Vertical distribution of zooplankton and active flux across an anticyclonic eddy in the Canary Island waters

The vertical distribution (0–900 m) of zooplankton biomass and indices of feeding (gut fluorescence, GF) and metabolism (electron transfer system, ETS) were studied across an anticyclonic eddy south of Gran Canaria Island (Canary Islands). Two dense layers of organisms were clearly observed during the day, one above 200 m and the other at about 500 m, coincident with the deep scattering layer (DSL). The biomass displacement due to interzonal migrants in the euphotic zone was more than 2-fold higher than that previously reported for the southern area of this archipelago. The gut flux estimated (0.14–0.44 mgC m⁻² d⁻¹) was similar to the values previously found in the Canaries. The respiratory flux outside the eddy (1.85 mgC m⁻² d⁻¹) was in the lower range of values reported for this area. Inside the eddy, migrant biomass and respiration rates were 2- and 4- fold higher than in the surrounding waters. Active flux mediated by diel vertical migrants inside the eddy (8.28 mgC m⁻² d⁻¹) was up to 53% of the passive carbon flux to the mesopelagic zone (15.8 mgC m⁻² d⁻¹). It is, therefore, suggested that the anticyclonic eddy enhanced both migration from deep waters and active flux.     

Influence of surface forcing on near-surface and mixing layer turbulence in the tropical Indian Ocean

An autonomous upwardly-moving microstructure profiler was used to collect measurements of the rate of dissipation of turbulent kinetic energy (ε) in the tropical Indian Ocean during a single diurnal cycle, from about 50 m depth to the sea surface. This dataset is one of only a few to resolve upper ocean ε over a diurnal cycle from below the active mixing layer up to the air–sea interface. Wind speed was weak with an average value of ~5 m s⁻¹ and the wave field was swell-dominated. Within the wind and wave affected surface layer (WWSL), ε values were on the order of 10⁻⁷–10⁻⁶ W kg⁻¹ at a depth of 0.75 m and when averaged, were almost a factor of two above classical law of the wall theory, possibly indicative of an additional source of energy from the wave field. Below this depth, ε values were closer to wall layer scaling, suggesting that the work of the Reynolds stress on the wind-induced vertical shear was the major source of turbulence within this layer. No evidence of persistent elevated near-surface ε characteristic of wave-breaking conditions was found. Profiles collected during night-time displayed relatively constant ε values at depths between the WWSL and the base of the mixing layer, characteristic of mixing by convective overturning. Within the remnant layer, depth-averaged values of ε started decaying exponentially with an e-folding time of 47 min, about 30 min after the reversal of the total surface net heat flux from oceanic loss to gain.     

Zooplankton associated with the oxygen minimum zone system in the northern upwelling region of Chile during March 2000

Zooplankton in the coastal upwelling region off northern Chile may play a significant biogeochemical role by promoting carbon flux into the subsurface OMZ (oxygen minimum zone). This work identifies the dominant zooplankton species inhabiting the area influenced by the OMZ in March 2000 off Iquique (20°S, northern Chile). Abundance and vertical distribution studies revealed 17 copepod and 9 euphausiid species distributed between the surface and 600 m at four stations sampled both by day and by night. Some abundant species remained in the well-oxygenated upper layer (30 m), with no evidence of diel vertical migration, apparently restricted by a shallow (40–60 m) oxycline. Other species, however, were found closely associated with the OMZ. The large-sized copepod Eucalanus inermis was found below the oxycline and performed diel vertical migrations into the OMZ, whereas the very abundant Euphausia mucronata performed extensive diel vertical migrations between the surface waters and the core of the OMZ (200 m), even crossing it. A complete assessment of copepods and euphausiids revealed that the whole sampled water column (0–600 m) is occupied by distinct species having well-defined habitats, some of them within the OMZ. Ontogenetic migrations were evident in Eucalanidae and E. mucronata. Estimates of species biomass showed a substantial (>75% of total zooplankton biomass) daily exchange of C between the photic layer and the OMZ. Both E. inermis and E. mucronata can actively exchange about 37.8 g C m⁻² d⁻¹ between the upper well-oxygenated (0–60 m) layer and the deeper (60–600 m) OMZ layer. This migrant biomass may contribute about 7.2 g C m⁻² d⁻¹ to the OMZ system through respiration, mortality, and production of fecal pellets within the OMZ. This movement of zooplankton in and out of the OMZ, mainly as a result of the migratory behavior of E. mucronata, suggests a very efficient mechanism for introducing large amounts of freshly produced carbon into the OMZ system and should, therefore, be considered when establishing C budgets for coastal upwelling systems.     

Fate of biogenic carbon in the upper 200 m of the central Greenland Sea

Sedimentation of particulate carbon from the upper 200–300 m in the central Greenland Sea from August 1993 to June 1995 was less than 2 g C m⁻² yr⁻¹. Daily rates of sedimentation of particulate organic carbon reached highest values of about 18 mg m⁻² d⁻¹ in fall 1994. For total particulate material, maximum rates of sedimentation of about 250 mg m⁻² d⁻¹ were recorded in spring and fall 1994. For chlorophyll equivalent, highest rates of sedimentation of about 140 μg m⁻² d⁻¹ were recorded in spring 1994. As reported in related investigations, the transient accumulation of DOC in surface waters during summer, as well as respiration and mortality of deep overwintering zooplankton stocks, appeared to dominate the fate of photosynthetically fixed organic carbon. The above processes may account for roughly 43 g C m⁻² in the upper 200 m of the central Greenland Sea. For comparison, the seasonal deficit in dissolved inorganic carbon was reported to be about 23 g C m⁻² in the upper 20 m of surface water, and estimates for new annual production were reported to be about 57 g C m⁻². In our investigation, the biological carbon pump was not unusually effective in transporting carbon out of the productive surface layer.     

Time-series observation of POC fluxes estimated from 234Th in the northwestern North Pacific

Time-series measurements of ²³⁴Th activities and particulate organic carbon (POC) concentrations were made at time-series stations (K1, K2, K3, and KNOT) in the northwestern North Pacific from October 2002 to August 2004. Seasonal changes in POC export fluxes from the surface layer (∼100 m) were estimated using ²³⁴Th as a tracer. POC fluxes varied seasonally from approximately 0 to 180 mg C m⁻² d⁻¹ and were higher in spring–summer than in autumn–winter. The export ratio (e-ratio) ranged from 6% to 55% and was also higher in spring–summer. Annual POC fluxes were estimated to be 31 g C m⁻² y⁻¹ in the subarctic region (station K2) and 23 g C m⁻² y⁻¹ in the region between the subarctic and subtropical gyres (station K3). POC fluxes and e-ratios in the northwestern North Pacific were much higher than those in most other oceans. The annual POC flux corresponded to 69% of annual new production estimated from the seasonal difference of the nutrient in the Western Subarctic Gyre (45 g C m⁻² y⁻¹). These results indicate that much of the organic carbon assimilated in the surface layer of the northwestern North Pacific is transferred to the deep ocean in particulate form. Our conclusions support previous reports that diatoms play an important role in the biological pump.     

Diapycnal nutrient fluxes on the northern boundary of Cape Ghir upwelling region

In this study we estimate diffusive nutrient fluxes in the northern region of Cape Ghir upwelling system (Northwest Africa) during autumn 2010. The contribution of two co-existing vertical mixing processes (turbulence and salt fingers) is estimated through micro- and fine-structure scale observations. The boundary between coastal upwelling and open ocean waters becomes apparent when nitrate is used as a tracer. Below the mixed layer (56.15±15.56 m), the water column is favorable to the occurrence of a salt finger regime. Vertical eddy diffusivity for salt (K_s) at the reference layer (57.86±8.51 m, CI 95%) was 3×10⁻⁵ (±1.89×10⁻⁹, CI 95%) m² s⁻¹. Average diapycnal fluxes indicate that there was a deficit in phosphate supply to the surface layer (6.61×10⁻⁴ mmol m⁻² d⁻¹), while these fluxes were 0.09 and 0.03 mmol m⁻² d⁻¹ for nitrate and silicate, respectively. There is a need to conduct more studies to obtain accurate estimations of vertical eddy diffusivity and nutrient supply in complex transitional zones, like Cape Ghir. This will provide us with information about salt and nutrients exchange in onshore–offshore zones.     

Biogeochemical cycling of N and Si during the mesoscale iron-enrichment experiment in the western subarctic Pacific (SEEDS-II)

Biogeochemical cycles of N and Si were examined in the surface mixed layer during the mesoscale iron-enrichment (IE) experiment in the high-nutrient low-chlorophyll (HNLC) western subarctic Pacific (SEEDS-II). Although the IEs increased nitrate uptake, silicic acid utilization was not stimulated. The nitrate drawdown in the iron-patch (IN-patch, 140.3 mmol m⁻² in the surface mixed layer, 0–30 m) was only 25% of the initial inventory, which was 1/3–2/5 of the previous IE experiments in the subarctic Pacific. This relatively weak response of nutrient drawdown to IEs was due to the high biomass of mesozooplankton (MZ) dominated by copepod Neocalanus plumchrus. Feeding of MZ (247.2 mmol m⁻² during Day 0–21 from the first IE) in the IN-patch was higher than the nitrate drawdown and prevented further development of the phytoplankton bloom. In the later period of the experiment (Day 14–21), the increase in the feeding activity and resultant decrease in phytoplankton biomass induced the accumulation of dissolved organic nitrogen (DON) and ammonium. Among total growth of MZ (81.6 mmol N m⁻²), 89% (72.8 mmol N m⁻²) was transported to the depth by the ontogenetic downward migration of N. plumchrus. Although silicic acid drawdown was not increased by the IEs, Si export flux increased by 2.7 times. The increase in Si export was also due to the increase in MZ, which egested faecal pellets with higher Si:N ratio and faster sinking speed than diatoms. The export efficiency (78% of new production) and total amount of export flux (143.8 mmol N m⁻², 1392 mmol C m⁻²) were highest records within the IE experiments despite weak responses of nutrient drawdown to the IE. During SEEDS-II, the high biomass of MZ reduced the phytoplankton response and nutrient drawdown to the IEs but via grazing and ontogenetic vertical migration accelerated the export flux as well as accumulations of dissolved forms of N. Results of the present and previous IE experiments indicate that the ecosystem and biogeochemical responses to IEs in the HNLC region are quite sensitive to the ecosystem components, especially for grazers of diatoms such as copepods and heterotrophic dinoflagellates. More attention needs to be paid to the ecosystem components and their biogeochemical functions as well as physical and chemical properties of the ecosystems in order to hindcast or forecast the impacts of changes in atmospheric iron deposition.     

High primary productivity and f-ratio in summer in the Ulleung basin of the East/Japan Sea

To better understand the cause of high summer primary productivity in the Ulleung Basin located in the southwest part of the East/Japan Sea, the spatial dynamics of primary, new, and regenerated productivities (PP, NP, and RP) were examined along the path of the Tsushima Warm Current system in summer 2008. We compared hydrographic and chemical parameters in the Ulleung Basin with those of the Kuroshio Current in the Western Pacific Ocean and the East China Sea. In summer, integrated primary productivity (IPP, 0.37–0.96 g C m⁻² d⁻¹) and integrated new productivity (INP, 26–221 mg N m⁻² d⁻¹) within the euphotic zone in the Ulleung Basin were higher than those in the East China Sea and the Western Pacific Ocean (0.17–0.28 g C m⁻² d⁻¹, 2−5 mg N m⁻² d⁻¹, respectively). In contrast, there was no pronounced spatial variation in integrated regenerated productivity (IRP, 43–824 mg N m⁻² d⁻¹). Strong positive correlations between IPP and INP (also the f-ratio), and between nitrate uptake rate in the mixed layer and nitrate upward flux through the top of pycnocline in summer in the Ulleung Basin imply that the high IPP was mainly supported by supply of nitrate from the underlying water in the euphotic zone. Shallowing of the pycnocline depth as the current enters the East/Japan Sea facilitates nitrate supply from the nutrient-replete cold water immediately below the pycnocline through nitrate upward flux. A subsurface maximum in PP at or above the pycnocline and a high f-ratio further support the importance of this source of nitrate for maintaining the high summer PP in the Ulleung Basin. In comparison, the high PP layer was observed at the surface in the following fall and spring in the Ulleung Basin. Our results demonstrate the importance of hydrographic features in enhancing PP in this oligotrophic Tsushima Warm Current system.     

Vertical distribution of zooplankton in Canary Island waters: implications for export flux

Zooplankton biomass, gut fluorescence and electron transfer system (ETS) activity were measured in vertical profiles (0–900 m) in two different size classes (<1 and >1 mm) in Canary Island waters. Both size fractions displayed a typical pattern of distribution with higher biomass, gut fluorescence and ETS in the shallower layers at night. By day, however, the vertical distribution varied between the size fractions, with higher biomass of the small fraction in the 0–200 m and a layer of large organisms at depth (∼500 m). For both size fractions, average ETS activity was higher by day than at night at depths between 200 and 600 m. Similarly, gut fluorescence was slightly higher by day below 200 m. The downward export of respiratory carbon was 1.92 and 4.29 mg C m⁻² d⁻¹ for samples obtained southwest of Gran Canaria Island and west of Tenerife Island respectively, being 2.68 mg C m⁻² d⁻¹ for the whole area. These values represented 16–45% (22–28% for the area) of the calculated passive particulate export production resulting from primary production. The estimated “gut flux” accounted for 0.35 (western zone) and 2.37 mg C m⁻² d⁻¹ (southwest of Gran Canaria), being 1.28 mg C m⁻² d⁻¹ for the whole area and represented between 3 and 25% (11–14% for the whole area) of the estimated passive particle export flux. These results agree with previous estimates and suggest that diel-migrant zooplankton can play an important role in the downward flux of carbon.     

Bioluminescence in the deep sea: Free-fall lander observations in the Atlantic Ocean off Cape Verde

A novel autonomous free-fall lander vehicle, with a capability down to 6000 m, was deployed off Cape Verde for studies on bioluminescence in the deep sea. The system was equipped with a high-sensitivity Intensified Silicon Intensified Target (ISIT) video camera, a programmable control-recording unit and an acoustic current meter with depth and temperature sensors. The ISIT lander was used in three modes: (1) free falling at 34 m min⁻¹, with the camera looking downwards at a mesh screen, recording impacts of luminescent organisms to obtain a vertical profile down to the abyssal sea floor, sampling at >100 l s⁻¹; (2) rotating, with the lander on the sea floor and the camera orienting to the bottom current using a servo-controlled turntable, impacts of luminescent organisms carried by the bottom current onto a mesh screen mounted 0.5 m in front of the camera were recorded to estimate abundance in the benthic boundary layer; (3) baited, with the camera focused on a bait placed on the sea floor.Profiles recorded abundance of luminescent organisms as 26.7 m⁻³ at 500–999 m depth, decreasing to 1.6 m⁻³ at 2000–2499 m and 0.5 m⁻³ between 2500 m and the sea floor at 4046 m, with no further detectable significant change with depth. Rotator measurements at a 0.5 m height above the sea floor gave a mean abundance of 0.47 m⁻³ in the benthic boundary layer at 4046 m and of 2.04 m⁻³ at 3200 m. Thirty five minutes after the bait was placed on the sea floor at 3200 m, bioluminescent fauna apparently arrived at the bait and produced luminescent displays at a rate of 2 min⁻¹. Moving, flashing light sources were observed and luminescent material was released into the bottom current.     

Vertical flux of particulate Al,Fe, Pb,and Ba from the upper ocean estimated from 234Th/238U disequilibria

The vertical sinking flux of particulate Al, Fe, Pb, and Ba from the upper 250 m of the Labrador Sea has been estimated from measurements of ²³⁴Th/²³⁸U disequilibrium and the respective metal/²³⁴Th ratios in >53 μm size particles. ²³⁴Th-derived particulate metal fluxes include in situ scavenged metals, labile lithogenic metals, and metals derived from external input (e.g., atmospheric supply). In contrast to the POC/²³⁴Th ratio, particle size-fractionated (0.4–10 μm, 10–53 μm, and >53 μm) Al/²³⁴Th, Fe/²³⁴Th and Pb/²³⁴Th, and Ba/²³⁴Th ratios generally increase with depth and exhibit no systematic change with particle diameter. Sinking fluxes of particulate Al (2.47–22.3 μmol m⁻² d⁻¹), Fe (2.69–16.3 μmol m⁻² d⁻¹), Pb (2.85–70 nmol m⁻² d⁻¹), and Ba (0.13–2.1 μmol m⁻² d⁻¹) at 50 m (base of the euphotic zone) and 100 m (base of the mixed layer) are largely within the range of previous sediment trap results from other ocean basins. Estimates of the upper ocean residence time of Al (0.07–0.28 yr) and Pb (0.8–2.9 yr) are short compared to previously reported values. The settling rate of >53 μm particles calculated from the ²³⁴Th data ranges from 14 to 38 m d⁻¹.     

Computations of the energy flux to mixing processes via baroclinic wave drag on barotropic tides

The geographical distribution of barotropic to baroclinic transfer of tidal energy by baroclinic wave drag in the abyssal ocean is estimated. Using tidal velocities from a state-of-the-art numerical tidal model, the total loss of barotropic tidal energy in the deep ocean (between 70°S and 70°N and at depths greater than 1000 m) is estimated to be about 0.7 TW (M₂) corresponding to a mean value of the energy flux (e) of 2.4×10⁻³ W/m². The distribution of e is however highly skewed with a median of about 10⁻⁶ W/m². Only 10% of the area is responsible for more than 97% of the total energy transfer.To assess the possible influence of the relatively coarse bathymetry representation upon the present estimate, complementary calculations using better resolved sea floor topography are carried out over a control area around the Hawaiian Ridge. There are no major differences between the results achieved using the two different bathymetry databases. Fluxes of about 16 GW or 6×10⁻³ W/m² are computed in both cases, and the main contributions to the total fluxes originate in the same range of e-values and cover equally large parts of the total area.It is not clear whether the present model is valid at flat or subcritical bottom slopes. However, for the Hawaiian region, only 2% of the total energy flux as calculated in the present study originates in areas of critical and subcritical slopes.     

Coccolithophorids on the continental slope of the Bay of Biscay – production,transport and contribution to mass fluxes

Coccoliths collected by sediment traps deployed on the slope of the Bay of Biscay (northeastern Atlantic), from June 1990 to August 1991, were examined to determine their contribution to the transport of carbonate on a mid-latitude continental margin. They also were used as tracers of particle transfer processes on this slope. Two traps located at 1900 m, respectively at 2300 (Mooring Site 1) and 3000 m (Mooring Site 2) water depths provided high-resolution (4–7 days) time-series samples covering a 14-month period at MS2 and a 3-month period at MS1. Coccoliths from 28 species were identified over the course of the experiment, among which Emiliania huxleyi was always dominant (relative abundance range: 59–93%). Total coccoliths number fluxes were high but variable, ranging from 390×10⁶ to 1610×10⁶ coccoliths m⁻² day⁻¹ at MS1, and from 58×10⁶ to 1500×10⁶ coccoliths m⁻² day⁻¹ at MS2. The time-weighted mean flux, calculated for the whole experiment at MS2, was 499×10⁶ coccoliths m⁻² day⁻¹. Estimate of coccoliths minimal contribution to total carbonate flux at 1900 m depth averaged 12%, which represented a weighted mean flux of 7.3 mg m⁻² day⁻¹ (2.7 g m⁻² yr⁻¹). Lateral transport of coccoliths resuspended from shelf and/or upper slope sediments seems to be the dominant transfer process to depth on this northeastern Atlantic slope. Nevertheless, the clear seasonal succession observed in the species composition implies that the deposition/resuspension/transport sequence is rapid (presumably less than a few months). Several short and unsmoothed signals directly issued from coccoliths bloom events also were recorded in our traps, a result that indicates rapid settling rates. The overall coccolith sedimentation processes appear as being quite diversified, but quantitative and qualitative analyses of aggregates collected by the traps suggest that they are important carriers of coccoliths in this margin environment.     

Biogenic fluxes in the Cariaco Basin: a combined study of sinking particulates and underlying sediments

The fluxes of total mass, organic carbon (OC), biogenic opal, calcite (CaCO₃) and long-chain C₃₇ alkenones (ΣAlk₃₇) were measured at three water depths (275, 455 and 930 m) in the Cariaco Basin (Venezuela) over three separate annual upwelling cycles (1996–1999) as part of the CARIACO sediment trap time-series. The strength and timing of both the primary and secondary upwelling events in the Cariaco Basin varied significantly during the study period, directly affecting the rates of primary productivity (PP) and the vertical transport of biogenic materials. OC fluxes showed a weak positive correlation (r²=0.3) with PP rates throughout the 3 years of the study. The fluxes of opal, CaCO₃ and ΣAlk₃₇ were strongly correlated (0.6<r²<0.8) with those of OC. The major exception was the lower than expected ΣAlk₃₇ fluxes measured during periods of strong upwelling. All sediment trap fluxes were significantly attenuated with depth, consistent with marked losses during vertical transport. Annually, strong upwelling conditions, such as those observed during 1996–1997, led to elevated opal fluxes (e.g., 35 g m⁻² yr⁻¹ at 275 m) and diminished ΣAlk₃₇ fluxes (e.g., 5 mg m⁻² yr⁻¹ at 275 m). The opposite trends were evident during the year of weakest upwelling (1998–1999), indicating that diatom and haptophyte productivity in the Cariaco Basin are inversely correlated depending on upwelling conditions.The analyses of the Cariaco Basin sediments collected via a gravity core showed that the rates of OC and opal burial (10–12 g m⁻² yr⁻¹) over the past 5500 years were generally similar to the average annual water column fluxes measured in the deeper traps (10–14 g m⁻² yr⁻¹) over the 1996–1999 study period. CaCO₃ burial fluxes (30–40 g m⁻² yr⁻¹), on the other hand, were considerably higher than the fluxes measured in the deep traps (∼10 g m⁻² yr⁻¹) but comparable to those obtained from the shallowest trap (i.e. 38 g m⁻² yr⁻¹ at 275 m). In contrast, the burial rates of ΣAlk₃₇ (0.4–1 mg m⁻² yr⁻¹) in Cariaco sediments were significantly lower than the water column fluxes measured at all depths (4–6 mg m⁻² yr⁻¹), indicating the large attenuation in the flux of these compounds at the sediment–water interface. The major trend throughout the core was the general decrease in all biogenic fluxes with depth, most likely due to post-depositional in situ degradation. The major exception was the relatively low opal fluxes (∼5 g m⁻² yr⁻¹) and elevated ΣAlk₃₇ fluxes (∼2 mg m⁻² yr⁻¹) measured in the sedimentary interval corresponding to 1600–2000 yr BP. Such compositions are consistent with a period of low diatom and high haptophyte productivity, which based on the trends observed from the sediment traps, is indicative of low upwelling conditions relative to the modern day.     

Contrasting life-histories,secondary production,and trophic structure of Peracarid assemblages of the bathyal suprabenthos from the Bay of Biscay (NE Atlantic) and the Catalan Sea (NW Mediterranean)

The life-histories and the secondary production of four dominant peracarid crustaceans (the mysids Boreomysis arctica and Parapseudomma calloplura, the amphipod Rhachotropis caeca, and the isopod Ilyarachna longicornis) in bathyal depths of the Bay of Biscay (NE Atlantic; between 383 and 420 m) and the Catalan Sea (Northwestern Mediterranean; between 389 and 1355 m) were established. Both the Atlantic and the Mediterranean populations of the major part of the target-species had two generations/year with mean cohort-production intervals (CPI) ranging from 5.5 mo for Ilyarachna longicornis to 6.3 mo for Parapseudomma calloplura. The Hynes method showed secondary production to vary in the Bay of Biscay between 0.113 mg DW m⁻² yr⁻¹ for I. longirostris and 3.069 mg DW m⁻² yr⁻¹ for P. calloplura, with P/B ratios between 4.57 (I. longirostris) and 7.93 (Boreomysis arctica). In the Catalan Sea, production varied between 0.286 mg DW m⁻² yr⁻¹ for I. longirostris and 1.096 mg DW m⁻² yr⁻¹ for P. calloplura, with P/B between 5.72 (I. longirostris) and 6.66 (P. calloplura). Application of two different empiric models to the whole peracarid assemblage gave similar levels of secondary production in both study areas (between 29.26 and 32.14 mgDWm⁻² yr⁻¹ in the Bay of Biscay; between 26.23 and 26.54 mg DW m⁻² yr⁻¹ in the Catalan Sea). From the analysis of gut contents of 22 species the dominant species in each study area were assigned to two basic trophic levels, detritus feeders and predators. Also, cumulative curves of dominance showed high diversity (low dominance) for peracarid assemblages distributed at mid-bathyal depths (524–693 m) both in the Bay of Biscay off Arcachon and in the Catalan Sea off Barcelona. We also discuss and compare, both within and between areas, how environmental features may explain the observed diversity patterns, the trophic structure, and the production results obtained for the suprabenthos assemblages.     

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.     

Seasonal and interannual variability in particle fluxes of carbon,nitrogen and silicon from time series of sediment traps at Ocean Station P, 1982–1993: relationship to changes in subarctic primary productivity

An extended time series of particle fluxes at 3800 m was recorded using automated sediment traps moored at Ocean Station Papa (OSP, 50°N, 145°W) in the northeast Pacific Ocean for more than a decade (1982–1993). Time-series observations at 200 and 1000 m, and short-term measurements using surface-tethered free-drifting sediment traps also were made intermittently. We present data for fluxes of total mass (dry weight), particulate organic carbon (POC), particulate organic nitrogen (PON), biogenic Si (BSi), and particulate inorganic carbon (PIC) in calcium carbonate. Mean monthly fluxes at 3800 m showed distinct seasonality with an annual minimum during winter months (December–March), and maximum during summer and fall (April–November). Fluxes of total mass, POC, PIC and BSi showed 4-, 10-, 7- and 5-fold increases between extreme months, respectively. Mean monthly fluxes of PIC often showed two plateaus, one in May–August dominated by <63 μm particles and one in October–November, which was mainly >63 μm particles. Dominant components of the mass flux throughout the year were CaCO₃ and opal in equal amounts. The mean annual fluxes at 3800 m were 32±9 g dry weight g m⁻² yr⁻¹, 1.1±0.5 g POC m⁻² yr⁻¹, 0.15±0.07 g PON m⁻² yr⁻¹, 5.9±2.0 g BSi m⁻² yr⁻¹ and 1.7±0.6 g PIC m⁻² yr⁻¹. These biogenic fluxes clearly decreased with depth, and increased during “warm” years (1983 and 1987) of the El Niño, Southern Oscillation cycle (ENSO). Enhancement of annual mass flux rates to 3800 m was 49% in 1983 and 36% in 1987 above the decadal average, and was especially rich in biogenic Si. Biological events allowed estimates of sinking rates of detritus that range from 175 to 300 m d⁻¹, and demonstrate that, during periods of high productivity, particles sink quickly to deep ocean with less loss of organic components. Average POC flux into the deep ocean approximated the “canonical” 1% of the surface primary production.     

Factors controlling silicon and nitrogen biogeochemical cycles in high nutrient,low chlorophyll systems (the Southern Ocean and the North Pacific): Comparison with a mesotrophic system (the North Atlantic)

A 1-D coupled physical-biogeochemical model is used to study the seasonal cycles of silicon and nitrogen in two High Nutrient Low Chlorophyll (HNLC) systems, the Antarctic Circumpolar Current (ACC) and the North Pacific Ocean, and a mesotrophic system, the North Atlantic Ocean. The biological model consists of nine compartments (diatoms, nano-flagellates, microzooplankton, mesozooplankton, two types of detritus, nitrate, ammonium and silicic acid) forced by irradiance, temperature, mixing and deep nitrate and silicic acid concentrations. At all sites, nanophytoplankton standing crop variations are low, in spite of variations in primary production, because of a “top–down” control by microzooplankton. Although nanophytoplankton sustain more than 60% of the annual primary production in these areas, their contribution to the export production does not exceed 1% of the total. The differences in the seasonal plankton cycle among these regions come mainly from differences in the dynamics of large phytoplankton (here diatoms). In the ACC, the chlorophyll maximum remains <1.5 mg m⁻³, as an unfavourable light/mixing regime and a likely trace-metal limitation keep diatoms from blooming. In the northeast Pacific, trace-metal limitation seems to keep diatoms from blooming throughout the year. In both these systems, light or iron limitations induce high Si/N uptake ratios. Incidentally these high Si/N uptake ratios lead to a net excess of silicic acid utilization over nitrate, and to a subsequent silicic acid limitation during the summertime. In the North Atlantic, under favourable light/mixing regime and nutrient-replete conditions at the onset of the growing period, diatoms outburst and sustain a bloom >3.5 mg Chl-a m⁻³. Thereafter, mesozooplankton grazing pressure and silicic acid limitation induce the collapse of the chlorophyll maximum and the persistence of lower chlorophyll concentrations in summer. Although the ACC and the North Pacific show HNLC features, they support a high biogenic silica production (1.9 and 1.07 mol Si m⁻² yr⁻¹) and export flux (0.79 and 0.61 mol Si m⁻² yr⁻¹), compared to the North Atlantic (production: 0.23 mol Si m⁻² yr⁻¹, export: 0.12 mol Si m⁻² yr⁻¹). The differences in Si production and export between the HNLC systems and the mesotrophic North Atlantic come from both higher Si concentrations and Si/N uptake ratios in the HNLC areas compared to the North Atlantic. Also, the low dissolution rate of biogenic silica compared to nitrogen degradation rate, and the inhibition of nitrate uptake by ammonium, reinforce the net excess of silicic acid utilization over nitrate. As a result, the model also illustrates the efficiency of the silica pump for the three sites: about 50% of the biogenic silica synthesized in the euphotic layer is exported out of the first 100 m, while only 4–11% of the particulate organic nitrogen escapes recycling in the surface layer.     

Particulate organic carbon export fluxes and size-fractionated POC/234Th ratios in the Ligurian,Tyrrhenian and Aegean Seas

Measurements of ²³⁴Th/²³⁸U disequilibria and particle size-fractionated (1, 10, 20, 53, 70, 100 μm) organic C and ²³⁴Th were made to constrain estimates of the export flux of particulate organic C (POC) from the surface waters of the Ligurian, Tyrrhenian and Aegean Seas in March–June 2004. POC exported from the surface waters (75–100 m depth) averaged 9.2 mmol m⁻² d⁻¹ in the Ligurian and Tyrrhenian Seas (2.3±0.5–14.9±3.0 mmol m⁻² d⁻¹) and 0.9 mmol m⁻² d⁻¹ in the Aegean Sea. These results are comparable to previous measurements of ²³⁴Th-derived and sediment-trap POC fluxes from the upper 200 m in the Mediterranean Sea. Depth variations in the POC/²³⁴Th ratio suggest two possible controls. First, decreasing POC/²³⁴Th ratios with depth were attributed to preferential remineralization of organic C. Second, the occurrence of maxima or minima in the POC/²³⁴Th ratio near the DCM suggests influence by phytoplankton dynamics. To assess the accuracy of these data, the empirical ²³⁴Th-method was evaluated by quantifying the extent to which the ²³⁴Th-based estimate of POC flux, P_POC, deviates from the true flux, F_POC, defined as the p-ratio (p-ratio=P_POC/F_POC=S_Th/S_POC, where S=particle sinking rate). Estimates of the p-ratio made using Stokes’ Law and the particle size distributions of organic C and ²³⁴Th yield values ranging from 0.93–1.45. The proximity of the p-ratio to unity implies that differences in the sinking rates of POC- and ²³⁴Th-carrying particles did not bias ²³⁴Th-normalized POC fluxes by more than a factor of two.     

Diel variations of the bathymetric distribution of zooplankton groups and biomass in Cap-Ferret Canyon,France

The bathymetric distribution, abundance and diel vertical migrations (DVM) of zooplankton were investigated along the axis of the Cap-Ferret Canyon (Bay of Biscay, French Atlantic coast) by a consecutive series of synchronous net hauls that sampled the whole water column (0–2000 m in depth) during a diel cycle. The distribution of appendicularians (maximum 189 individuals m⁻³), cladocerans (maximum 287 individuals m⁻³), copepods (copepods<4 mm, maximum 773 individuals m⁻³, copepods>4 mm, maximum 13 individuals m⁻³), ostracods (maximum 8 individuals m⁻³), siphonophores (maximum >2 individuals m⁻³) and peracarids (maximum >600 individuals 1000 m⁻³) were analysed and represented by isoline diagrams. The biomass of total zooplankton (maximum 18419 μg C m⁻³, 3780 μg N m⁻³) and large copepods (>4 mm maximum 2256 μg C m⁻³, 425 μg N m⁻³) also were determined. Vertical migration was absent or affected only the epipelagic zone for appendicularians, cladocerans, small copepods and siphonophores. Average amplitude of vertical migration was about 400–500 m for ostracods, some hyperiids and mysids, and large copepods, which were often present in the epipelagic, mesopelagic, and bathypelagic zones. Large copepods can constitute more than 80% of the biomass corresponding to total zooplankton. They may play an important role in the active vertical transfer of carbon and nitrogen.