Temporal and spatial variability of biogenic particles fluxes during the JGOFS northeast Atlantic process studies at 47°N, 20°W

Particle fluxes to 3100 m depth at 45°50′N, 19°30′W were measured using time-series sediment traps during a 17 month period encompassing 1989 and 1990 JGOFS spring bloom process studies in the northeast Atlantic. There was a marked intra-annual variability in fluxes of mass, particulate inorganic carbon (PIC), particulate organic carbon (POC) and opal, appearing as two major flux events in each year. In 1989, the first flux event represented the settlement of spring bloom-type material, whereas the second, in autumn, was heavily enriched in mucopolysaccharides. In 1990, in contrast, the two flux events comprised spring bloom-type material and arrived at depth at different times relative to the 1989 events. The intra- and interannual variability evident for all three biogenic components was most notable for POC: (i) the autumn 1989 event supplied twice as much POC to 3100 m as the earlier spring bloom settlement—a quite unexpected observation—and (ii) the annual average POC flux in 1989 was 3–4 times more than in 1990. A synthesis of process study datasets with sediment trap data enables an evaluation of the coupling of deep fluxes with surface-water events. Spatial variability of the 1989 deep flux events is assessed by comparing the sediment trap data reported here with those from a second site 100 km away (Honjo and Manganini,Deep-Sea Research II,40, 587–607, 1993). The timing and magnitude of the 1989 spring bloom settlement was indistinguishable in the two datasets, indicating no spatial variability in flux between these sites. In contrast, the autumn 1989 flux event was barely recorded at the second site. Given the biogeochemical importance of this latter event to deep waters, most notable in terms of its contribution to POC flux, this observation of deep-water mesoscale flux variability indicates a significant problem in determining regional carbon budgets. Construction of basin-scale budgets is a central goal of JGOFS and for this to be achieved further studies of mesoscale variability of particle flux are essential.     

Seasonal changes and biochemical composition of the labile organic matter flux in the Cretan Sea

Downward fluxes of labile organic matter (lipids, proteins and carbohydrates) at 200 (trap A) and 1515 m depth (trap B), measured during a 12 months sediment trap experiment, are presented, together with estimates of the bacterial and cyanobacterial biomasses associated to the particles. The biochemical composition of the settling particles was determined in order to provide qualitative and quantitative information on the flux of readily available organic carbon supplying the deep-sea benthic communities of the Cretan Sea. Total mass flux and labile carbon fluxes were characterised by a clear seasonality. Higher labile organic fluxes were reported in trap B, indicating the presence of resuspended particles coming from lateral inputs. Particulate carbohydrates were the major component of the flux of labile compounds (on annual average about 66% of the total labile organic flux) followed by lipids (20%) and proteins (13%). The biopolymeric carbon flux was very low (on annual average 0.9 and 1.2 gC m⁻² y⁻¹, at trap A and B). Labile carbon accounted for most of the OC flux (on annual average 84% and 74% in trap A and B respectively). In trap A, highest carbohydrate and protein fluxes in April and September, corresponded to high faecal pellet fluxes. The qualitative composition of the organic fluxes indicated a strong protein depletion in trap B and a decrease of the bioavailability of the settling particles as a result of a higher degree of dilution with inorganic material. Quantity and quality of the food supply to the benthos displayed different temporal patterns. Bacterial biomass in the sediment traps (on average 122 and 229 μgC m⁻² d⁻¹ in trap A and B, respectively) was significantly correlated to the flux of labile organic carbon, and particularly to the protein and carbohydrate fluxes. Cyanobacterial flux (on average, 1.1 and 0.4 μgC m⁻² d⁻¹, in trap A and B, respectively) was significantly correlated with total mass and protein fluxes only in trap A. Bacterial carbon flux, equivalent to 84.2 and 156 mgC m⁻² y⁻¹, accounted for 5–6.5% of the labile carbon flux (in trap A and B respectively) and for 22–41% protein pool of the settling particles. These results suggest that in the Cretan Sea, bacteria attached to the settling particles represent a potential food source of primary importance for deep-sea benthic communities.     

Sinking Particle Flux in the Subtropical Oligotrophic Northwestern Pacific from a Short-term Sediment Trap Experiment

Time-series sediment traps were deployed in the subtropical oligotrophic northwestern Pacific (SONP) at three depths from August to September 2015 to better understand vertical flux of sinking particles. Sinking particles were collected at 5-day intervals over the sediment trap deployment period. The average total mass flux at water depths of 400 m, 690 m, and 1,710 m was 9.1, 4.4, and 4.1 mg m^-2day^-1, respectively. CaCO₃ materials constituted 50 to 70% of sinking particles while in comparison particulate organic carbon (POC) constituted up to 20%. A synchronous variation of total mass flux was observed at the three depths, indicating that calcite-dominated particles sank from 400 to 1,710 m within a 5-day period. POC flux at these water depths was 2.4, 0.38, and 0.31 mg m^-2day^-1, respectively. Our results indicate low transfer efficiencies of 16% from 400 to 690 m and 13% for the 400 to 1,710 m depth range. The estimated transfer efficiencies were significantly lower than those observed at the K2 station in the northwest Pacific subarctic gyre, presumably because of a prevalence of pico-cyanobacteria in the SONP. Because cyanobacteria have a semi-permeable proteinaceous shell, they are more readily remineralized by bacteria than are siliceous phytoplankton in the northwest Pacific subarctic gyre. Continued surface water warming and expansion of the SONP will likely have a profound impact on ocean acidification in the northwest Pacific, possibly affecting the transfer efficiency of sinking POC to the deep-sea.     

A comparison of the quantity and composition of material caught in a neutrally buoyant versus surface-tethered sediment trap

The flux and composition of material caught using two different upper ocean sediment trap designs was compared at the Bermuda Atlantic Time-series Study site (BATS). The standard surface-tethered trap array at BATS was compared to a newly designed neutrally buoyant sediment trap (NBST). Both traps used identical cylindrical collection tubes. Of particular concern was the effect of horizontal flow on trap collection efficiency. In one experiment, mass, particulate organic carbon (POC) and particulate organic nitrogen (PON) fluxes were slightly lower (20–30%) in the NBST than in the standard BATS trap. In contrast, ²³⁴Th and fecal pellet fluxes were up to a factor of two to three lower in the NBST. In a second experiment, mass and POC fluxes decreased significantly with depth in the BATS surface-tethered trap, but not in the NBST. Different brine treatments had no measurable effect on collection efficiencies. A striking observation was that the swimmer “flux” was much larger in the standard BATS traps than in the NBST. Overall, these results show that different components of the sinking flux can be collected with differing efficiencies, depending upon how traps are deployed in the ambient environment.     

Particulate organic carbon supply to the sea bottom: stable carbon isotope ratio analysis of the sediment trap samples at the mouth of otsuchi bay,Northeastern Japan

An array of sediment traps was deployed for the analysis of the pattern of particulate organic carbon (POC) supply to the sea bottom in April, May and July 1988 at the mouth of Otsuchi Bay (about 80 m depth), Northeastern Japan.On the basis of a simple two-component mixing model using stable carbon isotope ratios, the POC flux was separated into marine planktonic and terrestrial components. Both the planktonic and terrestrial POC fluxes had maximum values at 30 m above the sea bottom throughout the three experiments. The planktonic POC flux showed a significant decrease with depth between 30 m and 10 m or 5 m above the bottom. Vertical supply of the planktonic POC and supply of the resuspended planktonic POC were estimated on the basis of regression lines between water depth and the planktonic POC flux in the depth range where the flux decreases with depth.Vertical supply of the planktonic POC and supply of the resuspended planktonic POC to the sea bottom were largest in May (52.1 mgC m^–2 d^–1 and 19.5 mgC m^–2 d^–1 at 5 m above the sea bottom), and horizontal supplies of the terrestrial POC were almost constant (31.9±3.5 mgC m^–2 d^–1 at 5 m above the bottom) throughout the three experiments.     

Controls of 234Th removal from the oligotrophic ocean by polyuronic acids and modification by microbial activity

To gain new insights into the variability of particulate organic carbon (POC) fluxes and to better understand the factors controlling the POC/²³⁴Th ratios in suspended and sinking particulate matter, we investigated the relationships between POC/²³⁴Th ratios and biochemical composition (uronic acids, URA; total carbohydrates, TCHO; acid polysaccharides, APS; and POC) of suspended and sinking matter from the Gulf of Mexico in 2005 and 2006. Our data show that URA/POC in sediment traps (STs), APS/POC in the suspended particles, and turnover times of particulate ²³⁴Th in the water column and those of bacteria in STs inside eddies usually increased with depth, whereas particulate POC/²³⁴Th (10–50 μm) and the sediment-trap parameters (POC flux, POC/²³⁴Th ratio, bacterial biomass, and bacterial production) decreased with depth. However, this trend was not the case for most biological parameters (e.g., phytoplankton and bacterial biomass) or for the other parameters at the edges of eddies or at coastal-upwelling sites.In general, the following relationships were observed: 1) ²³⁴Th/POC ratios in STs were correlated with APS flux, and these ratios in the 10–50 μm suspended particles also correlated with URA/POC ratios; 2) neither URA fluxes nor URA/POC ratios were significantly related to bacterial biomass; 3) the sum of two uronic acids (G2, glucuronic, and galacturonic acid, which composed most of the URA pool) was positively related to bacterial biomass; and 4) the POC/²³⁴Th ratios in intermediate-sized particles (10–50 μm) were close to those in sinking particles but much lower than those in > 50 μm particles. The results indicate that acid polysaccharides, though a minor fraction (~ 1%) of the organic carbon, act more likely as proxy compound classes that might contain the more refractory ²³⁴Th-binding biopolymer, rather than acting as the original ²³⁴Th “scavenger” compound. Moreover, these acid polysaccharides, which might first be produced by phytoplankton and then modified by bacteria, also influence the on-and-off “piggy-back” processes of organic matter and ²³⁴Th, thus causing additional variability of the POC/²³⁴Th in particles of different sizes.     

Benthic community responses to pulses in pelagic food supply: North Pacific Subtropical Gyre

Time-series measurements of particulate organic carbon (POC) and particulate nitrogen (PN) fluxes, sediment community composition, and sediment community oxygen consumption (SCOC) were made at the Hawaii Ocean Time-series station (Sta. ALOHA, 4730 m depth) between December 1997 and January 1999. POC and PN fluxes, estimated from sediment trap collections made at 4000 m depth (730 m above bottom), peaked in late August and early September 1998. SCOC was measured in situ using a free vehicle grab respirometer that also recovered sediments for chemical and biological analyses on six cruises during the 1-year study. Surface sediment organic carbon, total nitrogen and phaeopigments significantly increased in September, corresponding to the pulses in particulate matter fluxes. Bacterial abundance in the surface sediment was highest in September with a subsurface high in November. Sediment macrofauna were numerically dominated by agglutinating Foraminifera fragments with highest density in September. Metazoan abundance, dominated by nematodes was also highest in September. SCOC significantly increased from a low in February to a high in September. POC and PN fluxes at 730 m above bottom were significantly correlated with SCOC with a lag time of ⩽14 days, linking pelagic food supply with benthic processes in the oligotrophic North Pacific gyre. The annual supply of POC into the abyss compared to the estimated annual demand by the sediment community (POC:SCOC) indicates that only 65% of the food demand is met by the supply of organic carbon.     

Upper ocean export of particulate organic carbon and biogenic silica in the Southern Ocean along 170°W

Upper-ocean fluxes of particulate organic carbon (POC) and biogenic silica (bSi) are calculated from four US JGOFS cruises along 170°W using a thorium-234 based approach. Both POC and bSi fluxes exhibit large variability vs. latitude during the seasonal progression of diatom dominated blooms. POC fluxes at 100 m of up to 50 mmol C m⁻² d⁻¹ are found late in the bloom, and farthest south near the Ross Sea Gyre. Biogenic Si fluxes also peak late in the bloom as high as 15 mmol Si m⁻² d⁻¹, but this flux peak occurs at a different latitude, just south of the Antarctic Polar Front (APF), which is centered around 60°S along this cruise track. The ratios of both POC and bSi export relative to their production rates are large, suggesting an efficient biological pump at these latitudes. The highest relative bSi/POC flux ratios at 100 m are found just south of the APF, coincident with a bSi/POC flux peak seen in 1000 m traps during this same program by Deep-Sea Research II (Honjo et al., Deep-Sea Research II 47, 3521–3548). These data suggest that efficient export at these latitudes can support the high accumulation rates of bSi found in the sediments under and south of the APF, despite the generally low biomass and productivity levels in this region.     

Particle flux across the mid-European continental margin

Results are presented from particle flux studies using sediment trap and current meter moorings along a transect at the European continental margin at 49°N within the EU-funded Ocean Margin Exchange (OMEX) project. Two moorings were placed, at the mid- and outer slope in water depths of 1500 and 3660 m, with traps at 600 and 1050 m and at 580, 1440 and 3220 m, respectively. Residual currents at the mid-slope follow the slope contour, whereas seasonal off-slope flow was registered at the outer slope. At 600 m on the slope fluxes are similar to those in the abyssal North Atlantic. The flux of all components (bulk dry weight, particulate organic and inorganic carbon, lithogenic matter and opal) increased with water depth. Highest fluxes were recorded at 1440 m at the outer slope, where off-slope residual currents mediate particle export. The injection of biogenic and lithogenic particles below the depth of winter mixing results in the export of particles from shallower waters. Calculated lateral fluxes of particulate organic carbon exceed the primary flux by over a factor of 2 at 1440 m on the outer slope. Estimated lateral fluxes of suspended particulate matter in the water column and intermediate nepheloid layers at the outer slope are potentially large compared to sinking fluxes measured by sediment traps. A comparison is made of particle flux at three continental margin sites and two sites in the adjacent open North Atlantic, from which it is seen that bulk and organic matter flux increases exponentially with proximity to the shelf break. The percentage contribution of particulate organic carbon to biogenic fluxes increases from a mean of 5.7% in the abyssal N. Atlantic to 13.9% at the continental margins.     

Understanding the export of biogenic particles in oceanic waters: Is there consensus?

We examine progress towards a global view of oceanic export of particulate organic carbon (POC) and other nutrient elements (P, N, Si) from the surface (upper 100 m), through the subsurface, to the deep sea (>1000 m), focusing on syntheses published since 1999 and on the Joint Global Ocean Flux Study. Food-web structure is important, and surface and subsurface processes contribute similarly to determine the fraction of net primary production (NPP) reaching the deep sea. NPP by large cells generally favours high surface export of POC. Preferential remineralization of P and N (versus C) with depth is common, as is regional variation in subsurface POC flux attenuation.The role of mineral fluxes is complex. Annual mean fluxes of POC and minerals are correlated in global deep sediment trap records, but causality and the relative importance of different minerals depends on the assumptions made. Time-series observations at single sites can oppose the geographic trends, and their large seasonal variability in the contribution of POC to total flux is at odds with mechanistic models for POC transport by minerals. Despite generally positive correlations between biogenic carbonate and POC fluxes, the overall role of carbonate export is to decrease the transfer of carbon dioxide from the atmosphere to the ocean. Both autotrophs and heterotrophs produce minerals, and progress in separating these contributions is required for the deconvolution of mineral ballast and food-web effects.Many recent models suggest global surface POC export of ∼10 GTC/yr, despite widely varying biological complexity. This limits the usefulness of their prediction of ecosystem and carbon cycle responses to global change. Progress requires better observations for model validation, and more efforts to relate the models to the observed complexity, rather than to overly simplified global syntheses. We advocate more time-series stations targeting under-studied biogeochemical regions, development of automated in situ tools for study of the subsurface ocean, and increased emphasis on combining ecological and biogeochemical methods.     

Short-lived thorium isotopes (Th, Th) as indicators of POC export and particle cycling in the Ross Sea, Southern Ocean

Repeated measurements of depth profiles of ²³⁴Th (dissolved, 1–70 and >70 μm particulate) at three stations (Orca, Minke, Sei) in the Ross Sea have been used to estimate the export of Th and particulate organic carbon (POC) from the euphotic zone. Sampling was carried out on three JGOFS cruises covering the period from October 1996 (austral early spring) to April 1997 (austral fall). Deficiencies of ²³⁴Th relative to its parent ²³⁸U in the upper 100 m are small during the early spring cruise, increase to maximum values during the summer, and decrease over the course of the fall. Application of a non-steady-state model to the ²³⁴Th data shows that the flux of Th from the euphotic zone occurs principally during the summer cruise and in the interval between summer and fall. Station Minke in the southwestern Ross Sea appears to sustain significant ²³⁴Th removal for a longer period than is evident at Orca or Sei. Particulate ²³⁴Th activities and POC are greater in the 1–70 μm size fraction, except late in the summer cruise, when the >70 μm POC fraction exceeds that of the 1–70 μm fraction. The POC/²³⁴Th ratio in the >70 μm fraction exceeds that in the 1–70 μm fraction, likely due in part to the greater availability of surface sites for Th adsorption in the latter. Particulate ²³⁴Th fluxes are converted to POC fluxes by multiplying by the POC/²³⁴Th ratio of the >70 μm fraction (assumed to be representative of sinking particles). POC fluxes calculated from a steady-state Th scavenging model range from 7 to 91 mmol C m⁻² d⁻¹ during late January–early February, with the greatest flux observed at station Minke late in the cruise. Fluxes estimated with a non-steady-state Th model are 85 mmol C m⁻² d⁻¹ at Minke (1/13–2/1/97) and 50 mmol C m⁻² d⁻¹ at Orca (1/19–2/1/97). The decline in POC inventories (0–100 m) is most rapid in the southern Ross Sea during the austral summer cruise (Smith et al., 2000. The seasonal cycle of phytoplankton biomass and primary productivity in the Ross Sea, Antarctica. Deep-Sea Research II 47, 3119–3140. Gardner et al., 2000. Seasonal patterns of water column particulate organic carbon and fluxes in the Ross Sea, Antarctica. Deep-Sea Research II 47, 3423–3449), and the ²³⁴Th-derived POC fluxes indicate that the sinking flux of POC is 30–50% of the POC decrease, depending on whether steady-state or non-steady-state Th fluxes are used. Rate constants for particle POC aggregation and disaggregation rates are calculated at station Orca by coupling particulate ²³⁴Th data with ²²⁸Th data on the same samples. Late in the early spring cruise, as well as during the summer cruise, POC aggregation rates are highest in near-surface waters and decrease with depth. POC disaggregation rates during the same time generally increase to a maximum and are low at depth (>200 m). Subsurface aggregation rates increase to high values late in the summer, while disaggregation rates decrease. This trend helps explain higher values of POC in the >70 m fraction relative to the 1–70 m fraction late in the summer cruise. Increases in disaggregation rate below 100 m transfer POC from the large to small size fraction and may attenuate the flux of POC sinking out of the euphotic zone.     

Dissolved organic carbon in sediments from the eastern North Atlantic

Profiles of dissolved organic carbon (DOC) were measured in the pore water of sediments from 1000, 2000 and 3500 m water depth in the eastern North Atlantic. A net DOC accumulation in the pore waters was observed, which followed closely the zonation of microbial respiration in these sediments. The concentration of pore water DOC in the zone of oxic respiration was elevated relative to that in the bottom ocean water. The resulting upward gradient across the sediment–water interface indicated a steady state diffusive benthic flux, F_DOC, of 0.25–0.44 mmol m⁻² day⁻¹ from these sediments. Subsequent increase in the concentration of DOC in the pore water occurred only in the sediments from 1000 and 2000 m water depth that supported anoxic respiration, leading to a deep concentration maximum. By contrast, in the sediments from 3500 m water depth, a deep concentration minimum was measured, coincident with minimal postoxic respiration in this near-abyssal setting. The gradient-based F_DOC represented approximately 14% of the total remineralized organic carbon (TCR=sum of F_DOC and depth-integrated organic carbon oxidation rate) in the sediments from 1000 and 2000 m water depth, while it was 36% of the TCR in the sediments from 3500 m water depth. A covariance of particulate organic carbon (POC) and pore water DOC with depth in the sediments was evident, more consistently at the deepest site. While the covariance can be related to biotic processes in these sediments, an alternative interpretation suggests a possible contribution of sorption to the biotic control on sedimentary organic carbon cycling. The steady state diagenetic conditions in which this may occur can be conceivable for some organic-poor deep-sea locations, but direct evidence is clearly required to validate them.     

Export flux of particles at the equator in the western and central Pacific ocean

Export of particles was studied at the equator during an El Nin˜o warm event (October 1994) as part of the French ORSTOM/FLUPAC program. Particulate mass, carbon (organic and inorganic) (C), nitrogen (N), and phosphorus (P) export fluxes were measured at the equator in the western and central Pacific during two 6–7 day-long time-series stations located in the warm pool (TS-I at 0°, 167°E) and in the equatorial HNLC situation (TS-II at 0°, 150°W), using drifting sediment traps deployed for 48 h at four depths (between, approximately, 100 and 300 m).The particulate organic carbon (POC) fluxes at the base of the euphotic zone (0.1 % light level), were approximately four times lower at TS-I than at TS-11 (4.1 vs. 17.0 mmol C m^-2 day^-1). Conversely, fluxes measured at 300 m were similar at both sites (3.6vs. 3.7 mmol C m⁻² day⁻¹ at TS-I and TS-11, respectively). This change in export fluxes was in good agreement with food-web dynamics in the euphotic zone characterized by an increase in plankton biomasses and metabolic rates and a shift towards larger size from TS-1 to TS-II. The POC flux profiles indicated high remineralization (up to 78%) of the exported particles at TS-II, between 100 and 200 m in the Equatorial Undercurrent. According to zooplankton ingestion estimates from 100 – 300 m, 60% of this POC loss could be accounted for by zooplankton grazing. At TS-I, no marked increase of flux with depth was observed, and we assume that loss of particles was compensated by in-situ particle production by zooplankton. Fluxes of particulate nitrogen and phosphorus followed the same general patterns as the POC fluxes. The elemental and pigment composition of the exported particles was not very different between the two stations. In particular, the POCYN flux molar ratio at the base of the euphotic zone was low, 6.9 and 6.2 at TS-1 and TS-II, respectively.For particulate inorganic carbon (mainly carbonate) flux, values at the base of the euphotic zone averaged 0.9 mmol C m-2 day-1 at TS-I and 2.3 mmol C m-2 day-1 at TS-11 (corresponding to a 2.6-fold increase) and showed low depth changes at both stations.POC export flux (including active flux associated with the interzonal migrants) at the 0.1 % light level depth represented only 8% of primary production (1°C uptake) measured at TS-1 and 19% at TS-II. For the time and space scales considered in the present study, new primary production, as measured by the 15N method, was in good agreement with the total export flux in the HNLC situation, thus leading to negligible dissolved organic carbon (DOC) or nitrogen (DON) losses from the photic zone. Conversely, export flux was found to be only 50% (C units) and 60% (N) of new production in the oligotrophic system, either because of an overestimation by the 15N method or of a significant export of DOC and DON.Comparison with other oceanic regions shows that export flux in the warm pool was within the same range as in the central gyres. On the other hand, comparison with EgPac data in the central Pacific suggests that there is no straightforward relation between the magnitude of the export and surface nitrate concentrations.     

Downward fluxes of settling particles in the deep Cretan Sea (NE Mediterranean)

During the CINCS project (Pelagic–benthic Coupling IN the oligotrophic Cretan Sea—NE Mediterranean), a single mooring with two sediment traps (at 200 and 1515m water depth) and two current meters was deployed in the southern Cretan Sea margin at a depth of 1550 m. A second mooring deployed at the 500 m station was lost, as a result of fishing activities. The duration of the study was 12 months (November 1994 to November 1995) with sampling intervals of 15 or 16 days. The traps were retrieved, serviced and the sedimented material was collected every 6 months. In total, 48 samples were collected (24 from each trap) throughout the study period and fluxes of total particulate mass, opal, organic matter, carbonates, and lithogenic component were measured. Natural radionuclides (²¹⁰Po and ²¹⁰Pb) were determined for all trap samples. Total mass flux and the fluxes of four major constituents increased with depth, the total mass flux reaching values of nearly 550 mg m⁻² d⁻¹ at 1515 m and 187 mg m⁻² d⁻¹ at 200 m depth, following the same patterns observed in other experiments (ECOMARGE, SEEP-I, SEEP-II). The mean annual mass fluxes were 209 and 49.8 mg m⁻² d⁻¹ at the near bottom and near surface trap respectively. This suggests that lateral transport of particulate matter is of importance in the area. Total mass fluxes at the two depths were characterized by different seasonal fluctuations, although a general decreasing trend was observed from the I (winter) to the II (summer) deployment at both depths. This was mainly a result of reductions in aluminosilicate inputs during the summer dry period. At 200 m depth carbonates were more important during winter, because of a large carbonate input consisting mainly of coccoliths of Emiliania huxleyi, while during the summer decreased fluxes of carbonates and aluminosilicates resulted in a reduction of the mass flux. In contrast, at 1515 m depth the lithogenic component was the dominant component during the winter deployment, indicating a terrigenous input. During the summer period the decrease in mass flux was strongly effected by the decrease in aluminosilicates. There was a diminution in the organic carbon content with a concomitant increase in total mass flux, which, together with the almost negligible increase in the annual ²¹⁰Pb activity with depth and the increase of ²¹⁰Po activity with depth could be interpreted as indicating a contribution of resuspended material to the input at 1515 m. The complex mesoscale circulation of the Cretan Sea, consisting of a cyclone (east)–anticyclone (west) system, controls particle transfer in the area. This hydrodynamic system seems to move water masses towards the southern Cretan Sea margin, and consequently carry materials from the open sea to the upper slope and shelf.     

Was a carbon balance measured in the equatorial Pacific during JGOFS?

The likelihood that the carbon fluxes measured as part of the US-JGOFS field program in the equatorial Pacific ocean (EgPac) during 1992 yielded a balanced carbon budget for the surface ocean was determined. The major carbon fluxes incorporated into a surface carbon budget were: new production, particulate organic carbon (POC) and dissolved organic carbon (DOC) export, CaC0₃ export, C0₂ gas evasion, dissolved inorganic carbon (DIC) supply, and the time rate of charge. The ratio of the measured concentration gradients of DOC and DIC provided a constraint on the ratio of POC/DOC export. Uncertainties of ±30–50% for individual carbon flux measurements reduce the likelihood that a carbon balance can be measured during a JGOFS process-type study. As a benchmark, carbon fluxes were prescribed to yield a hypothetical surface carbon budget that was, on average, balanced. Given the typical errors in the individual carbon fluxes, however, there was only about a 30% chance that this hypothetical budget could be measured to be balanced to ±50%. Using this benchmark, it was determined that there was a 95 % chance that the carbon flux measurements yielded a surface DIC budget balanced (to ±50%) during El Nino conditions in boreal spring 1992, when the total organic carbon export rate was - 5 mmol C m-2 day- 1 and the POC export was 3 mmol C m⁻² day⁻¹. In boreal fall 1992, during cold period conditions, there was a 70% chance that the surface carbon DIC budget was balanced when the total organic carbon export rate was 20 mmol C m⁻² day⁻¹ and export was -13 mmol C m-2 day-'. The DOC to DIC concentration gradient ratio of - -0.15, measured in depth profiles down to 100m and in surface waters, was used as an important constraint that most (> 70%) of the organic carbon exported from the euphotic zone was POC rather than DOC. If a balanced surface DIC budget was used to test the compatibility of individual carbon fluxes measured during EgPac, then a three- to four-fold increase in total and particulate organic carbon export between spring and fall is indicated. This increase was not reflected in the POC loss rates measured by drifting sediment trap collections or estimated by²³⁴Th deficiencies coupled with the C/Th measured on suspended particles.     

Dynamics of particle export on the Northwest Atlantic margin

The Northwest Atlantic margin is characterized by high biological productivity in shelf and slope surface waters. In addition to carbon supply to underlying sediments, the persistent, intermediate depth nepheloid layers emanating from the continental shelves, and bottom nepheloid layers maintained by strong bottom currents associated with the southward flowing Deep Western Boundary Current (DWBC), provide conduits for export of organic carbon over the margin and/or to the interior ocean. As a part of a project to understand dynamics of particulate organic carbon (POC) cycling in this region, we examined the bulk and molecular properties of time-series sediment trap samples obtained at 968 m, 1976 m, and 2938 m depths from a bottom-tethered mooring on the New England slope (water depth, 2988 m). Frequent occurrences of higher fluxes in deep relative to shallower sediment traps and low Δ¹⁴C values of sinking POC together provide strong evidence for significant lateral transport of aged organic matter over the margin. Comparison of biogeochemical properties such as aluminum concentration and flux, and iron concentration between samples intercepted at different depths shows that particles collected by the deepest trap had more complex sources than the shallower ones. These data also suggest that at least two modes of lateral transport exist over the New England margin. Based on radiocarbon mass balance, about 30% (±10%) of sinking POC in all sediment traps is estimated to be derived from lateral transport of resuspended sediment. A strong correlation between Δ¹⁴C values and aluminum concentrations suggests that the aged organic matter is associated with lithogenic particles. Our results suggest that lateral transport of organic matter, particularly that resulting from sediment resuspension, should be considered in addition to vertical supply of organic matter derived from primary production, in order to understand carbon cycling and export over continental margins.     

Effects of the copepod community structure on fecal pellet flux in Kagoshima bay,a deep,semi-enclosed embayment

Seasonal changes in the shape and size composition of fecal pellets were investigated with sediment trap samples from 50 and 150 m in Kagoshima Bay to evaluate how the mesozooplankton community affects fecal pellet flux. Deep vertical mixing was evident in March, and thermal stratification was developed above 50 m in June, August and November. Chlorophyll a, suspended particulate organic carbon (POC) and copepod abundance were uniform throughout the water column during the seasonal mixing and concentrated above 50 m in the stratified seasons. Calanoids were the most predominant copepods in March and poecilostomatoids composed more than 45% of the copepod community in June, August and November. Fecal pellet fluxes at 50 and 150 m were the highest in March, nearly half of POC flux. The relative contribution declined considerably in the other months, especially for less than 4% of POC flux in August. The decline was corresponded to the predominance of cyclopoids and poecilostomatoids. Cylindrical pellets dominated the fecal matters at both depths throughout the study period, while larger cylindrical pellets nearly disappeared at 150 m in June, August and November. Copepod incubation revealed that cylindrical and oval pellets were egested by calanoids and the other copepods, respectively. We suggest that cylindrical fecal pellets produced by calanoid copepods contribute to feces flux but the predominance of poecilostomatoids and/or cyclopoids decreases feces flux via the increase of oval pellets and fragmentation of larger cylindrical pellets.     

Particulate carbon and nitrogen fluxes and compositions in the central equatorial Pacific

Mass, carbon, and nitrogen fluxes and carbon and nitrogen compositions were determined for particulate samples from plankton net tows, shallow floating sediment traps, intermediate and deep moored sediment traps, and sediment cores collected along 140°W in the central equatorial Pacific Ocean during the US JGOFS EqPac program. Mass, particulate organic carbon (POC), and particulate inorganic carbon (PIC) fluxes measured by the floating sediment traps during the Survey I (El Niño) and Survey II (non-El Niño) cruises follow essentially the same pattern as primary production: high near the equator and decreasing poleward. POC fluxes caught in free-floating traps were compared with alternative estimates of export fluxes, including ²³⁴Th models, new production, and other sediment trap studies, resulting in widely differing estimates. Applying ²³⁴Th corrections to the trap-based fluxes yielded more consistent results relative to primary production and new production. Despite factors of five differences in measured fluxes between different trap types, POC : ²³⁴Th ratios of trap material were generally within a factor of two and provided a robust means of converting modeled ²³⁴Th export fluxes to POC export fluxes. All measured fluxes decrease with depth. Trap compositional data suggest that mineral “ballasting” may be a prerequisite for POC settling. POC remineralization is most pronounced in the epipelagic zone and at the sediment–water interface, with two orders of magnitude loss at each level. Despite seawater supersaturation with respect to calcium carbonate in the upper ocean, 80% of PIC is dissolved in the epipelagic zone. Given the time-scale differences of processes throughout the water column, the contrasting environments, and the fact that only 0.01% of primary production is buried, sedimentary organic carbon accumulation rates along the transect are remarkably well correlated to primary production in the overlying surface waters. POC to particulate total nitrogen (PTN) ratios for all samples are close to Redfield values, indicating that POC and PTN are non-selectively remineralized. This constancy is somewhat surprising given conventional wisdom and previous equatorial Pacific results suggesting that particulate nitrogen is lost preferentially to organic carbon.     

Controls on the organic chemical composition of settling particles in the Northeast Atlantic Ocean

The organic matter of sinking particulate material collected in the Northeast Atlantic Ocean (ca. 49°N, 16°W) was investigated in order to determine temporal and depth-related variability in its composition. Three sediment traps were deployed at nominal depths of 1000 m (below the permanent thermocline), 3000 m (representing the deep-water fluxes) and at 4700 m, about 100 m above the seafloor (just above the benthic boundary layer). The samples span a 28-month sampling period from October 1995 until February 1998, each sample representing a period of between 7 and 28 days.Total organic carbon and total nitrogen contents decrease with depth, as did the absolute concentrations of most biochemicals measured in this study, such as intact proteins and individual lipids. However, concentrations of proteins relative to total organic carbon and total nitrogen did not show any significant change with depth, implying that they are not being rapidly degraded and so may provide an important supply of nitrogen to the benthos. Fluxes of protein, TN and TOC are significantly correlated at all depths.Lipid compositions vary temporally. During periods of high flux, particularly in the summer, the lipids are richer in ‘labile components’, namely unsaturated fatty acids and low molecular weight alcohols. During periods of low flux other compounds, such as sterols, steroidal ketones and a trisnorhopan-21-one are more abundant. One sample, taken close to the seafloor, was highly enriched in lipids, sterols and fatty acids in particular; this may represent detritus derived from bottom-dwelling invertebrates.     

The effect of pressure on leucine and thymidine incorporation by free-living bacteria and by bacteria attached to sinking oceanic particles

The effect of pressure on upper ocean free-living bacteria and bacteria attached to rapidly sinking particles was investigated through studying their ability to synthesize DNA and protein by measuring their rate of ³H-thymidine and ³H-leucine incorporation. Studies were carried out on samples from the NE Atlantic under the range of pressures (1–430 atm) encountered by sinking aggregates during their journey to the deep-sea bed. Thymidine and leucine incorporation rates per bacterium attached to sinking particles from 200 m were about six and ten times higher, respectively, than the free-living bacterial assemblage. The ratio of leucine incorporation rate per cell to thymidine incorporation rate per cell was significantly different between the larger attached (18.9:1) and smaller free-living (10.4:1) assemblages. The rates of leucine and thymidine incorporation decreased exponentially with increasing pressure for the free-living and linearly for attached bacteria, while there was no significant influence of pressure on cell numbers. At 100 atm leucine and thymidine incorporation rate per free-living bacterium was reduced to 73 and 20%, respectively, relative to that measured at 1 atm. Pressure of 100 atm reduced leucine and thymidine incorporation per attached bacterium to 94 and 70%, and at 200 atm these rates were reduced to 34 and 51%, respectively, relative to those measured at 1 atm. There was no significant uncoupling of thymidine and leucine incorporation for either the free-living or attached bacterial assemblages with increasing pressure, indicating that the processess of DNA and protein synthesis may be equally affected by increasing pressure. It is therefore unlikely that bacteria, originating from surface waters, attached to rapidly sinking particles play a role in particle remineralization below approximately 1000–2000 m. These results may help to explain the occurrence of relatively fresh aggregates on the deep-sea bed that still contain sufficient organic carbon to fuel the rapid growth of benthic micro-organisms; they also indicate that the effect of pressure on microbial processes may be important in oceanic biogeochemical cycles.