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Chronic oil pollution related to gas and oil drilling activities is increasing in the sea due to the rising offshore petroleum industry activity. Among marine organisms, zooplankton play a crucial role in the marine ecosystem and therefore understanding the effects of crude oil chronic exposure on zooplankton is needed to determine the impact of oil in marine environments. The present study reports on the effect of crude oil on adult northern krill, Meganyctiphanes norvegica, collected during three seasons. Their sensitivity to oil was examined with oil concentration of 0.01 versus 0.1 mg oil L− 1 and photo-modified oil in flowing seawater maintained in the dark for 2 weeks at in situ temperature. Oil (polycyclic aromatic hydrocarbons, PAHs) entered the krill (on average, 350 and 4400 μg·kg− 1 wet weight in low and medium oil treatments respectively) and a larger fraction of the krill exhibited digestive gland pathologies (enhanced apoptosis and pathology of digestive tubules) in oil treatments (27–80%) compared to a significantly lower fraction (7–13%) in treatments that received no oil. However, 2-week oil exposure at these concentrations did not significantly decrease survivorship or impair basic functioning such as feeding and respiration rates. Similarly, there were only limited changes in the transcription of 7 selected genes from head tissue. Additionally, although there was significant seasonal variation in krill total lipid content and fatty acid composition, there was no treatment effect on both these parameters, which suggests limited oxidative stress under experimental conditions. Furthermore, there was no significant treatment effect on two direct measures of oxidative stress (MDA: malondialdehyde and AOPP: advanced oxidation protein products) in any of the seasons. Nevertheless, histology clearly revealed enhanced digestive gland pathologies in krill even at low concentrations. Although krill with such pathologies continue to survive, their accumulation of PAHs may be transferred up the food chain, impacting their predators and the wider ecosystem.

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.

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.

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.

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.

Although organic matter (OM) settling on the seafloor is generally rapidly recycled, a key ecological process, large scale burial events manifest itself in the marine sedimentary record as organic carbon (C(org))-rich layers. Presently, this prevails under certain oceanic settings such as the oxygen minimum zones (OMZ) where OM accumulates in underlying sediments. A basic question that remains is as to what extent this C(org) accumulation reflects ecological "malfunctioning" or a shunting of ecological processes? Experimenting with eastern Arabian Sea OMZ sediment we found no evidence that C(org) accumulation here is not due to trophic satiation or to low tolerance of biota to severe oxygen depletion. However, we found direct evidence that suggests that the OMZ sediment C(org) has very low bioavailability that probably impairs biological transformation.In the first set of experiments, the impact of oxygenation on the benthic ecological functioning was examined by following the fate of fresh, highly degradable OM ((13)C-labelled diatoms) in intact sediment cores incubated for 7 days under normoxic versus suboxic bottom water conditions. Tracer organic matter assimilation (by bacteria and fauna) and respiration was evident and similar under both treatments and demonstrates that the benthic response was not hindered by severe oxygen depletion. Furthermore, relatively low biomass standing stock of fauna and bacteria, in spite of sediment high C(org) content, together with this clear uptake of fresh tracer OM suggest that the benthic community was not food saturated.In a second set of experiments, the bioavailability of in situ OMZ organic matter was determined directly through CO(2) production rate measurements in bottle sediment-water slurry incubations. In sharp contrast to fresh tracer algal carbon which had a half-life 0.07 years, the OMZ surficial sediment OM half-life was similar to 67 years already in very early diagenesis. Clearly, a distinct difference in functioning and indicates that a large fraction of OMZ sediment organic matter is evidently excluded from immediate first-hand biotic transformation but on its own represents a link between the "fast (biological)" and the "slow (geological)" carbon cycle along the continuum of OM recycling. Furthermore, while this rapid shift out of the "fast" biological cycle may be common or characteristic of large scale OM accumulation events, a comparison with an ancient C(org)-deposit suggest that the trigger mechanisms may not be uniform.

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.

Meiofauna (small-sized Metazoa and Foraminifera) may constitute a significant part of seafloor biomass and potentially play an important role in benthic metabolism. However, respiration measurements are limited and the methods used are diverse together complicating comparison or upscaling. Here we describe a novel glass micro-respiration chamber used to perform non-invasive measurements (built-in oxygen-sensitive fluorogenic membrane and stirrer) and together with direct organic carbon measurements report initial biomass-specific respiration rates of common intertidal meiofauna. Results indicate large differences between respiration rates of different taxa (biomass 0.7–5.2 µg C per individual) but very similar organic carbon biomass-specific respiration rates (1.6–2.5 µl O2 h-1 mgC-1 or on average 2.0 ± 0.3 µl O2 h-1 mgC-1). This new, rapid and accurate method allows the study of metabolic allometry of the different small-sized taxa and determining their functional role in benthic metabolism.

We investigated tolerance to, and activity of, nematodes in hypoxic and anoxic/partially sulphidic conditions and their ability for recovery after reoxygenation of anoxic sediment. To this end, sediments from an intertidal flat were incubated under oxic, suboxic and anoxic/partially sulphidic conditions for a 14-day period and the final density of nematodes, as a group, and of the most abundant species were assessed. In one treatment, oxygen was restored after anoxic incubation. The incorporation of 13C, originating from labeled algae added on top of the sediment, was taken as an indication of nematode activity. Short-term suboxic and anoxic/partially sulphidic conditions had similar structuring impact on the nematode community, reducing total densities by about one third. Survival in suboxic and anoxic/partially sulphidic conditions was species-specific. Daptonema setosum, D. tenuispiculum and Chromadora macrolaima, dominant in the oxic incubation, disappeared when the oxygen level was reduced. The density of the other dominant species was slightly reduced (Sabatieria pulchra), similar (Terschellingia communis) or even increased in the suboxic and anoxic conditions (Metachromadora vivipara). The activity level of these three species was, however, reduced under oxygen limitation. Our results are discussed in terms of the life-history strategies of these species.

In situ and on-board pulse-chase experiments were carried out on a sublittoral fine sand in the German Bight (southern North Sea) to investigate the hypothesis that sandy sediments are highly active and have fast turnover rates. To test this hypothesis, we conducted a series of experiments where we investigated the pathway of settling particulate organic carbon through the benthic food web. The diatom Ditylum brightwellii was labelled with the stable carbon isotope 13C and injected into incubation chambers. On-board incubations lasted 12, 30 and 132 h, while the in situ experiment was incubated for 32 h. The study revealed a stepwise short-term processing of a phytoplankton bloom settling on a sandy sediment. After the 12 h incubation, the largest fraction of recovered carbon was in the bacteria (62%), but after longer incubation times (30 and 32 h in situ) the macrofauna gained more importance (15 and 48%, respectively), until after 132 h the greatest fraction was mineralized to CO2 (44%). Our findings show the rapid impact of the benthic sand community on a settling phytoplankton bloom and the great importance of bacteria in the first steps of algal carbon processing. [KEYWORDS: 13C labelling experiment ; bacteria ; macrofauna ; mineralization ; sandy sediment]

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]