Nutrient exchange across the sediment-water interface in the Potomac River estuary

The flux of ammonia, phosphate, silica and radon-222 from Potomac tidal river and estuary sediments is controlled by processes occurring at the sediment-water interface and within surficial sediment. Calculated diffusive fluxes range between 0·6 and 6·5 mmol m^?2 day^?1 for ammonia, 0·020 and 0·30 mmol m^?2 day^?1 for phosphate, and 1·3 and 3·8 mmol m^?2 day^?1 for silica. Measured in situ fluxes range between 1 and 21 mmol m^?2 day^?1 for ammonia, 0·1 and 2·0 mmol m^?2 day^?1 for phosphate, and 2 and 19 mmol m^?2 day^?1 for silica. The ratio of in situ fluxes to diffusive fluxes (flux enhancement) varied between 1·6 and 5·2 in the tidal river, between 2·0 and 20 in the transition zone, and from 1·3 to 5·1 in the lower estuary. The large flux enhancements from transition zone sediments are attributed to macrofaunal irrigation. Nutrient flux enhancements are correlated with radon flux enhancements, suggesting that fluxes may originate from a common region and that nutrients are regenerated within the upper 10–20 cm of the sediment column.The low fluxes of phosphate from tidal viver sediments reflect the control benthic sediment exerts on phosphorus through sorption by sedimentary iron oxyhydroxides. In the tidal river, benthic fluxes of ammonia and phosphate equal one-half and one-third of the nutrient input of the Blue Plains sewage treatment plant. In the tidal Potomac River, benthic sediment regeneration supplies a significant fraction of the nutrients utilized by primary producers in the water column during the summer months.     

Anaerobic metabolism in lagoon sediments from Davies Reef,Great Barrier Reef

Sulphate reduction rates were measured in the lagoon sediments at Davies Reef, on the Great Barrier Reef, Australia. Sulphate reduction rates averaged 0·622 mmol sulphate m⁻² day⁻¹, over the 0–12 cm depth of sediment, compared to an average heterotrophic oxygen uptake rate by the sediment of 27 mmol oxygen m⁻² day⁻¹. Thus sulphate reduction to acid-volatile sulphide appeared to account for only about 5% of the total organic matter degradation, although the proportion may be greater if sulphate reduction to tin-reducible sulphide was taken into account. Although dissolved sulphate concentration in the sediment pore water was that of sea water, a large excess (equivalent to about 0·8% of the sediment dry weight) of precipitated sulphate was present in the sediment. This excess could not be attributed to precipitated strontium sulphate, and there was no exchange between the dissolved and precipitated sulphate. Methane formation in these superficial sediments was not detectable.     

Chlorophyll distribution in continental shelf sediments off West Palm Beach,Florida and west end,Bahamas

Measurements were made of chlorophyll-a and phaeophytin-a in calcareous sediments along transects off the east coast of Florida (75–190 m) and the west coast of Grand Bahama Island (170–300 m). Solvent partitioning showed that chlorophyll-a concentrations never exceeded 0·1 mg m^?2 at either location, most as degradation products. Total pigment concentrations (chlorophyll and phaeopigments) ranged from 0·18–1·83 mg m^?2 in sediments off Grand Bahama Island and 2·50 to 20·65 mg m^?2 off West Palm Beach. Pigments, expressed per gram dry weight of sediments, increased with depth across the Florida Continental Shelf. This is probably due to differences in sediment character between near-shore and off-shore sediments.     

Technetium: Uptake,organ distribution and loss in the mussel, Mytilus Californianus (Conrad) and the oyster Crassostrea gigas (Thunberg)

The bivalve molluscs Mytilus californianus and Crassostrea gigas were exposed to filtered seawater containing the isotope ^95mTc added as pertechnetate (TcO₄^?) to study the biokinetic behaviour of this element. Whole body concentration factors for both organisms at equilibrium were small (CF = 1·3–1·5); individual organ concentration factors were highest for the visceral mass in both. Loss rates determined both in the laboratory and under field conditions were biphasic for both molluscs; initial loss of Tc was rapid with slightly more than 50 % of the initial label being lost within 10 days. Thereafter, loss rates decreased and whole body biological half-lives ranged from 80 to 134 days. Neither I^? nor IO₃^? affected the uptake of Tc by M. californianus even when concentrations of these anions exceeded those normally present in seawater by two orders of magnitude.     

An investigation into technetium binding in sediments

Coastal environments with reducing waters and/or sediments represent potential sinks for ⁹⁹Tc discharged to sea. Here, we have examined estuarine sediments from four geochemically distinct locations that have been exposed to ⁹⁹Tc discharged from Sellafield. Both the relative uptake and the operationally defined sediment “component” that holds the Tc were investigated in order to establish whether particular biogeochemical processes are predominately responsible for reducing and binding Tc in sediments. Despite the artefacts that can pose problems for sequential extractions, this scheme [Int. J. Environ. Anal. Chem. 51 (1993) 187] appears to be robust with regard to Tc. The results show that relative uptake of Tc varied greatly between the sites, with the highest occurring at an almost permanently anoxic fjord (Mariager Fjord, Denmark), followed by a brackish, seasonally eutrophic fjord (Roskilde Fjord, Denmark), then a sub-oxic salt marsh (which receives particle bound radionuclides from a more reducing mud patch; Esk Estuary, UK) and finally sulfidic and iron-reducing muddy sandy sediments (Needle's Eye, Solway Firth, Scotland). High relative uptake at the fjords was explained by the greater mixing of Tc-labelled oxic seawater into the part of the system where reduction of TcO₄⁻ was possible. Uptake at Mariager Fjord was higher than at Roskilde Fjord, reflecting the highly reducing geochemical conditions in the water body. Low uptake at Needle's Eye may be related to the speciation of technetium at this site as the sequential extraction data suggest that Tc is associated with the carbonate phase here. Tc carbonates are largely soluble, and this is the first observation of Tc association with carbonate fractions in the natural environment. The other three sites showed that organic matter, in conjunction with reducing conditions, was very important for binding and retaining Tc in sediments. The specific role of sulfides in controlling Tc retention in sediments was unclear because sulfides and organic matter are leached simultaneously in the sequential extraction scheme we applied, but there was evidence that Tc was not associated with acid-volatile iron monosulfides.     

The transport of bacteria in the sediments of a temperate marsh

The number of bacteria in sediments, interstitial water and overlying tidal water of an oligohaline marsh system are about 10⁹, 10⁶ and 10⁶ cells cm^?3, respectively. Average cell size in the overlying water (about 0·06 μm³), is much smaller than that in sediments and interstitial water (about 0·18 μm³). Most bacterial cells in sediments are bound to sediment particles and less than 1% of the cells were displaced by percolating water through sediment columns. Concentration of bacteria in flooding tidal waters is generally higher than that in ebbing waters. Movement of bacterial biomass does not appear to be a significant mechanism of particulate organic transport in marsh sediments and marsh sediments do not appear to be a source of suspended bacteria for estuaries.     

Dynamics of phosphorus exchange between sediment and water in a gravel-bed river

ABSTRACT

Phosphorus (P) stores in gravel-bed rivers are released for uptake by periphyton when pH levels exceed 8.5. The Tukituki River has low alkalinity water and frequently experiences periphyton blooms, and daytime pH?>?9 during summer low-flows. We measured dissolved reactive P (DRP) and EPC₀, the water concentration of DRP at which no net release or sorption from the river bed occurs, in sediment samples from the Tukituki River subject to controlled pH levels before (2014) and after (2017) changes to two wastewater discharges that reduced P release to the river by 95%. DRP released from 2014 sediments at pH 8.5–10 were 30?±?10?mg/m³ above background (pH 8) whereas those released from 2017 sediments were 5?±?3?mg/m³ above background. EPC₀ levels in 2014 and 2017 were 11?±?6 and 7?±?2?mg/m³, respectively. Field estimates of released DRP calculated from continuous pH and the Redfield equation suggested that most of the readily available DRP released from sediments at elevated pH is derived from material attached to recently deposited sediment. Subsequently, a reduction in wastewater inputs or agricultural runoff should reduce sediment DRP stores, and hence sediment fluxes, within a few years and mitigate periphyton blooms in addition to directly lowering water column concentrations.     

Simultaneous exposure of English sole (Parophrys vetulus) to sediment-associated xenobiotics: Part 2—chronic exposure to an urban estuarine sediment with added 3H-benzo[a]pyrene and 14C-polychlorinated biphenyls

• 1.1. The accumulation of aromatic hydrocarbons (AHs) and polychlorinated biphenyls (PCBs) kv English sole (Parophrys vetulus) exposed for up to 108 days to an urban (test) sediment was compared to that for English sole exposed to sediment from a reference area. The concentrations of identified AHs and PCBs in the test sediment were 32 μg/g dry weight and 2·2 μg/g dry weight, respectively. English sole exposed to test and reference sediments for 108 days had biliary concentrations of aromatic compounds fluorescing at BaP wavelengths of 0·6 μg/g, wet weight and hepatic concentrations of PCBs of 1·4 ± 0·6 μg/g wet weight which were ten and eight times, respectively, as great as those in reference sole. These results show that accumulation of AHs and PCBs from sediment by English sole is a significant route of uptake in contaminated environments.
• 2.2. ³H-benzo(a)pyrene (BaP) and ¹⁴C-PCBs were added to the test sediment to compare the uptake and metabolism of BaP to that of PCBs by sole. Steady-state concentrations of ³H-BaP and ¹⁴C-PCBs in tissues and fluids were reached by 4 days and 14 days, respectively. At steady state, the level of covalent binding of ³H-BaP to hepatic proteins in test sole was six times greater than that of ¹⁴C-PCBs, and there was significantly greater relative formation of BaP-glutathione (GSH) conjugates than PCB-GSH conjugates. Thus, the amount of BaP metabolized to reactive metabolites was greater than that for PCBs, even though the concentration of ¹⁴C-PCBs in liver of test sole was five times as great as that of ³H-BaP.
• 3.3. Trace amounts of ³H-BaP were also added to the reference sediment to determine how simultaneous exposure of English sole to those contaminants already present in the test sediment affected the metabolism and disposition of BaP. The ³H-BaP concentration ratios for liver to sediment and bile to sediment, as well as proportions of biliary BaP-GSH conjugates in test sole, were significantly greater than in reference sole. Thus, the formation and accumulation of potentially toxic metabolites of a carcinogenic AH (BaP) in sole liver were increased by simultaneous exposure to contaminants present in an urban sediment.

     

The biokinetic behavior of Tc in juvenile Coho Salmon, Oncorhynchus kisutch (Walbaum)

Juvenile coho salmon, Onchorynchus kisutch, were exposed to ^95mTc in both saltwater and freshwater to study the biokinetic behavior of this element in an anadromous fish. In freshwater, equilibrium whole-body concentration factors (CF₅) ranged from 28 to 32 in two separate experiments, in sharp contrast to an equilibriumCF of < 5 in saltwater. While the uptake kinetics were unimodal in saltwater, in freshwater, biomodal uptake kinetics were observed.Technetium-95m retention was biphasic in freshwater depuration experiments, an initial rapid loss of 21% to 26% of the steady-state activity with an elimination half-life of 3 to 5 days was followed by a slower loss rate, with a whole-body elimination half-life ranging from 23 to 33 days. In saltwater, retention was monophasic with an elimination half-life of 5–16 days. Dramatic decreases in ^95mTc body burdens when freshwater-labeled fish are transferred to Tc-labeled saltwater indicate that the changes in physiology accompanying saltwater acclimation lead to the loss of ^95mTc.     

The biokinetic behavior of 95mTc in juvenile Coho Salmon,Oncorhynchus kisutch (Walbaum)

Juvenile coho salmon, Onchorynchus kisutch, were exposed to ^95mTc in both saltwater and freshwater to study the biokinetic behavior of this element in an anadromous fish. In freshwater, equilibrium whole-body concentration factors (CF₅) ranged from 28 to 32 in two separate experiments, in sharp contrast to an equilibriumCF of < 5 in saltwater. While the uptake kinetics were unimodal in saltwater, in freshwater, biomodal uptake kinetics were observed.Technetium-95m retention was biphasic in freshwater depuration experiments, an initial rapid loss of 21% to 26% of the steady-state activity with an elimination half-life of 3 to 5 days was followed by a slower loss rate, with a whole-body elimination half-life ranging from 23 to 33 days. In saltwater, retention was monophasic with an elimination half-life of 5–16 days. Dramatic decreases in ^95mTc body burdens when freshwater-labeled fish are transferred to Tc-labeled saltwater indicate that the changes in physiology accompanying saltwater acclimation lead to the loss of ^95mTc.     

The North-West European Shelf seas: The sea bed and the sea in motion. II. Physical and chemical oceanography,and physical resources: Edited by F. T. Banner,M. B. Collins and K. S. MassieElsevier Scientific Publishing Co., Amsterdam, 1980, xii+338 pp., £46·73

Total, chemical and biological oxygen demand of intertidal sediment cores from 12 stations in a mangrove swamp in southern Africa were measured under mean temperature and salinity conditions. In addition to measuring oxygen removed from water overlying cores, the uptake of oxygen from air overlying sealed cores was also determined. Total oxygen consumption ranged from 2·9 to 37·0 ml O₂ m^?2 h^?1 in water and from 22·1 to 81·6 ml O₂ m^?2 h^?1 in air. Chemical oxygen demand usually equalled or exceeded the total, underlining problems in the measurement of this parameter. Since oxygen is not present below a few millimeters in the sediment, it is concluded that oxygen diffusing from the overlying water or air is rapidly utilized at the surface and its uptake rate does not give any measure of metabolic activity deeper down. The oxygen content of the overlying water present during high tide may drop to relatively low levels due to this demand.     

The relationship between bacteria and micro-algae in the sediment of a Bay of Fundy mudflat

Bacterial abundance was studied from April to December 1979 in the sediments of a Bay of Fundy mudflat. Bacterial biomass ranged from 1·6 to 2·8 g C m⁻² in the top 5 cm and bacterial numbers normalized for the amount of organic carbon in the sediment were about 9 × 10¹¹ g⁻¹ C at the surface during most of the year. However, when a bloom of pennate diatoms occurred in the fall, bacterial numbers per gram carbon doubled. There was a correlation of r=0·83 between bacteria g⁻¹ C and chlorophyll a for all sampling dates at the high intertidal station. Sieved size fractions of surface sediment taken during a period of low micro-algal biomass and just after the micro-algal bloom showed a strong correlation (r=0·92) between the increases in bacterial and algal abundance for the fractions > 20 μm. We suggest that the apparent relation between benthic micro-algal and bacterial abundance on both a seasonal and within-sediment basis may be due to the release of extracellular material by the micro-algae.     

Degradation of organic pollutants in near shore marine sediments: Part I—breakdown of pulp mill fibre

Cellulose levels in the sediments of Loch Linnhe and Loch Eil were measured over a period of years following the introduction of pulp mill effluent to system. Cellulose levels in experimental sediment systems subjected to pulp fibre inputs at levels simulating various possible loch input levels were measured and compared. No permanent build up of cellulose in either the loch systems or the experimental systems was observed, even at high input levels.Pulp fibre degradation rates were measured experimentally using two different techniques. No difference was found between the degradation rates of wet and predried fibre.Degradation rates equivalent to 0·2 g dry fibre m^?2 day^?1 were found in areas of low fibre input and 0·5 g dry fibre m^?2 day^?1 in areas of high input were recorded. These rates were of the same order as, but somewhat lower than, the probable input rates to the sediments in Loch Eil. Speculations as to possible reasons for this are advanced.     

Deposition of organic material in a coral reef lagoon,One Tree Island,Great Barrier Reef

Deposition of organic material was measured at four sites on One Tree Island coral reef using fixed sediment traps. Although no reliable data were obtained for the reef crest area because of problems of resuspension, mean deposition in the backreef area amounted to some 4 g organic C m⁻² day⁻¹ whereas in the lagoon it was about 1·5 g C m⁻² day⁻¹. This amounted to mean nitrogen deposition rates of 160 and 95 mg N m⁻² day⁻¹, respectively. As primary production by turf algae, the principal producers at One Tree Island, has been estimated at about 2·3 g C m⁻² day⁻¹ for the whole reef system and the weighted mean carbon deposition is estimated at 2·2 g C m⁻² day⁻¹, it is clear that the carbon produced by plants is largely retained in the system. Nitrogen deposition, on the other hand, amounted to only about 60% of that produced by turf algae and it must be assumed that much of this leached into the water during sedimentation. Losses of nitrogen may be minimized by incorporation of dissolved nitrogen by pelagic microheterotrophs which may in turn be consumed by filter feeders before they leave the reef.     

Seasonal distribution of bacteria in salt marsh sediments in North Carolina

The number and size of bacteria at four depths (0–1, 5–6, 10–11 and 20–21 cm) in a North Carolina salt marsh were minotored by direct counts for 13 months. The number of bacteria reached a maximum of about 1·4 × 10¹⁰ cells cm^?3 at the sediment surface in October, corresponding to the period of Spartina alterniflora die-back. Cell numbers were lowest and most consistent throughout the year at the 20 cm depth of sediment. Cell volumes averaged 0·2 μm³ at the marsh surface and decreased with depth. Mean standing crop of bacteria to a depth of 20 cm of sediment was about 14 g bacterial carbon m^?2. In surface sediments bacteria contribute up to 15% and algae up to 10% of total living microbial biomass as estimated by adenosine triphosphate (ATP). Bacteria were the major biomass component at sediment depths of 5, 10 and 20 cm. At all depths the microbial community contributes < 4% total organic carbon and < 8% of total nitrogen.     

The effect of salinity on the microbial mineralization of two polycyclic aromatic hydrocarbons in estuarine sediments

Sediments from the lower Hudson River estuary and two other coastal environments were examined experimentally for their ability to mineralize (convert to CO₂) the polycyclic aromatic hydrocarbons (PAHs) naphthalene and anthracene over a range of salinities. Routine assays employed 1:1 (vol fresh sed:vol water) sediment slurrys in order to overcome natural variability in mineralization rates among replicates. Mineralization rates were stimulated by about 2·5 fold, compared to unslurried controls, while the coefficient of variation fell from 13% to 3·5%.Rates of naphthalene mineralization in surface sediments from along the mainstem of the Hudson River (salinities from 2 to 27%) ranged from 0·011 to 1·5 nmol cm⁻³ day⁻¹ (pool turnover [T_n] from 60 to 2040 days) with no discernible trends along the estuarine gradient. For two stations examined experimentally (mile point 5, salinity 23%; mile point 26, salinity 5%), microbial assemblages appeared acclimated to broad salinity variations as alter rates of mineralization compared to controls.Sediments from two upstream marshes of the Hudson (mile points 36 and 45) showed rates of naphthalene mineralization from 0·007 to 0·15 nmol cm⁻³ day⁻¹ (T_n from 14 to 368 days), while sediments from a third marsh in freshwater (mile point 76) had high rates (66 nmol cm⁻³ day⁻¹; T_n 40 days). For the two upstream marsh stations which rarely experienced salt intrusion, there was a substantial decrease in mineralization of naphthalene and anthracene with increasing salinity.Consistently high rates of naphthalene mineralization (780 to 1600 nmol cm⁻³ day⁻¹; T_n 5 to 6 days) were observed in petroleum contaminated sediments from Port Jefferson Harbor (PJH) on the north shore of Long Island. PJH has a relatively constant salinity regime (about 27%) and imposed decreases in salinity effected decreases in rates of naphthalene and anthracene mineralization. Lowest rates of naphthalene mineralization (0·003 to 0·004 nmol cm⁻³ day⁻¹; T_n from 714 days to 833 days) were found in sediments from two stations in the relatively pristine Carmans River estuary on the south shore of Long Island.The ability of increases or decreases in salinity to affect the rate of model PAH mineralization appeared to be dependent on the natural variation in the salinity regime from which a sample was obtained. Data from all the environments studied indicated a strong positive correlation between PAH concentration and the rates of mineralization of naphthalene. Rates of PAH mineralization in all environments examined appear to be primarily controlled by the extent of pollutant loading and not by natural variations in the salinity regime.     

Bioavailability and effects of sediment-bound TBT in deposit-feeding clams, Scrobicularia plana

Concentrations of tributyltin (TBT), dibutyltin (DBT), inorganic and total tin have been measured in water, sediments and deposit feeding clams, Scrobicularia plana, from 25 estuarine locations in England and Wales, in order to compare the bioavailability of different forms of the element. Abnormally high tin values in Scrobicularia from harbours and areas of high boating activity testify to the high bioaccumulation potential of organotins, notably TBT derived from anti-fouling paints, relative to inorganic tin.Infaunal deposit-feeding bivalves consistently contain higher concentrations of TBT than other benthic organisms indicating that sediments may be an important route for uptake. Significant correlations between sediment TBT values and residues in clams such as S. plana (r = 0·81, p < 0·001) support this hypothesis. Direct evidence of the bioaccumulation of sediment-bound TBT is provided from laboratory experiments in which Scrobicularia were exposed to TBT in water and in sediments, either separately or in combination. The results of these experiments clearly demonstrate a predominantly particulate component for TBT uptake in clams.The kinetics of TBT accumulation and elimination were studied in Scrobicularia exposed to sediments containing 0·3 and 1·0 μg/g TBT (as tin). Equilibrium concentrations in tissue are approached after 40 days.Sediments containing 10 μg/g TBT are acutely toxic to S. plana, although such concentrations are only likely to occur close to dockyards and large marinas. Preliminary laboratory and field observations suggest however, that clam populations could be affected at TBT concentrations in sediments of 0·3 μg/g and possibly lower.     

Deoxygenation and remineralization above the sediment-water interface; an in situ experimental study

An in situ chamber of volume 3881 and bottom area 0·64 m² was used to determine the flux of oxygen and inorganic nutrients across an estuarine sediment-water interface over a 65-day period. Over the first 7 days, oxygen uptake was 378 mg m^?2 day^?1 and the rates of ammonium and phosphate release were 2·22 and 0·34 mg at. m^?2 day^?1, respectively. The water became anoxic in 14 days.The rates of flux in a similar chamber containing only detritus recently settled from the water column were 371 mg m^?2 day^?1 (oxygen), 1·66 mg at. m^?2 day^?1 (ammonium) and 0 12 mg at. m^?2 day^?1 (phosphate), demonstrating that detritus contributes substantially to exchange across the sediment-water interface.The evolution of the two chambers was similar over the latter part of the experimental period. A third chamber containing only water exhibited very minor changes.The role of detritus in nutrient recycling at the sediment-water interface is discussed in relation to the productivity of shallow water bodies such as the estuary in which the experiment was conducted, which itself undergoes periodic deoxygenation during prolonged stratification. The measured flux of nitrogen across the interface was found to represent approximately 31% of the mean daily phytoplankton requirement.     

Cadmium accumulation,distribution and elimination in the bivalve Macoma balthica: Neither metallothionein nor metallothionein-like proteins are involved

Accumulation and elimination of cadmium was investigated in the tellinid clam, Macoma balthica (L.). Accumulation of Cd was linear in soft tissues (0·35 μg g⁻¹ day⁻¹ dry weight) throughout a 29-day exposure period at 100 μg Cd litre⁻¹. Loss of Cd, measured after transfer of experimentally-contaminated clams to clean seawater, was exponential and relatively slow (biological half-time, 70 days). In contrast, Cd uptake onto the shell of Macoma was characteristic of saturation kinetics, indicative of physical adsorption processes. Amounts of Cd accumulated by the shell were low, and the elimination rate rapid (half-time, 7 days) compared with soft tissues.Gel chromatographic profiles of cytosol extracts from control and experimental groups of M. balthica revealed that most of the Cd was bound to high molecular weight ligands, an unusual feature in bivalve molluscs. Although the proportion of cytosolic Cd associated with very low molecular weight ligands is small (< 15%) this pool may be important in regulating Cd fluxes in Macoma during uptake and elimination phases. No evidence was found of metallothionein (or similar metal-binding protein) involvement in Cd accumulation and storage in either field or laboratory exposed clams.The metabolism of Cd in M. balthica is different from that reported for other bivalve groups in several respects. In particular, the relatively slow rate of Cd accumulation in Macoma may be an important adaptive advantage for survival in contaminated areas and may compensate for the absence of a recognised detoxifying (MT) system in this species.     

Feeding strategies of deep-sea sub-Arctic macrofauna of the Faroe-Shetland Channel: Combining natural stable isotopes and enrichment techniques

The response of a sub-arctic, deep-sea macrofaunal community to a simulated food sedimentation event was studied by means of a stable isotope “pulse-chase” experiment. A food pulse was simulated by adding 500 mg C m⁻² of ¹³C-labelled diatoms, Chaetoceros radicans, to sediment cores retrieved from 1080 m in the Faroe-Shetland Channel. Carbon uptake by specific macrofaunal groups was quantified after 3 and 6 days of incubation. The carbon uptake of the dominant taxon (Polychaeta) was quantified at the genus-, and where possible, species-level, representing a data resolution that is rare in deep-sea tracer studies. The macrofaunal community reacted rapidly to the diatom addition, with 47% and 70% of the animals illustrating ¹³C-enrichment after 3 and 6 days, respectively. Approximately 95% of C uptake was located in the upper 2 cm due to the particularly shallow vertical distribution of the macrofaunal community and the nonexistent tracer subduction by burrowing species. Polychaetes of the families Ampharetidae and Cirratulidae were among the most heavily labelled with above background enrichment reaching 1300‰. Approximately 0.8 and 2.0 mg C m⁻² were processed by the macrofauna after 3 and 6 days, representing 0.2% and 0.4% of the added carbon, respectively. It was not possible to differentiate sub-surface deposit-feeding polychaetes from predator/scavenger- and omnivorous polychaetes using their natural ??¹⁵N signatures. However, the combination of natural abundance ??¹⁵N data and ¹³C-labelling experiments proved to be useful for elucidating trophic relations in deep-sea food webs. This study confirms that macrofauna play an active role in the short-term carbon cycling at bathyal depths even at sub-zero temperatures and highlights the need for detailed knowledge of the community structure in understanding carbon processing patterns and early diagenesis of organic matter in marine sediments.