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.     

Dynamics of carbohydrates in the Norwegian Sea inferred from monthly profiles collected during 3 years at 66°N, 2°E

Water samples were collected monthly for 3 years at 66°N, 2°E in the Norwegian Sea, 250 nautical miles off the Norwegian coast. Concentrations of mono- and polysaccharides were measured with the 2,4,6-tripyridyl-s-triazine (TPTZ) spectroscopic method. Total dissolved carbohydrates varied from 3.4 to 28.2 μM C of all samples and the ratio of carbohydrate to dissolved organic C (DOC) varied from an average of 14% at 0–25 m depth to 11% at 800–2000 m depth. This indicates that dissolved carbohydrates were a significant constituent of DOC in the Norwegian Sea. Polysaccharides varied from 0.4 to 21.5 μM C and monosaccharides from 0.7 to 11.7 μM C at all depths. The level of monosaccharides was relatively constant at 2.8–3.2 μM C below the euphotic zone, whereas polysaccharides showed more varying concentrations. Dissolved carbohydrates accumulated during the productive season, reaching maximum concentrations during summer although interannual differences were observed. A significant positive correlation between Chl a and soluble carbohydrate was found in one growing season with nutrient analyses. Average values for total carbohydrates were highest in the surface – 0 to 25 m – with 13.3 μM C and decreased to 8.4 μM C at 800–2000 m depth. The ratio of monosaccharides to polysaccharides exhibited a marked seasonal variation, increased from January to a maximum in June of 1.1, and declined to 0.5 in July.     

Purine and pyrimidine bases in marine particles in the Seto Inland Sea, Japan

Purine and pyrimidine bases in marine environmental particles collected in Harima-Nada, the Seto Inland Sea, Japan, were investigated by high performance liquid chromatography.Purines and pyrimidines concentrations varied from 0.3 to 9.3 μg l⁻¹ (n=20) for suspended matter, and 0.3 to 0.6 mg g⁻¹ (n=10) for sinking particles. A good correlation was found between chlorophyll a and purine+pyrimidine bases in suspended matter, indicating that these bases contained in suspended matter originated from phytoplankton.A comparison between several compositional data of the suspended matter and the sinking particles, namely CN ratio, composition of purines and pyrimidines, and percentages of the nitrogen bases relative to total particulate organic nitrogen, demonstrates that the sinking particles were different from suspended matter. Also, from the variety of purine and pyrimidine concentrations of marine particle samples, it was estimated that the decomposition rate of these bases seemed more rapid than decomposition rates of amino acids reported in our earlier study.     

Distribution of sterol and fatty alcohol biomarkers in particulate matter from the frontal structure of the Alboran Sea (S.W. Mediterranean Sea)

Sterol and fatty alcohol biomarkers were analyzed in suspended and sinking particles from the water column (20–300 m) of the Almeria–Oran frontal zone to characterize the biogenic sources and biogeochemical processes. Diatom- and haptophyte-related sterols were predominant at all sites and vertical distributions of sterol, and fatty alcohol biomarkers in sinking particles were markedly different from suspended particles. In contrast to the relatively fresh sinking particles with elevated concentrations of phyto- and zooplanktonic sterols, suspended particles were extensively degraded with increasing depth and exhibited a more terrestrial and zooplanktonic signature with depth.Sterol and alcohol biomarkers distributions and δ¹³C values from the jet core and the associated gyre of Atlantic waters showed a decoupling between the sinking particles of 100- and 300-m depth, demonstrating the influence of lateral advection in the frontal zone. In contrast, vertical transport of the particulate organic matter in Mediterranean waters was interpreted from the similar isotopic and molecular composition at both depths. The high abundance of phytosterols and phytol below the euphotic zone at 100 m signified that downwelling of biomass occurred on the downstream side of the gyre. The high concentrations of phytosterols and POC, in combination with the high phytosterols/phytol ratio, indicated the accumulation of detrital plant material in the oligotrophic Mediterranean waters near the frontal zone.A higher contribution of phytol in the sinking particles collected during the night at the surface of the jet and at the upstream side of the gyre provided evidence of diel vertical zooplankton migration and important grazing by herbivorous zooplankton.Carbon isotope ratios of sterols confirmed that the 24-ethylcholest-5-en-3β-ol, commonly associated with terrestrial sources, was a substantial constituent of the phytoplankton in this area. However, the more δ¹³C depleted values obtained for this compound in suspended particles suggested that there was some terrestrial contribution that only becomes evident after degradation of the more labile marine organic matter.     

Grazing impact of mesozooplankton in an upwelling region off northern California, 2000–2003

Mesozooplankton (>200 μm) grazing impact (% phytoplankton standing crop consumed d⁻¹) was determined by the gut fluorescence method during three springs (2000, 2001 and 2002) and two winters (2002 and 2003) in a coastal upwelling region off northern California. Wind events, in terms of both magnitude and duration, varied inter-annually and seasonally and included both upwelling-favorable and relaxation events. Grazing impact of mesozooplankton also varied inter-annually and seasonally, and was highest during June 2000 (mean=129% of standing crop d⁻¹), a prolonged period of wind “relaxation” and phytoplankton bloom. In contrast, mean grazing impact was lower during periods of stronger, more persistent winds, more active upwelling, greater cross-shelf transport, and lower chlorophyll concentration (25% and 38% in May–June 2001 and 2002, respectively). Wintertime conditions (January 2002 and 2003) were characterized by weakly upwelling or downwelling-favorable winds, low chlorophyll concentration, and lower mean mesozooplankton grazing impact (13% and 12%, respectively). The larger (>500 μm) size class contributed proportionally more to total mesozooplankton (>200 μm) grazing impact than the smaller (200–500 μm) size class during all sampling periods except spring 2002. These results suggest that mesozooplankton grazing impact is higher in spring than in winter, and that during the spring upwelling season, grazing is higher during periods of wind relaxation (weak upwelling) than during periods of stronger upwelling. Further, these results suggest an important role of mesozooplankton grazers on phytoplankton dynamics in the upwelling region off northern California.     

High potential activity of extracellular alkaline phosphatase in deep waters of the central Pacific

We found high potential activities of alkaline phosphatase associated with particles (0.2 μm or greater size fraction) in deep waters (1000–4000 m) of the central Pacific Ocean. The potential enzyme activity at depth (0.03 – 0.3 nM h⁻¹) was up to 50% of that at the surface (0–125 m). In contrast, activities of α- and β-d-glucosidase in the deep layer were low (generally less than 1 % of those in the upper layer), yielding up to two orders of magnitude difference in the ratio of alkaline phosphatase and α- and β-d-glucosidase activities with depth. It is unlikely that the phosphatase is actively produced by microorganisms inhabiting the deep-sea environment, where labile organic carbon supply is limited and phosphate concentration is high (2.4 – 3.0 AM). Instead, deep-water phosphatase is probably supplied by rapidly sinking particles and their subsequent fragmentation and dissolution. Different distributions of phosphatase and glucosidase indicate that sinking particles of phytoplankton origin are an important source of alkaline phosphatase enzymes in the deep sea.     

Microbial respiration in the aphotic zone of the Ross Sea (Antarctica)

In January–February 2001, we measured microbial biomass as ATP and community respiration as ETS activity of organisms < 200 μm in the aphotic zone of the Ross Sea. Microbial respiration amounted to 2.14 mmol C m^− 2 day^− 1 in the depth range 200–1000 m. Our daily estimates of carbon export are close to the daily percentage of net community production (NCP), removed as sinking biogenic particles from the upper 100 m in the entire Ross Sea, but lower than those of other oceanic systems. Comparing remineralization determined in this study with that obtained by sediment traps in the Ross Sea, it appeared that about 63% of organic carbon remineralized by respiration derived from POC pool. Such evidence highlighted POC source as the main organic fuel of the biological pump in the Ross Sea.     

Effects of hydrostatic pressure on microbial alteration of sinking fecal pellets

We used a new experimental device called PASS (PArticle Sinking Simulator) during MedFlux to simulate changes in in situ hydrostatic pressure that particles experience sinking from mesopelagic to bathypelagic depths. Particles, largely fecal pellets, were collected at 200 m using a settling velocity NetTrap (SV NetTrap) in Ligurian Sea in April 2006 and incubated in high-pressure bottles (HPBs) of the PASS system under both atmospheric and continuously increasing pressure conditions, simulating the pressure change experienced at a sinking rate of 200 m d⁻¹. Chemical changes over time were evaluated by measuring particulate organic carbon (POC), carbohydrates, transparent exopolymer particles (TEP), amino acids, lipids, and chloropigments, as well as dissolved organic carbon (DOC) and dissolved carbohydrates. Microbial changes were evaluated microscopically, using diamidinophenylindole (DAPI) stain for total cell counts and catalyzed reporter deposition-fluorescence in situ hybridization (CARD-FISH) for phylogenetic distinctions. Concentrations (normalized to POC) of particulate chloropigments, carbohydrates and TEP decreased under both sets of incubation conditions, although less under the increasing pressure regime than under atmospheric conditions. By contrast, dissolved carbohydrates (normalized to DOC) were higher after incubation and significantly higher under atmospheric conditions, suggesting they were produced at the expense of the particulate fraction. POC-normalized particulate wax/steryl esters increased only under pressure, suggesting biochemical responses of prokaryotes to the increasing pressure regime. The prokaryotic community initially consisted of 43% Bacteria, 12% Crenarchaea and 11% Euryarchaea. After incubation, Bacteria dominated (90%) the prokaryote community in all cases, with γ-Proteobacteria comprising the greatest fraction, followed by the Cytophaga–Flavobacter cluster and α-Proteobacteria group. Using the PASS system, we obtained chemical and microbial evidence that degradation by prokaryotes associated with fecal pellets sinking through mesopelagic waters is limited by the increasing pressure they experience.     

Adsorption of dissolved phosphate onto loess particles in surface and deep Eastern Mediterranean water

The objective of this study was to examine whether dissolved inorganic phosphate (DIP) is removed from the Eastern Mediterranean Sea by adhering to atmospherically deposited loess particles sinking through the water column. In a series of radiolable experiments, loess from the Negev Desert, treated in various ways, was added to surface (SSW) and deep (DSW) seawater spiked with ³²PO₄⁻³. It was shown that when fresh loess reaches the Mediterranean SSW approximately 1.3 μmol P/g are released (11% of the total P concentration). Biological activity and inorganic particles removed similar amounts of the tracer (30–40%) from SSW. It was estimated that about 0.2 μmol P/g of ‘aged loess' (proxy of particles sinking into DSW), were removed from poisoned SSW and DSW, while there was minor adsorption when either nothing or quartz powder was added. The adsorbed DIP accounts for approximately 15% of the released P and is equivalent to about 2% of the remaining P (11.17 μmol P/g loess). Therefore, the process of DIP removal by atmospherically derived particles exists, but due to the higher release of P, the result is a net addition of dissolved atmospheric phosphorus in seawater. It is postulated that in actual SSW where dust concentrations are much lower, biological uptake out-competes inorganic adsorption, although it was demonstrated that the decrease in loess particle concentrations tends to increase their adsorption capacity. As the loess descends into the DSW it continues to remove DIP and thus transports phosphate from the water column to the sediments. A preliminary quantitative estimate suggests that the process of DIP removal by loess particles cannot explain the phosphate ‘deficit' in the Eastern Mediterranean DSW characterized by unusually high DIN/DIP ratios (27) reported by others.     

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.     

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.     

Investigation of Cd, Cu, Ni and Th in the colloidal size range in the Gulf of Maine

Size-fractionated seawater samples were collected from the Gulf of Maine to determine the fraction (f_c/d) of total dissolved (< 1 μm) Cd, Cu, Ni and ²³⁴Th in the colloidal size range (1,000 nominal molecular weight, NMW, to 1 μm) using cross-flow filtration. Colloidal Cd, Cu and Ni represents < 1–7% of the total dissolved concentration in these shelf waters and increases with an increase in particle concentration. By comparison, results obtained for particle-reactive ²³⁴Th indicate that < 1–47% of total dissolved is associated with the colloidal size fraction. A revised relationship between the concentration of colloids (C_c) and suspended particles (C_p) is reported (log C_c = 0.66 log C_p −2.01 kg L⁻¹) and used to examine the dependence of f_c/d for these metals on the concentration of suspended particles for C_p = 0.01–100 mg L⁻¹. Results indicate that a significant fraction (˜ 10–30%) of Cd, Cu, Ni and ²³⁴Th in the traditionally defined ‘dissolved’ fraction may exist in the colloidal size range in regions characterized by high particle concentrations (C_p > 1–10 mg L⁻¹), such as in near-shore and estuarine waters.     

Seasonal and Spatial Variations of Total Mass Flux around Coral Reefs in the Southern Ryukyus,Japan

Total mass flux, size distribution of sediment particles and some chemical components such as total carbon (TC), total nitrogen (TN) and calcium carbonate (CaCO₃) were monitored monthly using a multi-cup sediment traps at seven coral reef sites (6 reef flat and 1 reef slope) of the Marine Protected Areas around Ishigaki, Kohama, Kuroshima and Iriomote Islands in the southern Ryukyus, Japan from September 2000 to September 2001. The size distribution of trapped sediments revealed mostly uni-modal fine sand to mud in the reef flat and gravelly to coarse sand in the reef slope. The total mass flux ranged between 0.54 to 872 gm⁻²d⁻¹, and showed a pronounced seasonality (high in summer-autumn and low in spring) at each site, which was consistent with the rainfall and typhoon regime. Exceptionally high values were observed on the reef slope (Iriomote) in February–March 2001 (1533 gm⁻²d⁻¹) owing to a large amount of bottom sediment re-suspension. On the reef flat (Todoroki South and North; Ishigaki), values obtained in July–August 2001 (872 gm⁻²d⁻¹) and August–September 2001 (800 gm^{− 2}d⁻¹) indicate the high terrestrial discharge from Todoroki River. Trapped sediment particles consist of CaCO₃ (1.2–27.1%) and a non-carbonate fraction (98.8–72.9%), which contains total carbon (4.9–26%), carbonate carbon (CO₂-C) (0.2–3.1%) and non-carbonate carbon (NC-C) (7.9–25.6%). Total nitrogen content was in the range 0.02–0.48%. TN is contained mainly in the carbonate fraction and NC-C may be contained in the non-carbonate fraction. The low TN/OC ratio of the trapped sediments suggests that they were mostly of terrestrial origin and that both fractions migrated. The high total mass flux derived from Todoroki River exceeded the threshold at which a lethal effect on coral community is caused. The results stress the importance of conducting seasonal studies of sedimentation over more than one year and at more than one location in south Japan coral reef ecosystems to gain an understanding of the processes controlling the total mass fluxes and their nutrients content, also to develop an awareness of how to prevent the damage of coral reef ecosystems and, if it does occur, to allow mitigation measures to be undertaken.     

Distributions of carbohydrates, including uronic acids, in estuarine waters of Galveston Bay

The concentrations of carbohydrates, including uronic acids, in dissolved (≤0.45μm) and colloidal (1 kDa—0.45 μm) phases were measured in estuarine waters of Galveston Bay, TX, in order to study their role in heavy metal detoxification. The concentrations of dissolved monosaccharides (MCHO) in Galveston Bay ranged from 13 to 62 μM-C, and those of dissolved polysaccharides (PCHO) ranged from 10 to 42 μM-C. On average, MCHO and PCHO contributed about 11% and 7% to dissolved organic carbon (DOC), respectively. The colloidal carbohydrates (CCHO) in Galveston Bay varied from 7 to 54 μM-C, and accounted for 9% to 24% of the colloidal organic carbon (COC), with an average value of 17%, suggesting that CCHO is abundant in the high molecular weight (HMW) fraction of DOC. The concentration of CCHO is generally significantly higher than that of PCHO. This result is attributed to entrainment of low molecular weight (LMW) carbohydrates into the retentate fraction during ultrafiltration. The concentration of total dissolved uronic acids (DUA) in the same samples varied from 1.0 to 8.3 μM-C, with an average value of 6.1 μM-C, while the colloidal uronic acids (CUA) ranged from 0.8 to 6.4 μM-C, with an average value of 4.8 μM-C. The concentrations of DUA are higher than the previously reported values in coastal waters. Furthermore, CUA represent a dominant component of DUA in Galveston Bay waters. More importantly, significant correlations of PCHO and DUA to dissolved Cu concentrations (≤0.45 μm) were found, suggesting that acid polysaccharides were produced in response to trace metal stressors.     

Effect of iron enrichment on the dynamics of transparent exopolymer particles in the western subarctic Pacific

Dynamics of transparent exopolymer particles (TEP) was studied during the first in situ iron-enrichment experiment conducted in the western subarctic Pacific in July–August 2001, with the goal of evaluating the contribution of TEP to vertical flux as a result of increased primary production following iron enrichment in open ocean ecosystems. Subsequent to the enhancement of phytoplankton production, we observed increase in TEP concentration in the surface layer and sedimentation of organic matter beneath it. Vertical profiles of TEP, chlorophyll a (Chl a) and particulate organic carbon (POC) were obtained from six depths between 5 and 70 m, from a station each located inside and outside the enriched patch. TEP and total mass flux were estimated from the floating sediment traps deployed at 200 m depth. Chl a and TEP concentrations outside the patch varied from 0.2 to 1.9 μg L⁻¹ and 40–60 μg XG equiv. L⁻¹, respectively. Inside the patch, Chl a increased drastically from day 7 reaching the peak of 19.2 μg L⁻¹ on day 13, which coincided with the TEP peak of 189 μg XG equiv. L⁻¹. TEP flux in the sediment trap increased from 41 to 88 mg XG equiv. m⁻² d⁻¹, with 8–14% contribution of TEP to total mass flux. This forms the basic data set on ambient concentrations of TEP in the western subarctic Pacific, and evaluation of the effect of iron enrichment on TEP.     

Distributions and upward fluxes of particulate matter near bottom in the southeastern Bering Sea Shelf

Time-series measurements of temperature, salinity, suspended matter and beam attenuation coefficient () were measured at four hour intervals for about two days in June/ July 1982 in the middle shelf region and the coastal region of the southeastern Bering Sea. Current meters were also moored at the same locations.Depth-time distributions of indicated that profiles of suspended matter resulted from a combined process of resuspension of underlying sediments and sinking of suspended particles. Average-values for all measurements for particles revealed that the upward transport of particles due to resuspension formed a boundary layer, with a thickness apparently related to scalar speed. The average-profiles of the particle volume concentration were assumed to result from a balance between the sinking and diffusive flux of particles under a steady state, and the upward fluxes were calculated. Within the boundary layer, values of the upward fluxes of particulate organic matter linearly decreased with the logarithm of distance from the bottom. Fluxes of organic carbon at the upper edge of the boundary layer were 0.375 gC·m^–2·day^–1 in the middle shelf region (18 m above the bottom, bottom depth=78m) and 0.484gC·m^–2·day^–1 in the coastal region (25 m above the bottom, bottom depth=33m), and fluxes of nitrogen in both regions were 0.067 gN·m^–2·day^–1. The flux of organic carbon obtained in the middle shelf region (18 m above the bottom) agreed approximately with the flux (0.416 gC·m^–2·day^–1) calculated by substituting primary production data into the empirical equation of Suess (1980).     

Distribution and sinking rates of phytoplankton, detritus, and particulate biogenic silica in the Laptev Sea and Lena River (Arctic Siberia)

Concentrations and sinking rates of particulate biogenic silica (BSi), chlorophyll a (chl a) and phaeopigments (phae) (< 3 μm, 3–10 μm, > 10 μm and total), as well as the abundances of the major phytoplankton species, were studied during September 1991 in the Eastern Laptev Sea and the lower Lena River (Siberian Arctic). The highest chl a concentrations were found in two major “new” production regimes of the study area: (1) a deep chl a maximum (5.8 mg chl a m⁻³) (formed by the diatom Chaetoceros socialis) at 30 m depth on the outer shelf of the northern Laptev Sea, and (2) in the Lena River, where the phytoplankton community was dominated by fresh water diatoms (1.5 to 4.5 mg chl a m⁻³). Elevated chl a concentrations were also found in the river plume phytoplankton community (dominated by brackish water diatoms), NE of the Lena delta. In the Laptev Sea, the low chl a (0.1 to 3 mg chl a m⁻³) and high phae concentrations (0.5 to 14 mg phae m⁻³) indicated that the phytoplankton community (dominated by picoplanktic algae and nanoflagellates) was already senescent and affected by grazing losses. Biogenic silica values were highest in the Lena River (4 to 17 μM) as compared to the low values found in the Laptev Sea (0.3 to 4 μM). The large chl a size fraction, phae and BSi in the Lena River samples revealed the highest measured sinking rates (1.4, 2.3, and 1.5 m d⁻¹, respectively). The formation of a strong halocline, decreasing turbulence, and possible nutrient deficiency resulted in death, disintegration and rapid sedimentation of fresh water diatoms. This was accompanied by a decrease in the BSi concentration and growth of the picoplanktic size fraction (< 3 μm) in the estuarine mixing zone (Gulf of Buorkhaya). Only a minor part of BSi was bound to intact diatom cells (< 3%) in the surface layer, most of which being apparently associated with detrital particles. In the Lena River, approximately 12% of the total silica was bound to BSi fraction, yet elsewhere in the Laptev Sea and in the estuarine mixing zone the BSi:total silica ratio was ≤ 5%. Thus, the results reflected the successional stage of a late summer phytoplankton community, characterized by dominance of small autotrophs and patchy distribution of senescent diatoms no longer able to affect the relative high levels of dissolved silica supplied by the Lena River.     

Th isotopes in the Santa Monica basin: Temporal variation,long-term mass balance and model rate constants

Distribution and flux of²³⁴Th,²³²Th and²³⁰Th in the water column of central Santa Monica basin observed over a period of seven years show seasonal and interannual variabilities. A steady-state model is applied to the integrated data to calculate long term average flux and model rate constants of Th isotopes. Mass balance calculations show that the basin acts like a closed system for short-lived²³⁴Th, but not for the long-lived isotopes²³⁰Th and²³²Th. Most²³⁰Th in the basin is transported from elsewhere. Of the incoming Th, 40–55% of the²³⁰Th and 14–26% of the²³²Th enter the surface water in dissolved form. In the upper 100 m, the residence time of dissolved Th with respect to adsorption onto suspended particulates, 70–80 days, is about one order of magnitude higher than the residence time of suspended particles with respect to aggregation into sinking particles, 7–10 days.     

Geochemistry of suspended matter from the Baltic Sea 2. Results of bulk trace metal analysis by AAS

In 1984, on a transect covering the whole Baltic Sea and parts of the adjacent North Sea, 160 water samples were taken and analysed for their concentrations of particulate and dissolved metals. In addition, the suspended materials were investigated for their elemental bulk composition.The particulate fractions represented from about 5% (Cd, Cu and Ni) to 50% (Fe and Pb) of the total (particulate plus dissolved) concentrations. For some elements (Ba, Cd, Cu, Pb and Zn), the particulate matter from the surface microlayer was enriched with respect to those suspended materials taken from 0.2 m depth. This could reflect the atmospheric input of metal-rich aerosols. In anoxic deep waters, maximum contents of Zn (6400 μg g⁻¹), Cu (1330 μg g⁻¹) and Cd (12 μg g⁻¹) were observed in the particulate matter, indicating sulphidic forms. On the other hand, under oxic conditions the distribution coefficients (K_d) decreased with the water depth (Cd, Fe and Pb).Relative to global background levels, the particulate matter contained metal “excesses” amounting to more than 90% of the total contents (Cd, Mn, Pb and Zn). Automated electron probe X-ray microanalysis (EPXMA) revealed that the elemental composition of sediments is mainly governed by post-depositional processes of early diagenesis and is only weakly related to the composition of suspended matter in the overlying water body. For instance, in relation to surface mud sediments of the central Baltic net-sedimentation basins, Zn, Cd, Cu and Mn had 30–100% higher levels in the suspended materials. The general pattern of metal contents of particulate matter taken from 10 m depth on a transect between the Bothnian Bay and the North Sea were—possibly as a result of anthropogenic inputs—rather similar for Pb, Zn and Cu. For Fe and Mn, the distribution patterns along the transect were probably governed by the natural loading characteristics and by the biogeochemistry of those elements.     

Particle-bound phosphorus along an urbanized coastal plain estuary

The distribution of particle-bound phosphorus in the suspended sediment of the Delaware Estuary was examined with a sequential chemical leaching technique. The phosphorus content of particles was highest in the tidal river (140–250 μmol g⁻¹) near major anthropogenic inputs. Despite this enrichment of river particles with phosphorus, suspended particles within the salinity gradient had a phosphorus content more similar to the world's average. Sequential chemical leaches revealed that particulate phosphorus was associated with organic matter, aluminum oxides, iron oxides, and apatite in all areas of the estuary. However, ‘excess’ particle-bound phosphorus in the tidal river was associated mainly with iron oxides (27%), aluminum oxides (23%), and organic matter (50%). Within the salinity gradient, particulate phosphorus associated with iron oxides, aluminum oxides, and apatite all decreased with increasing salinity. Estuarine mixing was simulated to determine whether the observed decreases in particle-bound phosphorus pools in field samples were due to release into solution. During simulated mixing, particulate phosphorus associated with iron and aluminum oxides decreased, but no change was observed in apatite-bound phosphorus. The results of the mixing study combined with the observed particle-bound phosphorus distributions suggest that phosphate concentrations along the Delaware Estuary may be partially ‘buffered’ by aluminum and iron oxide phases.