Deep-sea Stations Research Articles

Foraminiferal biodiversity associated with cold-water coral carbonate mounds and open slope of SE Rockall Bank (Irish continental margin—NE Atlantic)

Cold-water coral (CWC) ecosystems are hotspots of macro- and microfaunal biodiversity and provide refuge for a wide variety of deep-sea species. We investigated how the abundance and biodiversity of ‘live’ (Rose Bengal stained) foraminifera varies with, and is related to, the occurrence of CWC on the Rockall Bank (NE Atlantic). Qualitative and quantitative analyses were performed on 21 replicate samples from 8 deep-sea stations, including 4 stations on CWC-covered carbonate mounds at depths of 567–657 m, and 4 stations on the adjacent slope at depths of 469–1958 m where CWC were absent. This sampling strategy enabled us to demonstrate that sediments surrounding the living CWC were characterised by higher foraminiferal abundance and biodiversity than open-slope sediments from the same area. A total of 163 foraminiferal species was identified. The dominant species in CWC sediments were: Spirillina vivipara, Allogromiid sp. 1, Globocassidulina subglobosa, Adercotryma wrighti, Eponides pusillus, Ehrenbergina carinata, Planulina ariminensis, Trochammina inflata and Paratrochammina challengeri. Foraminifera were nearly absent in adjacent open slope areas subject to strong tidal currents and characterised by coarse grained deposits. We suggest that CWC create a heterogeneous three-dimensional substrate offering microhabitats to a diverse benthic foraminiferal community.

ANT XXIV/2 (SYSTCO) Hexactinellida (Porifera) and bathymetric traits of Antarctic glass sponges (incorporating ANDEEP-material); including an emendation of the rediscovered genus Lonchiphora

In this study, we present the hexactinellid sponges sampled in the Weddell-Sea during the ANT 24-2 SYSTCO expedition (30.10.2007–31.01.2008) on the RV Polarstern. All deep-sea stations sampled during this expedition showed comparably modest sponge colonization, with Hexactinellida and Demospongiae occurring in similar numbers of species and individuals. The hexactinellids sampled represent three abyssal species and six species from the deep shelf. Among the deep-sea glass sponges one species new to science was found, Lonchiphora antartica sp. nov., belonging to a poorly known genus, so far represented by only a single specimen known from the Sagami Bay, Japan. Herein we give the first detailed emended diagnosis of the genus and a description of the new species representing the genus as its first well-known specimen. Ecological analysis shows three clearly differentiated associations of hexactinellid sponges in the Antarctic Ocean, which replace each other with increasing depths. Sponge communities on the shelf are dominated by Rossella spp., those on the continental slope and in the lower bathyal mostly by Bathydorus spinosus, and the abyssal associations by Caulophacus spp. No distinct geographical distribution pattern of the Weddell-Sea Hexactinellida could be observed, apart from those caused by their bathyal ranges.

Live (stained) benthic foraminifera in the Whittard Canyon, Celtic margin (NE Atlantic)

Living (Rose Bengal stained) benthic foraminifera were investigated at 18 deep-sea stations sampled in the Whittard Canyon area (NE Atlantic). The stations were positioned along 4 bathymetric transects ranging from 300 to 3000 m depth: two along the main canyon axes (Western and Eastern branches) and two along adjacent open slopes (Western and Eastern slopes). The aim of this study was to assess changes of foraminiferal standing stock, composition and microhabitat in relation to the physico-chemical conditions prevailing at and below the sediment–water interface in various canyon and open-slope environments. Minimal oxygen penetration depths and maximal diffusive oxygen uptakes were recorded at upper canyon stations, suggesting a high mineralisation rate. This is confirmed by the high phytopigment concentrations measured in the sediment of the upper canyon axes. Foraminiferal abundance was positively correlated with diffusive oxygen uptake and phytopigment concentration in the sediment. This suggests a control of organic matter fluxes on the foraminiferal communities. Foraminiferal abundance was generally higher along the canyon axis compared to open-slope sites at comparable water depths. The species composition varied with water depth along all four transects, but was also different between canyon branches and adjacent slopes. The silty/sandy intercalations at many of the deeper canyon stations may have been rapidly deposited by fairly recent gravity flows. At station 51WB (3002 m), the faunal characteristics (strong dominance, shallow infaunal microhabitats) suggest that the foraminiferal community is in an early state of ecosystem colonisation after these recent sedimentation events, which would have supplied the important amounts of phytopigments.

The state of knowledge on deep-sea nematode taxonomy: how many valid species are known down there?

All available information from literature sources dealing with deep-sea nematode species was analyzed, in order to obtain an overview of the state of knowledge in deep-sea nematode taxonomy and answer the question of how many valid nematode species are known from the deep sea so far. One hundred and twenty-seven taxonomic and ecological literature sources reported a total of 638 valid species belonging to 175 genera and 44 families, from 474 deep-sea stations at depths of 400–8,380 m. This number is less than 16% of all known marine nematode species, whereas the deep sea comprises about 91% of the ocean bottom. Of these species, 71% were initially described from the deep sea. Most of the valid species have been reported from the North Atlantic, including the Mediterranean. The rest of the World Ocean, including the Pacific, Indian, Arctic and Antarctic oceans, is considerably less studied. The largest numbers of valid species were reported from the continental slope and the abyssal plains, while information on valid species from trenches, deep-sea canyons, and seamounts is extremely scanty. Some deep-sea families are much more investigated than others in proportion to their relative species abundances in the deep sea, i.e., the percentage of valid species from these families among all valid deep-sea species is much higher than the real percentage of species from these families reported in faunistic studies (e.g., Desmoscolecidae, Comesomatidae, Sphaerolaimidae, Benthimermithidae, Leptosomatidae, and Draconematidae). On the other hand, the families Xyalidae, Oxystominidae, and Monhysteridae were recognized as the most “underinvestigated,” as, in spite of their high species abundance in the deep sea, there are quite a few taxonomic studies on these taxa. Some deep-sea nematode species were reported from two or three oceans, and can be considered probable cosmopolitan species. Some number of probable eurybathic species were also found (the difference between minimum and maximum depth was from 1 km to more than 5 km).

The paramesochrid fauna of the Great Meteor Seamount (Northeast Atlantic) including the description of a new species of Scottopsyllus (Intermedopsyllus) Kunz (Copepoda: Harpacticoida: Paramesochridae)

Because of their potentially important role for the distribution of marine benthic organisms, seamounts have been the subject of focused interest on the part of marine biologists particularly during the past decade. One of the largest seamounts sampled so far is the Great Meteor Seamount (GMS), which is located in the North Atlantic. Although some of the most detailed investigations have been carried out on this particular seamount, it is still a little-known environment regarding benthic copepod diversity and ecology. Therefore, material from 14 stations collected in 1998 was investigated to address the following aspects: (1) species composition and diversity of the harpacticoid family Paramesochridae at the GMS; (2) faunistic comparison with other localities and Intermedopsyllus; and (3) revision of the worldwide distribution of known Paramesochridae. Of the 28 paramesochrid species determined from the GMS, 26 are new to science. The vast majority were found on the plateau; only two species were detected in the deep-sea stations. Other species found at the GMS are already known from East Atlantic deep-sea areas. In the frame of providing new taxonomical information for future research, Scottopsyllus (Intermedopsyllus) antoniae sp. nov. from the plateau of the GMS is described here. The new species can without any doubt be allocated to Scottopsyllus because of the 1-segmented endopods but 3-segmented exopods in P2–P3, the “paramecium”-like shape of P2–P3 endopods, and the 1-segmented exopods in the antenna and the mandible. The new species differs from its congeners mainly by retention of setae on the antennule, the reduced size of furcal seta VI, the fact that the proximal maxillar endite bears only 1 seta, and the loss of the maxillar endopod.

Live foraminifera from the open slope between Grand Rhône and Petit Rhône Canyons (Gulf of Lions, NW Mediterranean)

We present an ecological study of live (Rose Bengal stained) foraminifera from 6 deep-sea stations sampled on the open slope between the Grand Rhône Canyon and the Petit Rhône Canyon (eastern part of the Gulf of Lions, NW Mediterranean). The 6 stations describe a bathymetric transect from ∼350 to ∼2000 m depth. The main objective of our study is to investigate the changes of the foraminiferal density, composition and microhabitat along this transect in response to the physico-chemical conditions at and below the sediment–water interface. All our observations underline the general meso-oligotrophic character of our inter-canyon open-slope setting where low-quality organic matter originating from both marine and continental sources settles. The input of organic matter at the sediment–water interface leads to a classical succession of redox reactions within the sediment. The shallowest station (∼350 m) appears as an active sedimentary environment, where coarse sediments characterized by lower-quality organic matter and biogenic material accumulate. The 550-m-deep station presents bioturbated sediments with the highest concentration of labile organic compounds. The deeper stations, between about 750 and 2000 m deep, show decreasing sedimentation rates with water depth and are characterized by a background of low-quality organic matter. The foraminiferal changes recorded along the bathymetric transect are related to a complex association of physico-chemical parameters. We think that the quality of organic matter in the surficial sediment, as expressed by the lipid concentration, is the major parameter controlling the foraminiferal distribution at our open-slope stations. From the 550- to the 2000-m-deep station, the foraminiferal standing stocks and diversity decrease with depth, as a result of the increasing scarcity of labile organic compounds at the sediment–water interface. Oxygen concentration and penetration depth and the intensity of bioturbation seem to play only a secondary ecological role. Other, putative hydro-sedimentary processes (winnowing by strong bottom currents, sand-bed deposition) appear as additional parameters controlling the foraminiferal community structure. At the 350-m-deep station, the live foraminiferal fauna can be considered as a non-equilibrium assemblage thriving in frequently disturbed and food-impoverished sediments. At the 745- and 980-m-deep stations, the occurrence of suspensivorous epibenthic/epilithic species suggests the presence of strong bottom-water current velocities and the related suspension of organic particles.

Rich and rare—First insights into species diversity and abundance of Antarctic abyssal Gastropoda (Mollusca)

Abstract The abyssal depths of the polar oceans are thought to be low in diversity compared with the shallower polar shelves and temperate and tropical deep-sea basins. Our recent study on the gastropod fauna of the deep Southern Ocean gives evidence of the existence of a rich gastropod assemblage at abyssal depths. During the ANDEEP I and II expeditions to the southern Drake Passage, Northwestern Weddell Sea, and South Sandwich Trench, gastropods were collected by bottom and Agassiz trawls, epibenthic sledge, and multicorer, at 40 stations in depths between 127 and 5194 m. On the whole, 473 specimens, corresponding to 93 species of 36 families, were obtained. Of those, 414 specimens were caught below 750 m depth and refer to 84 (90%) benthic species of 32 (89%) families. Most families were represented by a single species only. The numerically dominant families were Skeneidae and Buccinidae (with 10 and 11 species, respectively), Eulimidae and Trochidae (with 9 species each), and Turridae (6 species). Thirty-Seven benthic deep-sea species (44%) were represented by a single specimen, and another 20 species (24%) were found at a single station, suggesting that more than two thirds of Antarctic deep-sea gastropod species are very rare or have a very scattered distribution. Of the 27 species occurring at two or more deep-sea stations, 14 were collected with different gear. Approximately half of the deep-water species are new to science or have been recently described. The present investigation increases the total number of recorded benthic Antarctic deep-sea gastropods (below 750 m) from 115 to 177. The previously known depth ranges have been extended, often considerably, for 31 species. The collected deep-sea gastropods comprise both eurybathic shelf species (29%) and apparently true deep-sea species (58%); some of the latter may belong to a so far unknown Antarctic abyssal fauna. Geographical ranges of the collected Antarctic benthic deep-sea gastropod species appear limited, and all these 84 species seem endemic to Antarctica south of the Polar Front. Comparing diversity and abundances based on epibenthic sledge samples, there is no clear relationship between Antarctic deep-sea gastropod abundance and species richness with depth. However, both Antarctic and adjacent deep-sea areas are still far from being adequately sampled to allow more comprehensive conclusions.

ABUNDANCE AND SIZE DISTRIBUTION DYNAMICS OF ABYSSAL EPIBENTHIC MEGAFAUNA IN THE NORTHEAST PACIFIC

The importance of interannual variation in deep-sea abundances is now becoming recognized. There is, however, relatively little known about what processes dominate the observed fluctuations. The abundance and size distribution of the megabenthos have been examined here using a towed camera system at a deep-sea station in the northeast Pacific (Station M) from 1989 to 2004. This 16-year study included 52 roughly seasonal transects averaging 1.2 km in length with over 35600 photographic frames analyzed. Mobile epibenthic megafauna at 4100 m depth have exhibited interannual scale changes in abundance from one to three orders of magnitude. Increases in abundance have now been significantly linked to decreases in mean body size, suggesting that accruals in abundance probably result from the recruitment of young individuals. Examinations of size-frequency histograms indicate several possible recruitment events. Shifts in size-frequency distributions were also used to make basic estimations of individual growth rates from 1 to 6 mm/month, depending on the taxon. Regional intensification in reproduction followed by recruitment within the study area could explain the majority of observed accruals in abundance. Although some adult migration is certainly probable in accounting for local variation in abundances, the slow movements of benthic life stages restrict regional migrations for most taxa. Negative competitive interactions and survivorship may explain the precipitous declines of some taxa. This and other studies have shown that abundances from protozoans to large benthic invertebrates and fishes all have undergone significant fluctuations in abundance at Station M over periods of weeks to years.

Chitinase gene diversity at a deep sea station of the east Pacific nodule province

The Pacific nodule province covered about 4.5 million km(2) in the east tropical Pacific with an abundance of polymetallic nodules at the seafloor. In view of the environmental protection and resource preservation, the survey of biodiversity was important during the reconnaissance and exploitation in this area. As one of the important component of the deep sea ecosystem, the microbial community in the Pacific nodule province was still largely unknown. The chitinolytic bacteria diversity in deep-sea sediment of a station within the Pacific nodule province was examined by molecular technology. A total of 18 chitinase genes were detected by a set of degenerate PCR primer specific for chiA gene fragment of family 18 chitinase. Most of them belonged to the Serratia-like chitinase. Eight genes had different amino acid sequences in the conserved motif, encompassing the catalytic site among the ChiA protein of family 18 glycosyl hydrolases, and clustered in an independent clade on the phylygenetic tree.

Spatial and temporal variations in deep-sea meiofauna assemblages in the Marginal Ice Zone of the Arctic Ocean

In order to understand the response of the deep-sea meiobenthos to a highly varying, ice-edge-related input of phytodetritus, we investigated the abundance and composition of the meiobenthos at the arctic long-term deep-sea station HAUSGARTEN (79°N, 4°E) along a bathymetric transect (1200–5500 m water depth) over 5 consecutive years (from 2000 to 2004) in relation to changes in environmental conditions. Results showed high sediment-bound pigment concentrations (chlorophyll a and degradation products) ranging from 4.5 to 41.6 μg/cm 3, and coinciding high meiobenthic densities ranging from 149±3 to 3409±525 ind/10 cm 2. Nematodes dominated the metazoan meiofaunal communities at every depth and time (85–99% of total meiofauna abundance), followed by harpacticoid copepods (0–4.6% of total meiofauna abundance). The expected pattern of gradually decreasing meiobenthic densities with increasing water depth was not confirmed. Instead, the bathymetric transect could be subdivided into a shallow area with equally high nematode and copepod densities from 1000 to 2000 m water depth (means: 2259±157 Nematoda/10 cm 2, and 50±4 Copepoda/10 cm 2), and a deeper area from 3000 to 5500 m water depth with similar low nematode and copepod densities (means: 595±52 Nematoda/10 cm 2, and 11±2 Copepoda/10 cm 2). Depth-related investigations on the meiobenthos at the HAUSGARTEN site showed a significant correlation between meiobenthos densities, microbial exo-enzymatic activity (esterase turnover) and phytodetrital food availability (chlorophyll a and phaeophytines). In time-series investigations, our data showed inter-annual variations in meiofauna abundance. However, no consistent relationship between nematode and copepod densities, and measures for organic matter input were found.

LAERTIS, a multidisciplinary station

LAERTIS, designed to collect environmental data from the deep-sea, is operated since 1999 and has been deployed several time at 4000m depth at the NESTOR site. Power and data were transferred through a 30-km electro-optical cable to the Shore Station. In this report, we describe briefly the LAERTIS instrumentation and present typical data that were collected successfully during those deployment demonstrating the importance of a deep-sea station permanently connected to shore.

Microbial Diversity at a Deep-Sea Station of the Pacific Nodule Province

The Pacific nodule province covers about 4.5 million km2 in the eastern tropical Pacific with abundance of polymetallic nodules. Microbes are believed to play large roles in the metal cycling in many environments, but the microbial community in the Pacific nodule province has never been studied. Phylogenetic studies based on 16S rRNA gene sequence analysis, together with bacterial cultivation were used to study the microbial populations in the Pacific nodule province (A core) deep-sea sediment. Bacterial 16S rRNA gene sequence analysisdemonstrated that Proteobacteria division mainly of γ-Proteobacteria dominated the microbial community of the nodule province A core. Among the γ-Proteobacteria, Shewanella species which were known as Fe(□), Mn(□) reducing bacteria were found prevalent. Besides Proteobacteria, Green nonsulfur bacteria, the candidate subdivision OP3, Cytophaga-Flexibacter-Bacteroides bacteria and novel unidentified strains were also detected. Archaeal 16S rDNA sequence analysis data and results from hybridization with crenarchaeotal marine group I specific probe revealed that all archaea detected at the station belong to Crenarchaeota nonthermophilic marinegroup I. Bacteria assigned to the gamma Proteobacteria wereisolated, none of them showed capability of manganese oxidation.

Ecophysiological properties of photosynthetic bacteria from the Black Sea chemocline zone

In May 1998, during the fifty-first voyage on board the research vessel Professor Vodyanitskii, a comparative study was conducted of the species diversity of green and purple sulfur bacteria in the water column of the chemocline zone at deep-sea stations and on the bottom surface of the Black Sea shallow regions. At three deep-sea stations, the accumulation of photosynthesizing bacteria in the chemocline zone at a depth of 85-115 m was revealed on the basis of the distribution of potential values of carbon dioxide light fixation. The location of the site of potential carbon dioxide light fixation suggests that the photosynthesis may be determined by the activity of the brown Chlorobium sp., revealed earlier at these depths. Enrichment cultures of brown sulfur bacteria were obtained from samples taken at the deep-sea stations. By morphology, these bacteria, assigned to Chlorobium sp., appear as nonmotile straight or slightly curved rods 0.3-0.5 x 0.7-1.2 microm in size; sometimes, they form short chains. Ultrathin sections show photosynthesizing antenna-like structures, chlorosomes, typical of Chlorobiaceae. The cultures depended on the presence of NaCl (20 g/l) for growth, which corresponds to the mineralization of Black Sea water. The bacteria could grow photoautotrophically, utilizing sulfide, but the Black Sea strains grew much more slowly than the known species of brown sulfur bacteria isolated from saline or freshwater meromictic lakes. The best growth of the strains studied in this work occurred in media containing ethanol (0.5 g) or sodium acetate (1 g/l) and low amounts of sulfide (0.4 mM), which is consistent with the conditions of syntrophic growth with sulfidogens. The data obtained allow us to conclude that the cultures of brown sulfur bacteria are especially adapted to developing at large depths under conditions of electron donor deficiency owing to syntrophic development with sulfate reducers. The species composition of the photosynthetic bacteria developing in the bottom sediments of shallow stations differed substantially from that observed at deep-sea stations. Pure cultures of the green Chlorobium sp. BS 1C and BS 2C (chlorobactin as the carotenoid), purple sulfur bacteria Chromatium sp. BS 1Ch (containing spirilloxanthine series pigments), and Thiocapsa marina BS 2Tc (containing the carotenoid okenone) were obtained from samples of sediments at shallow-water stations. Brown sulfur bacteria were absent in the sediment samples obtained from the Black Sea shallow-water stations 1 and 2.

Gelatinous invertebrate zooplankton of the South Adriatic: species composition and vertical distribution

The species composition and vertical distribution of gelatinous invertebrate zooplankton were investigated from April 1993 to June 1994 in the 0–1000 m water column at a deep-sea station in the northern part of the South Adriatic Pit. Fifty-seven species were identified: 11 hydromedusae, 13 calycophores, 3 ctenophores, 3 heteropods, 10 pteropods, 8 polychaetes and 9 chaetognaths. The pteropod Desmopterus papilio and the heteropod Protatlanta mediterranea were recorded for the first time in the Adriatic Sea. Data from this study differed from those of previous investigations in the South Adriatic as regards the numerically dominant polychaete and pteropod species. All investigated groups generally were more abundant in the upper 100 m and decreased with depth. Different vertical distributions of life stages were observed for those species that occupy a wide depth range: Persa incolorata, Solmissus albescens, Limacina inflata, Cymbulia peroni, Pelagobia longicirrata, Sagitta lyra and Sagitta decipiens.

Higher abundance of bacteria than of viruses in deep Mediterranean sediments.

The interactions between viral abundance and bacterial density, biomass, and production were investigated along a longitudinal transect consisting of nine deep-sea stations encompassing the entire Mediterranean basin. The numbers of viruses were very low (range, 3.6 x 10(7) to 12.0 x 10(7) viruses g(-1)) and decreased eastward. The virus-to-bacterium ratio was always < 1.0, indicating that the deep-sea sediments of the Mediterranean Sea are the first example of a marine ecosystem not numerically dominated by viruses. The lowest virus numbers were found where the lowest bacterial metabolism and turnover rates and the largest cell size were observed, suggesting that bacterial doubling time might play an important role in benthic virus development.

NEBALIA SCHIZOPHTHALMA, A NEW SPECIES OF LEPTOSTRACAN (MALACOSTRACA) FROM DEEP WATERS OFF THE EAST COAST OF THE UNITED STATES

Abstract A new species of leptostracan, Nebalia schizophthalma, is described from a single female specimen from one of the deep-sea stations along the Gay Head-Bermuda Transect. The new species differs from all other known species of the Leptostraca in that the distal margin of the eye is strongly invaginated. Other unusual features include a short, upturned ocular scale, a small indentation along the ventral margin of the carapace, and the absence of strong spinelike setae on the fourth article of the antennule. The new species represents only the fifth species of leptostracan known, and the second in the genus Nebalia, from the east coast of North America.

Microbial processes at the aerobic-anaerobic interface in the deep-water zone of the black sea

Chemical and key microbiological processes (assimilation of carbon dioxide, oxidation and formation of methane, and sulfate reduction) occurring at the aerobic-anaerobic interface in the deep-water zone of the Black Sea were investigated. Measurements were taken at depths from 90 to 300 m at intervals of 5–10 m. The integral rate of the dark assimilation of carbon dioxide varied from 120 to 207 mg C/(m2 day) with a maximum at the boundary of cyclonic currents. The organic matter (OM) formed from methane comprised less than 5% of the OM formed from carbon dioxide. A comparison between the rates of methane oxidation and methane production suggests that methane that is oxidized at depths from 100 to 300 m was formed in deeper water horizons. The maximum rate of sulfate reduction (1230 mg S/(m2 day)) was observed in the western halistatic region, and the minimum rate (490 mg S/(m2 day)), in the eastern halistatic region. The average rate of hydrogen sulfide production measured at three deep-sea stations amounted to 755 mg S/(m2 day), or 276 g S/(m2 year).

Viral density and virus-to-bacterium ratio in deep-sea sediments of the Eastern Mediterranean.

Viruses are now recognized as a key component in pelagic systems, but their role in marine sediment has yet to be assessed. In this study bacterial and viral densities were determined at nine deep-sea stations selected from three main sites (i.e., the Sporades Basin, the Cretan Sea, and the Ierapetra Trench at depths of 1,232, 1,840, and 4,235 m, respectively) of the Eastern Mediterranean. The three areas were characterized by different phytopigment and biopolymeric carbon concentrations and by changes in the protein and carbohydrate pools. A gradient of increasing trophic conditions was observed from the Sporades Basin (North Aegean) to the Ierapetra Trench (South Aegean). Viral densities (ranging from 1 x 10(9) to 2 x 10(9) viruses ml of sediment(-1)) were significantly correlated to bacterial densities (n = 9, r(2) = 0.647) and reached values up to 3 orders of magnitude higher than those generally reported for the water column. However, the virus-to-bacterium density ratio in deep-sea sediments was about 1 order of magnitude lower (range of 2 to 5, with a modal value of 2.6) than in pelagic environments. Virus density decreased vertically with depth in sediment cores at all stations and was below detection limits at the 10-cm depth of the abyssal sediments of the Ierapetra Trench. Virus density in the sediment apparently reflected a gradient of particle fluxes and trophic conditions, displaying the highest values in the Sporades Basin. The low virus-to-bacterium ratios and their inverse relationship with station depth suggest that the role played by viruses in controlling deep-sea benthic bacterial assemblages and biogeochemical cycles is less relevant than in pelagic systems.

Community structure and spatial heterogeneity of the deep-sea macrofauna at three contrasting stations in the tropical northeast Atlantic

A study of macrofauna community structure and spatial patterns was undertaken at three deep-sea stations during three EUMELI cruises in the tropical northeast Atlantic Ocean. The benthic response to contrasting regimes of organic carbon supply at the eutrophic (E), mesotrophic (M) and oligotrophic (O) stations (1700 m, 3 100 m and 4700 m depth, respectively) was evaluated through qualitative and quantitative analysis of the composition and structure of benthic macrofauna (> 250 μm) and discussed with regard to the biological and physical conditions. The mean densities of the total macrofauna, dominated by the Polychaeta, Tanaïdacea, Isopoda and Bivalvia, decreased with increasing depth from 5403 ind. m −2 at station E to 231.5 ind. m −2 at station O. From the 16 taxonomic groups represented at the E and M sites, only eight could be sampled at the O site. The density gradient can be related to the decreasing surface production, food supply and hydrographic conditions between the three stations. The comparison of the high densities encountered at station E with other studies underlines the particular feature of this station subjected to intense upwelling activity resulting in high organic carbon input. The patchy distribution of the macrofauna at stations E and M suggests that both physical and biologically-mediated disturbances create environmental heterogeneity favouring the aggregation of organisms. This process occurs at different observation scales. At a large scale, bottom current and upwelling activity induce high densities and faunal aggregations, displaying a degree of density-dependence. At the local scale, the complexity of the food web induces biologically-mediated disturbances, and the activity of major deposit-feeding taxa, like polychaetes, create sediment microhabitats favouring patchy distribution. Faunal patch sizes are variable among the taxa considered. Most are distributed in patches less than 1 m z. Our work contributes to the understanding of how the deep benthic communities are structured, particularly under the influence of environmental conditions in tropical latitudes.

Dynamic response of deep‐sea sediments to seasonal variations: A model

We present a dynamic, numerical model of early diagenetic processes that can be used to examine the response of different organic carbon mineralization pathways, concentration vs. depth profiles, and the resultant fluxes to seasonally varying carbon deposition. We show that there can be substantial temporal variability in sediment‐water fluxes as well as in the relative contribution of different organic carbon mineralization pathways and oxygen consumption processes in deep‐sea sediments. The timing and amplitude of the sediment response are most sensitive to the reactivity of the organic matter. We show that the fluxes of dissolved substances can be approximated by assuming that these constituents are at steady state with the carbon profiles.The model is used to reconcile a time series of community oxygen consumption rates, dissolved oxygen, nitrate, ammonia, and manganese profiles, and solid‐phase organic carbon profiles at a deep‐sea station in the Pacific. The organic carbon that is deposited is very reactive (7.2 yr‒1), which causes a fast response of the sediment community to seasonal pulses in carbon flux and significant temporal variation in oxygen and nutrient fluxes. The model predicts large seasonal variations in the relative importance of oxic (48–71%), denitrification (12–17%), and anoxic (16–37%) pathways. There is also substantial variation in the pore‐water profiles of oxygen, nitrate, ammonia, and other reduced substances over the seasonal cycle, indicating that seasonal variability should be taken into account when extrapolating from a single observation to yearly averaged values.