Further investigations on why POC concentrations differ in samples collected by Niskin bottle and in situ pump

Particulate organic carbon (POC) concentrations measured in bottles are often higher than those measured by in situ pumps when samples are taken concurrently. In previous work, we suggested that differential collection of zooplankton might explain this systematic discrepancy in POC between these small volume (bottle) and large volume (in situ pump) techniques. We have now further quantified the carbon contributed by zooplankton collected in the >70-μm particulate fractions from both bottles and pumps at sites in the Mediterranean Sea and Long Island Sound. Our results show that zooplankton abundance and lipid concentrations from zooplankton are one order of magnitude higher in the bottles than in the pumps, supporting the idea that part of the pump–bottle difference is due to collection of more zooplankton by the bottles. Particle washout off the 70-μm mesh used in the in situ pump may cause loss of some particles as well. However, zooplankton in the >70-μm fraction from the bottles contributed only about 1–2 μM POC, which cannot explain the up to 20 μM POC differences observed in this study. Thus, the mechanisms leading to such a large POC difference are still unclear and need to be further investigated. POC concentrations measured using microquartz filters were similar to those using glass fiber filters, suggesting that filter types cannot explain the higher POC observed in bottles, where glass fiber filters are normally used. Furthermore, we investigated several different pump inlet designs to determine how these might affect the ability of pumps to collect and retain large (>70 μm) particles, including zooplankton. The comparison among different pump inlets suggests that inlet design affects the efficiency and retention of large particles and that a sealed filter holder with a narrow right-angle tubular opening is the most efficient at catching/retaining zooplankton.     

Dissolved manganese and silicic acid in the Columbia River plume: A major source to the California current and coastal waters off Washington and Oregon

The spatial distributions of dissolved manganese and nutrients were examined in the Columbia River plume off Oregon and Washington during the summer of 2004 and 2005 as part of the River Influence on Shelf Ecosystems (RISE) program. Factors influencing the hydrochemical characteristics of the freshly formed and aged Columbia River plume were investigated. Hydrographic data and nutrient concentrations were used to delineate three distinct water sources for the Columbia River Plume: California Current surface water, coastal upwelled water, and Columbia River water. The warm, intermediate salinity, nutrient poor California Current water contains low levels of dissolved manganese (< 5 nM) and silicic acid (< 5 μM), and is depleted in nitrate. The cold, high salinity, nutrient rich, freshly upwelled water is highly variable (2–20 nM) in dissolved manganese and can be as high as  45 μM in silicic acid and  30 μM nitrate. The variable Columbia River has summer temperatures ranging from  13 to 24 °C, high silicic acid concentrations (ranging from  120 to 200 μM), and lower nitrate concentrations (ranging from  2 to 20 μM). During the summer, the concentrations of silicic acid and dissolved manganese can exceed 100 μM and 200 nM, respectively, in near-field Columbia River plumes. These values are markedly greater than those of surface coastal waters (even during upwelling conditions). As the plume advects and mixes, the concentrations of these two constituents remain relatively high within plume waters. The concentrations of dissolved manganese in the near-field plume vary with tidal amplitude, exhibiting much higher concentrations for a given salinity during spring tides than during neap tides. For example, the Columbia River plume at a salinity of 20 has a concentration of dissolved manganese of  240 nM during spring tides, as compared to only  60 nM during low amplitude tides. Silicic acid concentrations in the near-field plume remain relatively constant throughout the tidal month. Calculations indicate there is roughly an equivalent yearly delivery of dissolved manganese and silicic acid to the coastal waters off Oregon and Washington by upwelled waters and by the Columbia River plume.     

Benthic remineralization at the land–ocean interface: A case study of the Rhône River (NW Mediterranean Sea)

Biogeochemical processes in sediments under the influence of the Rhône River plume were studied using both in situ microelectrodes and ex situ sediment core incubations. Organic carbon (OC) and total nitrogen (TN) content as well as stable carbon isotopic composition of OC (δ¹³C_OC) were analysed in 19 surface sediments to determine the distribution and sources of organic matter in the Rhône delta system. Large spatial variations were observed in both the total O₂ uptake (5.2 to 29.3 mmol m⁻² d⁻¹) and NH₄⁺ release (−0.1 to −3.5 mmol m⁻² d⁻¹) rates at the sediment–water interface. The highest fluxes were measured near the Rhône River mouth where sedimentary OC and TN contents reached 1.81% and 0.23% respectively. Values of δ¹³C_OC ranged from −26.83‰ to −23.88‰ with a significant seawards enrichment tracing the dispersal of terrestrial organic matter on the continental shelf. The amount of terrestrial-derived OC reaches 85% in sediments close to the Rhône mouth decreasing down to 25% in continental shelf sediments. On the prodelta, high terrestrial OC accumulation rates support high oxygen uptake rates and thus indicating that a significant fraction of terrestrial OC is remineralized. A particulate organic carbon (POC) mass balance indicates that only 3% of the deposited POC is remineralized in prodelta sediments while 96% is recycled on the continental shelf. It was calculated that a large proportion of the Rhône POC input is either buried (52%) or remineralized (8%), mostly on the prodelta area. The remaining fraction (40%) is either mineralized in the water or exported outside the Rhône delta system in dissolved or particulate forms.     

Relevance of peat draining rivers in central Sumatra for the riverine input of dissolved organic carbon into the ocean

Sources and discharges of dissolved organic carbon (DOC) from the central Sumatran river Siak were studied. DOC concentrations in the Siak ranged between 560 and 2594 μmol l⁻¹ and peak out after its confluence with the river Mandau. The Mandau drains part of the central Sumatran peatlands and can be characterized as a typical blackwater river due to its high DOC concentration, its dark brown-coloured, acidic water (pH 4.4–4.7) and its low concentration of total suspended matter (12–41 mg l⁻¹). The Mandau supplies about half of the DOC that enters the Siak Estuary where it mixes conservatively with ocean water. The DOC input from the Siak into the ocean was estimated to be 0.3 Tg C yr⁻¹. Extrapolated to entire Indonesia the data suggest a total Indonesian DOC export of 21 Tg yr⁻¹ representing 10% of the global riverine DOC input into the ocean.     

Uncertainties in the preparation of Ra Mn fiber standards

Delayed coincidence counters (RaDeCC), used for measuring ²²³Ra and ²²⁴Ra preconcentrated from water onto MnO₂-impregnated acrylic fiber (“Mn-fiber”), require a standard Mn-fiber column that has a precisely known activity of ²²⁴Ra for calibration. This may be done by adding an aged ²²⁸Th standard solution to adsorb both ²²⁸Th and its daughter ²²⁴Ra quantitatively onto a Mn fiber. We used both seawater and deionized water (DIW) for testing the adsorption efficiency of Th and Ra onto Mn fibers. Our experimental results show that more than 50% of thorium (²³²Th and ²²⁸Th) breaks through the Mn-fiber column when DIW is used as a medium. However, near quantitative recoveries are obtained if filtered (0.45 μm) seawater is used to prepare the standard. In the case of pure DIW, the pH (initial pH  5.3) rises to > 10 after passing through the column while seawater (initial pH  7.8) changes to  7.2. Thus, the lack of thorium adsorption in DIW may be attributed to this huge increase of pH and the consequent formation of Th(OH)₄ and polyhydroxyl colloids. Based on these observations, we recommend that one should use either artificial seawater or natural seawater (which has negligible ²²⁴Ra and ²²⁸Th) as a loading solution after 0.45 μm filtration. In addition, the thorium adsorption efficiency should be confirmed either by thorium analysis of the effluent solution or long-term monitoring of the supported ²²⁴Ra on the Mn fiber using the RaDeCC. Similar cautions are likely necessary for making ²²³Ra standards by adsorption of ²²⁷Ac onto Mn fibers.     

Particulate organic carbon–Th relationships in particles separated by settling velocity in the northwest Mediterranean Sea

In order to better understand the relationship between the natural radionuclide ²³⁴Th and particulate organic carbon (POC), marine particles were collected in the northwestern Mediterranean Sea (spring/summer, 2003 and 2005) by sediment traps that separated them according to their in situ settling velocities. Particles also were collected in time-series sediment traps. Particles settling at rates of >100 m d⁻¹ carried 50% and 60% of the POC and ²³⁴Th fluxes, respectively, in both sampling years. The POC flux decreased with depth for all particle settling velocity intervals, with the greatest decrease (factor of 2.3) in the slowly settling intervals (0.68–49 m d⁻¹) over trap depths of 524–1918 m, likely due to dissolution and decomposition of material. In contrast the flux of ²³⁴Th associated with the slowly settling particles remained constant with depth, while ²³⁴Th fluxes on the rapidly settling particles increased. Taking into account decay of ²³⁴Th on the settling particles, the patterns of ²³⁴Th flux with depth suggest that either both slow and fast settling particles scavenge additional ²³⁴Th during their descent or there is significant exchange between the particle classes. The observed changes in POC and ²³⁴Th flux produce a general decrease in POC/²³⁴Th of the settling particles with depth. There is no consistent trend in POC/²³⁴Th with settling velocity, such as might be expected from surface area and volume considerations. Good correlations are observed between ²³⁴Th and POC, lithogenic material and CaCO₃ for all settling velocity intervals. Pseudo-K_ds calculated for ²³⁴Th in the shallow traps (2005) are ranked as lithogenic material opal <calcium carbonate <organic carbon. Organic carbon contributes 33% to the bulk K_d, and for lithogenic material, opal and CaCO₃, the fraction is 22% each. Decreases in POC/²³⁴Th with depth are accompanied by increases in the ratio of ²³⁴Th to lithogenic material and opal. No change in the relationship between ²³⁴Th and CaCO₃ was evident with depth. These patterns are consistent with loss of POC through decomposition, opal through dissolution and additional scavenging of ²³⁴Th onto lithogenic material as the particles sink.     

Dissolved organic carbon, nitrogen and phosphorus in the N-E Atlantic and the N-W Mediterranean with particular reference to non-refractory fractions and degradation

Dissolved organic carbon (DOC), dissolved organic nitrogen (DON) and dissolved organic phosphorus (DOP) measured in deep profiles in the N-E Atlantic and in the N-W Mediterranean in the period 1984–2002 are described. After accurate validation, they show close agreement with those previously published.Classic profiles were obtained, with concentrations decreasing in deep waters. In the Mediterranean and in the Atlantic comparable concentrations were found in the 1500–2000 m waters, 44–46 μmol l⁻¹ DOC, 2.6–2.8 μmol l⁻¹ DON and 0.02–0.03 μmol l⁻¹ DOP. In the surface layers, DOC concentrations were higher, but DON and DOP concentrations lower, in the Mediterranean than in the Atlantic, leading to higher element ratios in the Mediterranean. In autumn, values were, respectively, DOC:DON 17 vs. 14, DOC:DOP 950 vs. 500 and DON:DOP 55 vs. 35. The data suggest an increase in DOC and DON in the North Atlantic Central Water over 15 years, which may be linked to the North Atlantic climatic oscillations.Refractory DOM found in the 1500–2000 m layer exhibited C:N:P ratios of 1570:100:1. The labile+semi-labile (=non-refractory) DOM (nrDOM) pool was computed as DOM in excess of the refractory pool. Its contribution to total DOM above the thermocline in the open sea amounted to 25–35% of DOC, 30–35% of DON, and 60–80% of DOP. Element ratios of the nrDOM varied among stations and were lower than those of refractory DOM, except for C:N in the Mediterranean: nrDOC:nrDON 10–19, nrDOC:nrDOP 160–530 and nrDON:nrDOP 15–38. The specific stoichiometry of DOM in the Mediterranean led us to postulate that overconsumption of carbon is probably a main process in that oligotrophic sea.By coupling non-refractory DOM stoichiometry and relationships between the main DOM elements in the water column, the relative mineralization of C, N and P from DOM was studied. Below the thermocline, the preferential removal of phosphorus with regard to carbon from the semi-labile DOM can be confirmed, but not the preferential removal of nitrogen. In the ocean surface layers, processes depend on the oceanic area and can differ from deep waters, so preferential carbon removal seems more frequent. Bacterial growth efficiency data indicate that bacteria are directly responsible for mineralization of a high proportion of DON and DOP in the deep water.     

Assessment of excess nitrate development in the subtropical North Atlantic

Geochemical estimates of N₂ fixation in the North Atlantic often serve as a foundation for estimating global marine diazotrophy. Yet despite being well-studied, estimations of nitrogen fixation rates in this basin vary widely. Here we investigate the variability in published estimates of excess nitrogen accumulation rates in the main thermocline of the subtropical North Atlantic, testing the assumptions and choices made in the analyses. Employing one of these previously described methods, modified here with improved estimates of excess N spatial gradients and ventilation rates of the main thermocline, we determine a total excess N accumulation rate of 7.8 ± 1.7 × 10¹¹ mol N yr^− 1. Contributions to excess N development include atmospheric deposition of high N:P nutrients (adding excess N at a rate of 3.0 ± 0.9 × 10¹¹ mol N yr^− 1 for  38% of the total), high N:P dissolved organic matter advected into and mineralized in the main thermocline (adding excess N at 2.2 ± 1.1 × 10¹¹ mol N yr^− 1 for  28% of the total), and, calculated by mass balance of the excess N field, N₂ fixation (adding excess N at 2.6 ± 2.2 × 10¹¹ mol N yr^− 1 for  33% of the total). Assuming an N:P of 40 and this rate of excess N accumulation due to the process, N₂ fixation in the North Atlantic subtropical gyre is estimated at  4 × 10¹¹ mol N yr^− 1. This relatively low rate of N₂ fixation suggests that i) the rate of N₂ fixation in the North Atlantic is greatly overestimated in some previous analyses, ii) the main thermocline is not the primary repository of N fixed by diazotrophs, and/or iii) the N:P ratio of exported diazotrophic organic matter is much lower than generally assumed. It is this last possibility, and our uncertainty in the N:P ratios of exported material supporting excess N development, that greatly lessens our confidence in geochemical measures of N₂ fixation.     

Determination of picomolar iron in seawater by double Mg(OH)2 precipitation isotope dilution high-resolution ICPMS

A low-blank pre-concentration procedure is described for the analysis of picomolar iron (Fe) in seawater by isotope dilution high-resolution inductively coupled plasma mass-spectrometry (HR-ICPMS). The procedure uses a two-step Mg(OH)₂ co-precipitation procedure to extract Fe from a 50 ml seawater sample into a 100 μl 4% nitric acid (HNO₃) solution followed by HR-ICPMS measurement. The high pre-concentration ratio ( 500:1) achieved by the procedure minimizes the Fe blank due to ICPMS instrumental Fe background and results in a detection limit of  2 pM and a precision of  4% at the 50 pM Fe level. The measurement of a low-Fe seawater sample spiked with gravimetric Fe standard shows that the method can clearly distinguish 0.01 nM Fe from 0.02 nM Fe in seawater with high accuracy. The method is demonstrated by the analysis of dissolved Fe in the equatorial Pacific Ocean.     

A method to measure the isotopic (C) composition of dissolved organic carbon using a high temperature combustion instrument

The stable isotopes of dissolved organic carbon (DOC) are a powerful tool for distinguishing sources and inputs of organic matter in aquatic systems. While several methods exist to perform these analyses, no labs routinely utilize a high temperature combustion (HTC) instrument. Advantages of HTC instruments include rapid analysis, small sample volumes and minimal sample preparation, making them the favored devices for most routine oceanic DOC concentration measurements. We developed a stable carbon DOC method based around an HTC system. This method has the benefit of a simple setup, requiring neither vacuum nor high pressures. The main drawback of the method is a significant blank, requiring careful accounting of all blank sources for accurate isotopic and concentration values. We present here a series of experiments to determine the magnitude, source and isotopic composition of the HTC blank. Over time, the blank is very stable at  20 ng of carbon with a δ¹³C of − 18.1‰ vs. VPDB. The similarity of the isotopic composition of the blank and seawater samples makes corrections relatively minor. The precision of the method was determined by oxidizing organic standards with a wide isotopic and concentration range (− 9‰ to − 39‰; 18 μM to 124 μM). Analysis of seawater samples demonstrates the accuracy for low concentration, high salinity samples. The overall error on the measurement is approximately ± 0.8‰.     

Annual dynamics of pelagic primary production and respiration in a shallow coastal basin

The Wadden Sea (North Sea, Europe) is a shallow coastal sea with high benthic and pelagic primary production rates. To date, no studies have been carried out in the Wadden Sea that were specifically designed to study the relation between pelagic respiration and production by comparable methods. Because previous studies have suggested that the import of primary-produced pelagic organic matter is important for benthic Wadden Sea carbon budgets, we hypothesised that on an annual average the northern Wadden Sea water column is autotrophic. To test this hypothesis, we studied annual dynamics of primary production and respiration at a pelagic station in a shallow tidal basin (List Tidal Basin, northern Wadden Sea). Since water depth strongly influences production estimates, we calculated primary production rates per unit area in two ways: on the basis of the mean water depth (2.7 m) and on the basis of 1 m depth intervals and their respective spatial extent in the List Tidal Basin. The latter more precise estimate yielded an annual primary production of 146 g C m^− 2 y^− 1. Estimates based on the mean water depth resulted in a 40% higher annual rate of 204 g C m^− 2 y^− 1. The total annual pelagic respiration was 50 g C m^− 2 y^− 1. The P/R ratio varied between seasons: from February to October the water column was autotrophic, with the highest P/R ratio of 4–5 during the diatom spring bloom in April/May. In autumn and winter the water column was heterotrophic. On an annual average, the water column of the List Tidal Basin was autotrophic (P/R 3). We suggest that a large fraction of the pelagic produced organic matter was respired locally in the sediment.     

Experimental evidence for an effect of early-diagenetic interaction between labile and refractory marine sedimentary organic matter on nitrogen dynamics

In most natural sedimentary systems labile and refractory organic material (OM) occur concomitantly. Little, however, is known on how different kinds of OM interact and how such interactions affect early diagenesis in sediments. In a simple sediment experiment, we investigated how interactions of OM substrates of different degradability affect benthic nitrogen (N) dynamics. Temporal evolution of a set of selected biogeochemical parameters was monitored in sandy sediment over 116 days in three experimental set-ups spiked with labile OM (tissue of Mytilus edulis), refractory OM (mostly aged Zostera marina and macroalgae), and a 1:1 mixture of labile and refractory OM. The initial amounts of particulate organic carbon (POC) were identical in the three set-ups. To check for non-linear interactions between labile and refractory OM, the evolution of the mixture system was compared with the evolution of the simple sum of the labile and refractory systems, divided by two. The sum system is the experimental control where labile and refractory OM are virtually combined but not allowed to interact. During the first 30 days there was evidence for net dissolved-inorganic-nitrogen (DIN) production followed by net DIN consumption. (Here ‘DIN’ is the sum of ammonium, nitrite and nitrate.) After  30 days a quasi steady state was reached. Non-linear interactions between the two types of OM were reflected by three main differences between the early-diagenetic evolutions of nitrogen dynamics of the mixture and sum (control) systems: (1) In the mixture system the phases of net DIN production and consumption commenced more rapidly and were more intense. (2) The mixture system was shifted towards a more oxidised state of DIN products [as indicated by increased (nitrite + nitrate)/(ammonium) ratios]. (3) There was some evidence that more OM, POC and particulate nitrogen were preserved in the mixture system. That is, in the mixture system more particulate OM was preserved while a higher proportion of the decomposed particulate N was converted into inorganic N. It can be concluded that during the first days and weeks of early diagenesis the magnitude and composition of the flux of decompositional dissolved N-compounds from sediments into the overlying water was influenced by non-linear interactions of OM substrates of different degradability. Given these experimental results it is likely that the relative spatial distributions of OM of differing degradability in sediments control the magnitude and composition of the return flux of dissolved N-bearing compounds from sediments into the overlying water column.     

Environmental controls on phytoplankton community composition in the Thames plume, U.K.

The aim of this study was to investigate controls on the phytoplankton community composition and biogeochemistry of the estuarine plume zone of the River Thames, U.K. using an instrumented moored buoy for in situ measurements and preserved sample collection, and laboratory-based measurements from samples collected at the same site. Instrumentation on the moored buoy enabled high frequency measurements of a suite of environmental variables including in situ chlorophyll, water-column integrated irradiance, macronutrients throughout an annual cycle for 2001 e.g. nitrate and silicate, and phytoplankton biomass and species composition. The Thames plume region acts as a conduit for fluvial nutrients into the wider southern North Sea with typical winter concentrations of 45 μM nitrate, 17 μM silicate and 2 μM phosphate measured. The spring bloom resulted from water-column integrated irradiance increasing above 60 W h m^− 2 d^− 1 and was initially dominated by a diatom bloom mainly composed of Nitzschia sp. and Odontella sinesis. The spring bloom then switched after  30 days to become dominated by the flagellate Phaeocystis reaching a maximum chlorophyll concentration of 37.8 μg L^− 1. During the spring bloom there were high numbers of the heterotrophic dinoflagellates Gyrodinium spirale and Katodinium glaucum that potentially grazed the phytoplankton bloom. This diatom–flagellate switch was predicted to be due to a combination of further increasing water-column integrated irradiance > 100 W h m^− 2 d^− 1 and/or silicate reaching potentially limiting concentrations (< 1 μM). Post spring bloom, diatom dominance of the lower continuous summer phytoplankton biomass occurred despite the low silicate concentrations (Av. 0.7 μM from June–August). Summer diatom dominance, generally due to Guinardia delicatula, was expected to be as a result of microzooplankton grazing, dominated by the heterotrophic dinoflagellate Noctiluca scintillans, controlling 0.7–5.0 μm ‘flagellate’ fraction of the phytoplankton community with grazing rates up to 178% of ‘flagellate’ growth rate. The Thames plume region was therefore shown to be an active region of nutrient and phytoplankton processing and transport to the southern North Sea. The use of a combination of moorings and ship-based sampling was essential in understanding the factors influencing nutrient transport, phytoplankton biomass and species composition in this shelf sea plume region.     

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.     

Copper speciation in estuarine waters by forward and reverse titrations

The chemical speciation of copper in the estuarine waters of the Vigo Ria was determined by titrations with salicylaldoxime (reverse copper titrations) and with copper (forward titrations). The forward titrations quantified the concentrations of ligands present in excess whereas the reverse titrations demonstrated the presence of low concentrations of very strong binding ligands, approximately matching the copper concentration. The data obtained by the reverse titrations indicated that copper was about 10× stronger bound than data based on the usual forward titrations.The copper concentration in these ria waters was low at 5 nM with a minor mid-estuarine maximum of 8 nM. These copper levels are amongst the lowest reported for estuarine waters and therefore represent uncontaminated waters. The concentration of inorganic copper was very low across the ria at  10–100 fM, except at Bouzas harbour (salinity 35.5) where it was raised to  1 pM due to copper contamination, in waters affected by the port facilities, to total levels of 15 to 20 nM copper, exceeding the concentration of the very strong ligand detected by the reverse titrations.     

Temporal variability in oxygen and nutrient concentrations in the southern Aegean Sea and the Straits of the Cretan Arc

Nutrient and oxygen data collected in the southern Aegean Sea (Cretan Sea) and the straits of the Cretan Arc, during the four seasonal PELAGOS cruises in 1994–1995, are investigated and compared with data collected from 1987 to 1992 within the same area. During the cruises of the PELAGOS Project, nutrient enrichment of the intermediate layers of the Cretan Sea was observed, as a result of intrusion of ‘nutrient-rich, oxygen-poor’ Transition Mediterranean Water (TMW) compensating the Cretan Deep Water (CDW) outflow. TMW occupied the intermediate layers of the entire Cretan Sea. The concentrations of nutrients within this layer were often two times higher than those observed in the same area during previous studies undertaken before 1992 (increase 2.5 μmol/l of nitrate, 0.05 μmol/l of phosphate and 2.5μmol/l of silicate). The decrease of oxygen in this layer is about 0.8ml/l (35 μmol/l). Outflow of CDW occurs principally through the Antikithira and Kassos Straits (the two deeper straits of the Cretan Arc); it results in an increase of oxygen content but a decrease in the nutrient content of water in the deep and bottom layers outside the Cretan Sea. The major mesoscale features in the area have a major influence of the distributions and exchanges of nutrients and oxygen through the straits of the Cretan Arc. The surface and the intermediate layers were richer in nutrients and poorer in oxygen in spring (March 1994), than in autumn (September 1994).     

Black carbon in marine particulate organic carbon: Inputs and cycling of highly recalcitrant organic carbon in the Gulf of Maine

To increase our understanding of the roles of black carbon (BC), a highly sorptive and recalcitrant material, we measured BC concentrations and fluxes in marine particulate organic carbon (POC) out of the water column in the Gulf of Maine (GoM), a representative coastal area downwind of important BC sources of the Northeastern United States. Concentrations ranged from < 0.1 to 16 μg/L in the spring and late summer, typically contributing between 1 and 20% of the POC. Water-column export fluxes were near 10 g_BC/m²∙yr. These observations suggest that (a) up to 50% of the “molecularly uncharacterized” POC in this region's seawater is combustion-derived BC, and (b) the “bioavailabilities” of hydrophobic pollutants like polycyclic aromatic hydrocarbons (PAHs) would be influenced substantially by sorption to BC. The observed BC spatial distributions imply that a large part of the BC was carried offshore by wind and that much of it is accumulated in the coastal sediments. On a global scale, these results suggest the GoM and other coastal areas with similar BC loadings accumulate significant amounts of highly recalcitrant organic carbon that remineralizes on geological time scales in the world's oceans.     

Remotely sensed sea-surface chlorophyll and POC flux at Deep Gulf of Mexico Benthos sampling stations

Biweekly composite averages of the standing stock of sea-surface chlorophyll (SSC) were derived from SeaWiFS satellite ocean-color data at 44 benthic sampling stations occupied along the continental slope and rise by the Deep Gulf of Mexico Benthos (DGoMB) program. At the 22 DGoMB sites north of 26°N and west of 91°W in the NW Gulf of Mexico, annual average SSC was 0.19 mg m⁻³, ranging at most locations from annual highs of about 0.3 mg m⁻³ in November–February to lows of about 0.1 mg m⁻³ in May–August. Comparison of three years of SeaWiFS data (January 1998–December 2000) showed little inter-annual variation at these NW Gulf stations. In contrast, at the 22 NE Gulf sites north of 26°N and east of 91°W, SSC averaged 2.8 times higher than in the NW Gulf, showing also strong inter-annual variation. Maxima in the NE region occurred in November–February and also during summers. The summer maxima were associated with Mississippi River water transported offshore to the east and southward by anticyclonic eddies in the NE Gulf. The apparent increases in SSC in June–August at NE Gulf stations reached average monthly concentrations >50% greater than in November–February. Based on a primary productivity model and a vertical flux model, the calculated export of particulate organic carbon (POC flux reaching the seafloor) was estimated as 18 mg C m⁻² day⁻¹ at the 22 NE Gulf stations, and 9 mg C m⁻² day⁻¹ at the 22 NW Gulf stations. These estimates are comparable to fluxes measured by benthic lander by others in the DGoMB program, which may drive the differences in west versus east bathymetric zonation and community structure of macrobenthos that were sampled with large box corers by others in the DGoMB program.     

Seasonal variation and flux of dissolved nutrients in the Danshuei Estuary, Taiwan: A hypoxic subtropical mountain river

Comprehensive bimonthly field surveys were carried out from September 2000 to June 2002 to study the seasonal dynamics and the inter-annual variability of dissolved inorganic nitrogen (DIN; nitrate, nitrite and ammonium) and dissolved inorganic phosphorus (DIP) in a subtropical mountain river system, the Danshuei tributary, the largest urbanized estuarine system in Taiwan. The headwaters were found to be well aerated, saturated with oxygen, with low ambient DIN (<9 μM) and DIP (<0.2 μM) concentration. As the river flows through the city of Taipei, the river becomes hypoxic because re-aeration rates cannot keep up with elevated oxygen consumption, and the concentrations of DIP (7.53 μM) and DIN (390 μM) increase drastically. Conservative mixing was mostly observed for silicate while DIP and DIN mostly showed non-conservative removal characteristics. Silicate originates from weathering and erosion of bedrocks in the watershed, whereas nitrogen- and phosphorus-bearing nutrients come mainly from urban discharges. Ammonium is the predominant dissolved nitrogenous species, ranging from 10 to 1000 μM. The nutrient chemistry is complex and dynamic due to anthropogenic perturbations and reactions in the tidally mixed zone of strong redox gradients. On average, the annual loading rates of dissolved phosphate and dissolved inorganic nitrogen from the Danshuei River to the ocean are 0.1 and 3.2 Gmol/year, respectively, which represent 0.1% and 0.2% of the world's total river discharge of dissolved inorganic nitrogen and phosphate.     

Particulate barium fluxes and export production in the northwestern Mediterranean

Biogenic barium, mostly in the barite (BaSO₄) form, has been proposed as a tracer for export production in the ocean. Here we report on biogenic barium (Ba_xs) and particulate organic carbon (POC) fluxes from sediment traps deployed at the DYFAMED site in the Northwestern Mediterranean Sea. Ba_xs fluxes display average values of 37 ± 45 and 50 ± 58 μg/m²/d at 200 and 1000 m respectively, and are linearly correlated to POC fluxes (mean values of 7.9 ± 9.3 and 6.8 ± 6.8 mg C/m²/d at 200 and 1000 m). Export production estimates, calculated using published Ba_xs- or POC-based algorithms, all fall below or close to the lower limit of potential export values proposed in the literature. This work clearly demonstrates the usefulness of Ba_xs as a tracer of oceanic export production in the Northwestern Mediterranean Sea. However, development of a quantitative export production proxy requires a clear understanding of the underlying cause(s) for the observed spatial variations in the relationship between Ba_xs and POC fluxes. The present study confirms that the processes leading to barite formation differ between margin and open-ocean sites and probably account for much of the regional variability in the POC/Ba_xs ratio.