Heat,volume and chemical fluxes from submarine venting: A synthesis of results from the Rainbow hydrothermal field, 36°N MAR

High-temperature hydrothermal activity occurs in all ocean basins and along ridge crests of all spreading rates. While it has long been recognized that the fluxes associated with such venting are large, precise quantification of their impact on ocean biogeochemistry has proved elusive. Here, we report a comprehensive study of heat, fluid and chemical fluxes from a single submarine hydrothermal field. To achieve this, we have exploited the integrating nature of the non-buoyant plume dispersing above the Rainbow hydrothermal field, a long-lived and tectonically hosted high-temperature vent site on the Mid-Atlantic Ridge. Our calculations yield heat and volume fluxes for high-temperature fluids exiting the seafloor of ～0.5 GW and 450 L s^?1, together with accompanying chemical fluxes, for Fe, Mn and CH₄ of ～10, ～1 and ～1 mol s^?1, respectively. Accompanying fluxes for 25 additional chemical species that are associated with Fe-rich plume particles have also been calculated as they are transported away from the Rainbow vent site before settling to the seabed. High-temperature venting has been found to recur at least once every ～100 km along all slow-spreading ridges investigated to-date, with half of all known sites on the Mid-Atlantic Ridge occurring as long-lived and tectonically hosted systems. If these patterns persist along all slow- and ultraslow-spreading ridges, high-temperature venting of the kind reported here could account for ～50% of the on-axis hydrothermal heat flux along ～30,000 km of the ～55,000 km global ridge crest.     

Home range and diel behavior of the ballan wrasse,Labrus bergylta,determined by acoustic telemetry

Effective fisheries management needs to consider spatial behavior in addition to more traditional aspects of population dynamics. Acoustic telemetry has been extensively used to provide information on fish movements over different temporal and spatial scales. Here, we used a fixed-receiver array to examine the movement patterns of Labrus bergylta Ascanius 1767, a species highly targeted by the artisanal fleet of Galicia, NW Spain. Data from 25 individuals was assessed for a period of 71 days between September and November 2011 in the Galician Atlantic Islands Maritime-Terrestrial National Park. Fish were present within the monitored area more than 92% of the monitored time. The estimated size of individual home ranges, i.e. the area where fish spent most of their time, was small. The total minimum convex polygons area based on all the estimated positions was 0.133 ± 0.072 km², whereas the home range size estimated using a 95% kernel distribution of the estimated positions was 0.091 ± 0.031 km². The core area (50% kernel) was 0.019 ± 0.006 km². L. bergylta exhibited different patterns of movement in the day versus the night, with 92% of the fish detected more frequently and traveling longer distances during the daytime. In addition, 76% of the fish displayed a larger home range during the day versus during the night. The linearity index was less than 0.005 for all fish suggesting random movements but within a relatively small area, and the volume of intersection index between consecutive daily home ranges was 0.75 ± 0.13, suggesting high site fidelity. The small home range and the sedentary behavior of L. bergylta highlight the potential use of small MPAs as a management tool to ensure a sustainable fishery for this important species.     

Distribution of phosphine in the offshore area of the Southwest Yellow Sea,East Asia

Phosphine is a natural gaseous compound in the phosphorus biogeochemical cycle. This paper studies the spatial and temporal distributions of matrix-bound phosphine (MBP) and gaseous phosphine in the offshore area of the Southwest Yellow Sea, East Asia. The results show that MBP concentrations in marine surface sediments range from 0.69 ± 0.06 ng/kg (dry) to 179 ± 29 ng/kg (dry). Higher seasonal MBP concentrations in sediments are found in fall than in spring or winter in most sites. High MBP contents are observed in two fish-breeding areas. MBP concentrations decrease with distance to the coast, except in the southeast of the sampling area. MBP levels in marine sediments are found to be higher than those at several other places: freshwater sediments and soil, except eutrophic lakes. Gaseous phosphine contents in fall range from 0.14 ± 0.00 ng/m³ to 9.83 ± 0.97 ng/m³. No correlation is observed between MBP and gaseous phosphine.     

The role of crabs (Macrophthalmus japonicus) burrows on organic carbon cycle in estuarine tidal flat,Japan

The objective of this study is to elucidate the burrow structure and to clarify the role of burrows in material cycle in the tidal flat. In our work, we focused on the dominant species in muddy tidal flat, crab Macrophthalmus japonicus.Burrow structure of Macrophthalmus japonicus was investigated on a Katsuura river tidal flat in Tokushima prefecture, Japan, using in situ resin casting. Sampling was conducted in August 2006, and a total of 48 burrow casts were obtained. Burrows consisted mainly of J-shaped structures (98%) while the rest belonged to U-shaped structures (2%). The maximum measured burrow volume was 120 cm³ and wall surface area was 224 cm², while maximum burrow length and depth were 23.2 cm and 16.5 cm, respectively. Burrow volume and surface area were strongly correlated with carapace width of M. japonicus. Investigation of the individual number of M. japonicus in 13 quadrats (50 × 50 × 20 cm) was conducted using 2 mm sieve. The number of M. japonicus was 15–31 ind./m². Using cohort analysis we estimated that surface area of burrows was 0.07–0.15 m²/m².CO₂ emission rate was measured at the surface sediment during the period from June to December 2008. Results varied from 13.8 ± 2.2 to 49.4 ± 3.2 mg CO₂/m²/h, and organic carbon decomposition was 3.8 ± 0.6–13.5 ± 0.9 mg C/m²/h. This leads the increase of organic carbon decomposition by 1.1 times, because of the expansion of the tidal flat surface area by burrowing activity. Organic carbon decomposition in burrow walls therefore contributed to organic matter decomposition in the tidal flat. These results indicated that in situ activities of Macrophthalmus japonicus significantly influence the material cycle and it is important to consider the existence of burrow in order to understand the fluxes of materials and to evaluate the purification function of the tidal flat.     

Downward transport of organic carbon by diel migratory micronekton in the western equatorial Pacific:: its quantitative and qualitative importance

Using simultaneous sampling with a commercial-sized trawl, a zooplankton net, and a sediment trap, we evaluated the contribution of vertically migrating micronekton to vertical material transport (biological pump) at two stations (3°00′N, 146°00′E and 3°30′N, 145°20′E) in the western equatorial North Pacific. The gravitational sinking particulate organic carbon flux out of the euphotic zone was 54.8 mg C m⁻² day⁻¹. The downward active carbon flux by diel migrant mesozooplankton was 23.53 and 9.97 mg C m⁻² day⁻¹, and by micronekton 4.40 and 2.26mg C m⁻² day⁻¹ at the two stations. Assuming that the micronekton sampling efficiency of the trawl was 14%, we corrected the downward carbon flux due to micronekton respiration to 29.9 and 15.2mg C m⁻² day⁻¹, or 54.6 and 27.7% of the sinking particle flux at the two stations. The corrected micronekton gut fluxes were 1.53 and 0.97mg C m⁻² day⁻¹. The role of myctophid fish fecal matter as a possible food resource for deep-sea organisms, based on its fatty acid and amino acid analysis, is discussed.     

Deep-sea macrofaunal assemblages within the Benthic Boundary Layer of the Cap-Ferret Canyon (Bay of Biscay,NE Atlantic)

The Benthic Boundary Layer (BBL) assemblages from the Cap-Ferret Canyon (Bay of Biscay) were quantitatively sampled at two sites located within its main channel near mooring deployments (Mooring Sites MS 1: ca. 2400 m; MS 2: ca. 3000 m) with a suprabenthic sled equipped with four nets fishing at different heights above the bottom. The macrofaunal abundance above the sea-floor was mainly represented by Isopoda (42.2%), Amphipoda (19.0%), Euphausiacea (17.3%), Cumacea (13.5%), Mysidacea (2.8%) and Tanaidacea (2.6%). At both sampling sites, the highest total densities were generally recorded in the immediate vicinity of the sea floor (10–40 cm water layer), and a drastic decrease occurred higher in the BBL community. The BBL assemblages from the two sampling sites were similar in their faunal composition (major taxa), and their mean density estimates were not statistically different (MS 1 : 525.3 ind. 100 m⁻²; MS 2 : 283.3 ind. m⁻²) although the recorded values during each cruise were always lower at the deeper site. The BBL macrofauna abundance showed obvious temporal fluctuations at both sites, probably linked with a seasonal organic input from the euphotic zone (vertical flux) via phytodetritus deposition on the sea bottom.     

On the spatial scale needed for benthos community monitoring in the coastal North Sea

In community monitoring an attempt is made to identify long-term trends by regular sampling of selected sites. Since the benthos is reputed to be fairly sedentary, the spatial resolution is often reduced to single sites. However, members of many benthic invertebrate species have been found drifting across sedimentary seabeds in shallow waters. Transportation by currents may result in changes of their spatial pattern in the sediment, thereby changing local community composition. The quantitative importance of drifting was tested by repeated sampling of a 2-km² shallow (10 m) offshore area west of the island of Sylt (North Sea). Within the fortnight period between two samplings the benthic community composition had changed dramatically. Despite fairly calm weather, translocation of organisms by currents exceeded 1 km. In about half of the species, the spatial changes in abundance within these two weeks roughly equalled the average variation between consecutive years. This example suggests that community monitoring needs a wide spatial scale to discriminate long-term temporal changes from short-term variability. Extending the sampling area from 2 to 180 km² strongly reduced the variability of abundance estimates. However, only in a few species was the spatial distribution over the sampling sites found on one sampling date a suitable estimator for the spatial pattern found one or two months later, at the same sites. Instead the spatial patterns of the fauna changed strongly during a single month with a spatial scale of re-distribution exceeding several km in some species. At the same time the granulometric sediment composition changed, indicating changes of habitat quality. Hence, sampling of a large area, with random selection of the sampling sites on each sampling date, is suggested to yield the most reliable estimates of population development in the coastal North Sea. However, in view of the expected spatial scale of re-distribution during storm tides and the spatial variability of recruitment, even a 180-km² sampling area may be too small.     

Atmospheric input of dissolved inorganic phosphorus and silicon to the coastal northwestern Mediterranean Sea: Fluxes,variability and possible impact on phytoplankton dynamics

One hundred twelve rainwater samples collected from 1986 to 2003 at the signal station of Cap Ferrat (France, NW Mediterranean coast) were analysed for phosphate and silicate contents. This sampling site is affected by a European urban-dominated background material, with episodic Saharan dust inputs. The input of dissolved inorganic phosphorus (DIP) and dissolved inorganic silicon (DISi) was calculated. The most significant loadings of DIP and DISi were selected in order to assess their potential impact on phytoplankton dynamics, particularly in oligotrophic conditions, when surface waters are nutrient-depleted. The theoretical new production triggered by DIP and DISi inputs (NP_atmo) was estimated through Redfield calculations. The maximum theoretical DIP-triggered NP_atmo was up to 670 mg C m⁻² in October, at the end of the oligotrophic period (135 mg C m⁻³ in the 5 m-thick surface layer). During the same period, the daily integrated primary production measured at the DYFAMED site (NW Mediterranean Sea) was on average 219 mg C m⁻² d⁻¹ within the 0–100 m depth water column, while the mean daily primary production in the 5 m-thick surface layer was 1.6 mg C m⁻³ d⁻¹. However, high NP_atmo due to high DIP inputs might be episodically limited by lower DISi inputs, which may consequently lead to episodic preferential growth of non-siliceous phytoplanktonic species.     

Changes of coastal sedimentation in the Gulf of Tehuantepec,South Pacific Mexico,over the last 100 years from short-lived radionuclide measurements

Short-lived radionuclides (²¹⁰Pb and ¹³⁷Cs) were used to document sedimentation regime changes over the last ～100 years, from analysis of a single sedimentary sequence collected in the coastal zone of the Gulf of Tehuantepec, South Pacific of Mexico. Sedimentation rates found in the core Tehua II-21 varied from 0.03 to 0.21 cm yr^?1, whereas mass accumulation rates ranged from 0.05 to 0.29 g cm^?2 yr^?1. ¹³⁷Cs data validate results obtained from ²¹⁰Pb measurements and confirm significant changes in the sedimentation regime between 1950 and 1970. We associate these alterations to land use changes, including deforestation for agriculture development and industrialization, as well as the regimentation of the Tehuantepec River from the early 1960s to the late 1970s. This interval has been marked by a major demographic expansion that followed the industrial development of the area.     

Dissolved organic matter in the subterranean estuary of a volcanic island,Jeju: Importance of dissolved organic nitrogen fluxes to the ocean

We observed the origin, behavior, and flux of dissolved organic carbon (DOC), dissolved organic nitrogen (DON), colored dissolved organic matter (CDOM), and dissolved inorganic nitrogen (DIN) in the subterranean estuary of a volcanic island, Jeju, Korea. The sampling of surface seawater and coastal groundwater was conducted in Hwasun Bay, Jeju, in three sampling campaigns (October 2010, January 2011, and June 2011). We observed conservative mixing of these components in this subterranean environment for a salinity range from 0 to 32. The fresh groundwater was characterized by relatively high DON, DIN, and CDOM, while the marine groundwater showed relatively high DOC. The DON and DIN fluxes through submarine groundwater discharge (SGD) in the groundwater of Hwasun Bay were estimated to be 1.3 × 10⁵ and 2.9 × 10⁵ mol d^− 1, respectively. In the seawater of Hwasun Bay, the groundwater-origin DON was almost conservative while about 91% of the groundwater-origin DIN was removed perhaps due to biological production. The DON flux from the entire Jeju was estimated to be 7.9 × 10⁸ mol yr^− 1, which is comparable to some of the world's large rivers. Thus, our study highlights that DON flux through SGD is potentially important for delivery of organic nitrogen to further offshore while DIN is readily utilized by marine plankton in near-shore waters under N-limited conditions.     

The Temporal and Spatial Distribution of Foraminiferans in Marine Benthic Sediments of the Clyde Sea Area,Scotland

The numbers of benthic foraminiferans at four sites in the Clyde Sea area showed no consistent temporal variation throughout 1993. In the finest surface sediments, numbers ranged between 200 and 400 cells cm⁻³, compared to only 25–50 cells cm⁻³in the coarsest sediments. On two occasions, high populations of cells less than 63 μm were found in the surface layers. These were thought to represent recruitment peaks since these ‘ juvenile ’ cells grew rapidly when maintained in the laboratory. A total of 56 taxa were identified from the region, the greatest diversity being recorded in the finest sediments. Rose Bengal stained foraminiferans (i.e. presumed living) were found below the anoxic–oxic boundary. The fate of these cells was considered by examining their ability to migrate through fine sediments, and their capacity to survive (based on evidence of pseudopodial activity) periods of anoxia. This study has highlighted the numerical importance of foraminiferans, particularly in fine surface coastal sediments, but questions whether the high populations of ‘ stained ’ cells found in deeper sediments play a significant ecological role.     

Population ecology and habitat preferences of juvenile flounder Platichthys flesus (Actinopterygii: Pleuronectidae) in a temperate estuary

The European flounder Platichthys flesus is a widely distributed epibenthic species and an important component of demersal fish assemblages in the European Atlantic coastal waters. In Portuguese estuaries, this species reaches high densities, especially in Minho estuary (NW Iberian Peninsula, Europe), potentially playing an important role in the system's ecology. In this context, the population structure, production and the habitat use of juvenile P. flesus were investigated. Sampling took place monthly, from February 2009 until July 2010 along the entire estuarine gradient (5 sampling stations distributed in the first 29 km from the river mouth, with S1 located near the river mouth, S2 inside a salt marsh, S3 in a salinity transition zone, while S4 and S5 were located in the upper estuary). Flounder's density varied significantly among sampling stations and seasons (two-way PERMANOVA: p < 0.001), with the majority of the individuals being found during the spring (30.1%) and in S3 and S4 (72.6%). Males and females presented an even distribution, with a higher proportion of males observed during summer. Fish length also differed among sampling stations and seasons (two-way PERMANOVA: p < 0.001), with larger fishes being found in S1 during the autumn (168.50 ± 59.50 mm) and the smallest in S4 during the spring (33.80 ± 3.12 mm). Size classes associated differently with environmental variables, with larger juveniles being more abundant in the downstream areas of the estuary, whereas smaller juveniles were related to higher water temperatures, suggesting a habitat segregation of P. flesus of different sizes. The fish condition of P. flesus in Minho estuary was higher than in other systems, probably due to the dominance of juveniles on the population. Also, the densities found in this estuary were up to 32 times higher than in other locations, suggesting that Minho estuary is an important nursery area for the species. The estimated secondary production of P. flesus was lower than previous studies acknowledged in the system (0.037 g.WWm^− 2.year^− 1), indicating that the production estimates of this species in estuaries can vary considerably depending on of several factors such as the sampling year and strategy, population and fish size.     

Automated high resolution determination of the trace elements iron and aluminium in the surface ocean using a towed Fish coupled to flow injection analysis

A towed surface sampling device coupled to two automated flow injection analysis (FIA) systems is described. The towed system permits uncontaminated sampling of seawater from research vessels while underway at full speed. Coupling the sampler to the FIA systems permits automatic determination of Al and Fe in surface waters at natural levels at 5 min intervals, equivalent to ∼1.5 km spacing at a ship speed of 10 knots (5 m s⁻¹). Results from the tropical Atlantic indicate significant (50%) variation in concentrations of both Al and Fe on space scales of less than 90 km. The combined system facilitates surface mapping of large regions of the ocean for dissolved Al and Fe, thus identifying the sites and magnitude of eolian deposition to the surface ocean. In combination with the determination of nutrients and other biological parameters this permits the investigation of the role that eolian deposition plays in modifying surface water biogeochemical cycles.     

A mesoscale eddy driving spatial and temporal heterogeneity in the productivity of the euphotic zone of the northeast Atlantic

In this paper we show how different water masses from a similar geographic region provide an explanation for perturbations in the signal of declining productivity at the Porcupine Abyssal Plain (PAP) study site in the Northeast Atlantic. Furthermore we show that the passage of these different water masses is affected by the filamentary instabilities of a cyclonic eddy just southwest of the PAP site. We describe a high-resolution spatial hydrographic survey conducted with a towed instrument package, complemented by biogeochemical sampling. Maximum rates of primary production of 110 mmol C m^-2 d^-1 seen at the centre of the survey area were associated with the passage of an eddy filament and were enhanced 3 fold relative to far-field conditions (∼36 mmol C m^-2 d^-1). The rotation and stirring influence of the eddy resulted in the sequential passage of 3 distinct water masses past the observation point. This understanding of the lateral stirring around the site enabled us to explain the sharp changes observed in daily primary production rates and other biogeochemical parameters. The spatial survey also revealed a fluorescence maxima associated with the cyclonic eddy that was laterally displaced northwards away from the core, an observation supportive of recent modelling studies.     

Nucleic acid concentrations (DNA,RNA) in the continental and deep-sea sediments of the eastern Mediterranean: relationships with seasonally varying organic inputs and bacterial dynamics

In order to study temporal variations of the genetic material in the continental shelf and deep-sea sediments of the extremely oligotrophic Cretan Sea, samples were collected on seasonal basis from August 1994 to September 1995, with a multiple corer, at seven stations (from 40 to 1540 m depth). Surface sediments (0–1 cm) were sub-sampled and analyzed for nucleic acid content (DNA, RNA) and bacterial density. DNA concentrations in the sediments were high (on annual average, 25.0 μg g^-1) and declined with increasing water depth, ranging from 3.5 to 55.2 μg g^-1. DNA concentrations displayed wide temporal changes also at bathyal depths confirming the recent view of the large variability of the deep-sea environments. Also RNA concentrations decreased with increasing water depth (range: 0.4–29.9 μg g^-1). The ratio of RNA to DNA did not show a clear spatial pattern but was characterized by significant changes between sampling periods. DNA concentrations were significantly correlated with protein and phytopigment concentrations in the sediment, indicating a possible relationship with the inputs of primary organic matter from the photic layer. Bacterial densities were generally high (range: 0.9–4.6×10⁸ cells g^-1) compared to other deep-sea environments and decreased with increasing water depth. Estimates of the bacterial contribution to the sedimentary genetic material indicated that bacterial-DNA accounted, on annual average, for a small fraction of the total DNA pool (4.3%) but that bacterial-RNA represented a significant fraction of the total sedimentary RNA (26%). Bacterial contribution to nucleic acids increased, even though irregularly, with increasing depth. In deep-sea sediments, changes in RNA concentrations appear to be largely dependent upon bacterial dynamics. Estimates of the overall living contribution to the DNA pools (i.e. microbial plus meiofaunal DNA) indicated that the large majority (about 90%) of the DNA in continental and deep-sea sediments of the eastern Mediterranean was detrital. The non-living DNA pools reach extremely high concentrations up to 0.41 g DNA m^-2 cm^-1. Thus, especially in deep benthic habitats, characterized by low inputs of labile organic compounds, detrital DNA could represent a suitable and high quality food source or a significant reservoir of nucleic acid precursors for benthic metabolism.     

A re-appraisal of the total biomass and annual production of Antarctic krill

Despite much research on Euphausia superba, estimates of their total biomass and production are still very uncertain. Recently, circumpolar krill databases, combined with growth models and revisions in acoustics have made it possible to refine previous estimates. Net-based databases of density and length frequency (KRILLBASE) yield a summer distributional range of ～19×10⁶ km² and a mean total abundance of 8×10¹⁴ post-larvae with biomass of 379 million tonnes (Mt). These values are based on a standardised net sampling method but they average over the period 1926–2004, during which krill abundance has fluctuated. To estimate krill biomass at the end of last century we combined the KRILLBASE map of relative krill density around Antarctica with an acoustics-derived biomass estimate of 37.3 Mt derived for the Scotia Sea area in 2000 by the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR). Thus the CCAMLR 2000 survey area contains 28% of the total stock, with total biomass of ～133 Mt in January–February 2000. Gross postlarval production is estimated conservatively at 342–536 Mt yr^?1, based on three independent methods. These are high values, within the upper range of recent estimates, but consistent with the concept of high energy throughput for a species of this size. The similarity between the three production estimates reflects a broad agreement between the three growth models used, plus the fact that, for a given population size, production is relatively insensitive to the size distribution of krill at the start of the growth season. These production values lie within the envelope of what can be supported from the Southern Ocean primary production system and what is required to support an estimated predator consumption of 128–470 Mt yr^?1. Given the range of recent acoustics estimates, plus the need for precautionary management of the developing krill fishery, our net-based data provide an alternative estimate of total krill biomass.     

Ectoenzymatic activity in surface waters: A transect from the Mediterranean Sea across the Indian Ocean to Australia

The activities of two hydrolytic enzymes (leucine aminopeptidase and β glucosidase), belonging to the particle-bound enzymatic fraction, were measured in open-sea surface waters. Samples were collected along a transect crossing the Indian Ocean during the early NW monsoon period (November and December 2001). The latitudinal pattern of the ectoenzymatic activities highlighted a generally increasing trend of glycolysis approaching the equator, with significantly higher β glucosidase activity (0.79–3.00 nmol l⁻¹ h⁻¹) within the latitudinal range from 12°N to 16°S. In this area, the surface waters coming from the Indonesian Throughflow and the Bay of Bengal carry a considerable quantity of carbohydrates (38.9–41.9 μg l⁻¹), which stimulated glycolytic activity and its cell-specific rates scaled to bacterial abundance. On the other hand, in the Central Indian Ocean, the proteolytic activity was considerable (0.91–2.03 nmol l⁻¹ h⁻¹), although the particulate proteins did not show significant increases and the dissolved proteinlike signal was one of the lowest of the entire transect (0.7 mg l⁻¹ on average compared to the 1.4–1.6 mg l⁻¹ of the adjacent areas). Therefore, in this area, the two ectoenzymes studied did not respond to the same stimulatory effect (namely the specific substrate concentrations). The time needed for the hydrolysis of macromolecules within the particulate and dissolved organic substrate fractions, although these measures are affected by a number of assumptions starting with the potential nature of the ectoenzymatic determinations, confirms these observations. The Central Indian Ocean displayed the lowest values, from 8 to 26 days for particulate and dissolved organic carbon, respectively. As observed in the equatorial areas of the Atlantic Ocean, the relevant degradation activity of the central area of the Indian Ocean Basin suggests a notable heterotrophy based on a faster turnover of organic substrates.     

Hydrodynamic controls on cold-water coral growth and carbonate-mound development at the SW and SE Rockall Trough Margin,NE Atlantic Ocean

Long-term (⩽1-year) records obtained by seabed observatories (BOBO) and repeated (24-h) CTD casts show the presence of a highly energetic environment in and around two cold-water carbonate-mound provinces, on the Southwest and Southeast Rockall Trough (SW and SE RT) margin. Carbonate mounds, covered with a thriving coral cover, are embedded mainly in the Eastern North Atlantic Water (ENAW) and are observed in a confined bathymetric zone between 600 and 1000 m water depth. Cold-water corals seem to be restricted in their growth by temperature and food availability. The presence of living corals on top of the carbonate mounds appears linked to the presence of internal waves and tidal currents in the water column, and consequently carbonate mound structures are shaped by the local hydrodynamic regime. Mound clusters have an elongated shape perpendicular to the regional contours and corresponding to the direction of the highest current speeds. On the SW RT margin temperature, salinity and current speed reflect a diurnal tidal pattern, causing maximum temperature variations at 900 m depth of more than 3 °C. Current speeds up to 45 cm s⁻¹ occur, and a residual current of 10 cm s⁻¹ is directed along the slope to the southwest. At the SE RT margin the temperature of the bottom water fluctuates more than 1 °C with a semi-diurnal tidal cyclicity. Amplitudes of average and peak current speeds here are comparable with those measured on the southwest margin, but the residual current in this area is directed to the northeast. Tidal currents and internal waves at both margins force the formation of intermediate and bottom nepheloid layers and bring fresh food particles with increased velocity to the mounds. The distribution of corals in both mound areas is considered directly related to the presence of enhanced turbidity. An increase in temperature can be directly related to an increase in the amount of particles in the water column. Current velocity increases when a transition occurs from cold to warm waters. High current velocities prevent local sedimentation but provide sufficient food particles to the corals, so that the corals thrive at the mound summits.     

Isotopic constraints on the Si-biogeochemical cycle of the Antarctic Zone in the Kerguelen area (KEOPS)

Estimation of the silicon (Si) mass balance in the ocean from direct measurements (Si uptake-dissolution rates …) is plagued by the strong temporal and spatial variability of the surface ocean as well as methodological artifacts. Tracers with different sensitivities toward physical and biological processes would be of great complementary use. Silicon isotopic composition is a promising proxy to improve constraints on the Si-biogeochemical cycle, since it integrates over longer timescales in comparison with direct measurements and since the isotopic balance allows to resolve the processes involved, i.e. uptake, dissolution, mixing. Si-isotopic signatures of seawater Si(OH)₄ and biogenic silica (bSiO₂) were investigated in late summer 2005 during the KEOPS experiment, focusing on two contrasting biogeochemical areas in the Antarctic Zone: a natural iron-fertilized area above the Kerguelen Plateau (< 500 m water depth) and the High Nutrient Low Chlorophyll area (HNLC) east of the plateau (> 1000 m water depth). For the HNLC area the Si-isotopic constraint identified Upper Circumpolar Deep Water as being the ultimate Si-source. The latter supplies summer mixed layer with 4.0 ± 0.7 mol Si m^? 2 yr^? 1. This supply must be equivalent to the net annual bSiO₂ production and exceeds the seasonal depletion as estimated from a simple mixed layer mass balance (2.5 ± 0.2 mol Si m^? 2 yr^? 1). This discrepancy reveals that some 1.5 ± 0.7 mol Si m^? 2 yr^? 1 must be supplied to the mixed layer during the stratification period. For the fertilized plateau bloom area, a low apparent mixed layer isotopic fractionation value (?³⁰Si) probably reflects (1) a significant impact of bSiO₂ dissolution, enriching the bSiO₂ pool in heavy isotope; and/or (2) a high Si uptake over supply ratio in mixed layer at the beginning of the bloom, following an initial closed system operating mode, which, however, becomes supplied toward the end of the bloom (low Si uptake over supply ratio) with isotopically light Si(OH)₄ from below when the surface Si(OH)₄ pool is significantly depleted. We estimated a net integrated bSiO₂ production of 10.5 ± 1.4 mol Si m^? 2 yr^? 1 in the AASW above the plateau, which includes a significant contribution of bSiO₂ production below the euphotic layer. However, advection which could be significant for this area has not been taken into account in the latter estimation based on a 1D approach of the plateau system. Finally, combining the KEOPS Si-isotopic data with those from previous studies, we refined the average Si-isotopic fractionation factor to ? 1.2 ± 0.2‰ for the Antarctic Circumpolar Current.