Metazoan meiofauna biomass, grazing, and weight-dependent respiration in the Northern Gulf of Mexico deep sea

Metazoan meiofauna are ubiquitous in marine soft sediments and play a pivotal role in diagenesis of particulate organic matter. However, the relative importance of meiofauna to the function of deep-sea benthic boundary layer communities has not been resolved. Here, meiofauna biomass, respiration, and grazing on aerobic heterotrophic bacteria were estimated and compared to standing stocks and fluxes of other benthic components (e.g., bacteria and macrofauna). Biomass and respiration declined with depth. Highest biomass and respiration occurred in the proximity of the Mississippi River on the upper continental slope of the central Gulf of Mexico. Meiofauna required 7% of their biomass per day to meet their metabolic energy budget, compared to approximately 24% day⁻¹ in shallow water. Respiration accounted for 8–22% of whole sediment community respiration (SCOC), reflecting the importance of meiofauna in diagenesis, deep-sea carbon budgets, and global biogeochemical cycles.     

Meiofauna of the deep Eastern Mediterranean Sea: distribution and abundance in relation to bacterial biomass, organic matter composition and other environmental factors

Quantitative information on the abundance and biomass of metazoan meiofauna was obtained from samples collected at 15 deep-sea stations in the Eastern Mediterranean Sea (533–2400m). Meiofaunal abundance was compared to bacterial biomass and other environmental factors such as the total sedimentary organic matter content, the concentrations of the main biochemical classes of organic compounds (i.e. proteins, carbohydrates and lipids) and to ATP. To estimate the sedimentation potential of primary organic matter, sediment bound chloroplastic pigment equivalents (CPE) were assayed. Meiofaunal density was very low ranging from 4 ind.10cm⁻² (Station A4, 1658m depth) to 290 ind.10cm⁻² (Station A12, 636m depth). Nematodes were the numerically dominant taxon (68% of total meiofauna) and were usually confined to the top 6cm of the sediments. Total meiofaunal biomass ranged from 2.78μgC 10cm⁻² (Station A4) to 598.34μgC 10cm⁻² (Station 15A). There was a significant decrease in the density of metazoan meiofauna with water depth. Bacterial biomass largely dominated the total biomass (as the sum of bacterial and meiofaunal biomass) with an average of 73.2% and accounted for 35.8% of the living biomass (as ATP carbon) whereas meiofaunal biomass accounted only for 6.56%. Bacterial biomass was significantly related to the DNA concentrations of the sediment. A significant correlation between ATP concentration and CPE content was also found. No correlations were found between meiofauna, ATP and CPE, or between meiofauna and bacterial parameters. The significant relationship between meiofaunal density and the ratio of labile organic matter/total organic matter indicates that deep-sea meiofauna inhabiting an extremely oligotrophic environment (such as the Eastern Mediterranean) may be more nutritionally dependent upon the quality than on the quantity of sedimentary organic matter.     

Vertical and horizontal distribution of meiofauna in mangrove sediments in Transkei, southern Africa

The depth-distribution profiles of meiofauna in four transects in the Mngazana River, Transkei were studied during summer 1980. Highest densities [±1000(100 cm³)⁻¹] were encountered within the top 10 cm of the sediment. Nematodes dominated (80%) and the remainder was made up of ciliates, oligochaetes, gastrotrichs, and low numbers of polychaetes, copepods, kinorhynchs and various crustacean larvae. Among chemical parameters Eh correlated most consistantly with distribution, particularly at the lower tidal levels. Temperature and pH appeared to be of lesser importance. The maximum estimated depth of penetration was on average 72 cm at the HW levels; 32 at MW and 23 at LW. The mean dry biomass was estimated at 1073 mg m⁻²; 941 mg m⁻² and 196 mg m⁻² at these tidal levels respectively. The importance of preliminary studies designed to estimate the depth distribution of meiofauna is discussed.     

The biomass of macro- and interstitial fauna on clean and wrack-covered beaches in Western Australia

The benthic faunal spectrum including bacteria, protozoans, meiofauna, wrack epifauna and macrofauna, was quantitatively surveyed on two modally reflective, moderate energy, Western Australian beaches. The more exposed beach had coarser sand, no intertidal macrofauna and a poor interstitial fauna. The less exposed beach had a large deposit of wrack totalling 161 kg m^?1 dry mass concentrated on the lower shore. The amphipod Allorchestes compressa was abundant in the fresh wrack comprising most of the macrofauna. There were also fairly abundant small epifauna on the wrack. Dry biomass of macrofauna, epifauna, meiofauna, protozoans and bacteria was 0, 0, 15, 4 and 180 g m^?1 on the more exposed beach and 160, 3, 112, 9 and 901 g m^?1 on the less exposed beach with wrack. On the latter beach there was an inverse correlation between meiofaunal densities and the densities of protozoans and bacteria, suggesting grazing by the former on the latter. On both beaches meiofauna was concentrated in the mid- to upper beach, protozoans near the surface and bacteria in the mid- to lower beach. It is estimated that bacteria are responsible for most of the secondary production on both beaches.     

Changes in sediment processes across the western Irish Sea front

Sediment characteristics, sediment respiration (oxygen uptake and sulphate reduction) and sediment–water nutrient exchange, in conjunction with water column structure and phytoplankton biomass were measured at five stations across the western Irish Sea front in August 2000. The transition from thermally stratified (surface to bottom temperature difference of 2.3 °C) to isothermal water (14.3 °C) occurred over a distance of 13 km. The influence of the front on phytoplankton biomass was limited to a small region of elevated near surface chlorophyll (2.23 mg m⁻³; 50% > biomass in mixed waters). The front clearly marked the boundary between depositional sediments (silt/clays) with elevated sediment pigment levels (≈60 mg m⁻²) on the western side, to pigment impoverished (<5 mg m⁻²) sand, through to coarse sand and shell fragments on the eastern side. Maximal rates of sedimentary respiration on the western stratified side of the front e.g. oxygen uptake S2 (852 μmol O₂ m⁻² h⁻¹) and sulphate reduction at S1 (149 μmol SO₄²⁻ m⁻² h⁻¹), coupled to significant efflux of nitrate and silicate at the western stations indicate closer benthic–pelagic coupling in the western Irish Sea. Whether this simply reflects the input of phytodetritus from the overlying water column or entrapment and settlement of pelagic production from other regions of the Irish Sea cannot yet be resolved.     

Relationships between macroalgal biomass and nutrient concentrations in a hypertrophic area of the Venice Lagoon

Macroalgae biomass and concentrations of nitrogen, phosphorus and chlorophyll a were determined weekly or biweekly in water and sediments, during the spring-summer of 1985 in a hypertrophic area of the lagoon of Venice. Remarkable biomass production (up to 286 g m⁻² day⁻¹, wet weight), was interrupted during three periods of anoxia, when macroalgal decomposition (rate: up to 1000 g m⁻² day⁻¹) released extraordinary amounts of nutrients. Depending on the macroalgae distribution in the water column, the nutrients released in water varied from 3·3 to 19·1 μg-at litre⁻¹ for total inorganic nitrogen and from 1·8 to 2·7 μg-at litre⁻¹ for reactive phosphorus. Most nutrients, however, accumulated in the surficial sediment (up to 0·640 and to 3·06 mg g⁻¹ for P and N respectively) redoubling the amounts already stored under aerobic conditions, Phytoplankton, systematically below 5 mg m⁻³ as Chl. a, sharply increased up to 100 mg m⁻³ only after the release of nutrients in water by anaerobic macroalgal decomposition. During the algal growth periods, the N:P atomic ratio in water decreased to 0·7, suggesting that nitrogen is a growth-limiting factor. This ratio for surficial sediment was between 6·6 and 13·1, similar to that of macroalgae (8·6–12·0).     

An assessment of the spatial heterogeneity of environmental disturbance within an enclosed harbour through the analysis of meiofauna and nematode assemblages

Significant spatial heterogeneity in the abundance and composition of meiofaunal and nematode assemblages was described inside the Genoa-Voltri harbour (Genoa, Italy) in relation to variation in the main environmental variables. In harbour sediments characterized by low Eh values and high organic matter concentrations, total meiofauna abundance was lower (948 ± 919 ind 10 cm⁻²), nematode individual biomass was higher (0.17 ± 0.07 μg C), kinorhynchs and tanaids were completely absent, and the nematode assemblage was dominated by the genera Terschellingia, Sabatieria (pulchra group) and Paracomesoma. In contrast, in sediment characterized by lower levels of organic pollution, meiofaunal abundance was higher (1085 ± 737 ind 10 cm⁻²), nematode individual biomass was lower (0.11 ± 0.04 μg C), kinorhynchs and tanaids were present and the nematodes were dominated by the genera Desmodora, Daptonema, Anticoma and Halalaimus.Environmental disturbance as assessed by the analysis of meiofaunal and nematode assemblages and sediment environmental variables changed significantly over a scale hundreds of meters, but did not follow a gradient from the inner to the outer harbour. Analysis of nematode assemblages is proposed as a useful tool for the identification of environmental risk areas which may assist in the development of good planning, monitoring programmes and better management of harbour ecosystems.     

Macrofaunal density and biomass in the Campeche Canyon, Southwestern Gulf of Mexico

The composition, density and community structure of the benthic macrofauna were investigated in sediments of the Campeche Canyon in the SW Gulf of Mexico. Total macrofaunal density ranged from 9466±2736 ind m⁻² at the continental shelf station to 1550±195 ind m⁻² in the canyon. Density values significantly diminished with distance from the coast and depth; only a few stations in the center of the canyon displayed larger density values (E-37 with 4666±1530 ind m⁻², E-36 with 5791±642 ind m⁻² and E-26 with 6925±2258 ind m⁻²). Densities were positively correlated to organic nitrogen in the sediment (r=0.82) and coarse silt (r=0.43), and negatively with depth (r=−0.74) and distance from the coast (r=−0.68). At all stations, the polychaete worms contributed most to the multi-species community structure. The nematodes and Foraminifera displayed their highest densities in the center of the canyon. The biomass values declined significantly with depth. We conclude that the macrofauna density and biomass changed in response to organic matter contents in the sediment, both with distance from the coast and with depth.     

Evaluation of abyssal meiobenthos in the eastern central Pacific (Clarion-Clipperton fracture zone)

Meiobenthos were sampled from 17 stations in the abyssal deep-sea system of the central Pacific centered around 14°N, 130°W at depths 4960–5154m, during the Nixo 47 R/V Jean Charcot cruise. Meiofaunal density range from 45–89 ind. 10cm². Predominant taxa are nematodes (84–100%) and copepods (0–10%). Rotifera, Polychaeta, and Acarina also occur. Nematodes are uniformly distributed spatially with 45 species or so; Monhysteridae is the dominant taxon, and Syringolaimus sp. (Ironidae) co-occurs faithfully. Low biomass (0.4–70.6μg 10cm²) are attributed to supposed dwarfism of metazoan meiofauna and very high proportion (60–80%) of juveniles and pre-adult forms. The majority of protozoans and metazoans are detritus- or deposit-feeders; in addition symbiotic associations, coprophagy and gardening activities are frequent. In such an oligotrophic environment, low food supply may limit meiofaunal abundance, biomass and maturation, and to a lesser extent species richness.     

Mesozooplankton biomass and grazing responses to Cyclone Opal, a subtropical mesoscale eddy

As part of E-Flux III cruise studies in March 2005, plankton net collections were made to assess the effects of a cyclonic cold-core eddy (Cyclone Opal) on the biomass and grazing of mesozooplankton. Mesozooplankton biomass in the central region of Cyclone Opal, an area of uplifted nutricline and a subsurface diatom bloom, averaged 0.80±0.24 and 1.51±0.59 g DW m⁻², for day and night tows, respectively. These biomass estimates were about 80% higher than control (OUT) stations, with increases more or less proportionately distributed among size classes from 0.2 to >5 mm. Though elevated relative to surrounding waters south of the Hawaiian Islands (Hawai’i lee), total biomass and size distribution in Cyclone Opal were almost exactly the same as contemporary measurements made at Stn. ALOHA, 100 km north of the islands, by the HOT (Hawaii Ocean Time-series) Program. Mesozooplankton biomass and community composition at the OUT stations were also similar to ALOHA values from 1994 to 1996, preceding a recent decadal increase. These comparisons may therefore provide insight into production characteristics or biomass gradients associated with decadal changes at Stn. ALOHA. Gut fluorescence estimates were higher in Opal than in ambient waters, translating to grazing impacts of 0.11±0.02 d⁻¹ (IN) versus 0.03±0.01 d⁻¹ (OUT). Over the depth-integrated euphotic zone, mesozooplankton accounted for 30% of the combined grazing losses of phytoplankton to micro- and meso-herbivores in Opal, as compared to 13% at control stations. Estimates of active export flux by migrating zooplankton averaged 0.81 mmol C m⁻² d⁻¹ in Cyclone Opal and 0.37 mmol C m⁻² d⁻¹ at OUT stations, 53% and 24%, respectively, of the carbon export measured by passive sediment traps. Migrants also exported 0.18 mmol N m⁻² d⁻¹ (117% of trap N flux) in Cyclone Opal compared to 0.08 mmol N m⁻² d⁻¹ (51% of trap flux) at control stations. Overall, the food-web importance of mesozooplankton increased in Cyclone Opal both in absolute and relative terms. Diel migrants provided evidence for enhanced export flux in the eddy that was missed by sediment trap and ²³⁴Th techniques, and migrant-mediated flux was the major export term in the observed bloom-perturbation response and N mass balance of the eddy.     

Across shore variability and trophodynamic features of meiofauna in a microtidal beach of the NW Mediterranean

The across shore variability and trophodynamics of meiofauna were studied in a microtidal beach of the Thyrrenian Sea (NW Mediterranean). Two sites were sampled at Collelungo beach (Maremma Park, Italy) subjected to different regimes of sediment erosion and deposition. At each site, four levels were sampled in November 2002 and May 2003 along a transect from the supralittoral zone to the surf zone. Sediment cores were taken down to a depth of 10 cm and meiofaunal abundance and community structure were analyzed and related to the principal trophic resources (quantity and quality of organic matter, chlorophyll a, bacteria density and biomass).Meiofaunal abundance ranged between 14 ind. 10 cm⁻² and 716 ind. 10 cm⁻² in the top 0–10 cm of sediment. Abundance was lower in the surface (0–2 cm) than in the deeper (5–10 cm) sediment layers but no significant differences were found between the two sites.Multivariate BIOENV analysis showed that dryness, grain size (related to physical processes) and bacterial biomass, were the main variables explaining meiofauna distribution in these beaches. Meiofaunal densities and number of taxa were always higher at the swash level, while lower abundances were observed at the dry sampling level (+5 m). This across shore trend was also observed for the quality of the organic matter (PRT/CHO) and bacterial densities. Nematode assemblage structure at the swash sampling level showed a dominance of non-selective deposit feeders (1B), with Xyalidae as the dominant family (56%), followed by Thoracostomopsidae (14%) and Selachnematidae (12%).According to the findings, physical and biological variables at the swash level create optimal living conditions for the meiobenthos, making the swash a key area within the beach ecosystem, with potential implications within basic and applied ecological studies.     

Benthic primary production, respiration and remineralisation: in situ measurements in the soft-bottom Abra alba community of the western English Channel (North Brittany)

In situ measurements of ammonium and carbon dioxide fluxes were performed using benthic chambers at the end of spring and the end of summer in two soft-bottom Abra alba communities of the western English Channel (North Brittany): the muddy sand community (5 m, about 10% of surface irradiance) and the fine-sand community (19 m, about 1% of surface irradiance). High rates of ammonium regeneration were measured in the two communities at the end of summer (296.03±40.07 and 201.7±62.74 μmolN m⁻² h⁻¹, respectively) as well as high respiration rates (2.60±0.94 and 2.23±0.59 mmolC m⁻² h⁻¹, respectively). Significant benthic gross primary production (up to 6.11 mmolC m⁻² h⁻¹) was measured in the muddy sand community but no benthic primary production was measured in the fine-sand community. It suggests that microphytobenthic production values used in simulations previously published for these two communities were overestimated while values of community respiration were underestimated. The study confirms that this benthic system is heterotrophic and strengthens the idea that an important pelagic-benthic coupling is required for the functioning in such coastal ecosystems.     

Microbial community structure and biomass in surface waters during a Polar Front summer bloom along 170°W

As part of the US Joint Global Ocean Flux Study (US JGOFS) Southern Ocean Program, flow cytometry and epifluorescent microscopy were utilized to determine abundance, distribution and size structure of the microbial community in the Polar Front region during the summer biomass maximum. Surface samples were collected approximately every 10 km along 170°W during two N–S transects, separated in time by two weeks. Phytoplankton abundance and size structure varied with distinct latitudinal trends. Autotrophic biomass was lowest north of the Polar Front reflecting the dominance of small cells. The highest biomass (170 μg C l⁻¹) occurred at 65°S where the composition was strongly influenced by large centric diatoms. Farther south, the diatom community shifted to the dominance of smaller pennate diatoms. Total grazer biomass and size distributions followed similar patterns, ranging from 4 μg C l⁻¹ in the north to 52 μg C l⁻¹ in the south where larger (>20 μm) grazers were more abundant. Heterotrophic bacteria varied over an order of magnitude in abundance across the study site, with size generally increasing from north to south. In the second transect, phytoplankton biomass at 65°S was 50% lower, and grazer biomass and bacterial populations were slightly greater, indicating the decline of the bloom. The changes in biomass and community structure along 170°W and the reduction of phytoplankton standing stock at 65°S over time suggests adjacent, yet different, microbial systems in terms of carbon flux, spanning from primarily recycling to export-dominated.     

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.     

Compositions and transport of lipid biomarkers through the water column and surficial sediments of the equatorial Pacific Ocean

A systematic investigation of fluxes and compositions of lipids through the water column and into sediments was conducted along the U.S. JGOFS EgPac transect from l2°N to l5°S at 140°W. Fluxes of lipids out of the euphotic zone varied spatially and temporally, ranging from ≈0.20 – 0.6 mmol lipid-C m⁻² day⁻¹. Lipid fluxes were greatly attenuated with increasing water column depth, dropping to 0.002-0.06 mmol lipid-C m⁻² day⁻¹ in deep-water sediment traps. Sediment accumulation rates for lipids were ≈ 0.0002 – 0.00003 mmol lipid-C m⁻² day⁻¹. Lipids comprised ≈ 11–23% of C_org in net-plankton, 10–30% in particles exiting the euphotic zone, 2–4% particles in the deep EgPac, and 0.1-1 % in sediments. Lipids were, in general, selectively lost due to their greater reactivity relative to bulk organic matter toward biogeochemical degradation in the water column and sediment. Qualitative changes in lipid compositions through the water column and into sediments are consistent with the reactive nature of lipids. Fatty acids were the most labile compounds, with polyunsaturated fatty acids (PUFAs) being quickly lost from particles. Branchedchain C₁₅ and C₁₇ fatty acids increased in relative abundance as particulate matter sank and was incorporated into the sediment, indicating inputs of organic matter from bacteria. Long-chain C₃₉ alkenones of marine origin and long-chain C₂₀-C₃₀ fatty acids, alcohols and hydrocarbons derived from land plants were selectively preserved in sediments. Compositional changes over time and space demonstrate the dynamic range of reactivities among individual biomarker compounds, and hence of organic matter as a whole. A thorough understanding of biogeochemical reprocessing of organic matter in the oceanic water column and sediments is, thus, essential for using the sediment record for reconstructing past oceanic environments.     

Benthic microalgal biomass in sediments of Onslow Bay, North Carolina

Sediment samples were collected at stations along cross-shelf transects in Onslow Bay, North Carolina, during two cruises in 1984 and 1985. Station depths ranged from 11 to 285 m. Sediment chlorophyll a concentrations ranged from 0·06 to 1·87 μg g⁻¹ sediment (mean, 0·55), or 2·6–62·0 mg m². Areal sediment chlorophyll a exceeded water column chlorophyll a a at 16 of 17 stations, especially at inshore and mid-shelf stations. Sediment ATP concentrations ranged from 0 to 0·67 μg g⁻¹ sediment (mean, 0·28). Values for both biomass indicators were lowest in the depth range including the shelf break (50–99 m). Organic carbon contents of the sediments were uniformly low across the shelf, averaging 0·159% by weight. Photography of the sediments revealed extensive patches of microalgae on the sediment surface.Our data suggest that viable benthic microalgae occur across the North Carolina continental shelf. The distribution of benthic macroflora on the North Carolina shelf indicates that sufficient light and nutrients are available to support primary production out to the shelf break. Frequent storm-induced perturbations do not favour settling of phytoplankton, an alternative explanation for the presence of microalgal pigments in the sediments. Therefore, we propose that a distinct, productive benthic microflora exists across the North Carolina continental shelf.     

Settleable and Non-Settleable Suspended Sediments in the Ogeechee River Estuary, Georgia, U.S.A.

Suspended particle dynamics were investigated in the Ogeechee River Estuary during neap tide in July 1996. Samples were operationally separated into ‘ truly suspended ’ (settling velocity <0·006 cm s⁻¹) and ‘ settleable ’ (settling velocity >0·006 cm s⁻¹) fractions over the course of a tidal cycle to determine whether these two fractions were comprised of particles with differing biological and chemical characteristics. Total suspended sediment, organic carbon and nitrogen, chlorophyll a and phaeopigment concentrations were measured in each fraction, as well as rates of bacterial hydrolytic enzyme activity [β-1,4-glucosidase (βGase) and β-xylosidase (βXase)]. The majority of the suspended sediment (by weight) was in the truly suspended fraction; all measured parameters were largely associated with this fraction as well. When compared to the settleable material, the truly suspended material was significantly higher in % POC (5·7±0·6 vs. 3·9±1·8), % chlorophyll (0·07±0·02 vs. 0·03±0·01), % phaeopigment (0·030±0·006 vs. 0·018±0·012), and weight-specific maximal uptake rates (V_maxper mg suspended sediment) of both enzymes (1·8±0·4 vs. 0·7± 0·2 nmol mg⁻¹ h⁻¹βGase and 1·1±0·3vs . 0·3±0·2 nmol mg⁻¹ h⁻¹βXase), providing clear evidence for a qualitative distinction between the two fractions. These results are interpreted to mean that the more organic-rich, biologically active material associated with the suspended fraction is likely to have a different fate in this Estuary, as ‘ truly suspended ’ sediments will be readily transported whereas ‘ settleable ’ sediments will settle and be resuspended with each tide. These types of qualitative differences should be incorporated into models of particle dynamics in estuaries.     

Small benthic size classes along the N.W. European Continental Margin: spatial and temporal variability in activity and biomass

In the context of the European OMEX Programme this investigation focused on gradients in the biomass and activity of the small benthic size spectrum along a transect across the Goban Spur from the outer Celtic Sea into Porcupine Abyssal Plain. The effects of food pulses (seasonal, episodic) on this part of the benthic size spectrum were investigated. Sediments sampled during eight expeditions at different seasons covering a range from 200 m to 4800 m water depth were assayed with biochemical bulk measurements: determinations of chloroplastic pigment equivalents (CPE), the sum of chlorophyll a and its breakdown products, provide information concerning the input of phytodetrital matter to the seafloor; phospholipids were analyzed to estimate the total biomass of small benthic organisms (including bacteria, fungi, flagellata, protozoa and small metazoan meiofauna). A new term ‘small size class biomass' (SSCB) is introduced for the biomass of the smallest size classes of sediment-inhabiting organisms; the reduction of fluorescein-di-acetate (FDA) was determined to evaluate the potential activity of ester-cleaving bacterial exoenzymes in the sediment samples.At all stations benthic biomass was predominantly composed of the small size spectrum (90% on the shelf; 97–98% in the bathyal and abyssal parts of the transect). Small size class biomass (integrated over a 10 cm sediment column) ranged from 8 g C m⁻² on the shelf to 2.1 g C m⁻² on the adjacent Porcupine Abyssal Plain, exponentially decreasing with increasing water depth. However, a correlation between water depth and SSCB, macrofauna biomass as well as metazoan meiofauna biomass exhibited a significantly flatter slope for the small size classes in comparison to the larger organisms.CPE values indicated a pronounced seasonal cycle on the shelf and upper slope with twin peaks of phytodetrital deposition in mid spring and late summer. The deeper stations seem to receive a single annual flux maximum in late summer. SSCB and heterotrophic activity are significantly correlated to the amount of sediment-bound pigments. Seasonality in pigment concentrations is clearly followed by SSCB and activity. In contrast to macro- and megafauna which integrate over larger periods (months/years), the small benthic size classes, namely bacteria and foraminifera, proved to be the most reactive potential of the benthic communities to any perturbations on short time scales (days/weeks). The small size classes, therefore, occupy a key role in early diagenetic processes.     

Plankton dynamics and carbon flux in an area of upwelling off the coast of Morocco

A carbon flux study was carried out off the coast of Morocco, at 31°N, in a region characterized by the presence of a persistent cyclonic eddy. Two short-term (4 and 3 day) deployments of free-floating sediment traps were combined with water column sampling and rate process measurements as the ship followed the traps. For a period of 36 h between trap deployments, a hydrographic section was run along 31°30'N as part of a larger scale survey being carried out simultaneously on the R.V. A. von Humboldt. The first trap deployment was near the eastern margin of the eddy and the traps moved to the north and west in a frontal jet associated with its northern boundary. After the second deployment, which was at the recovery point of the first, the traps moved to the west and then to the southwest. Throughout the study, chlorophyll concentrations varied between 27 and 125 mg m⁻² (0–100 m), with highest concentrations in the upwelled water nearest the coast and in upwelled water generated within the cyclonic eddy. Particulate organic carbon (POC) and particulate organic nitrogen (PON) concentrations were relatively uniform (13.6±1.8 and 1.63±28 g m⁻² with phytoplankton carbon accounting for 16–85% of total POC. Bacterial carbon was 5% of total POC and mesozooplankton carbon concentrations were equivalent to 9% of total POC. Microzooplankton biomass was not assessed but POC:PON ratios in the water column were often high, suggesting there was sometimes a large detrital component in the POC. Primary production rates varied between 1.0 and 2.5 g C m⁻² day⁻¹. Bacterial consumption accounted for 50% of primary production. Metabolic rates suggested that copepods were ingesting more than 0.4 g C m⁻² day⁻¹. while filtration rates suggested that ingestion of phytoplankton carbon was only 0.2 g C m⁻²day⁻¹, even when phytoplankton constituted 85% of the POC. f-ratios (based on uptake rates for ¹⁵N-nitrate and ammonia) were between 0.1 and 0.4, and excretion by mesozooplankton could account for 40% of the daily ammonium uptake by phytoplankton. HPLC pigment analysis showed that when chlorophyll biomass was high, diatoms were dominant, whereas when it was low, small prymnesiophytes, chlorophytes and diatoms were all important. The composition of the fluoresecent pigments in material in the sediment traps indicated that intact phytoplankton and copepod faecal pellets were the main sources but the relative rates of sedimentation of pigment, POC and PON for the two trapping periods did not reflect differences that were observed in the overlying water column. This was likely to be the result of spatial heterogeneity and strong horizontal currents heterogeneity and strong horizontal currents within the euphotic zone. Thus, material collected at 100 m probably did not originate in the water column immediately overlying the traps and trapping efficiencies might also have been variable.     

Nutrient flux in the Rhode River: Tidal transport of microorganisms in brackish marshes

Concentrations of bacteria, chlorophyll a, and several dissolved organic compounds were determined during 11 tidal cycles throughout the year in a high and a low elevation marsh of a brackish tidal estuary. Mean bacterial concentrations were slightly higher in flooding (7·1 × 10⁶ cells ml⁻¹) than in ebbing waters (6·5 × 10⁶ cells ml⁻¹), and there were no differences between marshes. Mean chlorophyll a concentrations were 36·7 μg l⁻¹ in the low marsh and 20·4 μg l⁻¹ in the high marsh. Flux calculations, based on tidal records and measured concentrations, suggested a small net import of bacterial and algal biomass into both marshes. Over the course of individual tidal cycles, concentrations of all parameters were variable and not related to tidal stage. Heterotrophic activity measured by the uptake of ³H-thymidine, was found predominantly in the smallest particle size fractions (< 1·0 μm). Thymidine uptake was correlated with temperature (r = 0·48, P < 0·01), and bacterial productivity was estimated to be 7 to 42 μg Cl⁻¹ day⁻¹.