Description: The small sized organisms including prokaryotes (bacteria and archaea), protozoa and metazoan meiofauna (〈 250 µm) are the driving forces for biogeochemical fluxes in surficial deepsea sediments under oxic conditions. The relative proportion of small sized organisms increases along trophic gradients from eutrophy to oligotrophy or from the continental margin towards the mid oceanic deep-sea. They can consume up to 10% of freshly sedimented organic matter per day. The small sized fauna consumes and respires the largest part of organic matter, while macrofauna is instrumental in incorporating fresh detritus into the sediment, structuring the environment and thus facilitating microbial processes. Small organisms, in particular prokaryotes, can adapt to amount and quality of organic matter input. Under nutrient starvation probably a large proportion of the prokaryotic community is dormant and is reactivated during sedimentation events. On time scales of 7–10 days (metabolism) to 2–3 weeks (biomass increase) they can react to pulses of deposition of organic material. However, the history of food supply influences the speed of adaptation and effectiveness of growth. At stations close to continental margins estimates of organic matter input from sediment traps largely disagree with measurements of benthic respiration, carbon turnover or estimates obtained from geochemical modelling. This discrepancy is much smaller at mid-oceanic stations. Lateral inputs from productive shelf seas into the deep-sea are suspected to cause this discrepancy.

Description: The ocean margins contain a great variety of habitats and biological communities. Recent discoveries, such as deep-water coral reefs, show that these communities are poorly described and understood. However, observations have already indicated that benthic communities on ocean margins show high levels of spatial and temporal variation at all scales. The European continental margin is increasingly exploited for both biological resources (fisheries) and non-biological resources (oil, gas, minerals). Environmental management of the exploitation of continental margins requires an understanding of natural levels of variation inherent in biological communities that are potentially impacted by such activities. This paper presents a synthesis of the present knowledge of the spatial and temporal variation of slope communities. Priorities for future research and its technological development are discussed. The aim of this research is to provide a scientific basis for the environmental management of the continental slopes of Europe.

Description: We provide an overview of the role of biological processes in the Benthic boundary layer (BBL) and in sediments on the cycling of particulate organic material in the Goban Spur area (Northeast Atlantic). The benthic fauna, sediment and BBL characteristics were studied along a transect ranging from 208 to 4460 m water depth in different seasons over 3 years. Near-bottom flow velocities are high at the upper part of the slope (1000–1500 m), and high numbers of filter-feeding taxa are found there such that organic carbon normally passing this area during high flow conditions is probably trapped, accumulated, and/or remineralised by the fauna. Overall metabolism in shelf and upper slope sediments is dominated by the macrofauna. More than half of the organic matter flux is respired by macrofauna, with a lower contribution of metazoan meiofauna (4%) and anoxic and suboxic bacterial mineralisation (21%); the remainder (23%) being channelled through nanobiota and oxic bacteria. By its feeding activity and movement, the macrofauna intensely reworks the sediments on the shelf and upper slope. Mixing intensity of bulk sediment and of organic matter are of comparable magnitude. The benthos of the lower slope and abyssal depth is dominated by the microbiota, both in terms of total biomass (〉90%) and carbon respiration (about 80%). The macrofauna (16%), meiofauna (4%) and megafauna (0.5%) only marginally contribute to total carbon respiration at depths below 1400 m. Because large animals have a lower share in total metabolism, mixing of organic matter within the sediments is reduced by a factor of 5, whereas mixing of bulk sediment is one to two orders of magnitude lower than on the shelf. The food quality of organic matter in the sediments in the shallowest part of the Goban Spur transect is significantly higher than in sediments in the deeper parts. The residence time of mineralisable carbon is about 120 d on the shelf and compares well with the residence time of the biota. In the deepest station, the mean residence time of mineralisable carbon is more than 3000 d, an order of magnitude higher than that of biotic biomass.

Description: Rotifers, one of the smallest metazoans, are only seldom found in marine environments. Surprisingly, we discovered high abundances of at least two new species of rotifers settling in anoxic and highly sulphidic sediments associated with shallow gas hydrates (GH) at the southern crest of Hydrate Ridge off Oregon, NE Pacific, in a water depth of about 780 m. At basins adjacent to Hydrate Ridge, 1,285–2,304 m deep, we found rotifers co-occurring with the sulphide-oxidising bacteria Thioploca sp.

Description: As part of the large-scale, interdisciplinary deep-sea study “BIGSET”, the relationship between the monsoon-induced regional and temporal variability of POC deposition and the small-sized benthic community was investigated at several sites 2316–4420 m deep in the Arabian Sea during four cruises between 1995 and 1998. Vertical and horizontal distribution patterns of chloroplastic pigments (a measure of phytodetritus deposition), readily soluble protein and activity, and biomass parameters of the small-sized benthic community (Electron Transport System Activity (ETSA); bacterial ectoenzymatic activity (FDA turnover) and DNA concentrations) were measured concurrently with the vertical fluxes of POC and chloroplastic pigments. Sediment chlorophyll a (chl. a) profiles were used to calculate chl. a flux rates and to estimate POC flux across the sediment water interface using two different transport reaction models. These estimates were compared with corresponding flux rates determined in sediment traps. Regional variability of primary productivity and POC deposition at the deep-sea floor creates a trophic gradient in the Arabian Basin from the NW to the SE, which is primarily related to the activity of monsoon winds and processes associated with the topography of the Arabian Basin and the vicinity of land masses. Inventories of sediment chloroplastic pigments closely corresponded to this trophic gradient. For ETSA, FDA and DNA, however, no clear coupling was found, although stations WAST (western Arabian Sea) and NAST (northern Arabian Sea) were characterised by high concentrations and activities. These parameters exhibited high spatial and temporal variability, making it impossible to recognise clear mechanisms controlling temporal and spatial community patterns of the small-sized benthic biota. Nevertheless, the entire Arabian Basin was recognised as being affected by monsoonal activity. Comparison of two different transport reaction models indicates that labile chl. a buried in deeper sediment layers may escape rapid degradation in Arabian deep-sea sediments.

Description: During the large-scale deep-sea programme BIGSET in situ measurements of sediment community oxygen consumption (SCOC) were carried out during three cruises between 1995–1998 at five abyssal sites (3190–4450 m water depth) in the deep Arabian Sea in order to elucidate the regional and temporal variation of benthic carbon remineralisation. SCOC ranged from 0.9–6.3 mmol O2 m−2 d−1, with highest values in the western and northern Arabian Sea and lowest values in the southern Arabian Sea. For the central Arabian Sea intermediate oxygen uptake rates were detected. This regional pattern mirrors the overall regional pattern of primary productivity in surface waters and vertical particulate organic carbon (POC) flux at 1000 mab. Primary productivity in Arabian Sea surface waters and particulate flux into the deep-sea are controlled by the monsoon system and the flux maxima during the SW and NE monsoon are among the highest particle fluxes recorded in the deep open ocean. Highest flux rates were recorded in the western and northern Arabian Sea and decreased towards the central and southern Arabian Sea. SCOC at our western, northern and eastern Arabian Sea stations WAST, NAST and EAST were considerably higher than so far detected in other abyssal areas of the global oceans, and vertical POC flux can account for only 20–50% of benthic carbon remineralisation (BCR). Possible explanations for the high rates of BCR at these stations that are situated close to the continental margins are discussed: the accelerated deposition of very labile organic matter due to eolian dust particles, enhanced rain efficiencies, and lateral advection. A significant temporal variability in SCOC only could be detected at the eastern and western Arabian Sea stations WAST and EAST.

Description: Effects of monsoon-induced enhanced depositional regimes of particulate organic carbon (POC) on regional variability and distribution patterns and size spectra of metazoan meiofauna, particularly of nematodes, were investigated at five sites 3158–4414 m deep in the Arabian Sea. The sampling sites were subjected to different flux rates of POC. Total meiofaunal abundance ranged from 109 to 320 ind./10 cm2. Nematodes were the numerically most abundant taxon, with a relative abundance of 82.5–88.7%, followed by copepods and ostracods. Mean individual nematode biomass ranged from 0.0272 to 0.1033 μgC, and Mean nematode population biomass varied between 0.0026 and 0.0133 mgC/10 cm2. Mean nematode lengths ranged from 614.2 to 832.6 μm. The length distributions of nematodes at the different sites were typically skewed with the distributions extending into the longer size classes. At the sites with higher POC deposition rates, nematodes displayed deeper distributions in the sediment column (47.4–58.5% of nematodes in the top 1 cm layer of the sediment) in contrast to very shallow distributions at a site of low POC flux (75.1% of nematodes in the top 1 cm of the sediment). Regional variability of nematode biomass, size and vertical distribution was related to monsoon-driven gradients of POC- and chlorophyll a (chl. a) flux rates and bacterial biomass i.e. bioavailable organic carbon. This was in contrast to nematode abundance which did not correlate significantly with any of these environmental parameters. The differential pattern between biomass and abundance, distribution might be related to POC-dependent alterations in the species composition of the nematode assemblages at the different sites. The hypothesis of increased meiobenthic stocks due to monsoon-induced enhanced sedimentation could not be confirmed compared to data from other less productive oceanic regions. Nematode abundance and biomass in the Arabian Sea were similar to values obtained from the abyssal temperate NE-Atlantic.

Description: The distribution, biomass, and diversity of living (Rose Bengal stained) deep-sea benthic foraminifera (〉30 [mu]m) were investigated with multicorer samples from seven stations in the Arabian Sea during the intermonsoonal periods in March and in September/October, 1995. Water depths of the stations ranged between 1916 and 4425 m. The distribution of benthic foraminifera was compared with dissolved oxygen, % organic carbon, % calcium carbonate, ammonium, % silica, chloroplastic pigment equivalents, sand content, pore water content of the sediment, and organic carbon flux to explain the foraminiferal patterns and depositional environments. A total of six species-communities comprising 178 living species were identified by principal component analysis. The seasonal comparison shows that at the western stations foraminiferal abundance and biomass were higher during the Spring Intermonsoon than during the Fall Intermonsoon. The regional comparison indicates a distinct gradient in abundance, biomass, and diversity from west to east, and for biomass from north to south. Highest values are recorded in the western part of the Arabian Sea, where the influence of coastal and offshore upwelling are responsible for high carbon fluxes. Estimated total biomass of living benthic foraminifera integrated for the upper 5 cm of the sediment ranged between 11 mg Corg m-2 at the southern station and 420 mg Corg m-2 at the western station. Foraminifera in the size range from 30 to 125 [mu]m, the so-called microforaminifera, contributed between 20 and 65% to the abundance, but only 3% to 28% to the biomass of the fauna. Highest values were found in the central and southern Arabian Sea, indicating their importance in oligotrophic deep-sea areas. The overall abundance of benthic foraminifera is positively correlated with oxygen content and pore volume, and partly with carbon content and chloroplastic pigment equivalents of the sediment. The distributional patterns of the communities seem to be controlled by sand fraction, dissolved oxygen, calcium carbonate and organic carbon content of the sediment, but the critical variables are of different significance for each community.