Vertical Flux Of Particulate Organic Carbon Research Articles

Biological–physical oceanographic coupling influencing particulate organic matter in the South Yellow Sea

Using total suspended matter (TSM), particulate organic carbon (POC), and particulate nitrogen data, this study investigated the potential vertical POC flux and transport in the South Yellow Sea (SYS). The biogenic production and resuspension fraction (i.e., the proportion of resuspended particles in TSM) were estimated using an ecosystem model and a vertical mixing model. They were verified against reported sediment trap and primary productivity data. The estimates of resuspension fraction showed substantial uncertainty of 50% in summer likely owing to the potential errors of model parameter estimation and the influence of other unexplored biophysical processes such as biological degradation, upwelling, and monsoons; however, the estimates of resuspension fraction showed less uncertainty in other seasons (<20%). Few previous studies have considered the specific influence of resuspension on the dynamics and budget of particulate organic matter (POM) in the SYS. This study proposed a reasonably simple and effective method to address this issue, which was applied to systematic examination of the variation of vertical POM flux with the change of coupled biological–physical oceanographic processes along the Subei coast and in the SYS central basin. The influence of horizontal transport from the Subei coast to the central basin may cause an overestimation of >10% of the resuspension fraction. It will be necessary to acquire additional field data covering a larger spatiotemporal scale to establish an integrated network of the SYS carbon budget.

210Po-210Pb distribution and carbon export in the northern Gulf of Mexico continental slope

An important pathway for CO2 sinking in the ocean is via the “biological pump”, driven by the production of particulate organic carbon (POC) in the surface ocean and its subsequent export to the deeper ocean via vertical settling. Here we examine the vertical fluxes of POC in the continental slope of the northern Gulf of Mexico by utilizing the short-lived radionuclide pair 210Po - 210Pb. The total activities of 210Po and 210Pb ranged from 2.5 ± 0.35 to 16.52 ± 1.98 dpm 100 L-1 and from 3.47 ± 0.45 to 24.49 ± 3.42 dpm 100 L-1 respectively with the exception of higher activities of 210Po (87.36 ± 11.38 dpm 100 L-1) and 210Pb (32.25 ± 4.35 dpm 100 L-1) from the near bottom water in the hypersaline anoxic Orca basin. In the upper 150 m of the water column, the inventories of 210Pb were consistently greater than those of 210Po while the total overall water column inventories of 210Po and 210Pb were found to be near equilibrium. The 210Po fluxes varied between 9.8 ± 6.34 dpm m-2 d-1 and 30.7 ± 12.06 dpm m-2 d-1 with corresponding POC fluxes between 8.6 mg C m-2 d-1 and 37.65 mg C m-2 d-1. The fluxes showed a general decreasing trend further offshore, closely following a similar trend in net primary productivity across the slope. The export efficiency ratios “e-ratios” of the biological pump over this slope region was found to vary between 0.027 and 0.11 averaging at 0.05 ± 0.01, which is low. The 210Po based and the model-based e-ratio estimates were different but within the range of previous estimates.

Cross-shelf export of particulate organic carbon in the northern South China Sea: Insights from a 234Th mass balance

Cross-shelf transport of particulate organic carbon (POC) is an essential but poorly quantified process regulating the POC flux and the biological pump efficiency in marginal seas. Here, we estimated the cross-shelf export flux of 234Th and POC from the northern South China Sea (NSCS) shelf based on a dataset of 234Th in the water column and the underlying sediments over the shelf (bottom depth ≤ 200 m) and the adjacent slope (bottom depth > 200 m). Based on a full mass balance of 234Th throughout the water column and the underlying sediments, we found that besides radioactive production and decay, sedimentary accumulation and cross-shelf export associated with drifting particles were the most important processes in the 234Th budget. The input via horizontal shelf-slope water exchange and along-shelf water transport was a 234Th source to the shelf water, and neglecting this component would underestimate the actual vertical 234Th and POC fluxes by ~10–33%. The POC flux via vertical export from the euphotic zone was 26 ± 1 mmol m−2 d−1 while that via cross-shelf transport was 9.9 ± 3.4 mmol m−2 d−1. The cross-shelf exported POC presumably inject into the intermediate/deep waters and could be trapped for several decades in the SCS. Our results highlight the importance of cross-shelf POC transport in regulating carbon storage in the SCS on seasonal to longer time scales. A compilation of the case studies regarding cross-shelf transport suggests its role in the carbon cycle might be more important than previously recognized and needs to be re-evaluated.

Composition and vertical flux of particulate organic matter to the oxygen minimum zone of the central Baltic Sea: impact of a sporadic North Sea inflow

Abstract. Particle sinking is a major form of transport for photosynthetically fixed carbon to below the euphotic zone via the biological carbon pump (BCP). Oxygen (O2) depletion may improve the efficiency of the BCP. However, the mechanisms by which O2 deficiency can enhance particulate organic matter (POM) vertical fluxes are not well understood. Here, we investigate the composition and vertical fluxes of POM in two deep basins of the Baltic Sea (GB: Gotland Basin and LD: Landsort Deep). The two basins showed different O2 regimes resulting from the intrusion of oxygen-rich water from the North Sea that ventilated the water column below 140 m in GB, but not in LD, during the time of sampling. In June 2015, we deployed surface-tethered drifting sediment traps in oxic surface waters (GB: 40 and 60 m; LD: 40 and 55 m), within the oxygen minimum zone (OMZ; GB: 110 m and LD: 110 and 180 m) and at recently oxygenated waters by the North Sea inflow in GB (180 m). The primary objective of this study was to test the hypothesis that the different O2 conditions in the water column of GB and LD affected the composition and vertical flux of sinking particles and caused differences in export efficiency between those two basins. The composition and vertical flux of sinking particles were different in GB and LD. In GB, particulate organic carbon (POC) flux was 18 % lower in the shallowest trap (40 m) than in the deepest sediment trap (at 180 m). Particulate nitrogen (PN) and Coomassie stainable particle (CSP) fluxes decreased with depth, while particulate organic phosphorus (POP), biogenic silicate (BSi), chlorophyll a (Chl a) and transparent exopolymeric particle (TEP) fluxes peaked within the core of the OMZ (110 m); this coincided with the presence of manganese oxide-like (MnOx-like) particles aggregated with organic matter. In LD, vertical fluxes of POC, PN and CSPs decreased by 28 %, 42 % and 56 %, respectively, from the surface to deep waters. POP, BSi and TEP fluxes did not decrease continuously with depth, but they were higher at 110 m. Although we observe a higher vertical flux of POP, BSi and TEPs coinciding with abundant MnOx-like particles at 110 m in both basins, the peak in the vertical flux of POM and MnOx-like particles was much higher in GB than in LD. Sinking particles were remarkably enriched in BSi, indicating that diatoms were preferentially included in sinking aggregates and/or there was an inclusion of lithogenic Si (scavenged into sinking particles) in our analysis. During this study, the POC transfer efficiency (POC flux at 180 m over 40 m) was higher in GB (115 %) than in LD (69 %), suggesting that under anoxic conditions a smaller portion of the POC exported below the euphotic zone was transferred to 180 m than under reoxygenated conditions present in GB. In addition, the vertical fluxes of MnOx-like particles were 2 orders of magnitude higher in GB than LD. Our results suggest that POM aggregates with MnOx-like particles formed after the inflow of oxygen-rich water into GB, and the formation of those MnOx–OM-rich particles may alter the composition and vertical flux of POM, potentially contributing to a higher transfer efficiency of POC in GB. This idea is consistent with observations of fresher and less degraded organic matter in deep waters of GB than LD.

Elevated carbon flux in deep waters of the South China Sea

We measured particulate organic carbon (POC) fluxes from the euphotic zone into the twilight zone and deep waters (>1000 m) that occurred between the shelf and the basin in the South China Sea (SCS) and at the SouthEast Asia Time Series Station (SEATS) using floating sediment trap arrays. Additionally, selected sinking particles were imaged by scanning electron microscope (SEM) to reveal particle morphology and composition. Results showed large variations in POC fluxes with elevated values (32–104 mg-C m−2 d−1) below the euphotic zone and a trend towards lower values in the deep SCS. Vertical POC fluxes measured in deep waters between the shelf and the SCS basin were much higher than those estimated by Martin’s attenuation equation. These elevated POC fluxes in deep waters were attributed to lateral particle transport as opposed to enhanced settling out of the euphotic zone. SEM images of sinking particles at 150 m show abundant marine biogenic detritus, while those in deep waters contained a higher proportion of lithogenic material. A great deal of the spatial variability in POC fluxes across the twilight zone and deep waters of the SCS cannot be represented by current biogeochemical models.

Response of export production and dissolved oxygen concentrations in oxygen minimum zones to <i>p</i>CO<sub>2</sub> and temperature stabilization scenarios in the biogeochemical model HAMOCC 2.0

Abstract. Dissolved oxygen (DO) concentration in the ocean is an important component of marine biogeochemical cycles and will be greatly altered as climate change persists. In this study a global oceanic carbon cycle model (HAMOCC 2.0) is used to address how mechanisms of oxygen minimum zone (OMZ) expansion respond to changes in CO2 radiative forcing. Atmospheric pCO2 is increased at a rate of 1 % annually and the model is stabilized at 2 ×, 4 ×, 6 ×, and 8 × preindustrial pCO2 levels. With an increase in CO2 radiative forcing, the OMZ in the Pacific Ocean is controlled largely by changes in particulate organic carbon (POC) export, resulting in increased remineralization and thus expanding the OMZs within the tropical Pacific Ocean. A potential decline in primary producers in the future as a result of environmental stress due to ocean warming and acidification could lead to a substantial reduction in POC export production, vertical POC flux, and thus increased DO concentration particularly in the Pacific Ocean at a depth of 600–800 m. In contrast, the vertical expansion of the OMZs within the Atlantic is linked to increases POC flux as well as changes in oxygen solubility with increasing seawater temperature. Changes in total organic carbon and increase sea surface temperature (SST) also lead to the formation of a new OMZ in the western subtropical Pacific Ocean. The development of the new OMZ results in dissolved oxygen concentration of ≤ 50 µmol kg−1 throughout the equatorial Pacific Ocean at 4 times preindustrial pCO2. Total ocean volume with dissolved oxygen concentrations of ≤ 50 µmol kg−1 increases by 2.4, 5.0, and 10.5 % for the 2 ×, 4 ×, and 8 × CO2 simulations, respectively.

Export fluxes in northern Gulf of Mexico - Comparative evaluation of direct, indirect and satellite-based estimates

The northern Gulf of Mexico (NGOM) is one of the well-studied areas of global ocean, yet direct estimates of upper ocean particulate organic carbon (POC) fluxes from this region are limited. The present work reports vertical fluxes of POC from the oligotrophic region of NGOM utilizing short-lived radionuclide pairs 234Th/238U and 210Po/210Pb. In spite of the difference in time scale both 210Po and 234Th based estimates are in reasonable agreement with sinking POC fluxes, caught in sediment traps. POC flux estimates ranged between 22–41mgCm−2day−1 at 150m and 6–40mgCm−2day−1 at 250m. The average export efficiency at base of euphotic zone (Ez) was found to be 0.07±0.03 while the export ratio (T100) at 100m below euphotic zone was found to be 0.66±0.18 indicating that most of the attenuation of NPP in this region is set in the surface layer (low Ez ratio) and there is relatively little flux attenuation in the subsurface (high T100) which is typical for oligotrophic settings. Satellite based export efficiencies predicted by the Laws and Dunne models are on average found to be two times higher than the in situ observations while estimates from the Henson model were found to be similar or lower. This is consistent with the observation on a global scale where we find export estimates from in situ data to be consistently lower than those predicted by the Laws export model for the temperature range of 20–25°C. The discrepancy between modelled estimates and in situ measurements of POC fluxes highlights the fact that global empirical models of satellite based POC fluxes that only consider temperature are overly simple and may need further refinement for ocean biome specific scaling to accurately predict export fluxes.

Ecosystem function and particle flux dynamics across the Mackenzie Shelf (Beaufort Sea, Arctic Ocean): an integrative analysis of spatial variability and biophysical forcings

Abstract. A better understanding of how environmental changes affect organic matter fluxes in Arctic marine ecosystems is sorely needed. Here we combine mooring times series, ship-based measurements and remote sensing to assess the variability and forcing factors of vertical fluxes of particulate organic carbon (POC) across the Mackenzie Shelf in 2009. We developed a geospatial model of these fluxes to proceed to an integrative analysis of their determinants in summer. Flux data were obtained with sediment traps moored around 125 m and via a regional empirical algorithm applied to particle size distributions (17 classes from 0.08–4.2 mm) measured by an Underwater Vision Profiler 5. The low fractal dimension (i.e., porous, fluffy particles) derived from the algorithm (1.26 ± 0.34) and the dominance (~ 77%) of rapidly sinking small aggregates (< 0.5 mm) in total fluxes suggested that settling material was the product of recent aggregation processes between marine detritus, gel-like substances, and ballast minerals. Modeled settling velocity of small and large aggregates was, respectively, higher and lower than in previous studies within which a high fractal dimension (i.e., more compact particles) was consequential of deep-trap collection (~400–1300 m). Redundancy analyses and forward selection of abiotic/biotic parameters, linear trends, and spatial structures (i.e., principal coordinates of neighbor matrices, PCNM) were conducted to partition the variation of the 17 POC flux size classes. Flux variability was explained at 69.5% by the addition of a temporal trend, 7 significant PCNM, and 9 biophysical variables. The first PCNM canonical axis (44.5% of spatial variance) reflected the total magnitude of POC fluxes through a shelf-basin gradient controlled by bottom depth and sea ice concentration (p < 0.01). The second most important spatial structure (5.0%) corresponded to areas where shelf break upwelling is known to occur under easterlies and where phytoplankton was dominated by diatoms. Among biophysical parameters, bacterial production and northeasterly wind (upwelling-favorable) were the two strongest corollaries of POC fluxes (r2 cum. = 0.37). Bacteria were correlated with small aggregates, while northeasterly wind was associated with large size classes (> 1 mm ESD), but these two factors were weakly related with each other. Copepod biomass was overall negatively correlated (p < 0.05) with vertical POC fluxes, implying that metazoans acted as regulators of export fluxes, even if their role was minor given that our study spanned the onset of diapause. Our results demonstrate that on interior Arctic shelves where productivity is low in mid-summer, localized upwelling zones (nutrient enrichment) may result in the formation of large filamentous phytoaggregates that are not substantially retained by copepod and bacterial communities.

Seasonal contribution of living phytoplankton carbon to vertical fluxes in a coastal upwelling system (Ría de Vigo, NW Spain)

The aim of this study is to explore the contribution of living phytoplankton carbon to vertical fluxes in a coastal upwelling system as a key piece to understand the coupling between primary production in the photic layer and the transfer mechanisms of the organic material from the photic zone. Between April 2004 and January 2005, five campaigns were carried out in the Ría de Vigo (NW Iberian Peninsula) covering the most representative oceanographic conditions for this region. Measurements of particulate organic carbon (POC), chlorophyll- a (chl a), phaeopigments (phaeo), and identification of phytoplankton species were performed on the water column samples and on the organic material collected in sediment traps. The POC fluxes measured by the sediment traps presented no seasonal variation along the studied period ranging around a mean annual value of 1085±365 mg m −2 d −1, in the upper range of the previously reported values for other coastal systems. The fact that higher POC fluxes were registered during autumn and winter, when primary production rates were at their minimum levels points to a dominant contribution of organic carbon from resuspended sediments on the trap collected material. On the contrary, fluxes of living phytoplankton carbon (C phyto) and chl a clearly presented a seasonal trend with maximum values during summer upwelling (546 mg m −2 d −1 and 22 mg chl a m −2 d −1, respectively) and minimum values during winter (22 mg m −2 d −1 and 0.1 mg chl a m −2 d −1, respectively). The contribution of C phyto to the vertical flux of POC ranged between 2% and 49% in response to the pelagic phytoplankton community structure. Higher values of C phyto fluxes were registered under upwelling conditions which favour the dominance of large chain-forming diatoms ( Asterionellopsis glacialis and Detonula pumila) that were rapidly transferred to the sediments. By contrast, C phyto fluxes decreased during the summer stratification associated with a pelagic phytoplankton community dominated by single-cell diatoms and flagellates. Minimal C phyto fluxes were observed during the winter mixing conditions, when the presence of the benthic specie Paralia sulcata in the water column also points toward strong sediment resuspension.

Three-year assessment of particulate organic carbon fluxes in Amundsen Gulf (Beaufort Sea): Satellite observations and sediment trap measurements

Surface concentrations and vertical fluxes of particulate organic carbon (POC) were assessed in the Amundsen Gulf (southeastern Beaufort Sea, Arctic Ocean) over the years 2004 to 2006 by using ocean color remote-sensing imagery and sequential sediment traps moored over the ca. 400 m isobath. Environmental conditions (sea ice, wind) and oceanographic variables (temperature, salinity, fluorescence and currents) were investigated to explain the variability of POC data. Annual downward POC fluxes in 2004, 2005 and 2006 cumulated, respectively, to 3.3, 4.2 and 6.0 g C m −2 yr −1 at ∼100 m depth, and to 1.3, 2.2 and 3.3 g C m −2 yr −1 at ∼210 m depth. The fraction of settling POC attributable to autochthonous processes occurring at or next to ice break-up was estimated to be 75–84% of the 100 m annual fluxes and to be 61–75% of the 210 m fluxes. Over the three ice-reduced seasons, distinct scenarios between ice conditions, surface POC pools and vertical POC export at 100 m were identified: (1) in 2004, despite a normal ice break-up, a weak primary production was measured and low vertical fluxes were collected as old ice moved across the region; (2) in 2005, a lengthened ice-free period allowed an extended season of surface POC production near-shore, while an intermediate increase of vertical fluxes was recorded offshore; and (3) in 2006, a late ice melt gave rise to a pulsed ice edge bloom and to large vertical fluxes also associated with extra ice-flushed material. Linear regressions of vertical POC fluxes against satellite-derived surface POC concentrations suggested that the pelagic POC retention in the upper 100 m of the Amundsen Gulf ranged from ca. 70% to 90% depending on the timing of ice cover melt. Regardless of the inter-annual variability, the estimated fraction of the surface POC reservoir reaching the 210 m water depth was reduced to ∼5%. Therefore, as the Arctic Ocean warms up, our results support the expectation that the increasing extent of the seasonal ice zone will promote the POC pathways that benefit pelagic webs rather than benthic communities.

Vertical flux regulation by zooplankton in the northern Barents Sea during Arctic spring

Abstract The influence of zooplankton on vertical carbon export was investigated during three field investigations conducted on and off the northern Barents Sea shelf during bloom conditions in July 2003, 2004 and May 2005. Short-term patterns in vertical flux of organic matter were measured at high vertical resolution (eight depths) in the upper 200 m at 11 stations. Vertical flux of particulate organic carbon (POC) was highly variable between stations and depths, depending on water mass characteristics such as stratification, phytoplankton bloom development, and zooplankton-related activities. Detailed investigations of the vertical flux composition revealed that faecal pellets (FPs) produced by larger meso- and macrozooplankton comprised on average 20% of the vertical POC flux (average for all depths and stations). The relative importance of FP carbon (FPC) increased with depth, and at depths >60 m FPC comprised ∼30% of the vertical POC flux. The main contributors to the FPC flux varied depending on the prevailing water masses and the phytoplankton bloom stage. FPs produced by older copepodite stages of Calanus spp. dominated at most stations, while FP produced by appendicularians and euphausiids dominated at certain depths and stations. A conservative grazing estimate obtained by calculating the ingestion necessary to support the measured FPC at 50 m depth, suggests that average zooplankton community carbon ingestion equals the vertical POC export. This study clearly shows that zooplankton is important for vertical flux regulation. Zooplankton ingest POC in the range of 22–44% of the daily primary production, but accelerate the vertical flux through production of fast-sinking FP.

Particulate organic carbon fluxes on the slope of the Mackenzie Shelf (Beaufort Sea): Physical and biological forcing of shelf-basin exchanges

To investigate the mechanisms underlying the transport of particles from the shelf to the deep basin, sediment traps and oceanographic sensors were moored from October 2003 to August 2004 over the 300- and 500-m isobaths on the slope of the Mackenzie Shelf (Beaufort Sea, Arctic Ocean). Seasonal variations in the magnitude and nature of the vertical particulate organic carbon (POC) fluxes were related to sea-ice thermodynamics on the shelf and local circulation. From October to April, distinct increases in the POC flux coincided with the resuspension and advection of shelf bottom particles by thermohaline convection, windstorms, and/or current surges and inversions. Once resuspended and incorporated into the Benthic Nepheloid Layer (BNL), particles of shelf origin were transported over the slope by the isopycnal intrusion of the BNL into the Polar-Mixed Layer off-shelf. Offshore transport of the resuspended particles allowed them to settle over the slope. The resulting vertical POC flux at the shelf-basin boundary amounted to 1.0 g C m − 2 y − 1 or 58% of the annual POC flux over the 300-m isobath. Consistent with the resuspension of shelf sediments, POC fluxes in fall/winter were characterized by a high terrigenous fraction (25–60%), the dominance of small flagellate cells, and increasingly degraded fecal pellets with time. In late May–early June, a short-duration POC flux maximum characterized by high POC:PON ratio and more positive δ 13C resulted from the direct sinking of ice algae and transparent exopolymeric matter flushed from melting sea-ice. In July, a last sedimentation event coincided with the retreat of the sea-ice cover, phytoplankton production from a subsurface bloom, and the sinking of the intact fecal pellets of large herbivorous copepods and appendicularians. Our results confirm the importance of sea-ice thermodynamics and BNL resuspension in promoting the transfer of POC from the shelf to the deep basin in fall/winter. The actual contribution of the summer biological production to the shelf–basin flux of POC remains uncertain.

Export or retention? Copepod abundance, faecal pellet production and vertical flux in the marginal ice zone through snap shots from the northern Barents Sea

The balance between faecal pellet (FP) production and destruction that accelerates or diminishes vertical export has an effect on pelagic-benthic coupling, but is inadequately known. Production, export and retention of copepod FP were investigated in the marginal ice zone (MIZ) of the northern Barents Sea in July 2003. Older stages of Calanus finmarchicus and C. glacialis dominated the copepod biomass and FP production experiments revealed that more than 90% of the FP were produced in the upper 50 m where most of the copepods were located both day and night. Copepod pellets typically made up ∼10% of the vertical particulate organic carbon flux, and significantly less than what was produced by the copepod community. This implies a variable but significant retention of pellets. We suggest that retention of FP is caused partly by the zooplankton themselves and that retention of FP is the rule rather than the exception in the Barents Sea, particularly during non-bloom scenarios.

The fate of production in the central Arctic Ocean – top–down regulation by zooplankton expatriates?

We estimated primary and bacterial production, mineral nutrients, suspended chlorophyll a (Chl), particulate organic carbon (POC) and nitrogen (PON), abundance of planktonic organisms, mesozooplankton fecal pellet production, and the vertical flux of organic particles of the central Arctic Ocean (Amundsen basin, 89–88° N) during a 3 week quasi-Lagrangian ice drift experiment at the peak of the productive season (August 2001). A visual estimate of ≈15% ice-free surface, plus numerous melt ponds on ice sheets, supported a planktonic particulate primary production of 50–150 mg C m −2 d −1 (mean 93 mg C m −2 d −1, n = 7), mostly confined to the upper 10 m of the nutrient replete water column. The surface mixed layer was separated from the rest of the water column by a strong halocline at 20 m depth. Phototrophic biomass was low, generally 0.03–0.3 mg Chl m −3 in the upper 20 m and <0.02 mg Chl m −3 below, dominated by various flagellates, dinoflagellates and diatoms. Bacterial abundance (typically 3.7–5.3 × 10 5, mean 4.1 × 10 5 cells ml −1 in the upper 20 m and 1.3–3.7 × 10 5, mean 1.9 × 10 5 cells ml −1 below) and Chl concentrations were closely correlated ( r = 0.75). Mineral nutrients (3 μmol NO 3 l −1, 0.45 μmol PO 4 l −1, 4–5 μmol SiO 4 l −1) were probably not limiting the primary production in the upper layer. Suspended POC concentration was ∼30–105 (mean 53) mg C m −3 and PON ∼5.4–14.9 (mean 8.2) mg N m −3 with no clear vertical trend. The vertical flux of POC in the upper 30–100 m water column was ∼37–92 (mean 55) mg C m −2 d −1 without clear decrease with depth, and was quite similar at the six investigated stations. The mesozooplankton biomass (≈2 g DW m −2, mostly in the upper 50 m water column) was dominated by adult females of the large calanoid copepods Calanus hyperboreus and Calanus glacialis (≈1.6 g DW m −2). The grazing of these copepods (estimated via fecal pellet production rates) was ≈15 mg C m −2 d −1, being on the order of 3% and 20% of the expected food-saturated ingestion rates of C. hyperboreus and C. glacialis, respectively. The stage structure of these copepods, dominated by adult females, and their unsatisfied grazing capacity during peak productive period suggest allochthonous origin of these species from productive shelf areas, supported by their long life span and the prevailing surface currents in the Arctic Ocean. We propose that the grazing capacity of the expatriated mesozooplankton population would match the potential seasonal increase of primary production in the future decreased ice perspective, diminishing the likelihood of algal blooms.

Sensitivity of ocean carbon tracer distributions to particulate organic flux parameterizations

Vertical fluxes of particulate organic carbon (POC) from the euphotic zone to the deep sea are an important part of the carbon cycle in the oceans. In this study, oceanic fluxes of POC below the euphotic zone were simulated with the Hamburg ocean carbon cycle model (HAMOCC5.1) by using different POC‐flux parameterizations and compared with sediment trap data. Overall, the geochemical distributions in the deep sea showed a high sensitivity to the selection of POC flux parameterization. Below 2000 m in the oceans, differences between simulated and observed carbon tracers (PO4, Alk*) and model‐data differences of POC fluxes are lowest when a regionally variable POC flux parameterization is used. Specifically, model‐data differences are lowest in the subtropics when simulated vertical POC flux considers mineral ballasting, while they are lowest in the Arabian Sea and west coast of Africa when a mineral dust parameterization is used for the vertical POC flux.

Flocculation and phytoplankton cell size can alter 234Th-based estimates of the vertical flux of particulate organic carbon in the sea

The deficit of 234Th relative to its radioactive parent 238U in the surface ocean can yield reliable estimates of vertical Particulate Organic Carbon (POC) fluxes to deeper waters, but only when coupled with an accurate ratio of POC concentration to activity of 234Th on sinking matter. Assuming a simple partitioning of suspended phytoplankton mass between single cells and flocs, we calculate the ratio of the POC flux estimated from 234Th deficit to the actual POC flux ( p ratio, Smith, J.N., Moran, S.B., Speicher, E.A., in press. The p-ratio: a new diagnostic for evaluating the accuracy of upper ocean particulate organic carbon export fluxes estimated from 234Th/ 238U disequilibrium. Deep-Sea Research I.). The p ratios are calculated under the assumption that particle surface area is correlated with 234Th activity and particle volume is correlated with POC concentration. The value of the p ratio depends on the relative contributions of single cells and flocs to the vertical flux. When large single cells make up a significant fraction of the vertical flux, p ratios are less than one, meaning POC fluxes estimated from 234Th deficits underestimate actual POC fluxes. When large single cells are abundant but do not sink fast enough to contribute to vertical POC flux, p ratios are greater than one (up to 3 × overestimate). Factor analysis of the model indicates that altering the extent of flocculation in suspension and changing the density and maximum size of phytoplankton cells have the greatest effects on the p ratio. Failure to measure the properties of flocs when characterizing the ratio of POC to thorium on sinking matter potentially leads to large overestimation of the POC flux (over 20 ×). Failure to characterize the POC to thorium ratio of large particles, by, for example, destruction of phytoplankton cells in pumps, can lead to underestimation of POC flux. Estimates of POC flux should be most reliable in highly flocculated suspensions populated by small cells and rapidly sinking flocs. These conditions are often associated with intense phytoplankton blooms.

Water column particulate organic carbon modeled fluxes in the ice-frequented Southern Ocean

Nine sediment traps deployed in part of the ice-frequented Southern Ocean (Weddell Sea, Ross Sea, and Bransfield Strait) provide the necessary data to compare vertical annual particulate organic carbon (POC) fluxes with ambient ice regimes (annual mean ice concentration) as can be observed by satellite microwave radiometers. Sedimentation in these seasonally ice-covered regions is primarily dependent upon the mean annual ice concentration. We used a similar approach as the Greenland Sea Is Odden–Nordbukta model to develop a semi-empirical model for the Southern Ocean, and found that the vertical POC fluxes are a factor of two smaller than in the Greenland Sea. Applying the model for each individual trap for the years 1979 to 1999 shows that within the biological marginal ice zone (BMIZ), most of the sedimentation takes place within a mean annual ice concentration range of 9–<67%. This range accounts for 77% of the entire sedimentation as compared to 23% for the other sub-areas of the BMIZ, which account for 16% for the range 67–<85%, 2% for the range 85–100%, and 5% for the range 0–<9%. The inter-annual variations in flux are small (2%) ranging from 1.74 to 1.94 g m −2 year −1 at a depth of 500 m, with the fluxes of the western Weddell Sea being similar in magnitude as the Ross Sea (northwest and northeast). The mean annual amount of sedimentation for the years 1979 to 1999 is 32.45±1.06 Mt.

Downward fluxes of faecal material and microplankton at 2300m depth in the oceanic area off Coquimbo (30°S), Chile, during 1993–1995

Abstract Vertical fluxes of biogenic elements (carbon, carbonate) and their main sources (faecal material, foraminifers, and coccolithophorids) were studied during 1993–1995 with a sediment trap deployed at a depth of 2300 m in the oceanic area off the coastal upwelling system off Coquimbo, Chile (30°S). The average flux of particulate organic carbon (POC) was 6.8 mg m−2 d−1. Faecal material made the main contribution to the total POC flux, averaging 2.1 mg C m−2 d−1 (35%), followed by carbon-based tintinnids, with an average flux of 0.7 mg C m−2 d−1 (9.3%). Zooplankton faecal material made up 17% of the vertical POC flux in spring and 44% in the other seasons. Faeces from euphausiids and appendicularians contributed the most to total faecal carbon flux (65%), whereas the faeces of copepods and protists were relatively minor (10%). The vertical flux of microzooplankton was dominated by tintinnids, foraminifers, and coccolithophorids, which, respectively, averaged 61×103, 68×103, and 26×103 individuals m−2 d−1. Intra-annual variability in carbon flux seems to result mostly from seasonal changes in oceanographic conditions, such as Ekman transport offshore, which was highly correlated with the POC flux ( r 2 = 0.59 ). Inter-annual variability, on the other hand, might result partly from remote forcing. Other factors, such as surface pigment distribution and the composition and abundance of both zooplankton and phytoplankton in the study area, also are discussed. Average carbonate fluxes of foraminifers and coccosphere+coccolith were 56 and 7 mg m−2 d−1, respectively, or 73% and 11% of the total carbonate flux. The sinking flux of CaCO3 was significantly higher than that of POC throughout the year, averaging 90% of the total carbon and suggesting deep-water delivery of biogenic CaCO3 to be the main pathway for removing carbon from the upper-ocean biosphere in the study area.

The potential contribution of fecal pellets from large copepods to the flux of biogenic silica and particulate organic carbon in the Antarctic Polar Front region near 170°W

Abstract During a spring and a summer cruise, we determined the amounts of biogenic silica (bSi) and particulate organic carbon (POC) encapsulated into fecal pellets by the community of large copepods in the Antarctic Polar Front region, in order to estimate the potential contribution of sinking pellets to bSi and POC flux from the surface layer. Grazing by large copepods can be an important mechanism for enhancing vertical flux of POC and bSi because particles in the diatom size range are excellent foods for mesozooplankton, bSi is inert in copepod guts, and copepod fecal pellets (which have a chitinous membrane that retards dissolution of pellet contents) have sinking rates of 100s m d−1. Rates of pellet production were measured in deck incubations with natural seawater as food medium. BSi and POC contents of pellets were measured in the laboratory. These data were applied to measured abundances of large copepods and the resultant rates of bSi and POC pellet production were compared to flux measurements made by other investigators. Comparison between pellet production by the community of large copepods and flux at 100 m determined by thorium isotopes indicated that pellet production was equivalent to 22–63% of POC flux and 42–107% of bSi flux during the spring cruise. The potential contribution of pellets to flux at 100 m in the summer was considerably smaller, between 2% and 7% for POC and between 1% and 5% for bSi (with one exception at the southernmost station where there was a diatom bloom). We conclude that, in the spring especially, fecal pellets can contribute significantly to the bSi and POC sinking from the surface layer within the Antarctic Polar Front region. However, sediment traps deployed at a depth of 1000 m during the spring cruise indicated total fluxes of both bSi and POC were less than pellet production in the upper 100 m, indicating consumption or degradation of sinking material is significant between 100 and 1000 m. Fecal pellet production of POC was 4–10 times greater than the flux of POC at 1000 m and pellet production of bSi was 0.4–5 times the flux of bSi at 1000 m. Although pellets can contribute significantly to total flux, a large fraction of pellet material must be remineralized during sinking.

Calibration of sediment traps and particulate organic carbon export using 234 Th in the Barents Sea

Profiles of particulate and dissolved 234Th (t1/2=24.1 days) in seawater and particulate 234Th collected in drifting traps were analyzed in the Barents Sea at five stations during the ALV3 cruise (from June 28 to July 12, 1999) along a transect from 78°15′N–34°09′E to 73°49′N–31°43′E. 234Th/238U disequilibrium was observed at all locations. 234Th data measured in suspended and trapped particles were used to calibrate the catchment efficiency of the sediment traps. Model-derived 234Th fluxes were similar to 234Th fluxes measured in sediment traps based on a steady-state 234Th model. This suggests that the sediment traps were not subject to large trapping efficiency problems (collection efficiency ranges from 70% to 100% for four traps). The export flux of particulate organic carbon (POC) can be calculated from the model-derived export flux of 234Th and the POC/234Th ratio. POC/234Th ratios measured in suspended and trapped particles were very different (52.0±9.9 and 5.3±2.2 μmol dpm−1, respectively). The agreement between calculated and measured POC fluxes when the POC/234Th ratio of trapped particles was used confirms that the POC/234Th ratio in trap particles is representative of sinking particles. Large discrepancies were observed between calculated and measured POC fluxes when the POC/234Th ratio of suspended particles was used. In the Barents Sea, vertical POC fluxes are higher than POC fluxes estimated in the central Arctic Ocean and the Beaufort Sea and lower than those calculated in the Northeast Water Polynya and the Chukchi Sea. We suggest that the latter fluxes may have been strongly overestimated, because they were based on high POC/234Th ratios measured on suspended particles. It seems that POC fluxes cannot be reliably derived from thorium budgets without measuring the POC/234Th ratio of sediment trap material or of large filtered particles.