross-shelf/slope transects both north (Drake Passage) and south of the island (Bransfield Strait, off Potter Cove) at water depths ranging from 130 m to 2,000 m. For a quantitative inventory, a multibox corer was used at nine stations to collect mostly infaunal macrobenthos; at seven stations, seabed photography was employed concomitantly to survey the epibenthic megafauna. Macrofauna abundances ranged from 730 ind. m^–2 at 2,000 m to >14,000 ind. m^–2 at 100 m; biomass values varied between about 50 g wet mass m^–2 (6 g ash-free dry mass m^–2) at 2,000 m and about 950 g wet mass m^–2 (about 90 g ash-free dry mass m^–2) at 200 m. Densities were dominated everywhere by polychaetes, followed by bivalves, crustaceans and ophiuroids; in terms of biomass, krill and holothurians surpassed polychaetes at some stations. No significant differences between the northern and the southern transects in total abundance and biomass were obvious. Megafauna abundances were clearly higher south of King George Island, totalling about 110–150 ind. m^–2 on the shelf (235–330 m) and about 50 ind. m^–2 at the continental slope (750 m), whereas along the northern transect they reached values of only 21–31 ind. m^–2 on the shelf (130–430 m) and decreased at the continental slope (950 m) to about 5 ind. m^–2. A brittle star, Ophionotus victoriae, strongly dominated the southern-shelf epibenthos, with relative abundances of 70–95% and a biomass of about 40–80 g wet mass m^–2 (about 4–7 g ash-free dry mass m^–2), but was numerically less important at the slope (5%) where ammotheid pycnogonids prevailed (80%). Macro- and megabenthos distribution patterns were characterized by a pronounced shelf-slope gradient – in standing stock as well as in faunistic composition – but this resemblance was statistically not significant. This finding indicates that the spatial distributions of macrobenthos and megabenthos are primarily determined by a depth-dependent factor, most probably food supply, but apparently respond differently to secondary driving forces, possibly seabed features. Our results provide further evidence for the notion that there is no distinct latitudinal gradient in benthic abundance and biomass in the Southern Ocean between the South American Magellan region and high-Antarctic waters of the Weddell Sea.     

Late Holocene deep-sea benthic foraminifera from the Sea of Marmara

The benthic foraminiferal assemblages of two cores from the late-Holocene, organic-carbon-rich and carbonate-poor, deep-sea sediments of the eastern depression of the Sea of Marmara have been studied. They were deposited under high level of primary productivity and poorly oxygenated bottom-water conditions; they show low diversity and are dominated by a group of species adapted to an infaunal life style with wide bathymetric distribution (ca. 70–2000 m) in the Mediterranean Sea. Oxygen deficiency down to about 0.5 ml/l does not seem adversely to affect the rate of reproduction of the dominant species belonging to Melonis, Chilostomella, buliminids, and bolivinitids. Their distribution is primarily controlled by substrate conditions.Faunal similarities with fossil assemblages in association with some late-Quaternary sapropels and related facies from the eastern Mediterranean basins suggest that they were deposited under palaeo-oceanographic conditions closely similar to those of the modern Sea of Marmara.     

Benthic foraminiferal morphogroups of mid to outer shelf environments of the Late Jurassic (Prebetic Zone, southern Spain): Characterization of biofacies and environmental significance

Benthic foraminiferal assemblages belonging to a mixed carbonate–siliciclastic shelf succession of the Oxfordian (Upper Jurassic) in the Prebetic, southern Spain, were analyzed. The faunal data, obtained on thin sections of strongly lithified sediments, allowed the detailed differentiation of a foraminiferal morphogroups system to be applied to the interpretation of paleoenvironmental conditions. Eleven morphogroups were differentiated according to shell composition, general morphology, number of chambers and mode of coiling. Paleoenvironmental analysis combines these features with inferred life-style (epifaunal, shallow infaunal and deep infaunal) and feeding strategy (suspension-feeder, deposit-feeder, herbivores, etc.) of the foraminifera. Analogies and differences regarding previously established morphogroup systems could be outlined accordingly.

In the Prebetic Oxfordian, distribution trends of foraminiferal morphogroups allow for a comparison of mid to outer shelf environments occupying the South-Iberian paleomargin, indicating that nutrient supply was the most important factor controlling morphogroup development. The mid shelf settings are dominated by epifaunal active herbivores–phytodetritivores, in shallow to deep substrate position of the redox boundary. The outer shelf is dominated by agglutinated and infaunal detritivore-bacterial scavenger morphogroups revealing a deep substrate position of the redox boundary. Particularly favorable trophic conditions in mid shelf environments are related with higher phytodetritus input from more proximal and shallower shelf areas.     

Macrobenthic species composition and diversity in the Godthaabsfjord system, SW Greenland

The southwest Greenland coast is made up of large and deep sill fjords. On the shelf, a number of shallow banks separated by deep troughs are located 20–50 km from the coast. We collected three 0.1-m² van Veen grabs at nine stations along a transect spanning from the inner Godthaabsfjord influenced by glaciers, across the shallow Fyllas Bank and out to the slope of the continental shelf at approximately 1,000 m depth. Along this transect, we explored patterns of macrobenthic diversity, species composition, abundance and biomass. The sampled stations were very different in terms of environmental variables, resulting in large differences in species composition primarily related to differences in depth, silt–clay fraction and chl a content of the sediment (BIO-ENV analysis). Habitat differences also reduced species spatial ranges and the majority of species were found at only one (49%) or two (20%) stations and, consequently, species turnover or beta diversity was high and correlated to differences in depth, silt–clay fraction and median sediment grain size. Species richness and diversity were lowest in sites exposed to sediment disturbance: near the glaciers in the inner fjord (physical disturbance by mineral sedimentation) and at selected stations on the shelf (bioturbation by burrowing sand eel). Alpha diversity and richness were only weakly correlated to environmental parameters, indicating that alpha richness and diversity are influenced by several factors or that relationships are non-linear as was found for species richness and silt–clay fraction.