Macrofaunal Biomass Research Articles

Patterns of benthic fauna and benthic respiration on the celtic continental margin in relation to the distribution of phytodetritus

AbstractIn‐situ and shipboard measurements of sediment community oxygen consumption (SCOC), in conjunction with a study of the distributions of macro and megafauna and phytopigments, were used to determine and, where possible, explain the distribution of labile particulate organic matter (POM) on the NE Atlantic continental slope (Goban Spur, SW Ireland). A specific issue concerned the existence of depocentres of labile POM on the slope caused by lateral transport, a phenomenon that has been found previously in the NW Atlantic. The SCOC data from October 1993 and May 1994 showed a steady decrease with increasing water depth. SCOC values ranged from 5.4 mmol m−2 d−1 at the shelfbreak to 0.3 mmol m−2 d−1 at 4,500 m depth. No evidence was found for seasonal variation in SCOC. A clear seasonal signal was observed with regard to sediment phytopigments and phytopigment fluxes into sediment traps attached to the benthic lander. The upper‐ and mid‐slope values of both parameters were much higher in May 1994 than in October 1993 and August 1995. This is consistent with the normal spring bloom pattern; but because of the degraded state of the May phytodetritus in the near‐bottom water, reflected in the lack of a response in SCOC and the low chlorophyll‐a concentrations, it was concluded that the material was not derived from the overlying photic zone, but instead transported from elsewhere in the benthic nepheloid layer (BNL). In August 1995, the lower slope (>3,000 m) had received a strong and fresh phytodetritus pulse (3 g C m−2) forming a mucous layer on top of the sediment. Using phytopigments and sterols as molecular markers, it was shown that the pulse was derived from an offshore bloom with an important contribution by dinoflagellates. By contrast, no mucous layer was found on the upper slope stations in August 1995. Macrofauna biomass showed a distinct decline from the upper slope down to the lower slope conforming to the diminishing supply of labile POM. The total wet biomass of megafauna reached relatively high values at the lower slope (>3,500 m) owing to large motile sea cucumbers. The presence of these “vacuum‐cleaner” sea cucumbers is considered indicative of the occurrence of phytodetritus pulses. In spite of their assumed adaptation to periodic pulses, the estimated contribution by the sea cucumbers to the total benthic mineralization is minor. When combining data from different years/seasons we observed decoupling between the food supply to the lower slope and the upper and mid slopes. The major pulse to the former comes from an offshore summer bloom. The upper and mid slope appear to be fuelled by spring bloom material which is subsequently redistributed on the upper slope in a BNL. The quality of the seston in the BNL diminished in the offshore direction as indicated by the phytopigment concentrations.

Seasonal variations in faunal distribution and activity across the continental slope of the goban spur area (NE Atlantic)

Density, biomass and community structure of macrofauna were estimated together with several sediment characteristics at seven stations ranging from 208 m to 4460 m water depth along the OMEX transect in the Goban Spur area (NE Atlantic) during three seasons (October 1993, May 1994, and August 1995). Median grain size decreased with increasing water depth and showed no differences between the seasons. The percentages of organic carbon and total nitrogen were highest at mid-slope depths (∼1000 to 1500 m), and were significantly higher in August at the upper part of the slope to a depth of ∼1500 m. The C:N ratio in the surface layer amounted to ∼7 to 8 in May, ∼10 to 12 in August and ∼14 to 17 in October at all stations (except the deepest at 4460 m, where it was ∼11 in May and August), indicating arrival of fresh phytodetritus in May, and therefore seasonality in food input to the benthos. Densities of macrofauna decreased exponentially with increasing water depth. Significantly higher densities of macrofauna were found in May at the upper part of the slope to a depth of ∼1500 m. These differences were mainly due to high numbers of postlarvae of echinoids at the shallowest station and ophiuroids at the deeper stations. Biomass values also decreased with increasing water depth, but biomass was relatively high at the ∼1000 m station and low at ∼1500 m, due to relatively high and low mean weights of the individual macrofaunal specimens. No significant differences in biomass were found between the seasons. Respiration was high (15 to 20 mgC·m −2·d −1) in May at the upper part of the slope to a depth of ∼1000 m and low (1–3 mg C·m −2·d −1) at the deeper part. At the shallowest stations to a depth of ∼1000 m respiration was highest in May, at the mid-slope stations (∼1400–2200 m) it was highest in August, whereas the deepest stations (∼3600 to 4500 m) did not show any differences in respiration rates. In conclusion; seasonal variation in organic input is reflected in denstiy, community structure and activity of the macrofauna along the continental slope in the NE Atlantic.

Macroinfaunal biomass and energy flow in a shallow reef flat of the northwestern Philippines

Macrofaunal biomass of the Lucero reef flat in the northwestern Philippines accounted for 9 to 52% of total sediment organic matter, and did not exhibit any significant temporal trend. The polychaetes and crustaceans consistently alternated as biomass dominants; the latter group showed monthly and seasonal variations along with the chaetognaths, molluscs, chordates, and chelicerates, among the major groups (p<0.05). Faunal abundance correlated significantly with biomass. Salinity, mean sediment grain size, sediment heterogeneity, and total organic matter were found to significantly influence faunal biomass. The sandy substrate community was characteristically heterotrophic throughout the monitoring period, i.e., P/R<1. Hourly rates of net primary production (p n ) did not exhibit any significant diurnal pattern. Monthly comparisons yielded significant differences for estimates of daily gross primary production, P, and respiration, R. Values of P were relatively low, and ranged from 2240 (± 1526 S.D.) to 4890 (± 1377) mg O2 m−2 d−1 while R ranged from 3744 (± 1504) to 6879 (± 903) mg O2 m−2 d−1. R was lower during the dry warm months than the wet months. Multiple regression analyses indicate that primary production was a positive function of light intensity and temperature, and a negative correlate of salinity (adjusted R 2 = 0.2444, p< 0.05). Respiration (r) did not appear to relate with any environmental variable, with total macroinfaunal abundance nor with biomass. Results of the study suggest that other heterotrophic components of the sand community were probably responsible for most of the energy consumption, and that these may be dependent on external sources of organic matter.

Response of macro- and meiobenthos from a carbon-poor sand to phytodetrital sedimentation

Sandy sediments are widely distributed in marine systems. However, little is known of the impact of large amounts of organic matter deposition on the infauna of these low organic-content sediments. The purpose of this study was to determine whether infauna, adapted to living in low organic-content sediments, are able to cope with the deposition of large amounts of organic material. In laboratory microcosms, the response of macro- and meiobenthos was examined at three levels of phytodetrital addition (0, 100 and 200 g C m −2) to the surface of a carbon-poor (0.4–0.5% C by weight) sand (median grain size, 325 μm). At 3, 12 and 28 days after phytodetrital addition, changes in macrofauna and meiofauna abundance and macrofauna biomass, were noted. Abundance and biomass of macrofaunal taxa were generally either unaffected at even the highest addition level or a positive response was observed at the intermediate level. Capitella spp. exhibited the most striking positive response in the 100 g C m −2 treatment. However, signs of potential population decline were observed at the highest addition level. Unlike the macrofauna, nematodes and total meiofauna decreased significantly in the 200 g C m −2 treatment. Harpacticoid copepods were unaffected by phytodetrital addition with evidence of a recruitment pulse observed in the 100 g C m −2 treatment. The population dynamics of infauna from some organic-poor sandy sediments are likely limited by processes controlling organic matter input or residence time within the sediment, with little adverse effect of increases in organic matter deposition noticeable unless input is extremely high.

The Quantitative Distribution of Subtidal Macrozoobenthic Assemblages in Arcachon Bay in Relation to Environmental Factors: a Multivariate Analysis

The species composition and spatial distribution of macrozoobenthic assemblages in the subtidal, unvegetated areas of Arcachon Bay (a mesotidal coastal lagoon on the SW coast of France) were investigated through a quantitative survey of 18 stations, sampled during a winter period. Factorial correspondence analysis (FCA) of the distribution of main species among stations and automatic hierarchical classification identified seven species groups and five station groups or faunal assemblages, the latter being also characterized in terms of species constancy and fidelity. These assemblages were: (1) a Gastrosaccus spinifer –Nephtys cirrosacommunity on clean, medium sands in the deep entrance channel; (2) a fine –medium sand assemblage characterized by the amphipods Bathyporeia sarsiand Melita palmatain the two main tidal channels, which penetrates deeply in the bay along the hydrological axes of the system; (3) a very localized assemblage of epi- and infauna on a heterogeneous substratum; (4) a modified Abra albacommunity in the muddy, transverse channels; and (5) a facies of the latter dominated by the oligochaete Tubificoides benedeni.Multiple discriminant analysis including seven environmental variables revealed that the most important factors in the ordination of stations were the sediment parameters, especially organic matter and Zosteradebris, followed by the salinity range and water depth. The spatial organization of subtidal infauna in Arcachon Bay appears to be characteristic of a mesotidal coastal lagoon with important water exchanges with the ocean. Average abundance and biomass of macrofauna amounted to 1719 individuals m −2and 6 ·10 gm −2DW, respectively. Density, biomass and species richness were, on average, higher in muddy sands than in clean, sandy bottoms. Comparison of available quantitative data for Arcachon Bay shows that abundance and biomass of macrozoobenthos are lower in subtidal areas than in intertidal habitats.

Scales of coastal heterogeneity and benthic intertidal species richness, diversity and abundance

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 136:111-121 (1996) - doi:10.3354/meps136111 Scales of coastal heterogeneity and benthic intertidal species richness, diversity and abundance Archambault P, Bourget E Species richness, diversity, total biomass of the benthic macrofauna and macroflora, and the biomass of the 2 dominant taxa (Fucus spp. and Mytilusedulis) were examined in relation to coastal heterogeneity at different scales in the intertidal zone. The sampling design included randomness at all scales and replication of treatments. A 103 km portion of the south shore of the St. Lawrence Estuary, Canada, was divided into 1 km stretches of shore (stations)-the large scale-which were classified using a shore heterogeneity index (SHI) into 3 categories (low, medium and high). Species richness was qualitatively evaluated for each station and substratum heterogeneity on a 100 m (medium scale) was measured as covariate. At the smallest scale 4 types of surface (smooth, crevices of 1, 10 and 20 cm) were quantitatively sampled. Species richness tended to increase with SHI category but this tendency was not statistically significant. A multiple regression analysis was carried out to find which scale of heterogeneity was the most significant for defining species richness. Diversity in types of surface did not vary significantly among SHI categories. Our results show that large-scale heterogeneity explained a higher proportion of the variance in species richness than substratum heterogeneity on a 100 m scale. No statistically significant difference was found in total biomass, M. edulis and Fucus spp. biomass or percent cover among the SHI categories. At the small scale (types of surface), the abundance increased significantly from smooth surfaces to 20 cm crevices except for mussels, where abundance was higher in 10 cm crevices. The types of surface explained 42% of the variation in total biomass and 21% of that in Fucus spp. biomass. Variation in percent cover was explained by the types of surface (40%) and to a lesser extent by the SHI (7%). The present study showed that the scales which influenced abundance were smaller than 20 cm in the intertidal zone. Thus, our results indicated that 2 distinct spatial scales explained the variability within the same marine intertidal community, i.e. variability in species richness (scale of 1 km) and in abundance (types of surface; scale of &LT=20 cm). Shore heterogeneity . Topographical heterogeneity . Small- and large-scale heterogeneity . Species richness . Diversity . Abundance . Benthos . Sampling design Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 136. Publication date: June 06, 1996 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1996 Inter-Research.

Composition, Abundance, Biomass, and Production of Macrofauna in a New England Estuary: Comparisons among Eelgrass Meadows and Other Nursery Habitats

Quantitative suction sampling was used to characterize and compare the species composition, abundance, biomass, and secondary production of macrofauna inhabiting intertidal mud-flat and sand-flat, eelgrass meadow, and salt-marsh-pool habitats in the Nauset Marsh complex, Cape Cod, Massachusetts (USA). Species richness and abundance were often greatest in eelgrass habitat, as was macroinvertebrate biomass and production. Most striking was the five to fifteen times greater rate of annual macrofaunal production in eelgrass habitat than elsewhere, with values ranging from approximately 23–139 g AFDW m2 yr−1. The marsh pool containing widgeon grass (Ruppia maritima) supported surprisingly low numbers of macroinvertebrates, probably due to stressfully low dissolved oxygen levels at night during the summer. Two species of macroinvertebrates, blue mussels (Mytilus edulis) and to a lesser extent bay scallops (Argopecten irradians), used eelgrass as “nursery habitat.” Calculations showed that macroinvertebrate production is proportionally much greater than the amount of primary production attributable to eelgrass in the Nauset Marsh system, and that dramatic changes at all trophic levels could be expected if large changes in seagrass abundance should occur. This work further underscores the extraordinarily large impact that seagrass can have on both the structure and function of estuarine ecosystems. *** DIRECT SUPPORT *** A01BY070 00006

Benthic community structures in the North Sea

Coherent assemblages of marine benthic species have been recognized from the early twentieth century, and the classical papers of Petersen (1914, 1918) were based on studies of limited areas in the North Sea. In 1986, a synoptic survey of the North Sea north to 57°N was undertaken by a group of ten laboratories from seven North Sea countries. The results of this survey have recently been published (Heip et al., 1992a, b; Künitzer et al., 1992; Huys et al., 1992), and some of the results are summarized in this paper. The analysis of the macrofauna is based on slightly more than 700 taxa. In general, the North Sea macrofauna consists of northern species extending south to the northern margins of the Dogger Bank, and southern species extending north to the 100 m depth line. The central North Sea is an area of overlap of southern and northern species, especially around the 70 m depth contour. Consistent groupings of species are recognized that were summarized in seven faunal groupings. Macrofaunal body weight, density and diversity increase linearly towards the north. Macrofaunal biomass for the whole area averages 7 g adwt. m−2 and decreases from south to north. Distribution patterns and trends within the meiofauna were very different. Nematodes, which are the dominant taxon overall, are least abundant in the sandy sediments of the Southern Bight, then increase to a maximum around 53° 30′ N and slowly decrease again towards the north. Copepod density and diversity are highest in the Southern Bight, due to the presence of many interstitial species. A large number of species new to science were recorded by the North Sea Benthos Survey and about 1500 species are expected to occur. Copepods show very distinct assemblages according to water depth and sediment type. The contrasting patterns in latitudinal gradients of body weight and number of species of macro- and meiofauna can be only partially explained. Latitude and sediment characteristics, such as grain size and content in plant pigments, and water depth, determine part of the variance in species composition, density and biomass of the benthic fauna, but the patterns that are observed are different for different benthic groups, requiring careful consideration as to their use in biological monitoring procedures. Distributions are related to current patterns in the North Sea, annual temperature variations and availability of food. However, large parts of the variance in many parameters remain unexplained.

Community-level attributes of exploited and non-exploited rocky infratidal macrofaunal assemblages in Transkei

The rocky intertidal biota along the Transkei coast of southern Africa is subject to intense exploitation by the indigenous coastal inhabitants. To assess the putative ecological impact of such unconstrained foraging activities the infratidal macrofaunal assemblages at three exploited sites were contrasted with those at two adjacent non-exploited sites. Comparisons of community-level attributes were based on a variety of univariate, graphical and multivariate techniques. Two-way nested anovas revealed that trends in species richness and diversity were not consistent with the presence/absence of exploitation. The apparent conflict between these results and expectations based on Connell's intermediate disturbance hypothesis appear to be related to differences in the competitive abilities of “early” and “late” successional species on these shores. Macrofaunal biomass was the only univariate parameter to show significant treatment effects. Although the repeated pulse disturbance regime associated with exploitation differs from the sustained press regime imposed on pollution-impacted communities, their respective abundance/biomass curves show similar trends. Multivariate analyses, in the form of dendrograms, multidimensional scaling (MDS) ordinations and SIMPER routines, derived from Bray-Curtis measures of similarity based on abundance and biomass data, provided the most clear-cut discrimination between exploited and non-exploited sites. The difference in community structure appears to result from a reduction in the abundance of sessile filter-feeders, increase in abundance of certain seaweed-associated species, and decrease in biomass of exploited species and their associated fauna. Differences in community structure amongst sites within treatments were attributed to additional environmental variations. The putative effects of exploitation are also contrasted with those resulting from other forms of anthropogenic disturbance elsewhere.

Preliminary observations on biological communities at shallow hydrothermal vents in the Aegean Sea

Abstract Hydrothermal vents, at depths varying from the littoral zone to water depths of 115 m, have been explored around the islands of Milos and Santorini in the Hellenic Volcanic Arc. The biota were surveyed by scuba divers and with a remote operated vehicle as well as by soft-bottom sampling with grabs and corers. No species specific to hydrothermal areas were found at any of the sites investigated. Animals in the immediate vicinity of the vents belonged to opportunistic species, such as the polychaetes Capitella capitata, Microspio sp. and Spio decoratus . The nassariid gastropod Cyclope neritea was the dominant macrofaunal species found in the bacterial mat areas at Milos which overlay hot brine seeps. At all rocky hydrothermal sites deeper than 35 m water depth the echiuran Boniellia cf. viridis was observed. Around the periphery of the seeps the meiofaunal community was dominated by the nematode Onchlaimus camplyoceroides . Few nematodes were found in the centre of the hydrothermal brine areas. At Santorini the bacteria at the venting sites were dominated by iron bacteria, whereas at Milos large globular sulphur bacteria, Achromatium volutans , covered the hydrothermal brine seeps. Unlike deep-sea vents, little of the biomass production was due to symbiotic associations between animals and chemoautolithotrophic bacteria, although a few stilbotnematine nematodes were found at the periphery of the brine seeps at Milos. Macrofaunal biomass reached a maximum of 80 g m −2 around the vents, compared with ≥c 500 g m −2 at hydrothermal vents elsewhere. Much of the bacterial biomass appeared to be exported from the sites.

Biomass and abundance of macrofauna in intertidal sediments of Königshafen in the northern Wadden Sea

Intertidal sediments of Königshafen (Island of Sylt, North Sea) were sieved for mesofauna (>0.25 mm) and macrofauna (>1 mm) in spring and autumn 1990. Although sediments are coarser than in other parts of the Wadden Sea, the macrobenthic fauna was very similar but with a tendency towards higher species density, abundance and biomass. Taking into account the areal size of sandy flats, seagrass beds, mud flats and mussel beds, the average biomass is calculated to be 65 g ash-free dry weight m−2 The lugwormArenicola marina dominates the biomass (28%), followed by the bivalvesMytilus edulis (21%),Mya areanaria (16%),Cerastoderma edule (10%) and the mudsnailHydrobia ulvae (9%). While spring and autumn biomass are almost alike, abundance is highly variable and entirely dominated byH. ulvae. Mesofauna is mainly composed of oligochaetes, small and juvenile polychaetes. Abundance is similar to that of macrofauna, while biomass is only about 1 g m−2. Macrophyte biomass amounted to 9% of that macrofauna. In the course of the centurym mussel beds expanded while muddy areas declined. The concomitant effects on biomass presumably compensated each other.

Seasonal variation in sediment urea turnover in a shallow estuary

A seasonal study of sediment urea turnover rates was carried out in a shallow Danish estuary. Turnover rates decreased with sediment depth (0 to 16 cm) and were within the range 1.2 to 424.8 nrnol N cm-3 d-' . Integrated (0 to 16 cm depth) turnover rates varied between 1.5 and 16.9 nun01 N m-' d' with a maximum in July and a minimum in January. Urea turnover followed first-order rate kinetics, as the urea turnover rate constant, k,,,,, was independent of the urea concentration, and the urea turnover rates were positively related to the latter. The urea turnover rate and rate constant were positively related to temperature. Urea production was stimulated by availability of high quality organic material (low C/N) and temperature. Urea production rates and urea concentrations in the sediment reached a maximum in July, when C-mineralization, temperature, and the quality of organic material were maximal, and there was a high benthic macrofaunal biomass. Secondary maxima in urea concentrations matched major phytoplankton sedimentation events in spring and autumn. Urea-N accounted for 7 to 55 % of the total N pool (urea + NH,+ + NO,+ NO,') in the sed~ment surface (0 to 1 cm). The NH,+ pool was positively related to the urea turnover rate (CO to 16 cm). Urea turnover accounted for a major part of the NH,+ production in the sediment.

Benthic foraminiferal response to environmental change in the Skagerrak, northeastern North Sea

Abstract Studies of the benthic macrofauna (both quantitative and qualitative) conducted in the Skagerrak and Kattegat over a period of many years together with comparisons made with data collected in 1914 have led to the conclusion that the area is undergoing eutrophication. The general increase in biomass and densities of macrofauna has been attributed to an increased supply of degradable organic matter to the sediment. In this study, the effect of eutrophication on benthic Foraminifera is shown in a comparison of foraminiferal data from Recent sediment layers (the upper 2 cm of short sediment cores) with layers deposited more than 42 years B.P. (24–25 cm core interval). Foraminiferal densities are significantly lower in the Recent sediment with no large scale changes in the assemblage composition. It is proposed that the lower densities are a result of increased macrofaunal activity. The (paleo) environmental significance of this interrelated effect of food availability and biological interactions on f...

Variability of Dissolved Organic Carbon in Sediments of a Seagrass Bed and an Unvegetated Area within an Estuary in Southern Texas

Pore-water dissolved organic carbon (PWDOC) concentrations were examined in vegetated and bare sediments of aHalodule wrightii seagrass bed, and in a mud bottom sediment of a southern Texas estuary. Temporal variability was examined at diel (dawn and noon) and bimonthly time scales. Distribution patterns of PWDOC were compared with physical, chemical, and biological factors thought to exert control on PWDOC. Concentration of PWDOC, bacterial production, and resultant PWDOC turnover times displayed statistically significant spatial and temporal variability. Concentration of PWDOC ranged from 14 mg C 1−1 to 107 mg C 1−1 of pore water, or 9–71 μg C cm−3 wet sediment. PWDOC was more variable and was approximately 5 times higher than DOC concentrations in the water column. Low PWDOC concentrations (mean = 14.6 μg C cm−3) and high bacterial production rates (mean = 1.92 μg C cm−3 h−1) were observed at the mud station, whereas PWDOC concentrations were high (mean = 24.6 μg C cm−3) and bacterial production rates were low (mean = 0.43 μg C cm−3 h−1) at the bare station. PWDOC turnover times (Tt), assuming 50% bacterial growth efficiency (1–840 h) were shortest at the mud station (mean=13 h) and longest at the bare station (mean=180 h). In the overlying water column, Tt values were longer, ranging from 1,000–10,000 h. PWDOC concentrations were 25% higher in vegetated sediments than in neighboring bare sediments. This difference was probably due to inputs of labile photosynthetic excretia, since bacterial production rates in vegetated sediments displayed significant diel variability and were 4 times greater than that of bare sediments. Based upon the entire data set, PWDOC was significantly related to macrofaunal biomass, sediment POC, sediment C:N ratios, and oxygen metabolism, but was significantly correlated only to the latter two variables in stepwise multiple regression. Our findings suggest that organism activities and detrital quality are the major determinants controlling variability in PWDOC.

Seasonal variation in net benthic C-mineralization in a shallow estuary

Seasonal changes in net benthic C-mineralization were measured as sediment O2 uptake, as well as ΣCO2 flux from the sediment to the water column. Oxygen uptake varied between 26 and 181 mmol m−2 d−1 through the year, and ΣCO2 efflux varied between 17 and 132 mmol m−2 d−1. Both parameters were related to temperature, and the Q10 for O2 uptake and ΣCO2 efflux were 2.5 and 2.0, respectively. The ΣCO2/O2 ratio varied between 0.8 and 1.0 during most of the year. The ΣCO2/O2 ratio was, however, less than 0.8 in late summer and autumn, which might be due to reoxidation of accumulated reduced sulfur compounds and carbonate precipitation. Calcium carbonate deposition by molluscs seemed quantitatively important for the ΣCO2 efflux in September 1990 and August 1991. The ΣCO2/O2 ratio reached a maximum of 1.3 in mid-April during decomposition of benthic and sedimented microalgae. On an annual basis, all the oxidants which were reduced during anaerobic mineralization (e.g., Mn2+, Fe2+, HS−), apparently were reoxidized by O2, as illustrated by an annual ΣCO2/O2 ratio of 0.8. Benthic macrofauna stimulated C-mineralization, indicated by a positive relationship between O2 uptake and ΣCO2 efflux rates, respectively, and macrofauna biomass. Sediment chlorophyll-a+ pheopigment content and C/N ratio served as good indicators both of algal sedimentation from the water column and of the presence of benthic microalgae. The sediment parameters, including the organic carbon and nitrogen content, C/N ratio, and chlorophyll-a+ pheopigment content, clearly reflected the seasonal variation in sediment mineralization rates. There was a temporal phase displacement between input of organic matter and benthic C-mineralization, which resulted in a temporary build up of organic matter during periods with high primary production and/or low C-mineralization (spring, late autumn, winter). The accumulated organic matter was mineralized during periods when the actual supply of organic matter was insufficient (summer, early autumn).

Rates of nitrification, distribution of nitrifying bacteria and inorganic N fluxes in northern Bering-Chukchi shelf sediments

Spatial distribution of sediment nitrification rates and fluxes of ammonium and nitrate were measured in shelf sediments of the northern Bering and Chukchi seas. The sediments could be divided into three main areas depending on macrofaunal activity and input of organic nitrogen. Sediments underlying the highly productive Bering Shelf-Anadyr water (BSAS) were characterized by a high macrofaunal biomass and a high input of nitrogen-rich organic material. Tube-dwelling amphipods dominated in the sandy sediments of the northern Bering Sea, while bivalves dominated in the fine textured sediments of the Chukchi Sea. Sediments underlying the low productive Alaska Coastal Water (ACS) were characterized by low macrofaunal biomass and an input of lower quality organic material. Generally nitrification rates and nutrification potentials (NP) were highest in BSAS and lower in ACS. Nitrification rates of surface sediment, calculated from NP, accounted for 90% of the measured rates in ACS, but only 35–75% in BSAS. These data together with the distribution patterns of NP and pore water nitrate profiles implied, that most sediment nitrification was confined to the sediment surface in ACS and in BSAS bivalve sediments, while most sediment nitrification took place in the ventilated burrow walls of BSAS amphipod sediments. The NH 4 + efflux was five-fold greater from BSAS compared to ACS, whereas the estimated sediment net NH 4 + production was three-fold greater. The increase in NH 4 + efflux relative to net NH 4 + production could mostly be attributed to macrofaunal excretion. The NO 3 − flux between sediment and water column was correlated with NO 3 − concentrations in the bottom water. At concentrations higher than 10 μM NO 3 −, the flux was directed into the sediment and at lower concentrations out of the sediment. Spatial distribution of high bottom water NO 3 − concentrations correlated with high NH 4 + fluxes out of the sediment. This resulted in a lower net efflux of inorganic nitrogen from these sediments.

Biomass of macrobenthic tidal flat fauna of the Banc d'Arguin, Mauritania

The biomass of the benthic macrofauna of the tidal flats of the Banc d’Arguin, Mauritania, retained by a sieve of 0.6 mm, has been estimated. The total biomass amounts to about 17.0 g ash-free dry weight (AFDW) m-2 in the area sampled (about 300 km2). The large bivalve Anadara senilis is responsible for 8.1 g AFDW m-2, over 100 other species for another 8.9 gm-2. The largest biomasses are found near the continental shore; at the offshore tidal flats benthic filter-feeders are lacking. Seagrass beds, which occupy the larger part of the tidal flats, have an under average biomass. Sebkhas, extremely saline flats above mean high-water mark, contain no live animals. To sustain the consumption of benthic prey by the very abundant wintering shorebirds relatively low P/B ratios are sufficient.

Effect of the brown shrimp Crangon crangon L. on endobenthic macrofauna, meiofauna and meiofaunal grazing rates

The effect of natural densities (50 and 100 ind·m −2) of juvenile (<20 mm) Crangon crangon on abundance of endobenthic macrofauna and meiofauna and on meiofaunal grazing rates was investigated in two experiments using an outdoor flow-through system with sandy sediment. The experiments differed in duration (3 wk and 7.5 wk), and in time of the year (August – September and July – August). Macrofaunal biomass differed by a factor of 10 between the experiments, and was depressed by shrimp in both experiments. Neither total meiofaunal biomass, nor the biomass of the dominant taxon (nematodes), was significantly affected by the presence of Crangon in the shorter experiment. However, harpacticoid copepods and other meiofauna taxa (mainly ostracods, foraminiferans and juvenile bivalves) decreased in the presence of Crangon. In the longer experiment, no significant effect of Crangon on meiofauna was seen, and the biomass of most meiofaunal groups increased in all treatments. Meiofaunal grazing rates in microalgae and bacteria, measured with a dual-labelling method, using 14C-bicarbonate and [methyl- 3]]-thymidine as tracers, were 0.7 to 4.7% of algal biomass per day, and 0.8 to 7.9% of bacterial biomass per day. Generally, grazing rates were lower in the presence than in the absence of Crangon. In terms of carbon ingested, microalgae constituted a more important food source than bacteria for all three meiofaunal groups (nematodes, harpacticoids and other meiofauna). No individual growth of Crangon was found in the high-density treatment in the longer experiment, suggesting that intraspecific competition occurred. The difference between the two experiments in the effect of Crangon on meiofauna is thought to be caused by the large difference in macrofaunal biomass between the experiments; the shrimp ate mainly juvenile macrofauna in the longer experiment. Overall, the effect of Crangon on the sediment system was weak, suggesting that other factors (physical and chemical) are more important than epibenthic predation in setting the overall limits for production in this sediment.

Sediment characteristic of the continental shelf in the northern Gulf of Mexico

The Mississippi River is the largest river in the United States and discharges substantial amounts of nutrients, organic carbon, and sedi­ments onto the Texas-Louisiana continental shelf in the northern Gulf of Mexico. As a ba­sic part of an initiative to describe processes at the sediment-water interface in the northern Gulf of Mexico shelf ecosystem we measured sediment grain size and per cent total organic carbon. This paper reports sediment grain size characteristics and per cent carbon, and makes preliminary associations between these varia­bles and carbon utilization by the benthos (cal­culated from sediment oxygen consumption ra­tes) and macrofauna biomass.

Environmental effects of submarine seeping natural gas

It is suspected that most shallow reservoirs of natural gas vent to the surface to some degree. This seeping may be through diffusion of dissolved gas or by a flow of gas bubbles which entrain interstitial water during the rise through the sediments to the surface. Methane bubbles dissolved other gases, notably hydrogen sulphide and carbon dioxide, during their ascent. Under suitable temperature-pressure conditions gas hydrates may be formed close to or at the seabed Black suphide-rich sediments and mats of sulphur oxidizing bacteria are frequently observed close to the sediments surface at seep sites, including a sharp oxic/anoxic boundary. Animal species associated with these gas seeps include both species which obtain nutrition from symbiotic methane-oxidizing bacteria and species with symbolic sulphur-oxidizing bacteria. It is suspected that at some microseepage an enhanced biomass of meiofauna and macrofauna is supported by a food chain based on free-living and symbiotic sulphur-oxidizing and methane-oxidizing bacteria. The most common seep-related features of sea floor topography are local depressions including pockmark craters. Winnowing of the sediment during their creation leads to an accumulation of larger detritis in the depressions. Where the deprssions overlies salt diapirs they may be filled with hypersaline solutions. In some areas dome-shaped features are associated with seepage and these may be colonized by coral reefs. Other reefs, “hard-grounds”, columnar and disc-shaped protrusions, all formed of carbonate-cemented sediments, are common on the sea floor in seep areas. Much of the carbonate appears to be derived from carbon dioxide formed as a result of methane oxidation. The resulting hard-bottoms on the sea floor are often colonized by species not found on the neighboring soft-bottoms. As a result seep areas may be characterized by the presence of a rich epifauna.