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Ammonia beccarii is irregularly distributed in the subtidal sediment of the southern North Sea, with substantial numbers occurring as deep as 35 cm below the water-sediment interface. Deep infaunal specimens are insensitive to high oxygen concentrations (+/-225 {mu}M), and all specimens isolated from different depth intervals continued their normal activities (feeding and growth) when exposed to dysaerobic oxygen content (12.5 {mu}M). Specimens of E. excavatum, Q. seminulum, and E. scabra, when subjected to the same conditions, behave similarly to A. beccarii. These benthic foraminifera have very low oxygen requirements. The chambers of A. beccarii that are formed in situ at different depth intervals in the sediment have a wide range in the porosity (i.e., % of area occupied by pores) which is adequate for gas exchange under both high and low oxygen conditions. However, chambers formed in the laboratory under dysaerobic conditions have a significantly higher porosity (mainly due to larger pores) than do chambers constructed in well oxygenated water. Foraminifera live at the oxic-anoxic boundary throughout the sediment and therefore must occasionally be subjected to completely anoxic conditions. A. beccarii, E. excavatum, and Q. seminulum actively survived at least 24 h without oxygen, indicating that they are capable of facultative anaerobic metabolism.

The extraction of minerals from land-based mines necessitates the disposal of large amounts of mine tailings. Dumping and storage of tailings into the marine environment, such as fjords, is currently being performed without knowing the potential ecological consequences. This study investigated the effect of short-term exposure to different deposition depths of inert iron ore tailings (0.1, 0.5, and 3 cm) and dead subsurface sediment (0.5 and 3 cm) on a deep water (200 m) fjord benthic assemblage in a microcosm experiment. Biotic and abiotic variables were measured to determine structural and functional changes of the benthic community following an 11 and 16 day exposure with tailings and dead sediment, respectively. Structural changes of macrofauna, meiofauna, and bacteria were measured in terms of biomass, density, community composition and mortality while measures of oxygen penetration depth, sediment community oxygen consumption and 13C-uptake and processing by biota revealed changes in the functioning of the system. Burial with mine tailings and natural sediments modified the structure and functioning of the benthic community albeit in a different way. Mine tailings deposition of 0.1 cm and more resulted in a reduced capacity of the benthic community to remineralize fresh 13C-labeled algal material, as evidenced by the reduced sediment community oxygen consumption and uptake rates in all biological compartments. At 3 cm of tailings deposition, it was evident that nematode mortality was higher inside the tailings layer, likely caused by reduced food availability. In contrast, dead sediment addition led to an increase in oxygen consumption and bacterial carbon uptake comparable to control conditions, thereby leaving deeper sediment layers anoxic and in turn causing nematode mortality at 3 cm deposition. This study clearly shows that even small levels (0.1 cm) of instantaneous burial by mine tailings may significantly reduce benthic ecosystem functioning on the short term. Furthermore, it reveals the importance of substrate characteristics and origin when studying the effects of substrate addition on marine benthic fauna. Our findings should alert decision makers when considering approval of new deep-sea tailings placement sites as this technique will have major negative impacts on benthic ecosystem functioning over large areas.

The diet of cavity sponges on the narrow fringing reefs of Cura double dagger ao, Caribbean was studied. The origin and resources of the bulk food of these sponges, i.e., dissolved organic matter (DOM), were identified using stable carbon and nitrogen isotopes and fatty acid biomarkers. We found that phytoplankton and its derived DOM from the adjacent open sea and from reef overlying water is not the main source of food for most of the sponges examined nor is bacterioplankton. Interestingly, dual stable isotope signatures (delta(13)C(org), delta(15)N(org)) and fatty acid biomarkers appoint coral mucus and organic matter derived from crustose coralline algae (CCA) as probable food sources for encrusting sponges. Mucus-derived DOM may contribute up to 66% to the diet of examined sponges based on results of dual isotope mixing model analysis. The contribution of CCA (as purported representative for benthic algae) was smaller with values up to 31%. Together, mucus- and CCA-derived substrates contributed for 48-73% to the diet of sponges. The presence of the exogenous fatty acid 20:4 omega 6 in sponges, which is abundant in coral mucus of Madracis mirabilis and in CCA, highlights these reef-derived resources as sources of nutrition for DOM feeding cavity sponges. The relatively high concentrations of exogenous 20:4 omega 6 in all sponges examined supports our hypothesis that the bulk of the food of the cavity sponge community is reef-derived. Our results imply that cavity sponges play an important role in conserving food and energy produced within the reef.

Macrobenthic deposit feeders and bacteria compete for the same detrital food resources. We hypothesize that the spatial scale at which food is distributed in the sediment is an important factor determining the outcome of this competition. Macrobenthic deposit feeders are better adapted for fast consumption of food in concentrated patches, whereas diluted food can only be exploited by bacteria. This hypothesis was tested in an experiment in which a fixed quantity of isotopically labeled algal detritus was offered to a natural bacterial community and the polychaete worm Nereis (=Hediste) diversicolor, either as a concentrated patch or mixed through the sediment matrix. Worms dominated food uptake in the concentrated treatment, while bacterial uptake was much greater in the diluted treatment. The experiment demonstrated scale-based niche differentiation between these taxonomically distant groups. It also showed that worms spatially redistributed food and made it available to bacteria in that way. Together, these mechanisms may stimulate stable co-existence through a scale-based partitioning of resources.

Here we report on the trophodynamics of the bacteria-containing coral reef sponge Halisarca caerulea. The assimilation and respiration of the 13C-enriched substrates glucose, algal-derived dissolved and particulate organic matter (diatom-DOM and -POM), and bacteria were followed in 1- and 6-h incubations. Except for glucose, all substrates were readily processed by the sponge, with assimilation being the major fate. 13C-Enrichment patterns in fatty acid biomarkers revealed that sponge dissolved organic 13C assimilation was both direct and bacteria mediated as tracer carbon was recovered both in bacteria-specific and nonbacterial fatty acid. This is the first direct evidence of DOM incorporation by sponges. The present study demonstrates that the encrusting sponge H. caerulea feeds on both DOM and POM and given their dominant coverage of the largest coral reef habitat (coral cavities) it is proposed that organic matter assimilation by cryptic reef sponges may represent an important, largely overlooked ecological function. Quantitatively significant DOM processing may not be the exclusive function of the microbial world on coral reefs; sponges transform DOM to biomass, and thus retain and store organic matter in the reef system.

Håkon Mosby Mud Volcano (HMMV, SW Barents Sea slope, 1280 m) is one of the numerous cold methane-venting seeps existing along the continental margins. Analyses of video-guided core samples revealed extreme differences in the diversity and density of the metazoan meiobenthic communities associated with the different sub-habitats (centre, microbial mats, Pogonophora field, outer rim) of this mud volcano. Diversity was lowest in the sulphidic, microbial mat sediments that supported the highest standing stock, with unusually high densities (11000 ind. 10 cm–2) of 1 nematode species related to Geomonhystera disjuncta. Stable carbon isotope analyses revealed that this nematode species was thriving on chemosynthetically derived food sources in these sediments. Ovoviviparous reproduction has been identified as an important adaptation of parents securing the survival and development of their brood in this toxic environment. The proliferation of this single species in exclusive association with free-living, sulphide-oxidising bacteria (Beggiatoa) indicates that its dominance is strongly related to trophic specialisation, evidently uncommon among the meiofauna. This chemoautotrophic association was replaced by copepods in the bare, sulphide-free sediments of the volcano’s centre, dominated by aerobic methane oxidation as the chemosynthetic process. Copepods and nauplii reached maximum densities and dominance in the volcano’s centre (500 ind. 10 cm–2). Their strongly depleted carbon isotope signatures indicated a trophic link with methane-derived carbon. This proliferation of only selected meiobenthic species supported by chemosynthetically derived carbon suggests that, in addition to the sediment geochemistry, the associated reduced meiobenthic diversity may equally be related to the trophic resource specificity in HMMV sub-habitats.

We report the results of an integrated modeling and in situ isotope tracer experiment study examining the fate of bacterial carbon in an intertidal sediment. 13C-glucose was injected into the upper 10 cm of an intertidal sediment, which successfully tagged the bacterial community as evidenced by 13C enrichment of bacterial specific polar-lipid-derived fatty acids. Over a period of 4.5 months, 13C enrichment was monitored in sediment organic carbon, bacteria, meiobenthos, macrobenthos, and dissolved inorganic carbon. A mechanistic model accurately simulated label transfer among the biotic and abiotic compartments and was used to derive bacterial production and the loss processes grazing, mortality, respiration, and exchange. Bacterial production averaged 67 mmol C m-2 d-1, of which 8% was lost from the sediment by exchange processes, 3% was grazed by meiobenthos and 24% by macrobenthos. The primary fate of bacterial production was mortality (65%) and the released bacterial carbon was recycled back to dissolved organic carbon, resulting in recycling of carbon within the dissolved organic carbon-bacteria loop. Bacterial respiration was the main loss process from this loop. Although a significant fraction of bacterial carbon production was grazed, our results show that bacterial carbon is primarily a sink of organic carbon in the food web of intertidal sediments.

The deep eastern Mediterranean Sea is one of the most oligotrophic regions of the world’s oceans and is therefore highly suitable for investigating the response of a resource-limited deep-sea benthic community to food pulses. Additionally, the benthic response to organic matter (OM) deposition can differ according to the amount of OM entering the sediment. Here, we report on 36-h pulse‐chase experiments with 13 C-labeled diatoms as a tracer of labile OM. Two loads of OM, equivalent to 25 mg C m 22 (low-carbon [L] experiment) and 250 mg C m 22 (highcarbon [H] experiment), were injected into chambers of a benthic lander. The fate of the added carbon was followed in total community respiration and bacterial assimilation (through 13 C enrichment in bacteria-specific fatty acids). A benthic response was clearly evident in both experiments, but carbon turnover rates were higher with a higher OM addition. However, although a larger amount of carbon was assimilated by bacteria in the H experiment, bacterial assimilation accounted for a relatively lower proportion of total carbon processing compared with the L experiment. This result could indicate a modified bacterial response, a larger role of other biotic components in the high-load experiment, or both. This study demonstrates that an impoverished deep-sea benthic community rapidly responds to an input of fresh OM. Furthermore, the carbon flow pathways can differ according to the amount of OM entering the sediment. Consequently, comparison of benthic responses across systems can only be made in cases in which similar quantities and qualities of OM have been used. With the discovery of life in the deep ocean and the first profound studies of deep-sea fauna (e.g., with the HMS Por

The herbivorous, detrital and microbial pathways are major components of marine food webs. Although it is commonly recognized that these pathways can be linked in several ways, elucidating carbon transfers between or within these pathways remains a challenge. Intertidal flat communities are driven by a wide spectrum of organic matter sources that support these different pathways within the food web. Here we reconstruct carbon pathways using inverse analysis based on mass balancing, stable isotope signatures and tracer data. Data were available on biomass, total carbon production and processing, integrated diet information from stable isotope signatures and the transfer of recently produced carbon through the food web from an isotope tracer study. The integration of these data improved the quality of the inverse food web reconstruction considerably, as demonstrated explicitly by an uncertainty analysis. Deposition of detritus (detrital pathway) from the water column and subsequent assimilation and respiration by bacteria and to a lesser extent by microbenthos (microbial pathway) dominated the food web. Secondary production was dominated by bacteria (600 mg C m-2 d-1), but transfer to higher trophic levels was limited to 9% and most bacterial carbon was recycled back to dissolved organic carbon (DOC) and detritus. Microbenthos secondary production (77 mg C m-2 d-1) was supported by DOC (73%) and detritus (26%) and was entirely transferred up the food web. The higher trophic levels consisting of nematodes, meiobenthos (copepods, ostracods and foraminifera) and macrobenthos fed highly selectively and relied primarily on microphytobenthos and pelagic primary production (herbivorous pathway). Deposit feeding is a common feeding mode among these sediment dwelling fauna, but detritivory was negligible due to this selective feeding. This strong resource selectivity implies that the herbivorous and detrital-microbial pathways function more or less autonomously, with limited interaction.

The deep-sea floor has long been considered a 'food desert' but recent observations suggest that episodic inputs of relatively fresh organic matter (phytodetritus) occur and that benthic processing of this material may be rapid. Although the responses of the total community in terms of oxygen consumption and of some individual benthic groups have been identified, the quantitative role of the different groups in the short-term response remains largely unknown. We examined the short-term response in major benthic compartments in an in situ experiment in the NE Atlantic (2170 m water depth) using C-13-enriched diatoms as a tracer of labile carbon. Within 35 h, 6 mg C m(-2) was processed by the benthos, with the majority of the processed carbon recorded as respiration (45%). Among the fauna retained on a 300 mum sieve, Foraminifera were rapid consumers which, together with Bacteria, accounted for 50% of the processing. Therefore, although Bacteria dominate long-term carbon mineralization (as suggested by their general dominance in the benthic biomass), some faunal components, in this case Foraminifera, may play a central role in the rapid initial processing of fresh organic carbon in deep-sea sediments., The deep-sea floor has long been considered a ‘food desert’ but recent observations suggest that episodic inputs of relatively fresh organic matter (phytodetritus) occur and that benthic processing of this material may be rapid. Although the responses of the total community in terms of oxygen consumption and of some individual benthic groups have been identified, the quantitative role of the different groups in the short-term response remains largely unknown. We examined the short-term response in major benthic compartments in an in situ experiment in the NE Atlantic (2170 m water depth) using 13C-enriched diatoms as a tracer of labile carbon. Within 35 h, 6 mg C m-2 was processed by the benthos, with the majority of the processed carbon recorded as respiration (45%). Among the fauna retained on a 300 µm sieve, Foraminifera were rapid consumers which, together with Bacteria, accounted for 50% of the processing. Therefore, although Bacteria dominate long-term carbon mineralization (as suggested by their general dominance in the benthic biomass), some faunal components, in this case Foraminifera, may play a central role in the rapid initial processing of fresh organic carbon in deep-sea sediments.

Chemoautotrophy has been little studied in typical coastal marine sediments, but may be an important component of carbon recycling as intense anaerobic mineralization processes in these sediments lead to accumulation of high amounts of reduced compounds, such as sulfides and ammonium. We studied chemoautotrophy by measuring dark-fixation of 13C-bicarbonate into phospholipid derived fatty acid (PLFA) biomarkers at two coastal sediment sites with contrasting sulfur chemistry in the Eastern Scheldt estuary, The Netherlands. At one site where free sulfide accumulated in the pore water right to the top of the sediment, PLFA labeling was restricted to compounds typically found in sulfur and ammonium oxidizing bacteria. At the other site, with no detectable free sulfide in the pore water, a very different PLFA labeling pattern was found with high amounts of label in branched i- and a-PLFA besides the typical compounds for sulfur and ammonium oxidizing bacteria. This suggests that other types of chemoautotrophic bacteria were also active, most likely Deltaproteobacteria related to sulfate reducers. Maximum rates of chemoautotrophy were detected in first 1 to 2 centimeters of both sediments and chemosynthetic biomass production was high ranging from 3 to 36 mmol C m(-2) d(-1). Average dark carbon fixation to sediment oxygen uptake ratios were 0.22±0.07 mol C (mol O2)(-1), which is in the range of the maximum growth yields reported for sulfur oxidizing bacteria indicating highly efficient growth. Chemoautotrophic biomass production was similar to carbon mineralization rates in the top of the free sulfide site, suggesting that chemoautotrophic bacteria could play a crucial role in the microbial food web and labeling in eukaryotic poly-unsaturated PLFA was indeed detectable. Our study shows that dark carbon fixation by chemoautotrophic bacteria is a major process in the carbon cycle of coastal sediments, and should therefore receive more attention in future studies on sediment biogeochemistry and microbial ecology.

Sediments collected from the northwestern Adriatic Sea (Mediterranean) were exposed to anoxic bottom-water conditions for more than 2 mo in order to examine the resistance of dominant meiobenthic taxa to prolonged anoxia. Copepods appeared to be most sensitive to anoxia, with densities being reduced to zero within 11 d. Compared to oxic conditions, densities of both nematodes and soft-shelled foraminifera were significantly lower under prolonged anoxia. In contrast, total hard-shelled foraminiferal densities did not differ significantly. This differential response resulted in a change in the meiobenthic community structure which was apparent after 1 mo. The change is reflected, and can be followed, in the foraminifera:nematode ratio, which is proposed as a bio- indicator of prolonged anoxia. The results clearly demonstrate that, among the meiobenthos, hard-shelled foraminifera are most resistant to prolonged anoxia. However, conditions resulting from the anoxic treatment (e.g. decreased biological interactions) were beneficial to some foraminiferal genera. This resulted in a shift in faunal patterns eventually leading to a strong reduction in foraminiferal generic diversity. Results also indicated that details of these trends may not be evident if only the part of the foraminiferal assemblage retained on a 63 mu m sieve is examined. [KEYWORDS: anoxia; meiobenthos; foraminifera; experiments Deep-sea sediments; southern north-sea; pore-water oxygen; sulfide gradients; organic-matter; microhabitats; tubes; microelectrode; diversity; tolerance]

Burial of organic matter (OM) plays an important role in marine sediments, linking the short-term, biological carbon cycle with the long-term, geological subsurface cycle. It is well established that low-oxygen conditions promote organic carbon burial in marine sediments. However, the mechanism remains enigmatic. Here we report biochemical quality, microbial degradability, OM preservation and accumulation along an oxygen gradient in the Indian Ocean. Our results show that more OM, and of biochemically higher quality, accumulates under low oxygen conditions. Nevertheless, microbial degradability does not correlate with the biochemical quality of OM. This decoupling of OM biochemical quality and microbial degradability, or bioavailability, violates the ruling paradigm that higher quality implies higher microbial processing. The inhibition of bacterial OM remineralisation may play an important role in the burial of organic matter in marine sediments and formation of oil source rocks.

The bacterial loop, the consumption of dissolved organic matter (DOM) by bacteria and subsequent transfer of bacterial carbon to higher trophic levels, plays a prominent role in pelagic food webs. However, its role in sedimentary ecosystems is not well documented. Here we present the results of isotope tracer experiments performed under in situ oxygen conditions in sediments from inside and outside the Arabian Sea's oxygen minimum zone (OMZ) to study the importance of the microbial loop in this setting. Particulate organic matter, added as phytodetritus, was processed by bacteria, protozoa and metazoans, while dissolved organic matter was processed only by bacteria and there was very little, if any, transfer to higher trophic levels within the 7 day experimental period. This lack of significant transfer of bacterial-derived carbon to metazoan consumers indicates that the bacterial loop is rather inefficient, in sediments both inside and outside the OMZ. Moreover, metazoans directly consumed labile particulate organic matter resources and thus competed with bacteria for phytodetritus.

We have elucidated the trophic interactions in the foodweb of sediments from and close to the oxygen minimum zone (OMZ) of the Arabian Sea. Sediment cores from inside (885 m depth) and outside (1791 m depth) the OMZ were manipulated onboard by adding C-13-enriched phytodetritus. The incorporation of phytodetritus by the benthic community was quantified after incubating for 7 days. To assess the effect of bottom-water oxygenation on the processing of organic matter, the oxygen concentration in the overlying water of the incubated cores was also manipulated. Biomass values inside and outside the OMZ were comparable for bacteria (1068 and 1276 mg C m(-2)) and macrofauna (2528 and 3263 mg C m(-2)), but not for meiofauna (63 and 1338 mg C m(-2)). Uptake values in percentage of total added tracer were 0.8 and 0.5% for bacteria suboxic and oxic treatments inside the OMZ, and 0.5 and 12% for suboxic and oxic treatments outside the OMZ. Macrofauna uptake accounted for 17.4 and 4.4% in the suboxic and oxic treatments inside the OMZ, and only for 0.1% and 13% respectively outside the OMZ. Respiration accounted for 13% of total tracer added inside the OMZ for both treatments, 4.6 and 6.8% for oxic and suboxic treatments outside the OMZ, respectively. Our results show that phytodetritus is most efficiently processed at in situ oxygen conditions, that foraminifera and bacteria remain active both under elevated and lowered bottom-water oxygen levels and that macrofauna was present in high abundance and showed high tracer uptake.

We sampled the Darwin mud volcano (MV) for meiofaunal community and trophic structure in relation to pore-water geochemistry along a 10 m transect from a seep site on the rim of the crater towards the MV slope. Pore-water profiles indicated considerable variation in upward methane (CH4) flow among sediment cores taken along the transect, with highest flux in the seep sediment core, gradually decreasing along the transect, to no CH4 flux in the core taken at a 5 m distance. Low sulphate concentrations and high levels of total alkalinity and sulphide (H2S) suggested that anaerobic oxidation of methane (AOM) occurred close to the sediment surface in the seep sediment core. High H2S levels had a genus- and species-specific impact on meiofaunal densities. Nematode genus composition varied gradually between sediment cores, with the genus Sabatieria dominating almost all sediment cores. However, genus diversity increased with increasing distance from the seep site. These limited data suggest that the community structure of seep meiofauna is highly dependent on local (a)biotic habitat characteristics, and a typical seep meiofaunal community cannot be delineated. Stable isotope values suggested the nematode diet up to 10 m from the seep site included thiotrophic carbon. The thicker hemipelagic sediment layer (photosynthetic carbon), the increased trophic diversity, and the heavier nematode d13C farther from the seep site suggest a decrease in thiotrophy and an increase in photosynthetic carbon in the nematode diet.

We report the results of a detailed investigation on the trophoecology of two dominant meiofaunal species at the Håkon Mosby Mud Volcano (HMMV), a deep-sea cold methane-venting seep. Analyses of fatty acids (FAs) and their stable carbon isotopes were used to determine the importance of chemosynthetic nutritional pathways for the dominant copepod species (morphologically very similar to Tisbe wilsoni) inhabiting the volcano’s centre and the abundant nematode Halomonhystera disjuncta from the surrounding microbial mats. The strong dominance of bacterial biomarkers (16:1?7c, 18:1?7c and 16:1?8c) coupled with their individual light carbon isotopes signatures (d13C ranging from -52 to -81‰) and the lack of symbiotic relationships with prokaryotes (as revealed by molecular analyses and fluorescent in situ hybridisation) indicated that chemosynthetically derived carbon constitutes the main diet of both species. However, the copepod showed a stronger reliance on the utilisation of methanotrophic bacteria and contained polyunsaturated FAs of bacterial origin (20:5?3 and 22:6?3 with isotope signatures d13C < -80‰). Instead, the FA profiles of H. disjuncta suggested that sulphide-oxidising bacteria constituted the main diet of this nematode. Therefore, HMMV can be regarded as a persistent deep-sea cold seep, allowing a chemosynthesisbased trophic specialisation by the dominant meiofaunal species inhabiting its sediments. The present investigation, through the determination of the fatty acid profiles, provides the first evidence for trophic specialisation of meiofauna associated with sub-habitats within a cold seep.

Organic matter input and processing was studied in 2 contrasting sediments (Stn 115FINE and Stn 330COARSE) in the southern North Sea. The sediments are subjected to similar hydrodynamic conditions, but Stn 115FINE underlies a high turbidity zone, making it a fine, low-permeability sediment. Monthly data on chlorophyll a (chl a), d13C and d15N of particulate organic matter in the water column and sediment showed that the algal spring bloom deposition created a strong vertical gradient of sedimentary chl a at Stn 115FINE. Macrobenthic biomass (78 ± 60 g Cm-2, mean ± SD) was dominated by suspension feeders, suggesting biological mediation of the organic matter input. In contrast, the offshore Stn 330COARSE is a coarse, high-permeability sediment in which chl a penetrated centimeters deep due to physically mediated input. The macrobenthic community, low in biomass (3.8 ± 2.4 g Cm-2), was dominated by mobile polychaetes and epibenthic amphipods, which is characteristic of physically disturbed sediments. Overall, sediment characteristics played an important but indirect role in the organic matter input and processing. At Stn 115FINE, a large macrobenthic community developed that mediated the input of organic matter to the sediment through herbivore and predatory pathways. At Stn 330COARSE, in contrast, organic matter input seemed to be dominated by physical processes. Overall, the fraction of algal carbon degraded in the sediment was higher at Stn 115FINE than at Stn 330COARSE, indicating that the physical input at Stn 330COARSE was less efficient than the biological input at Stn 115FINE.

We report the results of an in situ tracer experiment in an intertidal sediment, where bacterial carbon was tagged with stable carbon–isotope label, after the injection of 13C-glucose. The appearance of label in bacteria (based on label incorporation in bacteria-specific, phospholipid-derived fatty acids) and subsequent transfer to meiobenthos (group level) and macrobenthos (species level) was followed for 36 days. The label dynamics of benthic taxa were either fitted with a simple-isotope model or evaluated against enrichment in bacteria, to derive the importance of bacterially derived carbon for the meiobenthos and macrobenthos. Although selective uptake of bacteria was evident, as 2.4 times more bacterial carbon was grazed as expected from indiscriminate feeding, bacterial carbon accounted on average for only 0.08 and 0.11 of the carbon requirements of meiobenthic and macrobenthic taxa, respectively. Additionally, the contribution of bacterial carbon to total carbon requirements did not depend on the living/feeding depth in the sediment or organism size (evaluated over a size range of four orders of magnitude). The observed overall low contribution of bacterial carbon implies that most intertidal benthic fauna depend primarily on other carbon resources that may assert a stronger control on the structure of intertidal-sediment communities.

The impact of oxygen on the vertical distribution of an intertidal nematode community was investigated in a manipulation experiment with sediments collected from the Oosterschelde (The Netherlands). The vertical distribution of nematodes was examined in response to sediment inversion in perspex cores with or without the presence of an air-flushed silicone tube introducing oxygen to the bottom sediment layer. Following an incubation of 1 week, peak densities of nematodes were recorded in the deep layers of the sediment in cores with subsurface oxygenation. In contrast, this concentration of nematodes was absent in the cores that lacked bottom oxygenation and the majority of the total assemblage was still located in anoxic sediment layers. This suggests that oxidised sediments with traces of oxygen represent favourable conditions for nematodes and indicates that these nematodes are tolerant to short-term anoxia and burial. Observations on the species level suggest that oxygenation, primarily through its direct impact on geochemical properties of the sediment, may be an important factor governing the subsurface activity of nematodes. [KEYWORDS: nematodes ; vertical distribution ; estuarine sediments ; experiment - oxygen]

Clarification of the factors involved in the formation of unusual ancient organic carbon- rich deposits (like eastern Mediterranean sapropels) is central in understanding oceanic carbon cycling. The role of oxygenation remains a subject of controversy primarily due to two major uncertainties: (1) it is unknown if ancient organic-rich deposits reflect an accumulation of refractory organic matter (OM) or oxygenation-related aberrant sediment OM recycling, and (2) although marine OM degradation may be slower under anoxic conditions, its ultimate impact on organic carbon (Corg) preservation over geological time remains unclear. Here we provide direct experimental evidence that the Corg in eastern Mediterranean S1 sapropels (deposited .5 ka) is still highly reactive and that a shutdown in labile organic matter degradation under anoxic conditions played a key role in the formation of these deposits.

The vertical profiles of dominant metazoan meiobenthic taxa were studied at four. contrasting sites ill the Adriatic Sea in relation to macrofaunal activity and oxygen gen penetration depth in the sediment. Vertical profiles of copepods were related to, the oxygen penetration depth and showed peak densities in the upper cm or deeper in sediments with a broader oxic zone. In contrast, nematodes penetrated much deeper into the sediment and no direct relationship with oxygen penetration depth was evident. Nematode penetration depths were related to or reflected the differences in macrofaunal activity or mixing at the four sites which appears to be an important factor governing the subsurface activity of nematodes. [KEYWORDS: Adriatic Sea; meiobenthos; nematodes; vertical distribution; oxygenation; bioturbation Nematode communities; diagenetic processes; northeast atlantic; benthic meiofauna; abyssal-plain; ne atlantic; water; shelf; meiobenthos; abundance]

Benthic activity was examined at three stations (18 m water depth) in the northwestern Adriatic Sea. Carbon mineralisation rates, as based on sediment oxygen consumption rates, ranged from 54 to 89 g C m(-2) y(-1). The relatively high carbon mineralisation rates, large macrofaunal biomass (9 to 16 g C m(-2)) and macrofaunal production (11 to 19 g C m(-2) y(-1)) provide evidence of high organic-matter input and intense benthic-pelagic coupling. This is further supported by the high dominance of the suspension-feeding bivalve Corbula gibba, which accounts for 52 to 63% of the total annual macrofaunal biomass production. Although the infaunal distribution of total macrofauna showed a sharp decline in densities and biomass with depth into the sediment, different patterns within the dominant taxa were observed. Whilst the bivalve Corbula gibba and the amphipod Ampelisca sp. were restricted to the surface layer, other species such as the dominant bivalve Mysella sp. and the gastropod Hyala sp. were not confined to a specific depth level and the majority of the populations occurred deeper than 5 cm into the sediment. Bioturbation, based on the occurrence of macrofauna, extended to at least 20 cm. Nematodes and foraminifera together formed 80 to 90% of the meiofaunal community in the upper 5 cm of the sediment. Annual mean densities ranged from 3.40 to 6.07 x 10(6) ind. m(-2). Maximum abundance of meiofauna was not encountered at the station where maximum macrofaunal activity was recorded, and this could reflect the negative effect of biological interaction on meiofaunal densities in areas that have a high food supply. [KEYWORDS: northwestern Adriatic Sea; eutrophication; sediment oxygen consumption; carbon flux; benthic-pelagic coupling; bioturbation; macrofauna; meiofauna; biotic interactions Long-term eutrophication; northeastern north-sea; east china sea; continental-shelf; marine-sediments; organic-matter; respiration rates; margin sediments; nutrient fluxes; amazon shelf]

Benthic foraminifera are dominant members of tb meiofauna, commonly occurring below the anoxic-oxic interface in marine sediments. The absence of oxygen in marine coastal sediments is often correlated with the formation of hydrogen sulphide. In this study the tolerance of benthic foraminifera (from the northwestern Adriatic Sea) to hydrogen sulphide was examined experimentally. Although the foraminiferal assemblage exhibited a high tolerance to short-term exposure (21 d), prolonged exposure to sulphidic conditions (66 d with a final concentration of 12 mu M dissolved hydrogen sulphide) resulted in a significant reduction of total foraminiferal densities with time. Reproduction was evident under oxic conditions but none of the genera proliferated under sulphidic conditions. This implies that tolerance of sulphidic conditions was restricted to survival and that sulphide may be a prominent distributional factor for benthic foraminifera. [KEYWORDS: hydrogen sulphide; meiofauna; foraminifera; laboratory experiments Macoma-balthica bivalvia; northern adriatic sea; pore-water oxygen; differential response; thiobiotic meiofauna; subsurface activity; continental-margin; body-size; sulfide; microhabitats]

We examined the ability of foraminifera in taking up freshly deposited algal carbon in intertidal estuarine sediments. Lyophylized C-13-labelled axenic Chlorella was added to intact sediment cores containing natural benthic biota. The response of the system as a whole was rapid; similar to 5% of the added carbon was respired to CO2 within 6 h. Bacteria assimilated similar to 2 to 4 % of the added carbon within 12 h. Among the foraminifera, the dominant foraminifer Ammonia exhibited rapid uptake and it is estimated that similar to 1 to 7% of the added carbon was ingested within 3 to 53 h. This rapid, substantial uptake by Ammonia and the significant contribution of foraminifera to meiobenthic abundance and biomass (37 and 47% respectively) suggest that foraminifera play an important role in the carbon cycle in these sediments. [KEYWORDS: meiobenthos; benthic foraminifera; bacterial biomarkers; algal carbon; carbon cycle; CO2; C-13 labelling Deep-sea; organic-carbon; sedimentation event; metazoan meiofauna; adriatic sea; fatty-acid; phytodetritus; community; microalgae; deposition]