Subtidal Marine Sediments Research Articles

Distribution of terrestrial organic material in intertidal and nearshore marine sediment due to debris flow response efforts

We examined the distribution and processing of terrestrial organic material, derived from the disposal of material from a massive debris flow event following a major wildfire in a coastal California (USA) catchment in intertidal and nearshore subtidal marine sediments. Organic matter biomarkers, pyrogenic carbon and lignin phenols, were used to trace the distribution of terrestrial debris material in marine environments. In intertidal sediments located <1 km east of the debris deposition site, pyrogenic carbon values did not significantly change and lambda values, a lignin measure, decreased over time, indicating little lateral transport of the disposed material. In subtidal sediment, pyrogenic carbon and lambda values were greatest in 20 m water depths indicating transport and deposition of this material nearshore. An additional lignin measure indicative of degradation suggested terrestrial organic material degradation in subtidal sediment decreased with distance from shore. Terrestrial biomarkers demonstrated that the disposed material was not detected in the top 20 cm of intertidal sediment but was retained in subtidal sediment offshore of the disposal site. Results suggest coastal management should incorporate consideration of the effects of debris disposal activities on nearshore benthic communities and biogeochemical cycling.

The impact of mobile demersal fishing on carbon storage in seabed sediments.

Subtidal marine sediments are one of the planet's primary carbon stores and strongly influence the oceanic sink for atmospheric CO2. By far the most widespread human activity occurring on the seabed is bottom trawling/dredging for fish and shellfish. A global first‐order estimate suggested mobile demersal fishing activities may cause 0.16–0.4 Gt of organic carbon (OC) to be remineralized annually from seabed sediment carbon stores (Sala et al., 2021). There are, however, many uncertainties in this calculation. Here, we discuss the potential drivers of change in seabed sediment OC stores due to mobile demersal fishing activities and conduct a literature review, synthesizing studies where this interaction has been directly investigated. Under certain environmental settings, we hypothesize that mobile demersal fishing would reduce OC in seabed stores due to lower production of flora and fauna, the loss of fine flocculent material, increased sediment resuspension, mixing and transport and increased oxygen exposure. Reductions would be offset to varying extents by reduced faunal bioturbation and community respiration, increased off‐shelf transport and increases in primary production from the resuspension of nutrients. Studies which directly investigated the impact of demersal fishing on OC stocks had mixed results. A finding of no significant effect was reported in 61% of 49 investigations; 29% reported lower OC due to fishing activities, with 10% reporting higher OC. In relation to remineralization rates within the seabed, four investigations reported that demersal fishing activities decreased remineralization, with three reporting higher remineralization rates. Patterns in the environmental and experimental characteristics between different outcomes were largely indistinct. More evidence is urgently needed to accurately quantify the impact of anthropogenic physical disturbance on seabed carbon in different environmental settings and to incorporate full evidence‐based carbon considerations into global seabed management.

Microplastics in Galway Bay: A comparison of sampling and separation methods

Microplastics, an emerging pollutant, are recognised as having a ubiquitous distribution in the environment. Currently several benthic sampling tools are being employed to collect subtidal marine sediment, however, there are no comparative studies on the efficiency of these tools to sample for microplastics or the subsequent extraction methods of microplastics from these marine sediments. This study addresses these knowledge gaps by comparing commonly applied benthic sampling tools (Van Veen grab, box corer, gravity corer) and a variety of density separation methods (elutriation column, sodium chloride solution, sodium tungstate dihydrate solution) for microplastic collection and processing.Each sampling tool was tested at the same station and the collected sediment was used to assess the extraction performance for the different density separation techniques. No statistically significant differences were found between the concentrations of microplastics extracted for any of the sampling tools. However, there were significant differences between the density separation methods using sodium tungstate dihydrate and sodium chloride solution and the elutriation method. This preliminary study provides evidence that the sampling tools tested are both suitable and proficient at determining the abundance of microplastics in sediments. Sodium tungstate dihydrate proved to be a novel and feasible option for dense liquid separation of microplastics in subtidal marine sediments. These results will allow for more confidence in data quality when comparing future surveys applying different benthic sampling tools.

Phylogeny and systematics of Protodrilidae (Annelida) inferred with total evidence analyses.

Protodrilidae is a group of small, superficially simple-looking annelids, lacking chaetae and appendages, except for two prostomial palps. Originally considered to be one of the primitive "archiannelid" families, its affinity within Annelida is still highly debated. Protodrilids are found worldwide in the interstices of intertidal and subtidal marine sediments. Despite their simple appearance they constitute one of the most species-rich interstitial families, with 36 described species in two genera, Protodrilus and the gutless Astomus. Here we present the first phylogenetic study of Protodrilidae employing five gene fragments, 55 morphological characters and 73 terminals (including seven outgroups) analysed under direct optimization and parsimony as well as model-based methods. The large data set includes all 36 described species of Protodrilidae (17 of which are represented only by the morphological partition) as well as 30 undescribed or uncertain species (represented by both morphology and molecules). This comprehensive, inclusive and combined analysis revealed a new perspective on the phylogeny of Protodrilidae: the family is shown to contain six cosmopolitan subclades, each supported by several morphological apomorphies, and with the genus Astomus consistently nested among the other five clades rather than next to these. Consequently, the diagnosis of Protodrilus is emended, Astomus remains unchanged and the four remaining lineages are diagnosed and named Megadrilus n. gen, Meiodrilus gen. nov., Claudrilus n. gen and Lindrilus gen. nov. Character transformations showed that large size and presence of pigmentation, oviducts and eyes are plesiomorphies of the family, retained in Protodrilus, Megadrilus gen. nov. and Lindrilus gen. nov. These features are secondarily lost in the gutless Astomus with epidermal uptake of nutrients, as well as in Meiodrilus gen. nov. and some species of Claudrilus n. gen, with smaller size correlated to life in interstices of finer sediments.

Contrasting macrobenthic activities differentially affect nematode density and diversity in a shallow subtidal marine sediment

By bioturbating and bio-irrigating the sea floor, macrobenthic organisms transport organic matter and oxygen from the surface to deeper layers, thereby extending the habitat suitable for smaller infauna. Next to these engineering activities, competition, disturbance and predation may also affect the spatial distribution of these smaller organisms. In a controlled laboratory experiment, we studied the effects of 3 functionally different macrobenthic species on the vertical distribution of nematodes. Abra alba, a suspension-deposit feeding bivalve reworking the sediment randomly, Lanice conchilega, a suspension-deposit feeding, tube-irrigating polychaete and Nephtys hombergii, a burrowing predatory polychaete, were added in single-species treatments to sediment from a coastal subtidal station in the Belgian part of the North Sea, sieved (1 mm) to remove macrofauna. After 14 d, the control treatment without macrobenthos was found to be detrimental to nematode density and diversity, which points to the importance of macrobenthic engineering to sustain the smaller components of the food web. Nematode densities were highest at the sediment surface in all treatments, but subsurface density peaks were observed in A. alba (to 3 cm depth) and L. conchilega (to 7 cm depth) microcosms. In the A. alba treatment, the dominant non-selective deposit feeders and the epistrate feeders shifted downwards probably to avoid disturbance and exploitative competition by the bivalve siphons at the surface, while they might have benefited from the faecal pellets deposited in the subsurface. In the L. conchilega treatment, the several dominant species were redistributed over depth layers, indicating polychaete-mediated habitat extension from surface into depth. Nematode communities seemed hardly affected by the presence of N. hombergii. These results reveal that functionally contrasting macrobenthic engineering effects shape nematode communities in different ways, which may maintain the role of nematodes in ecosystem functioning. The present study therefore highlights the need for conservation of macrobenthic functional diversity.

Sublethal Predation and Regeneration in Two Onuphid Polychaetes: Patterns and Implications

We examined sublethal predation in the polychaete Diopatra cuprea, an important ecosystem engineer of intertidal and shallow subtidal marine sediments in the western Atlantic. D. cuprea commonly loses its antennae and portions of its anterior to predator attacks; these lost body portions are subsequently regenerated. We asked (i) if the intensity of sublethal predation differs for D. cuprea populations in Virginia versus Florida, (ii) if sublethal predation varies temporally in each region, and (iii) if sublethal predation influences activity and tube-building rates. Within Florida, we also drew comparisons between D. cuprea and the closely related onuphid Americonuphis magna. Surprisingly, we found that sublethal predation is more intense in Virginia than in Florida, likely making substantial contributions to secondary productivity. Within Florida, A. magna experienced more antennal loss than D. cuprea and is incapable of anterior regeneration. D. cuprea activity and tube-building rates are strongly influenced by anterior loss, but more subtly influenced by antennal loss. Given the observed rates of sublethal predation and population densities in Virginia versus Florida, sublethal predation is an important factor influencing D. cuprea populations and their associated communities.

Geochemical behavior of trace elements in sub-tidal marine sediments of the Belgian coast

High resolution profiles of trace elements (Fe, Mn, Co, As, Cu, Cr, Ni and Pb) were assessed using the DET (Diffusive Equilibrium in Thin films) and DGT (Diffusive Gradients in Thin films) techniques in silty, organically enriched, sub-tidal sediments of the Belgian coast during late winter and spring 2008. The general chemical properties of the sediments such as dissolved oxygen, pH, Eh and sulfide profiles, controlling precipitation/mobilization reactions, were determined with electrodes (pH and Eh) and microelectrodes (oxygen) and AgI-DGT probes (sulfide). Most trace elements show subsurface maxima and low concentrations beneath 8 cm of depth. The main physicochemical parameters controlling the vertical concentration profiles are dissolved oxygen and redox potential in the surface sediment and sulfide in the deeper sediment layers. Thermodynamic equilibrium calculations have been carried out verifying which solid phases can explain the dissolved trace metal concentrations. Seasonal variations of trace elements have been observed during the sampling period and sedimentation of fresh particulate organic matter (POM) derived from phytoplankton blooms appear to be the main cause of this temporal variability. Flux calculations based on DGT profiles (these fluxes are minimum ones) show that exchange fluxes of trace metals in February are slightly higher than in April. In addition, “DGT pistons” were deployed at the sediment water interface (SWI) to accumulate labile ions from below. This way all labile ions, binding onto the DGT Chelex resin, are pumped out of the pore waters and the solid sediment phase (only the mobile fraction). These results are a direct estimation of the amount of trace elements that can be released from the upper sediment to the water column (in the range of 4.4·10 − 5 to 0.10 mmol·m − 2 ·d − 1 for Co, Pb, Cr, As, Cu, Ni, Fe and Mn).

Study of the diversity of culturable actinomycetes in the North Pacific and Caribbean coasts of Costa Rica

In this study, 137 actinomycetes were isolated from subtidal marine sediments in the North Pacific and Caribbean coasts of Costa Rica. Bioinformatics analysis of the 16S rRNA gene sequences assigned the isolates to 15 families and 21 genera. Streptomyces was the dominant genus while the remaining 20 genera were poorly represented. Nearly 70% of the phylotypes presented a coastal-restricted distribution whereas the other 30% were common inhabitants of both shores. The coastal tropical waters of Costa Rica showed a high diversity of actinomycetes, both in terms of the number of species and phylogenetic composition, although significant differences were observed between and within shores. The observed pattern of species distribution might be the result of several factors including the characteristics of the ecosystems, presence of endemic species and the influence of terrestrial runoff.

Laser Diffraction and Dry-Sieving Grain Size Analyses Undertaken on Fine- and Medium-Grained Sandy Marine Sediments: A Note

Subtidal marine sediments from the Belgian continental shelf were measured using laser diffractometry and dry-sieving techniques. The laser diffractometry approach provided a coarser and less-sorted particle size distribution, compared with the dry-sieving technique. The 5th, 10th, 16th, 50th, 84th, 90th, and 95th percentiles, 180–710 μm cumulative mass/volume percentages, mean grain size, and sorting measured using both techniques were correlated, with r2 > 0.76 (p < 0.001). Skewness and kurtosis were poorly correlated between both techniques. For the purpose of conversion of data from one type of measurement to the other, for sandy sediments a global approximation can be obtained with the proposed relationships.

Isolation and characterization of novel marine-derived actinomycete taxa rich in bioactive metabolites.

A unique selective enrichment procedure has resulted in the isolation and identification of two new genera of marine-derived actinobacteria. Approximately 90% of the microorganisms cultured by using the presented method were from the prospective new genera, a result indicative of its high selectivity. In this study, 102 actinomycetes were isolated from subtidal marine sediments collected from the Bismarck Sea and the Solomon Sea off the coast of Papua New Guinea. A combination of physiological parameters, chemotaxonomic characteristics, distinguishing 16S rRNA gene sequences, and phylogenetic analysis based on 16S rRNA genes provided strong evidence for the two new genera (represented by strains of the PNG1 clade and strain UMM518) within the family Micromonosporaceae. Biological activity testing of fermentation products from the new marine-derived actinomycetes revealed that several had activities against multidrug-resistant gram-positive pathogens, malignant cells, and vaccinia virus replication.

A novel shear vane used to determine the evolution of hydraulic dredge tracks in sub-tidal marine sediments

A novel shear vane is described which can be used to record the shear strength of discrete depth horizons of a variety of marine sediment types in situ. This vane, or modifications of it, has great potential in allowing measurement of in situ shear strength without requiring any samples to be removed from the sea bed, a process which can often destroy the fine structure of the sediment. The vane was used to monitor the change in sediment shear strength caused by a hydraulic dredge, which was used to fish razor clams ( Ensis arcuatus) in a sheltered bay within the Clyde Sea area, Scotland. Data collected using this apparatus provided valuable and immediate information on the stratification of the sea bed post-dredging and allowed the evolution of the dredge track to be regularly monitored by divers over a period of 100 d.

Degradation rates of low molecular weight PAH correlate with sediment TOC in marine subtidal sediments

The degradation rate of low molecular weight (LMW) polycyclic aromatic hydrocarbons (PAH) in subtidal marine sediments was found to correlate with sediment total organic carbon (TOC) in stations sampled two or more times after the North Cape No. 2 fuel oil spill. With 2.5–5 months between samplings, stations with lower sediment TOC had lower fractions of LMW PAH remaining at the time of the second sampling. Apparent first-order degradation rate constants calculated for each station varied by nearly an order of magnitude between stations with a range of TOC from 0.4% to 7.3%. The correlation of degradation rate with sediment TOC can be used to provide improved and site-specific predictions of the initial time-course of LMW PAH concentrations in sediments after oil spills.

Rates and regulation of microbial iron reduction in sediments of the Baltic-North Sea transition

The rates and pathways of anaerobic carbon mineralization processes were investigated at seven stations, ranging from 10 to 56 m water depth, in the Kattegat and Belt Sea, Denmark. Organic carbon mineralization coupled to microbial Mn and Fe reduction was quantified using anaerobic sediment incubation at two stations that were widely separated geographically within the study area. Fe reduction accounted for 75% of the anaerobic carbon oxidation at the station in the northern Kattegat, which is the highest percentage so far reported from subtidal marine sediment. By contrast, sulfate reduction was the dominant anaerobic respiration pathway (95%) at the station in the Great Belt. Dominance of Fe reduction was related to a relatively high sediment Fe content in combination with active reworking of the sediment by infauna. The relative contribution of Fe reduction to anaerobic carbon oxidation at both stations correlated with the concentration of poorly crystalline Fe(III), confirming that the concentration of poorly crystalline Fe(III) exerts a strong control on rates of Fe reduction in marine sediments. The dependence of microbial Fe reduction on concentrations of poorly crystalline Fe(III) was used to quantify the importance of Fe reduction at sites where anaerobic incubations were not applied. This study showed that Fe reduction is an important process in anaerobic carbon oxidation in a wider area of the seafloor in the northern and eastern Kattegat (contribution 60 – 75%). By contrast, Fe reduction is of little significance (6 – 25%) in the more coarse-grained sediments of the shallower western and southern Kattegat, where a low Fe content was an important limiting factor, and in fine-grained sediments of the Belt Sea (4 – 28%), where seasonal oxygen depletion limits the intensity of bioturbation and thereby the availability of Fe(III). A large fraction of the total deposition of organic matter in the Kattegat and Belt Sea occurs in the northern Kattegat, and we estimate 33% of benthic carbon oxidation in the whole area is conveyed by Fe reduction.

Sulfate reduction and methanogenesis in a Thioploca-dominated sediment off the coast of Chile

Continental shelf sediments of the central Chile upwelling area are dominated by the presence of dense mats of the filamentous, sulfur-depositing bacterium Thioploca spp. We examined rates and pathways of S and methane cycling in these sediments along a transect from the Bay of Concepción to the continental slope. Sulfate reduction rates (170–4670 nmol cm −3 d −1) were equal to or exceeded rates reported for other subtidal marine sediments. Elemental S and pyrite were the dominant end-products of sulfate reduction in Thioploca mats on the continental shelf, whereas, in the highly-reducing, Beggiatoa-dominated sediments of the nearby Bay of Concepción, acid-volatile S was the principal end-product. Dissolved organic C values were lowest at the stations with the highest sulfate reduction rates and increased offshore. Sediment porewater methane concentrations in all surface sediments were low (<12 nmol cm −3), and methane production rates at the station most dominated by Thioploca were extremely low ( <0.5 nmol cm −3 d −1). Low methane production rates and concentrations were matched by low methane oxidation rates (<0.1 nmol cm −3 d −1). Radio-tracer studies showed that methane production was almost exclusively from methylamines, substrates which are noncompetitive with sulfate reduction, rather than from acetate or CO 2/H 2. Bacterial MPN (most probable number) counts also indicated the presence of a methylotrophic population of methanogens. Surprisingly, high numbers of autotrophic acetogenic bacteria were found, suggesting that the bacterial population involved in anaerobic DOC degradation is more complex than expected. In spite of the high sulfate reduction rates, sulfide concentrations in the shelf and slope were low or undetectable (<0.5 μM), and sulfate concentrations were never depleted below bottom water levels down to depths of 25–30 cm. Calculations suggest that Thioploca were oxidizing a maximum of 35% of sulfide production—not enough to prevent sulfate depletion. Either other sulfide oxidizers were also present or transient hydrodynamic conditions coupled with bioturbation resulted in oxidation of the sediments.

Nitrogen-fixing, photosynthetic, anaerobic bacteria associated with pelagic copepods

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 12:105-113 (1997) - doi:10.3354/ame012105 Nitrogen-fixing, photosynthetic, anaerobic bacteria associated with pelagic copepods Proctor LM Purple sulfur bacteria are photosynthetic, anaerobic microorganisms that fix carbon dioxide using hydrogen sulfide as an electron donor; many are also nitrogen fixers. Because of their requirements for sulfide or organic carbon as electron donors in anoxygenic photosynthesis, these bacteria are generally thought to be limited to shallow, organic-rich, anoxic environments such as subtidal marine sediments. We report here the discovery of nitrogen-fixing, purple sulfur bacteria associated with pelagic copepods from the Caribbean Sea. Anaerobic incubations of bacteria associated with full-gut and voided-gut copepods resulted in enrichments of purple/red-pigmented purple sulfur bacteria while anaerobic incubations of bacteria associated with fecal pellets did not yield any purple sulfur bacteria, suggesting that the photosynthetic anaerobes were specifically associated with copepods. Pigment analysis of the Caribbean Sea copepod-associated bacterial enrichments demonstrated that these bacteria possess bacteriochlorophyll a and carotenoids in the okenone series, confirming that these bacteria are purple sulfur bacteria. Increases in acetylene reduction paralleled the growth of purple sulfur bacteria in the copepod enrichments, suggesting that the purple sulfur bacteria are active nitrogen fixers. The association of these bacteria with planktonic copepods suggests a previously unrecognized role for photosynthetic anaerobes in the marine S, N and C cycles, even in the aerobic water column of the open ocean. Marine purple sulfur bacteria · Pelagic copepods · Gut bacteria · Nitrogen fixation Full text in pdf format NextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 12, No. 2. Publication date: April 10, 1997 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 1997 Inter-Research.

Effects of trace metals on infaunal species composition in polluted intertidal and subtidal marine sediments near a lead smelter, Spencer Gulf, South Australia

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 135:123-135 (1996) - doi:10.3354/meps135123 Effects of trace metals on infaunal species composition in polluted intertidal and subtidal marine sediments near a lead smelter, Spencer Gulf, South Australia Ward TJ, Hutchings PA The infauna of intertidal and subtidal marine sediments at 18 sites near a large lead-zinc smelter in Spencer Gulf, South Australia, was sampled twice about 6 mo apart. The sampled habitats were bare intertidal mudflats, intertidal seagrass (Zostera sp.) beds, subtidal seagrass (Posidonia spp.) beds, and unvegetated sediments at 5 to 10 m depth. The infauna consisted of 372 identifiable taxa, mainly polychaetes, molluscs and crustaceans. The number of taxa at each site and time showed no simple relationship with the concentrations of trace metals in sediments, habitat type or time of sampling. However, the patterns of distribution, analysed using frequencies of occurrence of taxa derived from multivariate classification and ordination techniques, were related to the sediment concentrations of As, Cu, Mn, Pb, Sb and Zn. The extremely high concentrations of these trace elements (probably mainly Pb and Zn) appeared to affect both the abundances and distribution of many infaunal species. The multivariate techniques could detect which species were affected in the most metal-polluted sites (the intertidal sites). However, in the less-polluted subtidal sites, patterns in species composition were detected but only a few individual species could be unambiguously correlated to the presence of the metals. The polluted intertidal sediments, which had some of the highest metal concentrations ever recorded in marine sediments (Pb up to 5270 ug g-1 and Zn up to 16700 ug g-1), supported 42 taxa. The polychaetes Heteromastus filiformis, Capitella 'capitata' and Glycera americana, and the crustacean Tanais dulongii were very abundant in these sites; they apparently can selectively exploit the metal-contaminated conditions. C. 'capitata' and T. dulongii were found almost exclusively in these sites. In contrast, 15 species of polychaetes, 5 crustaceans and 4 molluscs found elsewhere in the study area, representing 26, 20, and 17% of these intertidal taxa respectively, were not found in the metal-polluted sites. No taxonomic selectivity in the effects of the metals was detected amongst the 3 major groups-polychaetes, molluscs and crustaceans. Future studies may, therefore, be able to sample indicator species from only one of these major groups to determine the spatial extent of ecological effects or to monitor metal effects in this area. Metals . Infauna . Species composition . Biological effects . Monitoring . Smelter Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 135. Publication date: May 17, 1996 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1996 Inter-Research.

Measurement of biologically available organic material in intertidal sediments subject to inputs of sea coal or colliery wastes

A method for the measurement of biologically available organic matter in intertidal or subtidal marine sediments subject to an input of sea coal or colliery wastes is described. The method involves boiling sediment in hydrogen peroxide (H 2O 2) for 4 h at 160°C to dissolve non-colliery waste organic matter which is then recorded as weight lost after rinsing and drying. Samples are then ashed at 495°C to determine the weight of coal or colliery wastes present. Although H 2O 2 shows a slight reaction with colliery wastes (overall mean of 0.86% weight loss), it is appropriate to use this technique for the determination of organic matter available to sediment-dwelling biota. We show that shore organic content should not be determined merely by ashing sediment, even on shores which do not appear to contain wastes, since even visibly ‘clean’ shores in north-east England generally contain some coal fragments in their sediment. Our method is suitable anywhere where biologically available organic matter needs to be measured independently of waste content, e.g. in terrestrial systems close to centres of mining activity.

Thiosulfate and sulfite distributions in porewater of marine sediments related to manganese, iron, and sulfur geochemistry

Depth distributions of thiosulfate (S 2O 3 2−) and sulfite (SO 3 2−) were measured in the porewaters of a Danish salt marsh and subtidal marine sediments by HPLC analysis after derivatization with DTNP [2,2'-dithiobis(5-nitropyridine)]. The distributions were compared to the redox zonation as indicated by Eh and Mn 2+, Fe 2+ and H 2S distributions. Concentrations of S 2O 3 2− varied from below detection (<50 nM) to 600 nM while SO 3 2− concentrations generally were 2–3 times higher, 100–1500 nM. Depth distributions of the two species were roughly similar. Lowest concentrations were found in the oxidized zone, including both the oxic surface layer and the suboxic zone of intense manganese and iron reduction, and concentrations tended to increase through the suboxic and into the reduced, sulfidic zone. The similarity of SO 3 2− and S 2O 3 2− profiles suggested a close coupling of the cycling of the two species. Rates of consumption were suggested as the main factor governing their distribution. Rapid turnover times for S 2O 3 2− and H 2S of 4 and 1.1 h, respectively, were estimated for the upper 0–1 cm of a subtidal sediment.

Annual in situ carbon dioxide and oxygen flux across a subtidal marine sediment

Dissolved carbon dioxide release (445 l CO2 m−2 year−1) measured with chambers placed in situ over subtidal sediments at Eastern Passage, N.S. exceeded oxygen uptake by 2·5 times. Patterns of seasonal changes in flux were similar for both gases with maximum rates at times of highest temperature between May and July. Estimates of algal pigments in surface sediments were not correlated with seasonal changes in gas flux. If concentrations of organic carbon in surface sediment layers vary around steady-state levels, and if mineralization of organic carbon decreases exponentially over time, the ratio of carbon dioxide loss to average concentrations of organic carbon in the upper 1 cm of sediment implies that decomposition is 95% complete in less than 2 years. If carbon dioxide is derived from the upper 10-cm sediment layer, estimates of turnover time increase to 15 years.

Vertical distribution of sulfate reduction, methane production, and bacteria in marine sediments

The vertical distribution of porewater chemistry, and the numbers and activities of methane‐producing bacteria (MPB) and sulfate‐re‐ducing bacteria (SRB) were examined in shallow subtidal marine sediment cores. Sulfate concentrations decreased sharply below 5 cm in three of the four cores collected, and methane levels were highest (2500 to 3500 nmol/ml) immediately below the depth where sulfate was depleted. The fourth core (BM‐2) showed little sulfate depletion with depth and contained low levels of methane (less than 15 nmol/ml). Upon extended incubation at 20°C, rates of methano‐genesis were highest immediately below the high sulfate zone and decreased with depth. Sulfate reduction rates were highest in surface layers and decreased sharply with depth. The respiratory index [Rl = 14CO2/(14CO2 = 14CH4)] for [2‐14C] acetate metabolism was high in the presence of high levels of sulfate and decreased sharply in the core sections low in sulfate. In the high sulfate BM‐2 core, meth‐anogenesis was not detected and an Rl of 1.0 was calculated at all depths where radioactivity was detected. Bacterial counts revealed that populations of MPB were 1 to 2 orders of magnitude less than SRB at all depths. Numbers did not decrease sharply below surface sediments, but the activities of these groups did decrease. These results demonstrated that sulfate reduction dominated over methane production in regions of high sulfate, while the relative importance of methanogenesis increased in low sulfate regions. The magnitude of the rates of either process, however, was more related to depth in the sediment core than to sulfate concentration.