Pelagic Flux Research Articles

Effects of a Submesoscale Oceanographic Filament on Zooplankton Dynamics in the Arctic Marginal Ice Zone

Submesoscale structures, characterized by intense vertical and horizontal velocities, potentially play a crucial role in oceanographic dynamics and pelagic fluxes. Due to their small spatial scale and short temporal persistence, conditions for in situ measurements are challenging and thus the role of such structures for zooplankton distribution is still unclear. During RV Polarstern expedition PS107 to Arctic Fram Strait in July/August 2017, a submesoscale filament was detected, which initiated an ad hoc oceanographic and biological sampling campaign. To determine zooplankton taxonomic composition, horizontal and vertical distribution, abundance and biomass, vertical MultiNet hauls (depth intervals: 300–200–100–50–10–0 m) were taken at four stations across the filament. Zooplankton data were evaluated in context with the physical-oceanographic observations of the filament to assess submesoscale physical-biological interactions. Our data show that submesoscale features considerably impact zooplankton dynamics. While structuring the pelagial with distinct zooplankton communities in a vertical as well as horizontal dimension, they accumulate abundance and biomass of epipelagic species at the site of convergence. Further, high-velocity jets associated with such dynamics are possibly of major importance for species allocation and biological connectivity, accelerating for instance processes such as the ‘Atlantification’ of the Arctic. Thus, submesoscale features affect the surrounding ecosystem in multiple ways with consequences for higher trophic levels and biogeochemical cycles.

Aliphatic hydrocarbons and triterpenes of the Congo deep-sea fan

Hydrocarbons were analyzed in sediments from the Congo River deep-sea fan, from the Congo River, and in sinking particles collected by sediment traps 40m above the sediment. Studied sites encompassed three lobes of decreasing age of formation along the canyon: sites A, F and C and a another lobe system, disconnected from the active channel since 4 ka, Site E. Terrestrial long-chain odd n-alkanes were dominant in all sediments of the lobe system. Unsaturated terpenoids sourced by higher plants, such as gammacerene, lupene, ursene and oleanene, were also detected. At site C, characterized by high accumulation rates (10–20cmyr−1), the organic matter spends less time in the oxic layer than at other sites and high phytadiene concentrations 10–17μggOC−1) evidenced recent terrestrial and phytoplanktonic remains reworked in anaerobic conditions. In these sediments, organic carbon-normalized concentrations of terrestrial alkanes and terpenoids were several fold higher than in the lobe sediments with lower accumulation rates (sites A and F), arguing for a more rapid degradation of terrestrial hydrocarbons than bulk organic carbon in the first steps of pre-diagenesis. Ample variations in the contributions of biomarkers from higher plants, ferns, bacteria and angiosperms, indicate an heterogeneous contribution of the soil and vegetation detritus delivered to the Congo lobe sediments. Lower concentrations in terrestrial hydrocarbons at site E, 45km away from the active canyon, indicated that river particles are still admixed to the dominant marine organic matter. Diploptene and hop-7(21)-ene have a dual origin, from terrestrial and marine microorganisms. Scatter in their relationship to gammacerene argues for a contribution of marine microorganisms, in addition to soils-sourced microorganisms. The close distribution patterns of diploptene, hop-21-ene, hop-7(21)ene and neohop-13(18)-ene is in line with the hypothesis of sequential clay-catalyzed isomerisation of bacterial hopenes. Terrestrial biomarker accumulation fluxes at site C are one order of magnitude higher than vertical pelagic flux, demonstrating the magnitude of the inputs delivered through turbiditic transport in the submarine canyon. Crude oil contamination was evidenced at the disconnected site E (UCM, C21 to C26 tricyclic diterpanes, CPI) and, in smaller amounts, in some sediments from sites A and C. It may be related to marine crude oil extraction and transport. A short-chain mode of alkanes with an even predominance is evidenced in sediments of the lobe complex and likely sources, crude oil, microorganisms and ferns, are discussed.

Optimizing recovery of eutrophic estuaries: Impact of destratification and re-aeration on nutrient and dissolved oxygen dynamics

Widespread and global efforts to improve degraded coastal ecosystems, especially those experiencing hypoxia, warrant a renewed focus on understanding and quantifying restoration trajectories. We describe a whole-ecosystem experiment to manipulate dissolved oxygen concentrations using large-scale destratification aeration that was used to document the biogeochemical response of a small estuary to changes in oxygen availability. The experiment was successful in creating oxic and anoxic bottom water conditions at a spatial scale much larger than that encompassed by the aerators. After a period of anoxic conditions, the return of oxygenated bottom water by destratification resulted in rapid decreases in sediment phosphate fluxes, uptake of nitrate and nitrite, and an increase in simulated denitrification rates. Bottom water nutrient concentrations responded near-simultaneously to these changes to benthic–pelagic fluxes. The rapidity with which the ecosystem responded to increases in bottom water oxygen confirms the critical role of dissolved oxygen concentrations in modulating nutrient cycling. This result also provides insight into the likely response of hypoxic and anoxic estuaries to remediation of oxygen deficits at the sediment–water interface.

Years are not brothers: Two-year comparison of greenhouse gas fluxes in large shallow Lake Võrtsjärv, Estonia

Summary Two-year comparison of methane (CH 4 ) and carbon dioxide (CO 2 ) fluxes from different habitat zones in Lake Vortsjarv revealed strong zonal, seasonal and annual variabilities. On average, largest emissions of both gases per unit area occurred in the helophyte zone. Fluxes of CO 2 exceeded those of CH 4 in both pelagic and littoral zones. In both years, emission peaks occurred in early spring and late autumn, but were considerably higher in 2010, which caused a more than twenty times higher net efflux. Annual differences in carbon emission were mostly accounted for by variable pelagic CO 2 fluxes caused by different ice and snow conditions in spring and phytoplankton growth patterns in summer and autumn. On an annual basis, carbon uptake predominated over release in the pelagic zone in 2009, whereas an opposite balance occurred in 2010. An almost continuous emission from the helophyte zone outbalanced the pelagic uptake in 2009, resulting in a positive net efflux from the lake as a whole. In 2010, when the pelagic area was also mostly emitting carbon, the efflux from the helophyte zone constituted 36% of the total. Multiple linear regression analysis showed that the pelagic flux of CO 2 could be best described by opposite changes in phytoplankton biomass and pH ( R 2 = 0.68), whereas the best descriptors for the whole lake methane emission ( R 2 = 0.52) were sediment temperature (positive), saturation level of dissolved oxygen and depth (both negative). Strong annual differences in net C emissions of Vortsjarv confirm the necessity of inclusion longer observation period, compared to widely used one year/ice-free period, in cases of lakes in climate zones with large seasonal variabilities. The results also confirm the need for including GHG emissions from helophytes ( e.g . Phragmites australis ) to the lake’s net GHG budget, even if they cover a relatively small area.

Swept under the carpet: organic matter burial decreases global ocean biogeochemical model sensitivity to remineralization length scale

Abstract. Although of substantial importance for marine tracer distributions and eventually global carbon, oxygen, and nitrogen fluxes, the interaction between sinking and remineralization of organic matter, benthic fluxes and burial is not always represented consistently in global biogeochemical models. We here aim to investigate the relationships between these processes with a suite of global biogeochemical models, each simulated over millennia, and compared against observed distributions of pelagic tracers and benthic and pelagic fluxes. We concentrate on the representation of sediment–water interactions in common numerical models, and investigate their potential impact on simulated global sediment–water fluxes and nutrient and oxygen distributions. We find that model configurations with benthic burial simulate global oxygen well over a wide range of possible sinking flux parameterizations, making the model more robust with regard to uncertainties about the remineralization length scale. On a global scale, burial mostly affects oxygen in the meso- to bathypelagic zone. While all model types show an almost identical fit to observed pelagic particle flux, and the same sensitivity to particle sinking speed, comparison to observational estimates of benthic fluxes reveals a more complex pattern, but definite interpretation is not straightforward because of heterogeneous data distribution and methodology. Still, evaluating model results against observed pelagic and benthic fluxes of organic matter can complement model assessments based on more traditional tracers such as nutrients or oxygen. Based on a combined metric of dissolved tracers and biogeochemical fluxes, we here identify two model descriptions of burial as suitable candidates for further experiments and eventual model refinements.

Coupling early-diagenesis and pelagic biogeochemical models for estimating the seasonal variability of N and P fluxes at the sediment–water interface: Application to the northwestern Adriatic coastal zone

An integrated numerical model for the simulation of both pelagic and benthic biogeochemical processes was obtained by fully coupling a transient early diagenesis model with a reaction-transport pelagic biogeochemical model. The integrated model was applied to a shallow coastal area of the Northern Adriatic Sea, approximately 1400 km 2 wide, with the aim of investigating: 1) the seasonal dynamics of the fluxes of macronutrients, namely N and P, at the sediment–water interface; 2) the spatial variability in both sediment concentration profiles and benthic–pelagic fluxes of NH 4 +, NO 3 −, DIP and O 2. Model results concerning the pelagic state variables compare well with field data. The outputs of the benthic biogeochemical sub-model were found to be consistent with a set of literature data on sediment biogeochemistry collected within the model domain. Even though these comparisons should not be regarded as a formal validation, they provide evidence that the model correctly simulates the main patterns of both primary production and organic matter mineralisation. Model results indicate a strong coupling between benthic and pelagic systems, since the oxygen penetration depth is small (2–4 mm) and the sediment oxygen demand, up to 20 mmol m − 2 day − 1 , is high. This is also confirmed by the importance of fluxes of macronutrients from sediments, 17.5 10 6 kg N year − 1 and 1.7 10 6 kg P year − 1 on the whole model domain, which accounted for approximately 14% and 50% of the total N and P reaching the water column on a yearly basis. Fluxes of dissolved inorganic compounds at the SWI are higher in the proximity of the coastline, where the sedimentation rate of organic matter is also more elevated. The integrated model could be further applied for research investigations and management purposes. To this regard, its use for the assessment of aquaculture impacts on sediment geochemistry and nutrient recycling looks appropriate.

Large-scale variation in macrofaunal communities along the eastern Nordic Seas continental margin: A comparison of four stations with contrasting food supply

A hypothesis-driven approach was used to examine the relative importance of pelagic and advective organic flux as drivers of macrofaunal community structure at mid-slope (900–1400 m) depths along the eastern Nordic Seas margin. Ice-free stations were sampled on the Vøring Plateau (VP, mainly pelagic flux) and Bear Island Fan (BIF, major downslope advection). Seasonally ice-covered stations with episodic inputs of fresh phytodetritus were located on the Svalbard Margin (SM, major current-driven advective input), and Yermak Plateau (YP, low advective input). Macrofaunal biomass was higher at stations with seasonal ice cover, suggesting a greater supply of bioavailable organic matter in the marginal ice zones. At VP, BIF and SM, box corer and multiple corer samples gave broadly consistent community data. Very large infauna in box corer samples from YP indicated the existence of a faunal component absent or not sampled at the other three stations. This faunal component and higher mean body size for 18 out of 23 major taxa at YP relative to VP was consistent with a hypothesis relating body size to interannual variability of sinking flux, but was confounded by a depth difference between the two stations. Consistent with predictions, taxa associated with subsurface ‘caching’ of fresh organic matter were important community elements at stations experiencing episodic pelagic input, but not at the advection-dominated BIF. Current-driven advective flux was associated with high suspension-feeder biomass at BIF, but not at SM. The predicted high representation of subsurface deposit-feeders at BIF was not found. Sipunculans ( Nephasoma sp.) known to rapidly subduct fresh organic matter were found at all stations but were most abundant at VP. Comparison with data from the mid-1980s showed a large difference in estimated macrofaunal abundance on the Vøring Plateau, but good agreement in biomass. Community patterns on the Vøring Plateau were generally consistent with earlier data. Standing stock estimates from the Svalbard Margin and Yermak Plateau also agreed well with previous data. Results suggest that large-scale contrasts in organic flux can be used to predict some aspects of bathyal macrofaunal community composition in the Nordic Seas.

Dispersal of natural and anthropogenic lead through submarine canyons at the Portuguese margin

Submarine canyons represent natural conduits for preferential transport of particulate material, including anthropogenic contaminants, from coastal zones directly to the deep sea. To assess related dispersal of natural and anthropogenic lead (Pb), we analyzed Pb concentrations and stable isotope ratios in surface sediments and sediment trap particulate material from the Portuguese margin Nazaré and Setúbal/Lisbon canyons. Geochemical data are integrated with previously obtained data on near-bottom hydrodynamics and processes and pathways of sediment transport. The two canyon systems are located in close geographic proximity to each other, but represent contrasting settings in terms of sediment input and down-canyon sediment transport processes. Concentration–isotope diagrams and three-isotope plots ( 206Pb/ 207Pb vs. 208Pb/ 206Pb) suggest binary mixing between natural and anthropogenic end members. The inferred isotopic signature of pollutant Pb ( 206Pb/ 207Pb=1.143 [1.134–1.149, 95% confidence interval]) is most consistent with industrial Pb; ongoing influence from gasoline Pb additives is at most of minor importance. Two proposed natural end members most likely bracket the isotopic signature of natural Pb. Accordingly, binary mixing calculations indicate that on average 20–45% vs. 35–55% of total Pb is derived from anthropogenic sources in the Nazaré and Setúbal–Lisbon canyon systems, respectively. Enhanced anthropogenic influence in the latter area is consistent with its proximity to heavily populated and industrialized areas and with sediment input from the Tagus and Sado rivers, potential major carriers of pollutant particles. In both canyon systems, the anthropogenic component generally decreases with increasing water depth. Isotopic signatures of sediment trap particulate material are generally consistent with surface sediment data at similar water depth, but show large variability in the upper Nazaré canyon and major deviations from surface sediments in the lower canyon. In the lower canyon, Pb isotopic ratios of sediment trap particulate material mostly reflect low pelagic fluxes from the overlying water column, whereas surface sediment signatures are dominated by episodic down-canyon mass transport events. Such gravity flows appear to incorporate older (pre-industrial) material masking the isotopic signature of pollutant Pb. Large variability in the upper canyon reflects continuous sediment resuspension by bottom currents. Stronger average bottom currents are associated with higher 206Pb/ 207Pb ratios of sediment trap particulate material and hence decreased influence of pollutant Pb. This may reflect preferential resuspension of natural Pb at the canyon floor and/or additional remobilization of older, less-polluted sediment in adjacent areas such as the canyon walls.

Temporal variation of currents, particulate flux and organism supply at two deep-sea hydrothermal fields of the Azores Triple Junction

In order to provide information on the production and the distribution of hydrothermal particulate and biological material produced by deep-sea hydrothermal vents, three moorings with sediment traps and currentmeters were deployed for one year in the Azores Triple Junction region. Two moorings were positioned close to two hydrothermal vent fields at the Mid-Atlantic Ridge (Rainbow, 36°14′N, 33°54′W, 2250 m depth and Lucky Strike 37°17′N, 32°17′W, 1600 m depth). A third was moored out of the influence of the Rainbow vents representing the reference of the pelagic flux in this area. At the Rainbow field, particulate flux (range 341–634 g m −2 d −1) showed a high temporal variation controlled essentially by the current direction. The particles were characterized by a high concentration of total sulfur (16.4%) and copper (8.6%) and a low concentration of organic carbon (1.4%). Bivalves collected in the trap belonged to the hydrothermal mytilid Bathymodiolus azoricus and were all in the prejuvenile prodissoconch II stage of their development. Their density decreased strongly between July and September, indicating a possible discontinuous reproduction of this species. The particles at Lucky Strike were rich in barium and the calculated flux was lower than at Rainbow (range 52–308 g m −2 d −1) because of the translucent fluid poor in particles emitted at this shallower station. However, a peak in the flux was observed in May corresponding to the seasonal phytoplankton bloom. Two different bivalve species were sampled in this trap. The most abundant was B. azoricus. However, their development stages showed higher variations compared to the Rainbow trap. The abundance of mussels in the trap was very low throughout the year and does not allow one to show significant spawning periodicity.

Influence of suspended scallop cages and mussel lines on pelagic and benthic biogeochemical fluxes in Havre-aux-Maisons Lagoon, Îles-de-la-Madeleine (Quebec, Canada)

An in situ experiment was done in July 2004 to test and compare the influence of suspended bivalve cultures (1- and 2-year-old blue mussels (Mytilus edulis) and sea scallops (Placopecten magellanicus)) on biogeochemical fluxes in the water column and at the benthic interface in Havre-aux-Maisons Lagoon (Quebec, Canada). Aquaculture structures increased the pelagic macrofaunal biomass (PMB) and acted as an oxygen sink and nutrient source in the water column under dark conditions. Although PMB was lower in scallop culture, the influence of scallop cages on pelagic fluxes was similar to or greater (nitrate and nitrite) than that of mussel lines. Sediments were organically enriched, and benthic macrofaunal abundances were decreased in mussel culture zones relative to the control zone, but such an effect was not observed in the scallop zone. Nevertheless, benthic oxygen demand did not vary among culture types and control zones. Benthic nutrient fluxes were greatest beneath aquaculture structures. Both pelagic and benthic interfaces may modify oxygen and nutrient pools in culture zones in Havre-aux-Maisons Lagoon. The contribution of aquaculture structures to oxygen, ammonium, and phosphate pools may be a function of PMB and type. While aquaculture structures had an important role on nitrate and nitrite cycling, silicate turnover was mainly driven by benthic mineralization of biodeposits.

Cross-shelf biogeochemical characteristics of sediments in the central Benguela and their relationship to overlying water column hypoxia

Data from two cross-shelf sediment sampling cruises were used to explain reasons for the sediment biogeochemical variability in respect of carbon, nitrogen and sulphur, and how the cycling of these elements governs the biogeochemistry of the overlying water through their control of the redox conditions. The spatial extent of this benthic–pelagic flux link is limited to the innershelf mud belt system on the Namibian shelf. The inshore mud belt is the primary deposition area of the carbon and nitrogen new production export flux. The offshore organic-rich zones are thought to be relict particulate organic matter originating from the inshore mud belt rather than from an overlying pelagic source. These data were used to set up a multi-layer sediment model that was used through sensitivity analyses to elucidate the input characteristics that result in the most significant feedbacks on hypoxia in the overlying water. The analyses showed that, although the new production flux is a requirement to drive an oxygen demand in the sediments, the onset and persistence of anoxia may depend critically on a low-oxygen boundary condition threshold. This is thought to be a key differentiating factor between systems that, despite comparable carbon export fluxes, are characterised by a persistent hypoxia/anoxia signal and those that are characterised by episodic hypoxia events. It was concluded that sediment oxygen demand and methane and 'sulphide' emissions from the central Benguela sediments are responses to external hypoxia boundary conditions rather than the local drivers of oxygen variability.

A bio-physical coastal ecosystem model for assessing environmental effects of marine bivalve aquaculture

A simple lower trophic level, bio-physical marine ecosystem model is developed for the purpose of assessing the environmental effects of bivalve aquaculture in coastal embayments. The ecosystem box model includes pelagic and benthic components and describes the cycling of a most-limiting nutrient. The pelagic compartment is comprised of phytoplankton, zooplankton, nutrients and detritus. These populations interact following predator–prey dynamics and biogeochemical processes. Mixing processes within the bay, and exchange of waters with the adjacent open ocean, are included. The pelagic ecosystem is coupled to a simple benthos containing a dynamically active organic matter pool. Benthic–pelagic coupling includes episodic resuspension, remineralization, sinking, and permanent burial. A population of grazing bivalves is superimposed on this system as a diagnostic variable. The model is applied to a coastal bay and used to determine how bivalve populations affect nutrient cycling in the ecosystem. This is done by examining changes in the standing stock of the various populations, as well as associated nutrient (mass) fluxes, for cases both with and without intensive bivalve culture. It was demonstrated that bivalves divert production from the pelagic to benthic food webs. Phytoplankton and detritus are depleted from the water by bivalve filter feeding and biodeposited to the benthos as fecal matter. This organic loading causes order of magnitude changes in the benthic detrital pool and the associated benthic–pelagic fluxes. It was also shown that water motion and mixing is important in structuring the ecological dynamics in the bay. To faciliate future applications and observational studies, a retrospective analysis of parameter identifiability and uncertainty was also undertaken.

BENTHIC FORAMINIFERA AND PORE-WATER OXYGEN PROFILES: A RE-ASSESSMENT OF SPECIES BOUNDARY CONDITIONS AT THE WESTERN IBERIAN MARGIN

The depth distribution of living (Rose Bengal-stained) benthic foraminifera in near-surface sediments was analyzed along 4 short cores from the western Iberian Margin and Gulf of Cadiz at water depths from 800 to 1920 m. Pore-water oxygen, Chloroplastic Pigment Equivalent (CPE), and total organic carbon (C org ) concentrations were measured in the same or adjacent cores. These values were used to constrain the limiting oxygen and nutrient levels that defined the ranges in sediment depth and abundance maxima of foraminiferal species. The population density showed a good correlation with pore-water oxygenation indicating that oxygen is a limiting factor for endobenthic foraminiferal assemblages at mid-depths between 800 and 1200 m. At the deep-water site, however, the fauna appeared nutrient limited. More than half of the benthic species occupied extended sediment depth habitats and a wide range of oxygen levels among the sites studied. Thirty-nine of 103 species occurred in two or more cores and showed distinct abundance maxima, 36 in the uppermost 2.0 cm where the CPE concentrations were high. Living foraminifers were not found at depths where the CPE-values were lower than 0.1 μg/g. This level indicates trophic conditions critical for the survival of endobenthic species. The frequency distribution of the lower oxygen range limits of all species showed maxima at the high oxic/low oxic (3 ml/l), low oxic/suboxic (1.5 ml/l), and suboxic/dysoxic (0.3 ml/l) boundaries and thus depicted the ecological significance of these previously recognized ecolimits. Only 30 % of the species display consistent patterns in relation to pore water oxygen. Dysoxic indicators were Globobulimina affinis, Chilostomella ovoidea and Bathysiphon capillare. Many species previously designated as suboxic indicators were ubiquitous or preferred microhabitats at oxic levels. Boundary conditions for the dysoxic indicator G. affinis were pelagic flux rates higher than 3.5 g C m −2 yr −1 , a habitat depth below the homogeneously bioturbated layer in areas outside of high productivity zones, and CPE concentrations of 0.1 to 1.0, preferentially 0.7 to 0.8 μg/g. The distribution pattern suggested that G. affinis thrives mainly on dysoxic bacteria and actively seeks this nutrient resource.

Modelling the pelagic nitrogen cycle and vertical particle flux in the Norwegian sea

A 1D Eulerian ecosystem model (BIological Ocean Model) for the Norwegian Sea was developed to investigate the dynamics of pelagic ecosystems. The BIOM combines six biochemical compartments and simulates the annual nitrogen cycle with specific focus on production, modification and sedimentation of particles in the water column. The external forcing and physical framework is based on a simulated annual cycle of global radiation and an annual mixed-layer cycle derived from field data. The vertical resolution of the model is given by an exponential grid with 200 depth layers, allowing specific parameterization of various sinking velocities, breakdown of particles and the remineralization processes. The aim of the numerical experiments is the simulation of ecosystem dynamics considering the specific biogeochemical properties of the Norwegian Sea, for example the life cycle of the dominant copepod Calanus finmarchicus. The results of the simulations were validated with field data. Model results are in good agreement with field data for the lower trophic levels of the food web. With increasing complexity of the organisms the differences increase between simulated processes and field data. Results of the numerical simulations suggest that BIOM is well adapted to investigate a physically controlled ecosystem. The simulation of grazing controlled pelagic ecosystems, like the Norwegian Sea, requires adaptations of parameterization to the specific ecosystem features. By using seasonally adaptation of the most sensible processes like utilization of light by phytoplankton and grazing by zooplankton results were greatly improved.

Late Glacial and Holocene palaeocurrents around Rockall Bank, NE Atlantic Ocean

Core samples and hydrographic data from 50° to 60°N and 15° to 25°W in the Northeast Atlantic Ocean are used to study changes in current activity from the last glacial to present. We establish a new method to distinguish between the effects of changing bottom current speed and varying size input by drawing upon sediment flux data and detailed grain size analysis. Changes in current speed are recorded by the mean size of the sortable (coarse) noncarbonate silt component, which increases with current vigor by winnowing away of fines. Our method involves the definition of an ideal sortable silt “input function”, recorded at a site accumulating unmodified pelagic flux only. The ideal profile is then compared with records from other sites to determine the history of current speed at a range of water depths. The upper surface of Southern Source Water (SSW) probably shoaled during the glacial, as indicated by the covariation of sortable silt records from sites presently covered by SSW and Lower North Atlantic Deep Water (LNADW). The data suggest that production of LNADW was suppressed during the glacial, increased following the last glacial maximum, and declined at the start of Termination 1A. A second pulse of production occurred immediately before the Younger Dryas event. Intermediate waters were generally faster‐flowing during the glacial and may have expanded their depth domain, such that a single glacial intermediate water mass covered depths from 1100 to more than 2000 m.

Pelagic fluxes and sedimentation

In February 1992 there was a two day meeting of the Marine Studies Group dealing with pelagic fluxes and sedimentation. Day 1 covered several topics while Day 2 focused on BOFS (Biogeochemical Ocean Flux Study). The BOFS work is being published elsewhere and so the present thematic set and the meeting summary below concentrates on the topics covered during Day 1. The floor of the ocean is the site of deposition for particles from any source that are transported down through the water column. The sediments record both about past activities in the overlying waters and about past inputs to those waters. Present-day oceanographic processes are analogues for the past, but the data must be interpreted correctly. To do that some idea is needed of what happens to particles on their way down, and what modifications to the record take place upon sedimentation. Amongst the main themes of the meeting were the variability of the fluxes, and the coupling of the surface waters with the bottom sediments. Although the story that can be gained from isotopes provides extra details, there is still a great deal of information to be gleaned from detailed examination of the trace-metal content of particulates. R. Chester showed how the trace-metal composition of particulate material undergoes modification during its journey from the sea surface to the sea-floor. What is essentially a ‘fingerprint’ of solid-state speciation can be determined by looking at which carrier phase contains which trace element. It was shown that organically bound copper in

Microbial and meiofaunal abundance, redox potential, pH and shear strength profiles in deep sea Pacific sediments

Vertical profiles of microbial and meiofaunal abundances, and of redox potential, pH, water content and shear strength have been measured in sediment subcores from seven box cores collected in the southern Central Pacific and the findings have been related to potential effects on pelagic fluxes and sedimentation. Microbial and meiofaunal abundance decreased exponentially with sediment depth. Micro-organisms were found throughout the depth of the core to 40 cm. Meiofauna were only present in the top 5 cm to 10 cm. Meiofaunal metazoans decreased more rapidly than foraminiferans. Redox potential was always positive, decreased below the surface and often rose again deeper in the sediment. pH showed no clear cut trend with depth. Shear strength increased with sediment depth and showed occasional anomalous fluctuations. Water content decreased progressively downwards with the widest scatter in the top 5–10 cm. The most important implications of these results for the water column above are likely to be biologically mediated organic mineralization and nutrient regeneration into the water column, and biodeposition of heavy metals and the binding of particulates at and above the sediment—water interface by extracellular polymeric material, which will in turn lead to differential patterns of sedimentation.

Bioturbation in deep sea Pacific sediments

Bioturbation structures have been recorded in sediment cores from 5000 m to 5300 m water depth collected from the Pacific Ocean using subcores from spade box cores. Shear strength and Eh profiles were also taken. The results have been related to pelagic fluxes and sedimentation. Bioturba-tion structures consist of holes and chambers of various sizes running vertically and horizontally, whose abundance, size and shapes have been measured. Values did not decrease exponentially with sediment depth but end abruptly at depths ranging from 30 cm to 45 cm. In many instances there were subsurface peaks of vertical and horizontal burrows. There was also evidence of lateral galleries. Large deep in filled burrows were observed. These consisted of light and dark brown sediment having different shear strengths. Eh profiles showed that the sediment was aerobic, with slightly lower values near the sediment surface, possibly associated with meiofaunal and bacterial activity. The most important implications of this work for pelagic processes are likely to be increased transfer of dissolved and particulate material between the sediment and the water column due to the physical network of burrows within the sediment and increased surface roughness caused by burrow openings. Mucus binding of particulates and faecal pelletization are also likely to be important. All of these effects will be most noticeable at high burrow densities.

Sedimentation of pteropods in the Norwegian Sea in autumn

Pteropod vertical distribution on the Vøring Plateu (eastern Norwegian Sea) was recorded during a 3 week drifting experiment during August 1988. Parallel to sampling of hydrographical, chemical and biological properties of the water column, sediment traps recorded vertical pelagic flux at five depth strata. Pteropods ( Limacina retroversa) dominated the zooplankton and reached maximum values (>13,600 individuals m −3) in the upper 25 m of the water column; the size spectrum shifted from small (< 1 mm) towards large (> mm) specimens during the study. Vertical flux at 100 m depth increased from 600 mg dry weight m −2 day −1 to 1000 mg dry weight m −2 day −1 at the end of the experiment. Trap material during the first sampling intervals consisted primarily of phytoplankton and protozoans aggregated within pteropod feeding nets. Thereafter, the increase in flux rates was associated with empty shells of pteropods in the size class 1–3 mm indicating mortality prior to mass sinking. Particle flux during the investigation period totalled 0.2 g organic Cm −2 and 1.4 g carbonate m −2 equivalent to 8 and 15% of annual flux recorded in the area in 1988. Through their feeding, reproduction and subsequent mortality pteropods were the main contributors to vertical particle flux in the eastern Norweigian Sea during 1988.

Settling of particles in a hydrothermal vent field (East Pacific Rise 13°N) measured with sediment traps

Samples from nine sediment traps moored in the vicinity of an active hydrothermal vent on a segment of the East Pacific Rise (EPR), near 13°N, were collected in 1987 during the Hydronaut cruise. The results show variability in the distribution and composition of the particle flux to the area immediately surrounding the sea-floor vents. The settling of material measured at three distances from the “Totem” vent field demonstrates a sharp decrease in total flux with increasing distance from the source. The amount of particles collected with sediment traps at the vent field is 5.5 times greater than the pelagic flux measured on this site Comparison of elemental fluxes determined in the near- and far-field experiments reveals the spatial distribution of vent effluent. The sulpur particle flux decreases by a factor of 300 between the near-field station and the seamount station 15 km southeast of the vent. In contrast, the pelagic carbonate flux appears equally distributed throughout the studied area. Few organic particles produced by the vent ecosystem are reaching the seafloor only 80 m away. Organic output from vent sites to their surroundings in the central graben, or the ridge crest, via large settling particles appear negligible compared to the pelagic flux from surface waters. The production of organic matter by the benthos near the vent because this organic flux is too diluted by the mineral fraction rich in sulphur.