Tropical Northeast Atlantic Research Articles

A quantitative study on the terrestrial input to the tropical Northeast Atlantic

Eolian dust and riverine discharge are identified as two key components of terrestrial input to the oceans. They supply micronutrients to the oceans and modify marine carbon biogeochemistry and global climate through dust-land-ocean interactions. However, it is challenging to accurately constrain regional terrestrial inputs in the past, with currently available models and geochemical proxies. The present study utilizes sedimentary wtCaCO3% records to estimate lithogenic fluxes. The depth-dependance of CaCO3 preservation in the Holocene and Last Glacial Maximum (LGM) sediments in two major basins of the tropical Northeast Atlantic Ocean is described using a carbonate dissolution model. Results show that during the LGM, reduced dust deposition and slight drops of fluvial input are found in the Canary Basin and Cape Verde margins, respectively. To supplement, carbonate deposition during the LGM indicates that the deep subtropical Northeast Atlantic is seized by more sluggish deep waters relative to today.

Dynamics of currents and biological scattering layers around Senghor Seamount, a shallow seamount inside a tropical Northeast Atlantic eddy corridor

Biophysical interactions of the mesopelagic fauna and hydrodynamic features were investigated at a shallow, tall seamount in the tropical Northeast Atlantic based on data from three inter-disciplinary surveys collected between 2009 and 2015. Senghor Seamount is located close to the equator in a unique but underexplored oceanographic setting, where classical seamount effects are expected to be small. Shipboard Acoustic Doppler Current Profiler (ADCP) and remote sensing data revealed flow phenomena strongly varying over different temporal and spatial scales. The quasi-steady, longer period flow at the seamount was largely unchanged by flow-topography interaction with the exception of a persistent and recurring shadow zone of weaker currents downstream of the summit. Senghor Seamount is exposed to energetic mesoscale eddy activity in surrounding waters with potentially large physical and biological implications. At higher frequencies, tidal currents interacting with the seamount generated large-amplitude internal waves propagating horizontally and vertically away from the summit. Analysis of acoustic backscatter data and micronekton biomass displayed prominent near-surface night time and deep (>400 m) day time scattering layers associated with the diurnal vertical migration of the mesopelagic fauna. An intense and previously unreported aggregation of acoustic scatterers was detected in the depth range 150–300 m inside a radius of 3 km along the upper slopes of the seamount summit. ADCP measurements over a full semi-diurnal tidal cycle suggest that biophysical interactions were present in near-seafloor waters around the upper seamount as evidenced by plankton distributions that were modified by topographic blocking during descent and aggregation in waters characterised by internal waves and enhanced tidally-driven energy dissipation.

Aerosol time-series measurements over the tropical Northeast Atlantic Ocean: Dust sources, elemental composition and mineralogy

The North Atlantic receives the largest dust loading of any of the world's oceans due to its proximity to North African deserts and prevailing wind patterns. The supply of biologically important trace elements and nutrients via aerosols has an important influence on biogeochemical processes and ecosystems in this ocean region. In this study we continuously sampled aerosols between July 2007 and July 2008 at the Cape Verde Atmospheric Observatory (CVAO), which is situated on an island group close to the North African continent and under the Saharan/Sahelian dust outflow path. The aim of our work was to investigate temporal variations in aerosol concentration, composition and sources in the Cape Verde region over a complete seasonal cycle, and for this purpose we undertook mineralogical and chemical (42 elements) analyses of the aerosol samples and air mass back-trajectory calculations. Aerosol samples were also collected during a research cruise in the (sub-) tropical Northeast Atlantic Ocean in January 2008.The concentration of atmospheric Al, a proxy for mineral aerosol concentration, at CVAO was in the range of 0.01–66.9μgm−3 (maximum on 28–30 January 2008) with a geometric mean of 0.76μgm−3. It showed distinct seasonal variations, with enhanced Al concentrations in winter (geometric mean 1.3μgm−3), and lower concentrations in summer (geometric mean 0.48μgm−3). These observations have been attributed to dust transport occurring in higher altitude air layers and mainly north of the Cape Verde during summer, while in winter the dust transport shifts south and occurs in the lower altitude trade winds with consequent greater influence on the Cape Verde region. The elemental composition of the aerosols closely agreed with mean upper crustal abundances, with the exception of elements with pronounced anthropogenic sources (e.g. Zn and Pb) and major constituents of sea water (Na and Mg). Mineral analysis showed that clays were the most abundant mineral fraction throughout the whole sampling period, with an increase in quartz and clays during strong dust events and an associated decrease in calcite. This could have important implications for the estimation of release of for example Fe from mineral dust with clays having a higher Fe solubility than quartz.The elemental composition and mineralogy of aerosol samples collected during the cruise were indistinguishable from those collected at the CVAO during the same period, although mean atmospheric Al was 65% higher at the CVAO than those measured on the ship due to the irregular and uneven nature of dust transport.Air mass back-trajectories showed an important role for southern source regions of the North African deserts during summer, with 92.5% of the samples indicating a contribution from the Sahel. Significantly elevated ratios of V, Ni, Cu, Zn, Cd and Pb with Al were present in samples originating from the Sahel compared with samples with a more northerly origin. This was likely due to enhanced anthropogenic emissions related to the greater population densities in the Sahel compared with the less developed Saharan regions further north.Ratios of other elements and trends in rare earth elements could however not be used to distinguish differences in source regions. Similar source material compositions, the mixing of dust from different regions during transport, and the pooling of samples over a 1–3day collection period appear to have diluted specific signals from source regions.

Effects of hook and bait in a tropical northeast Atlantic pelagic longline fishery: Part II—Target, bycatch and discard fishes

The incidental bycatch of sea turtle in tuna and swordfish fisheries is currently recognized as one of the major threats to the populations of these species. Therefore a number of mitigation measures have been tested, particularly for longline fisheries targeting swordfish. As mitigation measures may also affect the fish catches, it is important to quantify these impacts both at the ecological and socio-economic levels. Between August 2008 and December 2011, a total of 202 experimental pelagic longline sets were carried out in the Tropical Northeast Atlantic Ocean. The combination J-hook baited with squid (traditionally used by the fishery) was compared against two circle hooks (one non-offset and one with 10° offset) and mackerel bait. Catches per unit effort (CPUE) were calculated and compared between the different hook style and bait combinations for all target, bycatch and discarded fish species. In addition, a GLM (generalized linear model) was applied for swordfish Xiphias gladius and blue shark Prionace glauca (two main target species) and bigeye thresher Alopias superciliosus (most discarded species). The swordfish catches were negatively affected when changing from the traditional gear (J-style hooks baited with squid) to one of the experimental combinations, with the bait type having a stronger influence than the hook style on this reduction. However, the overall target species CPUE and the value of the retained catch (VPUE, value per unit of effort) were not significantly affected, due to an increase on the blue shark CPUE. Furthermore, the hook style and the bait type did not seem to influence the at-haulback mortality rates of most discarded species, which were highly species-specific. Given the apparent lack of impact on the overall value of the retained catch, the use of circle hooks baited with mackerel on this particular fishery and region would be highly beneficial for sea turtle conservation, without affecting the economic viability of the fishery.

Effects of hook and bait in a tropical northeast Atlantic pelagic longline fishery: Part I—Incidental sea turtle bycatch

The interaction between tuna fisheries and sea turtles is commonly recognized as one of the major threats and causes for the decline of sea turtle populations. Within the tuna and swordfish fisheries, the incidental sea turtle bycatch is usually more frequent from longline fisheries targeting swordfish. Therefore it is important to test possible mitigation measures, particularly in areas where fishing activities and high abundance of these species overlaps, as is the case of the Tropical Northeast Atlantic Ocean. Between August 2008 and December 2011, a total of 202 experimental pelagic longline sets were carried out in that region (latitude: 11–22°N, longitude: 20–38°W). The aim was to test the effects of changing the traditional J-style hooks (10° offset) baited with squid used by the fishing industry, against two circle hooks (one non-offset and one with 10° offset) and mackerel bait. Four sea turtle species were captured, with the leatherback Dermochelys coriacea comprising most of the bycatches (BPUE, bycatch per unit of effort using the traditional configuration of 0.990 turtles/1000 hooks), followed by three hardshell species: the loggerhead Caretta caretta and the olive ridley Lepidochelys olivacea (BPUE=0.165 turtles/1000 hooks), and the Kemp ridley Lepidochelys kempii (BPUE=0.024 turtles/1000 hooks). In general, the sea turtle interactions in the fishery can be reduced by changing from the traditional gear to one of the experimental combinations. However, those reductions were species-specific, with the leatherback bycatches reduced only when changing from J-style to the non-offset circle hook, while for the hardshell turtles both the hook style (using a circle hook, with or without offset) and the bait (using mackerel) significantly reduced the incidental bycatches. Hooking location was also species-specific, with most hardshell specimens hooked by the mouth and esophagus, while leatherbacks were mostly hooked externally by the flippers. Most of the sea turtles were captured and released alive with the mortality rates independent of the hook style and bait type used. A reduction of 55% in leatherback incidental bycatches can be expected in this fishery by changing from J-style to circle hooks, whereas for the hardshell species a 50–59% reduction can be obtained by changing to circle hooks (respectively with and without offset), and a 55% reduction by using mackerel bait.

Oxygen variance and meridional oxygen supply in the Tropical North East Atlantic oxygen minimum zone

The distribution of the mean oceanic oxygen concentration results from a balance between ventilation and consumption. In the eastern tropical Pacific and Atlantic, this balance creates extended oxygen minimum zones (OMZ) at intermediate depth. Here, we analyze hydrographic and velocity data from shipboard and moored observations, which were taken along the 23°W meridian cutting through the Tropical North East Atlantic (TNEA) OMZ, to study the distribution and generation of oxygen variability. By applying the extended Osborn–Cox model, the respective role of mesoscale stirring and diapycnal mixing in producing enhanced oxygen variability, found at the southern and upper boundary of the OMZ, is quantified. From the well-ventilated equatorial region toward the OMZ core a northward eddy-driven oxygen flux is observed whose divergence corresponds to an oxygen supply of about 2.4 μmol kg−1 year−1 at the OMZ core depth. Above the OMZ core, mesoscale eddies act to redistribute low- and high-oxygen waters associated with westward and eastward currents, respectively. Here, absolute values of the local oxygen supply >10 μmol kg−1 year−1 are found, likely balanced by mean zonal advection. Combining our results with recent studies, a refined oxygen budget for the TNEA OMZ is derived. Eddy-driven meridional oxygen supply contributes more than 50 % of the supply required to balance the estimated oxygen consumption. The oxygen tendency in the OMZ, as given by the multidecadal oxygen decline, is maximum slightly above the OMZ core and represents a substantial imbalance of the oxygen budget reaching about 20 % of the magnitude of the eddy-driven oxygen supply.

Drivers of diel and regional variations of halocarbon emissions from the tropical North East Atlantic

Abstract. Methyl iodide (CH3I), bromoform (CHBr3) and dibromomethane (CH2Br2), which are produced naturally in the oceans, take part in ozone chemistry both in the troposphere and the stratosphere. The significance of oceanic upwelling regions for emissions of these trace gases in the global context is still uncertain although they have been identified as important source regions. To better quantify the role of upwelling areas in current and future climate, this paper analyzes major factors that influenced halocarbon emissions from the tropical North East Atlantic including the Mauritanian upwelling during the DRIVE expedition. Diel and regional variability of oceanic and atmospheric CH3I, CHBr3 and CH2Br2 was determined along with biological and physical parameters at six 24 h-stations. Low oceanic concentrations of CH3I from 0.1–5.4 pmol L−1 were equally distributed throughout the investigation area. CHBr3 and CH2Br2 from 1.0 to 42.4 pmol L−1 and to 9.4 pmol L−1, respectively were measured with maximum concentrations close to the Mauritanian coast. Atmospheric CH3I, CHBr3, and CH2Br2 of up to 3.3, 8.9, and 3.1 ppt, respectively were detected above the upwelling, as well as up to 1.8, 12.8, and 2.2 ppt at the Cape Verdean coast. While diel variability in CH3I emissions could be mainly ascribed to oceanic non-biological production, no main driver was identified for its emissions over the entire study region. In contrast, biological parameters showed the greatest influence on the regional distribution of sea-to-air fluxes of bromocarbons. The diel impact of wind speed on bromocarbon emissions increased with decreasing distance to the coast. The height of the marine atmospheric boundary layer (MABL) influenced halocarbon emissions via its influence on atmospheric mixing ratios. Oceanic and atmospheric halocarbons correlated well in the study region, and in combination with high oceanic CH3I, CHBr3 and CH2Br2 concentrations, local hot spots of atmospheric halocarbons could solely be explained by marine sources. This conclusion is in contrast to previous studies that hypothesized elevated atmospheric halocarbons above the eastern tropical Atlantic to be mainly originated from the West-African continent.

Distributions of particulate Hemebin the Atlantic and Southern Oceans—Implications for electron transport in phytoplankton

Concentrations of hemeb, the iron‐containing component ofb‐type hemoproteins, ranged from < 0.4 to 5.3 pM with an average of 1.18 ± 0.8 pM (± 1σ;n = 86) in the Iceland Basin (IB), from < 0.4 to 19.1 pM with an average of 2.24 ± 1.67 pM (n = 269) in the tropical northeast Atlantic (TNA) and from 0.6 to 21 pM with an average of 5.1 ± 4.8 pM (n = 34) in the Scotia Sea (SS). Hemebconcentrations were enhanced in the photic zone and decreased with depth. Hemebconcentrations correlated positively with chlorophylla(chla) in the TNA (r = 0.41,p < 0.01,n = 269). Hemebdid not correlate with chlain the IB or SS. In the IB and SS, stations with high‐chlorophyll and low‐nutrient (Fe and/or Si) concentrations exhibited low hemebconcentrations relative to particulate organic carbon (< 0.1 μmol mol−1), and high chla:hemebratios (> 500). High chla:hemebratios resulted from relative decreases in hemeb, suggesting proteins such as cytochrome b6f, the core complex of photosystem II, and eukaryotic nitrate reductase were depleted relative to proteins containing chlorophyll such as the eukaryotic light‐harvesting antenna. Relative variations in hemeb, particulate organic carbon, and chlacan thus be indicative of a physiological response of the phytoplankton community to the prevailing growth conditions, within the context of large‐scale changes in phytoplankton community composition.

Pronounced subsurface cooling of North Atlantic waters off Northwest Africa during Dansgaard–Oeschger interstadials

Millennial-scale Atlantic meridional overturning circulation (AMOC) variability has often been invoked to explain the Dansgaard–Oeschger (DO) events. However, the underlying causes responsible for millennial-scale AMOC variability are still debated. High-resolution U37K′ and TEX86H temperature records for the last 50kyr obtained from the tropical Northeast (NE) Atlantic (core GeoB7926-2, 20°13′N, 18°27′W, 2500m water depth) show that distinctive DO-type subsurface (i.e. below the mixed layer: >20m water depth) temperature oscillations occurred with amplitudes of up to 8°C in the tropical NE Atlantic during Marine Isotope Stage 3 (MIS3). Statistical analyses reveal a positive relationship between the reconstructed substantial cooling of subsurface waters and prominent surface warming over Greenland during DO interstadials. General circulation model (GCM) simulations without external freshwater forcing, the mechanism often invoked in explaining DO events, demonstrate similar anti-phase correlations between AMOC and pronounced NE Atlantic subsurface temperatures under glacial climate conditions. Together with our paleoproxy dataset, this suggests that the vertical temperature structure and associated changes in AMOC were key elements governing DO events during the last glacial.

Rapid biological oxidation of methanol in the tropical Atlantic: significance as a microbial carbon source

Abstract. Methanol is the second most abundant organic gas in the atmosphere after methane, and is ubiquitous in the troposphere. It plays a significant role in atmospheric oxidant chemistry and is biogeochemically active. Large uncertainties exist about whether the oceans are a source or sink of methanol to the atmosphere. Even less is understood about what reactions in seawater determine its concentration, and hence flux across the sea surface interface. We report here concentrations of methanol between 151–296 nM in parts of the oligotrophic North Atlantic, with corresponding microbial uptake rates between 2–146 nM d−1, suggesting turnover times as low as 1 day (1–25 days) in surface waters of the oligotrophic tropical North East Atlantic. Methanol is mainly (≥97%) used by microbes for obtaining energy in oligotrophic regions, which contrasts with shelf and coastal areas where between 20–50% can be used for cell growth. Comparisons of microbial methanol oxidation rates with parallel determinations of bacterial leucine uptake suggest that methanol contributes on average 13% to bacterial carbon demand in the central northern Atlantic gyre (maximum of 54%). In addition, the contribution that methanol makes to bacterial carbon demand varies as a power function of chlorophyll a concentrations; suggesting for concentrations <0.2 μg l−1 that methanol can make a significant contribution to bacterial carbon demand. However, our low air to sea methanol flux estimates of 7.2–13 μmol m−2 d−1 suggest that the atmosphere is not a major methanol source. We conclude that there must be a major, as yet unidentified, in situ oceanic methanol source in these latitudes which we suggest is sunlight driven decomposition of organic matter.

Response and impact of equatorial ocean dynamics and tropical instability waves in the tropical Atlantic under global warming: A regional coupled downscaling study

[1] A regional coupled model is used for a dynamic downscaling over the tropical Atlantic based on a global warming simulation carried out with the Geophysical Fluid Dynamics Laboratory CM2.1. The regional coupled model features a realistic representation of equatorial ocean dynamical processes such as the tropical instability waves (TIWs) that are not adequately simulated in many global coupled climate models. The coupled downscaling hence provides a unique opportunity to assess their response and impact in a changing climate. Under global warming, both global and regional models exhibit an increased (decreased) rainfall in the tropical northeast (South) Atlantic. Given this asymmetric change in mean state, the regional model produces the intensified near-surface cross-equatorial southerly wind and zonal currents. The equatorial cold tongue exhibits a reduced surface warming due to the enhanced upwelling. It is mainly associated with the increased vertical velocities driven by cross-equatorial wind, in contrast to the equatorial Pacific, where thermal stratification is suggested to be more important under global warming. The strengthened upwelling and zonal currents in turn amplify the dynamic instability of the equatorial ocean, thereby intensifying TIWs. The increased eddy heat flux significantly warms the equator and counters the effect of enhanced upwelling. Zonal eddy heat flux makes the largest contribution, suggesting a need for sustained monitoring of TIWs with spatially denser observational arrays in the equatorial oceans. Overall, results suggest that eddy heat flux is an important factor that may impact the mean state warming of equatorial oceans, as it does in the current climate.

Composite analysis of dust impacts on African easterly waves in the Moderate Resolution Imaging Spectrometer era

This study examines the synoptic scale impacts of African dust on easterly waves in the tropical northeast Atlantic. Moderate Resolution Imaging Spectrometer aerosol optical depth (AOD), National Oceanic and Atmospheric Administration products, and National Center for Environmental Prediction reanalysis fields in the Atlantic main hurricane development region (MDR) form the basis for statistical analysis of a limited set of cases objectively selected for the 2000–2008 hurricane seasons when thresholds are exceeded for sea surface temperature (SST), easterly wind shear, cyclonic vorticity, and upward motion. After ranking African easterly waves by AOD, the top (dusty) and bottom (clean) cases are studied as composite differences. African dust and subsidence cause temperatures to warm ∼3°C in the 700 hPa layer, while SSTs cause temperatures to cool, stabilizing the atmosphere. Increased AOD and strong (10 m s−1) 600 hPa easterly winds limit cloud efficiency through shear and oversupply of condensation nuclei. Vertical section composites demonstrate that warm dry subsident air coincides with the African dust plume in the latitudes 18°N–30°N. Hurricane reanalysis data indicate that higher AOD in the MDR reduces chances for the intensification of African easterly waves.

Significance of non‐sinking particulate organic carbon and dark CO2 fixation to heterotrophic carbon demand in the mesopelagic northeast Atlantic

It is generally assumed that sinking particulate organic carbon (POC) constitutes the main source of organic carbon supply to the deep ocean's food webs. However, a major discrepancy between the rates of sinking POC supply (collected with sediment traps) and the prokaryotic organic carbon demand (the total amount of carbon required to sustain the heterotrophic metabolism of the prokaryotes; i.e., production plus respiration, PCD) of deep‐water communities has been consistently reported for the dark realm of the global ocean. While the amount of sinking POC flux declines exponentially with depth, the concentration of suspended, buoyant non‐sinking POC (nsPOC; obtained with oceanographic bottles) exhibits only small variations with depth in the (sub)tropical Northeast Atlantic. Based on available data for the North Atlantic we show here that the sinking POC flux would contribute only 4–12% of the PCD in the mesopelagic realm (depending on the primary production rate in surface waters). The amount of nsPOC potentially available to heterotrophic prokaryotes in the mesopelagic realm can be partly replenished by dark dissolved inorganic carbon fixation contributing between 12% to 72% to the PCD daily. Taken together, there is evidence that the mesopelagic microheterotrophic biota is more dependent on the nsPOC pool than on the sinking POC supply. Hence, the enigmatic major mismatch between the organic carbon demand of the deep‐water heterotrophic microbiota and the POC supply rates might be substantially smaller by including the potentially available nsPOC and its autochthonous production in oceanic carbon cycling models.

Differential responses ofProchlorococcusand SAR11-dominated bacterioplankton groups to atmospheric dust inputs in the tropical Northeast Atlantic Ocean

The metabolic responses of indigenous dominant bacterioplankton populations to additions of dust were examined in the tropical northeast Atlantic. Subsurface seawater samples were treated with dust, added directly or indirectly as a 'leachate' after its rapid dissolution in deionized water. Samples were incubated at ambient temperature and light for up to 24 h and microbial metabolic responses were assessed by (35)S-methionine ((35)S-Met) uptake. Prochlorococcus and low nucleic acid (LNA) cells were sorted by flow cytometry to determine their group-specific responses. Sorted cells were also phylogenetically affiliated using FISH. The high-light-adapted ecotype II dominated the Prochlorococcus group and 73+/-14% of LNA prokaryotes belonged to the SAR11 clade of Alphaproteobacteria. Both Prochlorococcus and LNA cells were metabolically impaired by the addition of dust (40+/-28% and 37+/-22% decrease in (35)S-Met uptake compared with controls, respectively). However, LNA bacterioplankton showed minor positive responses to dust leachate additions (7+/-4% increase in (35)S-Met uptake), while the metabolic activity of Prochlorococcus cells decreased in the presence of dust leachate by 16+/-11%. Thus, dust dissolution in situ appears to be more deleterious to Prochlorococcus than SAR11-dominated LNA bacterioplankton and hence could initiate a compositional shift in the indigenous bacterioplankton.

Changes in iron speciation following a Saharan dust event in the tropical North Atlantic Ocean

Concentrations of dissolved iron (DFe) and Fe-binding ligands were determined in the tropical Northeast Atlantic Ocean (12–30°N, 21–29°W) as part of the UK-SOLAS (Surface Ocean Lower Atmosphere Study) cruise Poseidon 332 (P332) in January–February 2006. The surface water DFe concentrations varied between 0.1 and 0.4 nM with an average of 0.22 ± 0.05 nM ( n = 159). The surface water concentrations of total Fe-binding ligands varied between 0.82 and 1.46 nM with an average of 1.11 ± 0.14 nM ( n = 33). The concentration of uncomplexed Fe-binding ligands varied between 0.64 and 1.35 nM with an average of 0.90 ± 0.14 nM ( n = 33). Thus, on average 81% of the total Fe-binding ligand concentration was uncomplexed. The average logarithmic conditional stability constant of the pool of Fe-binding ligands was 22.85 ± 0.38 with respect to Fe 3+ ( n = 33). A transect (12°N, 26°W to 16°N, 25.3°W) was sailed during a small Saharan dust event and repeated a week later. Following the dust event, the concentration of DFe increased from 0.20 ± 0.026 nM ( n = 125) to 0.25 ± 0.028 ( n = 17) and the concentration of free Fe-binding ligands decreased from 1.15 ± 0.15 ( n = 4) to 0.89 ± 0.10 ( n = 4) nM. Furthermore, the logarithmic stability constants of the Fe-binding ligands south of the Cape Verde islands were distinctively lower than north of the islands. The absence of a change in the logarithmic stability constant after the dust event south of the Cape Verde islands suggests that there was no significant atmospheric input of new Fe-binding ligands during this dust event.

Variation in structure and biomass of the benthic communities at three contrasting sites in the tropical Northeast Atlantic

Three major size classes of the benthic metazoan communities (megafauna, macrofauna and meiofauna) were investigated simultaneously at 3 sites in the tropical Northeast Atlantic. The sites varied in their level of surface primary productivity (eutrophic, mesotrophic and oligotrophic). Results for 3 cruises in 3 seasons (Eumeli 2, winter 1991; Eumeli 3, autumn 1991; Eumeli 4, spring 1992) are given for each size class in terms of taxonomic composition, density and biomass. The benthic communities did not exhibit evidence of seasonal variation, but there were substantial differences in the quantitative structure of the metazoan communities between the 3 sites. Total metazoan density and biomass, as well as density and biomass of each size class, decreased with increasing depth and decreasing food supply, suggesting that food limitation is the major factor controlling benthic standing stocks. The comprehensive study across the 3 major benthic metazoan size classes reveals that each component responds differently to the variation in food input, leading to a variation in the quantitative structure of the benthic metazoan fauna. The largest size class, megafauna, dominated biomass when the food input was high (eutrophic site), macrofauna was predominant in intermediate conditions (mesotrophic site), and meiofauna dominated where the food input was the lowest and depth the greatest (oligotrophic site). However, within a single size group, some taxa did not follow the trend of decreasing biomass with decreasing food supply, suggesting that food availability is not the only factor governing the structure of the benthic fauna. The influence of abiotic and biotic factors likely to affect the benthic communities was analysed. Physical conditions and biological features can also play a significant role in structuring abyssal fauna.

Formula omitted] activity ratio in oceanic settling particles: implications regarding the use of barium as a proxy for paleoproductivity reconstruction

Activities of the natural radionuclides 226Ra ( T 1 2 =1602 years ) and 22sRa ( T 1 2 =5.75 years ) in particulate matter are reported from sediment traps deployed in the tropical northeast Atlantic. 228Ra 226Ra activity ratios in settling particles are compared to those found in the water column in order to specify the origin of radium incorporated in particulate matter, and consequently that of barium, since the two elements are known to display close geochemical behaviours in the oceans. Whereas 228Ra 226Ra activity ratios in the water column display very large variations with depth, they remain nearly constant in particles, with values close to those found in the upper 250 m. These results clearly demonstrate that particles acquire their radium and, by inference, their barium, in the upper water column and that there is very little exchange with the dissolved phase as they settle to the bottom.

Sedimentary organic matter and micro-meiobenthos with relation to trophic conditions in the tropical northeast Atlantic

The biological and chemical characteristics of sediments sampled during the EUMELI cruise in the tropical northeast Atlantic are presented. These sediments are representative of three different trophic conditions prevailing in surface waters off the Mauritanian coast: EUtrophic, MEsotrophic and o LIgotrophic. The benthic response to surface primary production, considered as the main supply of biogenic material to the sea floor, was evaluated through the qualitative and quantitative analysis of: (i) the composition of the sedimentary organic matter; (ii) the biomass of the micro- and meiobenthos; and (iii) the benthic metabolic activity. The bacterial biomass (17.5 μg cm −2 at the oligotrophic site and 64.5 μg cm −2 at the mesotrophic site) the meiofauna biomass (1.12, 4.34 and 9.63 μg cm −2 at the oligo-, meso- and eutrophic sites, respectively), the respiratory potential (ETS) and the utilization of labeled amino acids by heterotrophic microbenthos are germane tracers of the benthic response. However, their relative variations between the three different sites are reduced in comparison to the carbon fluxes measured or estimated at the sediment-water interface. Sedimentary organic carbon and biopolymer analysis show still further reduced variations between the three environments. In order to better distinguish the three different oceanographic provinces, it is more suitable to integrate the various properties investigated over depth in sediment than to consider surficial characteristics. We conclude from our multi-parameter approach that micro-meiobenthos may be more efficient utilizers of the vertical carbon supply than their counterparts in rich shallower zones.

Fluxes and balance of 210Pb in the tropical northeast Atlantic

Activities of the natural radionuclide 210Pb in particulate matter are reported from sediment traps deployed in the tropical northeast Atlantic at two sites, oligotrophic and mesotrophic, of the French EUMELI programme (EUtrophic, MEsotrophic and oLIgotrophic). Whereas 210Pb specific activities in settling particles do not vary with time at the oligotrophic site, they exhibit marked temporal variations at the mesotrophic site. Relationships between activities of 210Pb xs (i.e. that scavenged from seawater) and major constituents of trapped particles are examined in order to determine the influence of particle composition on 210Pb partitioning between solution and particles. In this oceanic area, 210Pb scavenging is directly related to the composition of particulate matter, and micro-particles of aluminosilicate are shown to be the primary phases carrying this radionuclide from the water column to the sea floor. Particulate organic carbon (POC) fluxes also appear to play an important role. 210Pb xs fluxes are linearly related to POC fluxes, as observed elsewhere, up to 10 mg m −2 day −1, but are limited to near-constant values when POC fluxes are higher than 10–20 mg m −2 day −1. This limit is ascribed to the depletion of the suspended aluminosilicate particle standing crop in the water column. To assess the relative importance of vertical and horizontal transport on scavenging processes, an oceanic 210Pb mass balance is drawn up for the tropical northeast Atlantic. This budget clearly demonstrates that this area of the tropical Atlantic eastern margin constitutes a significant sink for 210Pb.