Structure and composition of the consolidated mud tube of Maldane sarsi (Polychaeta: Maldanidae)

Maldanid polychaetes can be important components of marine benthic communities, playing significant roles in particle subduction or sediment irrigation. Many maldanids are known to inhabit tubes consisting of sediments consolidated by mucus; the structure and composition of these tubes, and their potential impact on benthic environments, are poorly known. We examined the three-dimensional organization of Maldane sarsi tubes, using CT scanning together with analyses of sediment grain size and concentrations of Fe, Mn, organic carbon and bacteria in tube material. M. sarsi tubes consist of stacks of individual consolidated mud disks, surrounding a dense, continuous, inner tube. The tubes of M. sarsi contained fewer fine particles than surrounding sediments, and greater concentrations of Fe, Mn, organic carbon and bacteria, especially in the inner zone. These distributions suggest that tube irrigation affects Fe and Mn oxidation and precipitation in a narrow zone surrounding M. sarsi, and that mucous secretion and potential feeding activities (the hoeing of surface sediments) lead to increases in organic carbon and bacteria in the inner, and deepest parts of the tube. The finding of relict tubes, buried at up to 15 cm depth, indicates a relatively high longevity for these structures and suggests a potential importance in biogeochemical cycling.     

Distribution and quality of sedimentary organic matter on the Aquitanian margin (Bay of Biscay)

During the ECOFER experiment (French ECOMARGE program), surficial sediments were sampled on the Aquitanian margin with box corers and analyzed to determine the quantity and quality of organic matter. Sediments from the margin are enriched in organic carbon (mean value 1.35%) in comparison to deep-sea and shelf sediments, due to a fine grain-size sedimentation. As sedimentation rates are high, the margin appears to be an organic depocenter. Some preferential organic enrichment zones were identified in the Cap-Ferret Canyon. There is a supply of continental material from the Gironde estuary, but marine contribution seems more possible than Adour or spanish rivers. No seasonal variations of organic matter were observed at the surface of sediments, suggesting mineralization processes of labile organic matter: average organic carbon consumption was evaluated to 9.0 g C m⁻² yr⁻¹. Rapid biological mineralization processes are lower than on the Mediterranean margin, mainly related to significant differences in water temperature. The great width of the canyon, its distance from the continent, and the current circulation pattern prevent any precise recording of the variable organic inputs to the sediment and favor nepheloı̈d transport, resuspension and shelf break processes, which wipe out any print of fresh material input. An organic carbon budget indicates that an equilibrium between organic inputs and organic mineralization+accumulation is not obtainable. The supply of suspended matter could have been minor during the year in question, and sedimentation rates are still imprecise.     

Pathways of organic matter through food webs of diverse habitats in the regulated Nakdong River estuary (Korea)

The benthic macroinvertebrates of the Nakdong River estuary were sampled at three different habitats: two salt marsh (Scirpus triqueter and Phragmites australis) beds and a bare intertidal flat. Fishes were sampled in the main channel. The trophic importance of marsh vascular plants, microphytobenthos, and riverine and channel particulate organic matter to macroinvertebrate and fish production was studied using stable carbon and nitrogen isotope tracers. There was a dramatic change in coverage of macrophytes (salt marshes and seagrass) after the construction of an estuarine barrage in 1987 in the Nakdong River estuary, with the S. triqueter bed increasing, the P. australis bed decreasing, and Zostera marina habitats being nearly lost. Although the invertebrate δ¹³C were within a narrower range than those of the primary producers, the values varied considerably among consumers in these habitats. However, the isotope signatures of consumers showed similarities among different habitats. Cluster analysis based on their isotopic similarity suggested that the isotope variability among species was related more to functional feeding groups than to habitats or taxonomic groups. While δ¹³C values of suspension feeders were close to that of the channel POM (mainly phytoplankton), other benthic feeders and predators had δ¹³C similar to that of microphytobenthos. Isotopic mixing model estimates suggest that algal sources, including microphytobenthos and phytoplankton, play an important role in supporting the benthic food web. Despite the huge productivity of emergent salt marshes, the contribution of the marsh-derived organic matter to the estuarine food webs appears to be limited to some nutrition for some invertebrates just within marsh habitats, with little on the bare intertidal flats or in the channel fish communities. Isotope signatures of the channel fishes also confirm that algal sources are important in supporting fish nutrition. Our findings suggest that benthic and pelagic microalgae made a large contribution to consumer diets, while marsh plants may not have a large role in supporting food webs in this estuarine system.     

Nursery fidelity,food web interactions and primary sources of nutrition of the juveniles of Solea solea and S. senegalensis in the Tagus estuary (Portugal): A stable isotope approach

Stable carbon and nitrogen isotopes were used to assess site fidelity of Solea solea and Solea senegalensis juveniles, to investigate food web interactions and to determine the dominant nutrient pathways in two nursery areas in the Tagus estuary, Portugal. Samples of water from the main sources and from the nursery areas and respective saltmarsh creeks were collected for isotope analysis, as well as sediment, benthic microalgae, saltmarsh halophytes, S. solea, S. senegalensis and its main prey, Nereis diversicolor, Scrobicularia plana and Corophium spp. While site fidelity was high in 0-group juveniles, it was lower for 1-group juveniles, possibly due to an increase in mobility and energy demands with increasing size. Analysis of the food web revealed a complex net of relations. Particulate organic matter from the freshwater sources, from each nursery's waters and saltmarsh creeks presented similar isotopic composition. Sediment isotopic composition and saltmarsh halophytes also did not differentiate the two areas. All components of the food web from the benthic microalgae upwards were isotopically different between the nursery areas. These components were always more enriched in δ¹³C and δ¹⁵N at the lower nursery area than at the nursery located upstream, appearing as if there were two parallel trophic chains with little trophic interaction between each other. A mixture of carbon and nitrogen sources is probably being incorporated into the food web. The lower nursery area is more dependent upon an isotopically enriched energy pathway, composed of marine particulate organic matter, marine benthic microalgae and detritus of the C₄ saltmarsh halophyte Spartina maritima. The two nursery areas present a different level of dependence upon the freshwater and marine energy pathways, due to hydrological features, which should be taken into account for S. solea and S. senegalensis fisheries and habitat management.     

On the oxidation and burial of organic carbon in sediments of the Iberian margin and Nazaré Canyon (NE Atlantic)

As a contribution to the EC-OMEX-II program, sediment carbon and nitrogen budgets are presented for the Iberian Margin (northeastern Atlantic). The budgets for degradable organic carbon and associated nitrogen were calculated from sediment and pore water properties, using a steady-state version of a numerical coupled diagenetic model, OMEXDIA. Data were collected throughout the major upwelling period along five transects, four of which were located on the open margin and one positioned in a major submarine canyon, the Nazaré Canyon.A comparison of in situ oxygen profiles measured with monocathodic microelectrodes and with Clark type microelectrodes showed that monocathodic electrodes overestimate the oxygen concentration gradient near the sediment–water interface. This artifact probably results from the loss in sensitivity of the monocathodic microelectrode during profiling. Shipboard time course measurements with Clark type electrodes demonstrated transient conditions upon sediment retrieval on deck and indicated enhanced rates of oxygen consumption in the surface sediment, presumably as a result of lysis or exudation of oxidisable substrates by infauna. As a result, oxygen fluxes calculated from shipboard oxygen profiles overestimated in situ fluxes by up to a factor of 5 for water depths >1000 m.The sediments from the canyon and from a depositional area on the shelf were enriched in organic carbon (3–4.5 wt%) relative to the open margin stations (0.5–2 wt%) and showed C/N ratios exceeding Redfield stoichiometry for marine organic matter, indicating there was deposition of organic carbon of terrestrial origin in these areas. The oxidation of organic carbon on the open margin declined from ˜11 gCm⁻²y⁻¹ on the shelf to 2 gCm⁻²y⁻¹ at 5000 m water depth, and was dominated by aerobic oxidation. The reactivity of the degradable organic carbon at the time of deposition was <2.5 y⁻¹ on the shelf, and declined to <0.5 y⁻¹ offshore. The burial of refractory organic carbon at the stations along the open margin transects also declined with increasing water depth from ˜5 gCm⁻²y⁻¹ on the shelf to <1 gCm⁻²y⁻¹ at 2000 m depth, whereas the burial of particulate inorganic carbon declined from ˜20 gCm⁻²y⁻¹ to <5 gCm⁻²y⁻¹. A comparison of the estimated total organic carbon deposition and predicted delivery for the shelf suggest that 58 to 165 gCm⁻²y⁻¹ is oxidized in the water column, laterally advected, or focused into one of the canyons.Anaerobic oxidation, denitrification and, therefore, total oxidation of organic carbon was enhanced within the canyon relative to the open margin. Total organic carbon oxidation decreased with water depth from 22 gCm⁻²y⁻¹ at the head of the canyon to 3 gCm⁻²y⁻¹ over its fan. The reactivity of the organic carbon deposited in the canyon was lower than those of the shelf stations, suggesting that the canyon is being enriched in older, laterally advected organic matter. The burial of refractory organic carbon in sediments from the Nazaré Canyon was considerably higher than in the sediments from the open margin; it also decreased with depth from 20 gCm⁻²y⁻¹ at 343 m to ˜2.5 gCm⁻²y⁻¹ at 4298 m water depth. The burial of particulate inorganic carbon was slightly lower than that of refractory organic carbon.The burial of refractory organic carbon and the deposition of degradable organic carbon were both positively correlated with the sedimentation rates for the Iberian Margin, and indicated burial efficiencies were 0.6 to 48%. A single trend for burial efficiency versus sedimentation rate for both the canyon and the open margin indicates that the sedimentation rate was the master variable for the geographical distribution of organic carbon oxidation and carbon preservation on the NW Iberian Margin.     

Organic matter budget in the Southeast Atlantic continental margin close to the Congo Canyon: In situ measurements of sediment oxygen consumption

A study of organic carbon mineralization from the Congo continental shelf to the abyssal plain through the Congo submarine channel and Angola Margin was undertaken using in situ measurements of sediment oxygen demand as a tracer of benthic carbon recycling. Two measurement techniques were coupled on a single autonomous platform: in situ benthic chambers and microelectrodes, which provided total and diffusive oxygen uptake as well as oxygen microdistributions in porewaters. In addition, sediment trap fluxes, sediment composition (Org-C, Tot-N, CaCO₃, porosity) and radionuclide profiles provided measurements of, respectively input fluxes and burial rate of organic and inorganic compounds.The in situ results show that the oxygen consumption on this margin close to the Congo River is high with values of total oxygen uptake (TOU) of 4±0.6, 3.6±0.5 mmol m⁻² d⁻¹ at 1300 and 3100 m depth, respectively, and between 1.9±0.3 and 2.4±0.2 mmol m⁻² d⁻¹ at 4000 m depth. Diffusive oxygen uptakes (DOU) were 2.8±1.1, 2.3±0.8, 0.8±0.3 and 1.2±0.1 mmol m⁻² d⁻¹, respectively at the same depths. The magnitude of the oxygen demands on the slope is correlated with water depth but is not correlated with the proximity of the submarine channel–levee system, which indicates that cross-slope transport processes are active over the entire margin. Comparison of the vertical flux of organic carbon with its mineralization and burial reveal that this lateral input is very important since the sum of recycling and burial in the sediments is 5–8 times larger than the vertical flux recorded in traps.Transfer of material from the Congo River occurs through turbidity currents channelled in the Congo valley, which are subsequently deposited in the Lobe zone in the Congo fan below 4800 m. Ship board measurements of oxygen profiles indicate large mineralization rates of organic carbon in this zone, which agrees with the high organic carbon content (3%) and the large sedimentation rate (19 mm y⁻¹) found on this site. The Lobe region could receive as high as 19 mol C m⁻² y⁻¹, 1/3 being mineralized and 2/3 being buried and could constitute the largest depocenter of organic carbon in the South Atlantic.     

Fisheries for orange roughy (Hoplostethus atlanticus) on seamounts in New Zealand

Major commercial fisheries for orange roughy (Hoplostethus atlanticus) occur on seamount features, which are widely distributed throughout the New Zealand region. When the fishery developed in the late 1970s to early 1980s, it occurred mainly on flat bottom, but over time has become more focused on seamounts. In the 1995–1996 fishing year, it is estimated that about 70 % of the catch of orange roughy within the New Zealand EEZ was taken from seamounts. Seamounts on the Chatham Rise have been fished for over ten years. Examination of commercial catch and effort data show strong declines in catch rates over time, and a pattern of serial depletion of seamount populations, with the fishery moving progressively eastwards to unfished seamounts along the southern margins of the Rise. Catch rates on seamounts in other regions of New Zealand have also generally shown a similar pattern of rapid decline. There is growing concern over the impact of trawling on seamounts, and the effects this can have on the benthic habitat and fauna, and the long-term sustainability of associated commercial fisheries.     

Importance of seagrass as a carbon source for heterotrophic bacteria in a subtropical estuary (Florida Bay)

A stable carbon isotope approach was taken to identify potential organic matter sources incorporated into biomass by the heterotrophic bacterial community of Florida Bay, a subtropical estuary with a recent history of seagrass loss and phytoplankton blooms. To gain a more complete understanding of bacterial carbon cycling in seagrass estuaries, this study focused on the importance of seagrass-derived organic matter to pelagic, seagrass epiphytic, and sediment surface bacteria. Particulate organic matter (POM), seagrass epiphytic, seagrass (Thalassia testudinum) leaf, and sediment surface samples were collected from four Florida Bay locations with historically different organic matter inputs, macrophyte densities, and primary productivities. Bulk (observed and those reported previously) and compound-specific bacterial fatty acid δ¹³C values were used to determine important carbon sources to the estuary and benthic and pelagic heterotrophic bacteria. The δ¹³C values of T. testudinum green leaves with epiphytes removed ranged from −9.9 to −6.9‰. Thalassia testudinum δ¹³C values were significant more enriched in ¹³C than POM, epiphytic, and sediment samples, which ranged from −16.4 to −13.5, −16.2 to −9.6, and −16.7 to −11.0‰, respectively. Bacterial fatty acid δ¹³C values (measured for br14:0, 15:0, i15:0, a15:0, br17:0, and 17:0) ranged from −25.5 to −8.2‰. Assuming a −3‰ carbon source fractionation from fatty acid to whole bacteria, pelagic, epiphytic, and sediment bacterial δ¹³C values were generally more depleted in ¹³C than T. testudinum δ¹³C values, more enriched in ¹³C than reported δ¹³C values for mangroves, and similar to reported δ¹³C values for algae. IsoSource mixing model results indicated that organic matter derived from T. testudinum was incorporated by both benthic and pelagic bacterial communities, where 13–67% of bacterial δ¹³C values could arise from consumption of seagrass-derived organic matter. The IsoSource model, however, failed to discriminate clearly the fraction of algal (0–86%) and mangrove (0–42%) organic matter incorporated by bacterial communities. These results indicate that pelagic, epiphytic, and sediment surface bacteria consumed organic matter from a variety of sources. Bacterial communities incorporated consistently seagrass-derived organic matter, the dominant macrophyte in Florida Bay, but seagrass δ¹³C values alone could not account fully for bacterial δ¹³C values.     

The trophic position of the alien crab <Emphasis Type="Italic">Rhithropanopeus harrisii</Emphasis> (crustacea decapoda panopeidae) in the Taman Bay,Sea of Azov community

This work concerns the trophic web positioning of the alien crab Rhithropanopeus harrisii and other common marine invertebrate species and fishes in the benthic ecosystem of the shallows of Taman Bay, Sea of Azov. The base of the trophic web in this system is composed of phytoplankton, macrophytes (algae and marine grasses), and reeds that use atmospheric carbon for photosynthesis. Analysis of the isotopic composition of nitrogen and carbon has shown that although marine grasses are dominating primary producers in the shallows of the bay, primary consumers (such as Cerastoderma glaucum, Porifera gen. sp., Gammarus aequicauda, Deshayesorchestia deshayesii and Idotea balthica) only partially use this organic source; instead, they use a combination of different sources of primary production. It has been shown that the food source of the alien crab is primarily of animal origin. In Taman Bay, R. harrisii is on the same trophic level as other carnivores/scavengers: benthic fishes Syngnathus nigrolineatus, Gobius spp. and native crab Pilumnus hirtellus and shrimp Palaemon adspersus.     

Transfer of organic carbon on the Moroccan Atlantic continental margin (NW Africa): productivity and lateral advection

Land—ocean transfer of sediment and organic matter along the Moroccan Atlantic margin (NW Africa) seems to have been very effective during the last 130 ka. In a marine core from this region, we found total organic carbon (TOC) values ranging from 0.3 to 1.7 dry wt% of bulk sediments. These relatively high values are fairly unusual, as the core was recovered from an open-ocean environment that is currently oligotrophic. In order to explain this trend, more typical of an upwelling eutrophic setting, three processes were evaluated: (1) in situ primary production associated with the extension of the Cape Ghir upwelling filament, (2) bottom water conditions that may favour organic carbon preservation and (3) lateral organic carbon advection. The site occasionally experienced more eutrophic conditions, especially during termination I; here, we recorded a relative high abundance of the planktonic foraminifer Globigerina bulloides, suggesting high primary production. However, given the absence of correlation between TOC and G. bulloides records, high TOC storage cannot be attributed exclusively to primary production. Preservation factors such as bottom water ventilation are also ruled out. Lateral TOC advection seems to be the most plausible process. Today, lateral advection and offshore transport of nutrients and organic matter characterize the study region. However, the triggering mechanisms deserve further investigation. Different controlling factors influencing the mobilization and advection of organic carbon from coastal upwelling sites to the deep basin are discussed. The correlation found between down-core TOC and sea-level changes suggests sea-level fluctuations as the most effective mechanism driving nepheloid layer detachment and seaward material transport.     

Seasonal Changes in Quality and Quantity of Food Available for Benthic Suspension-feeders in the Golfo Marconi (North-western Mediterranean)

Suspended particulate matter was collected, from the water layer at 10 cm above the sediments, over a period of 13 months in the Golfo Marconi (Ligurian Sea, NW Mediterranean). Measurements of seston concentration as well as the elemental (particulate organic carbon and nitrogen; POC and PON, respectively) and biochemical composition (lipids, proteins, carbohydrates, DNA) of particulate organic matter were carried out to assess quality and quantity of food potentially available to benthic suspension-feeders. Particulate organic matter showed wide qualitative and quantitative variations during the sampling year. Seston concentrations and POC did not reflect the quantity and quality of the food available to benthic suspension-feeders. The biopolymeric fraction of particulate matter (C-BPF, i.e. the sum of lipid, protein and carbohydrate carbon) was mostly composed of phytoplankton (which accounted for about 60% of C-BPF). The ratio of C-BPF to POC was utilized as a measure of the fraction which had the potential to be more readily available to consumers. Suspended organic matter showed higher values of the C-BPF:POC ratio during spring, and lower values in summer and autumn–winter. Quantitative estimates of the energy content of the suspended particulate matter were obtained from its biochemical composition. Bacterial dynamics were significantly related to changes in phytoplankton biomass. Bacteria accounted for a significant fraction of the biopolymeric carbon pool (annual average about 15%) and of the total particulate DNA (21·5%), thus enhancing the nutritional value of the particulate organic matter. The results achieved in this study indicate that the biochemical composition of the particulate matter provides additional information on the origin, quality and characteristics of the seston more readily available to benthic suspension-feeders.     

Sedimentary labile organic carbon and pore water redox control on species distribution of benthic foraminifera: A case study from Lisbon-Setúbal Canyon (southern Portugal)

Rose Bengal stained benthic foraminifera were studied from 11 cores collected along two depth transects off southern Portugal: one in the Lisbon-Setúbal Canyon and the other along the canyon edge. The total standing stocks and distribution of foraminifera were investigated in relation to sediment and pore water geochemistry. Nitrate was used as a redox indicator, sedimentary chlorophyll a and CPE (chloroplastic pigment equivalents) contents as a measure of labile organic matter, and total organic carbon as a measure of bulk organic matter availability.The canyon sediments were enriched in organic carbon and phytopigments at all water depths in comparison with the canyon edge. Water depth seemed to control sedimentary phytopigment content, but not total organic carbon. No significant correlation was seen between pigment and total organic carbon content.The abundance of calcareous foraminifera correlated with the phytodetritus content, whereas a weaker correlation was observed for the agglutinated taxa. Therefore, calcareous foraminifera appear to require a fresher food input than agglutinated taxa. The foraminiferal species composition also varied with pigment content and nitrate penetration depth in the sediment, in line with the TROX concept. Phytopigment-rich (surficial CPE content >20 μg/cm³) sediments with a shallow nitrate penetration depth (∼1 cm depth) were inhabited by generally infaunal species such as Chilostomella oolina, Melonis barleeanus and Globobulimina spp. As the nitrate penetration increased to ∼2 cm depth in sediment and the pigment content remained relatively high (>15 μg/cm³), Uvigerina mediterranea and Uvigerina elongatastriata became dominant species. With declining CPE content and increasing nitrate penetration depth, the foraminiferal assemblages changed from the mesotrophic Cibicides kullenbergi-Uvigerina peregrina assemblage to the oligotrophic abyssal assemblage, mainly consisting of agglutinated taxa.     

The partitioning of organic carbon fluxes and sedimentary organic matter decomposition rates in the ocean

The partitioning of annual organic carbon fluxes from five stations located in the vicinity of the Pacific-Antarctic Ridge and the Peru continental margin suggests that 35–85% of the total near-bottom organic carbon flux is utilized at or near the sediment-water interface. These estimates have large uncertainties, but illustrate that assessments of organic carbon utilization can be made by several stepwise approaches which are generally applicable to a wide spectrum of marine environments.In one approach, the mineralization of organic carbon from the sediments was predicted from both sedimentary organic carbon and pore water nutrient profiles with comparable results. Neglecting sediment mixing, the rate constants of the anoxic sediments off Peru range from 0.1 × 10^?3 to 4 × 10^?3 y^?1, and rate constants derived for oxic SW Pacific sediments range from 3 × 10^?4 to 7 × 10^?4 y^?1. As with other values reported for sulfate reducing sediments by Toth and Lerman (1977) and for oxic central Pacific sediments by Müller and Mangini (1980), log-log plots of rate constants vs. sedimentation rate define two parallel linear relationships for oxic and anoxic sediments, respectively. The apparently enhanced rates for oxic environments may result from large benthic organisms which redistribute a portion of the available detritus and in doing so convert it into more easily accessible and metabolizable organic matter. In low-oxygen environments, bottom feeders and infauna are less abundant and more likely to irrigate rapidly accumulating sediments.     

Comparison of benthic foraminifera inside and outside a sulphur-oxidizing bacterial mat from the present oxygen-minimum zone off Pakistan (NE Arabian Sea)

Assemblages of live (Rose-Bengal-stained) and dead benthic foraminifera and stable oxygen and carbon isotopic composition of live benthic foraminifera were studied in and outside a bacterial mat composed of the large sulphur-oxidizing bacteria Thioploca and Beggiatoa from the oxygen-minimum zone off Pakistan (NE Arabian Sea). Two cores from the same Multicorer retrieved a bacterial mat and ambient sediment. The dominant species (Globobulimina affinis, G. turgida, Bolivina pacifica, B. pseudopunctata, Uvigerina peregrina and Buliminella tenuata) in both cores are characteristic for dysoxic oxygen minimum zone conditions. The most significant difference between the two cores is the reduced number of stained benthic foraminifera (SBF) in the top 0.5 cm of the bacterial mat. Faunal densities of stained species are more than four times higher in the sediment surface sample (0–0.5 cm) outside the bacterial mat, at a distance of only 1.5 m. All stained species, however, observed outside the Beggiatoa/Thioploca mat were also observed in the core with the mat. Two species, Virgulinella fragilis and Bolivina dilatata, occur exclusively in the core with the bacterial mat. The diversity within the bacterial mat core is thus slightly higher than outside. Furthermore, the abundances of the species Buliminella morgani, B. tenuata and Alliatina primitiva are substantially higher in the bacterial mat than outside. Globobuliminids, on the other hand, seem to prefer the conditions outside the bacterial mat and are five times more frequent in the core taken outside the bacterial mat. Benthic foraminifers inhabit a broader microhabitat range outside the bacterial mat (∼5 cm) than within (3.5 cm). A marked decrease in SBF abundances was observed at the level of a black sulphur-rich layer which is interpreted to mark the shallow redox front below the bacterial mat. Stable carbon isotope analyses on live benthic foraminifera do not support a relation of the investigated Beggiatoa/Thioploca mat to a constant or seasonal seepage of methane at the continental slope off Pakistan. Surprisingly, however, stable oxygen isotope values of many species and especially of U. peregrina decrease with depth, which calls into question the suitability of U. peregrina as a recorder of bottom-water δ¹⁸O.     

Stable isotopes in deep-sea living (stained) foraminifera from the Mozambique Channel (eastern Africa): multispecies signatures and paleoenvironmental application

Oxygen and carbon isotopes (δ¹⁸O and δ¹³C) have been investigated in carbonate tests of deep-sea foraminifera living in the Mozambique Channel (eastern Africa) to understand how environmental constraints (e.g., organic matter, oxygenation) control the intra- and interspecific variability of isotopic signatures. 197 living individuals, including eight different species, from various microhabitats within the sediment were sorted from sediment samples gathered at two stations on the Malagasy upper slope. Results show that the δ¹⁸O values of foraminiferal taxa were not controlled by microhabitat pattern. They presented tremendous and intriguing intraspecific variability that is not explained by the classical ontogenetic effect. The δ¹³C values of infaunal foraminiferal taxa do not show a 1:1 relationship with the bottom water δ¹³C DIC and do not present a constant offset from it; instead, they appear to be mainly controlled by a microhabitat effect. The lower δ¹³C values of shallow, intermediate, and deep infaunal taxa at the deeper station compared to those seen at the shallower station reflect the enhanced exportation of sedimentary organic matter at the sediment–water interface, and its related mineralization within the upper sediments. The ?δ¹³C between shallow/very shallow infaunal species (i.e., Hoeglundina elegans, Uvigerina hispida) and intermediate/deep infaunal species (i.e., Melonis barleeanus, Globobulimina barbata) permits insight into (1) the exportation of organic matter to the seafloor and (2) the various degradation pathways for organic detritus in the benthic environments off NW Madagascar.     

Benthic organic carbon flux and oxygen penetration reflect different plankton provinces in the Southern Ocean

For the investigation of organic carbon fluxes reaching the seafloor, oxygen microprofiles were measured at 145 sites in different sub-regions of the Southern Ocean. At 11 sites, an in situ oxygen microprofiler was deployed for the measurement of oxygen profiles and the calculation of organic carbon fluxes. At four sites, both in situ and ex situ data were determined for high latitudes. Based on this data set as well as on previous published data, a relationship was established for the estimation of fluxes derived by ex situ measured O₂ profiles. The fluxes of labile organic matter range from 0.5 to 37.1 mg C m^?2 d^?1. The high values determined by in situ measurements were observed in the Polar Front region (water depth of more than 4290 m) and are comparable to organic matter fluxes observed for high-productivity, upwelling areas like off West Africa. The oxygen penetration depth, which reflects the long-term organic matter flux to the sediment, was correlated with assemblages of key diatom species. In the Scotia Sea (～3000 m water depth), oxygen penetration depths of less than 15 cm were observed, indicating high benthic organic carbon fluxes. In contrast, the oxic zone extends down to several decimeters in abyssal sediments of the Weddell Sea and the southeastern South Atlantic. The regional pattern of organic carbon fluxes derived from microsensor data suggests that episodic and seasonal sedimentation pulses are important for the carbon supply to the seafloor of the deep Southern Ocean.     

Spatio-temporal changes in the distribution of phytopigments and phytoplanktonic groups at the Porcupine Abyssal Plain (PAP) site

We have made a comprehensive study of pigment distributions and microscopically determined phytoplankton abundances within the Porcupine Abyssal Plain (PAP) location in the North Atlantic to better understand phytoplankton variability, and make some suggestions regarding the composition of the material falling to the sea bed and its impacts on benthic organisms such as Amperima rosea. The area has been the focus of many studies of ocean fluxes and benthic communities over recent years, but little attention has been given to the spatio-temporal variability in the surface waters. Dawn casts over a 12-day period at the PAP mooring site (48.83°N 16.5°W) revealed the presence of only one species, the diatom Actinocyclus exiguus, at bloom concentrations for just 5 days. Smaller populations of other diatoms and the dinoflagellates Gymnodinium and Gyrodinium were also present at this time. Following this 5-day interval, a mixed population of small–sized dinoflagellates, prymnesiophytes, prasinophytes, chrysophytes and cyanobacteria occurred. It is clear from concomitant CTD/bottle surveys that rapid changes in phytoplankton community structure at a fixed time series position do not necessarily reflect a degradation or manifestation of one particular species but rather represent the movement of eddies and other water masses within very short timescales. These cause substantial variability in the species class and size fraction that may explain the variability in carbon export that has been seen at the PAP site. We also make some suggestions on the variable composition of the material falling to the seabed and its impact on benthic organisms such as Amperima rosea.     

Downward particle fluxes within different productivity regimes off the Mauritanian upwelling zone (EUMELI program)

A 2-yr record of downward particle flux was obtained with moored sediment traps at several depths of the water column in two regions characterized by different primary production levels (mesotrophic and oligotrophic) of the eastern subtropical North Atlantic Ocean. Particle fluxes, of ∼71–78% biogenic origin (i.e. consisting of CaCO₃, organic matter and opal) on average, decrease about six-fold from the mesotrophic site (highest fluxes in the North Atlantic) nearer the Mauritanian margin (18°30′N, 21°00′W) to the remote, open-ocean, oligotrophic site (21°00′N, 31°00′W). This decrease largely reflects the difference in total primary production between the two sites, from ∼260 to ∼110 g organic C m⁻² yr⁻¹. At both sites, temporal variability of the downward particle flux seems to be linked to westward surface currents, which are likely to transport seaward biomass-rich water masses from regions nearer the coast. The influence of coastal upwelling is marked at the mesotrophic site. The large differences between the 1991 and 1992 records at that site, where carbon export is large, underscore the interest of long-term studies for export budget estimates. The different productivity regimes at the two sites seem to induce contrasting downward modes of transport of the particulate matter, as shown in particular by the faster settling rates and the higher E ratio (particulate organic carbon export versus total primary production) estimated at the mesotrophic site.     

Carbon flows in the benthic food web at the deep-sea observatory HAUSGARTEN (Fram Strait)

The HAUSGARTEN observatory is located in the eastern Fram Strait (Arctic Ocean) and used as long-term monitoring site to follow changes in the Arctic benthic ecosystem. Linear inverse modelling was applied to decipher carbon flows among the compartments of the benthic food web at the central HAUSGARTEN station (2500 m) based on an empirical data set consisting of data on biomass, prokaryote production, total carbon deposition and community respiration. The model resolved 99 carbon flows among 4 abiotic and 10 biotic compartments, ranging from prokaryotes up to megafauna. Total carbon input was 3.78±0.31 mmol C m⁻² d⁻¹, which is a comparatively small fraction of total primary production in the area. The community respiration of 3.26±0.20 mmol C m⁻² d⁻¹ is dominated by prokaryotes (93%) and has lower contributions from surface-deposit feeding macro- (1.7%) and suspension feeding megafauna (1.9%), whereas contributions from nematode and other macro- and megabenthic compartments were limited to <1%. The high prokaryotic contribution to carbon processing suggests that functioning of the benthic food web at the central HAUSGARTEN station is comparable to abyssal plain sediments that are characterised by strong energy limitation. Faunal diet compositions suggest that labile detritus is important for deposit-feeding nematodes (24% of their diet) and surface-deposit feeding macrofauna (∼44%), but that semi-labile detritus is more important in the diets of deposit-feeding macro- and megafauna. Dependency indices on these food sources were also calculated as these integrate direct (i.e. direct grazing and predator–prey interactions) and indirect (i.e. longer loops in the food web) pathways in the food web. Projected sea-ice retreats for the Arctic Ocean typically anticipate a decrease in the labile detritus flux to the already food-limited benthic food web. The dependency indices indicate that faunal compartments depend similarly on labile and semi-labile detritus, which suggests that the benthic biota may be more sensitive to changes in labile detritus inputs than when assessed from diet composition alone. Species-specific responses to different types of labile detritus inputs, e.g. pelagic algae versus sympagic algae, however, are presently unknown and are needed to assess the vulnerability of individual components of the benthic food web.     

Food web structure of the coastal area adjacent to the Tagus estuary revealed by stable isotope analysis

The identification of energy sources, pathways and trophic linkages among organisms is crucial for the understanding of food web dynamics. Stable isotopes were used to identify the trophic level of food web components and track the incorporation of organic matter of different origins in the coastal ecosystem adjacent to the Tagus estuary. It was shown that the river Tagus is a major source of organic carbon to this system. Also, the wide difference in δ¹³C among the primary consumers allowed the identification of the pelagic and the benthic energy pathways. The maximum trophic level observed was 2.4 for Sepia officinalis. This value is indicative of a short food web. It was concluded that the diet of the upper trophic level species relies directly on the lower food web levels to a considerable extent, instead of relying mostly on intermediate trophic level species. Moreover, the δ¹⁵N values of primary consumers were very close to that of particulate organic matter, probably due to poorly known processes occurring at the basis of the food web. This lowers the trophic length of the whole food web. Reliance on benthic affinity prey was high for all upper trophic level secondary consumers.