The distribution and preferential biological uptake of cadmium at 6°W in the Southern Ocean

The spatial and temporal distribution of cadmium (Cd) and phosphate in the Southern Ocean are related to biology and hydrography. During a period of 18 days between transects 5/6 and 11, a phytoplankton spring bloom developed in the Polar Frontal region. Upper water Cd concentrations were not depleted and ranged from 0.2 to 0.8 nM at about 10 m depth. These relatively high Cd concentrations are attributed to upwelling of Upper Circumpolar Deep Water (0.5–1.2 nM in the core) in combination with low biological productivity (0.2 to 0.3 mg m⁻³ chlorophyll-a, 0.3 g C m⁻² d⁻¹). Total particulate Cd concentrations at 40 m depth were between 0.02 and 0.14 nM with the maximum in concentration in the Polar Frontal region. Most of the particulate Cd at this depth (85–94%) was detected in the first phase of a sequential chemical leaching treatment which includes adsorbed Cd as well as Cd incorporated in algae. The Polar Frontal region was characterized by minima in Cd concentration and Cd/phosphate ratio of seawater at both transects; values were the lowest at transect 11 after development of the spring bloom which was dominated by diatoms. This decreasing Cd/phosphate ratio in seawater during spring bloom development was attributed to preferential Cd gross uptake which more than compensated the process of preferential Cd recycling. Within the Upper Circumpolar Deep Water, Cd showed a maximum in concentration similar to that of the major nutrients. Both the Cd concentration and the Cd/phosphate ratio of the deeper water increased in southern direction, from 0.4 to 0.7 nM and from 0.2 to 0.3 nM/μM, respectively. Antarctic Intermediate Water has a Cd concentration of 0.21 nM with a Cd/phosphate ratio of 0.10 nM/μM. In Antarctic Bottom Water, Cd concentrations ranged from 0.60 to 0.82 nM.     

The fluorescence of dissolved organic matter in seawater

A total of 28 vertical profiles of seawater fluorescence was measured in the Sargasso Sea, the Straits of Florida, the Southern California Borderlands, and the central Pacific Ocean. In all cases, surface seawater fluorescence was low as a result of photochemical bleaching which occurs on the timescale of hours. Fluorescence of deep water was 2–2.5 times higher than that of surface waters, and was constant, implying a long residence time for fluorescent organic matter, possibly of the order of thousands of years. Fluorescence correlates well with nutrients (NO₃⁻, PO₄³⁻) in mid-depth waters (100–1000 m) in the Sargasso Sea and the central North Pacific, consistent with results in the central Pacific and the coastal seas of Japan. This suggests that regeneration or formation of fluorescent materials accompanies the oxidation and remineralization of settling organic particles.The various sources and sinks of fluorescent organic matter in the global oceans are assessed. The major sources are particles and in situ formation; rivers, rain, diffusion from sediments, and release from organisms are minor sources. The major sink is photochemical bleaching.     

The behaviour of dissolved Cd, Co, Zn, and Pb in North Atlantic near-surface waters (30°N/60°W–60°N/2°W)

In September 1993 (M26) and June/July 1996 (M36), a total of 239 surface samples (7 m depth) were collected on two transects across the open Atlantic Ocean (224 samples) and northwest European shelf edge area. We present an overview of the horizontal variability of dissolved Cd, Co, Zn, and Pb in between the northwest and northeast Atlantic Ocean in relation to salinity and the nutrients. Our data show a preferential incorporation of Cd relative to P in the particulate material of the surface ocean when related to previously published parallel measurements on suspended particulate matter from the same cruise. There is a good agreement with results recently estimated from a model by Elderfield and Rickaby (Nature 405 (2000) 305), who predict for the North Atlantic Ocean a best fit for α_Cd/P=[Cd/P]_POM/[Cd/P]_SW of 2.5, whereas the approach of our transect shows a α_Cd/P value of 2.6. The Co concentrations of our transects varied from <5 to 131 pmol kg⁻¹, with the lowest values in the subtropical gyre. There were pronounced elevations in the low-salinity ranges of the northwest Atlantic and towards the European shelf. The Co data are decoupled from the Mn distribution and support the hypothesis of marginal inputs as the dominant source. Zinc varied from a minimum of <0.07 nmol kg⁻¹ to a maximum of 1.2 and 4.8 nmol kg⁻¹ in regions influenced by Labrador shelf or European coastal waters, respectively. In subtropical and northeast Atlantic waters, the average Zn concentration was 0.16 nmol kg⁻¹. Zinc concentrations at nearly three quarters of the stations between 40°N and 60°N were <0.1 nmol kg⁻¹. This suggests that biological factors control Zn concentrations in large areas of the North Atlantic surface waters. The Pb data indicated that significant differences in concentration between the northwest and northeast Atlantic surface waters presently (1996) do not exist for this metal. The transects in 1993 and 1996 exhibited Pb concentrations in the northeast Atlantic surface waters of 30 to 40 pmol kg⁻¹, about a fifth to a quarter of the concentrations observed in 1981. This decline is supported by our particle flux measurements in deep waters of the same region.     

Trace metal distributions across the continental shelf near Otago Peninsula, New Zealand

The distributions of the trace metals iron (Fe), copper (Cu) and cadmium (Cd) along with hydrological parameters (salinity, temperature and reactive phosphate) across the New Zealand continental shelf near Otago Peninsula have been studied. This is a region in which the Subtropical Convergence (STC), a major oceanic front separating subtropical and subantarctic waters, is uniquely located close to land, permitting an examination of the influence of terrestrial sources of Fe and Cu on oceanic waters containing excess micronutrients. Acid-soluble (110 nmol kg⁻¹) and dissolved (6.3 nmol kg⁻¹) Fe concentrations were highest over the central shelf, and decreased rapidly across the mixing zone of the STC to about 5 nmol kg⁻¹ for both forms. The distribution of acid-soluble and dissolved Cu were similar to their counterparts for Fe. Depth-concentration profiles for acid-soluble Fe and Cu suggest resuspension of shelf sediments is the main source. The ratio of oxine-labile to acid-soluble Fe varied from 0.03 to 0.26, with the highest values found in the near surface waters. Oxine-labile Fe and Cu also decreased in concentration in a seawards direction, and with depth, indicating the influence of near surface processes on the reactivity of these elements. Cd concentrations across the continental shelf were very low (<200 pmol kg⁻¹) and exhibited no clear spatial trend and no correlation with phosphate. Comparison of the Cd/P ratio across the shelf indicated that the waters in this region were strongly depleted in Cd relative to P. Phosphate concentrations were lowest in neritic water and increased in the seawards direction because of mixing with nutrient-rich Subantarctic Surface Water.     

Stable carbon isotope distribution of particulate organic matter in the ocean: a model study

The stable carbon isotopic composition of particulate organic matter in the ocean, δ¹³C_POC, shows characteristic spatial variations with high values in low latitudes and low values in high latitudes. The lowest δ¹³C_POC values (−32‰ to −35‰) have been reported in the Southern Ocean, whereas in arctic and subarctic regions δ¹³C_POC values do not drop below −27‰. This interhemispheric asymmetry is still unexplained. Global gradients in δ¹³C_POC are much greater than in δ¹³C_DIC, suggesting that variations in isotopic fractionation during organic matter production are primarily responsible for the observed range in δ¹³C_POC. Understanding the factors that control isotope variability is a prerequisite when applying δ¹³C_POC to the study of marine carbon biogeochemistry. The present model study attempts to reproduce the δ¹³C_POC distribution pattern in the ocean. The three-dimensional (3D) Hamburg Model of the Oceanic Carbon Cycle version 3.1 (HAMOCC3.1) was combined with two different parametrizations of the biological fractionation of stable carbon isotopes. In the first parametrization, it is assumed that the isotopic fractionation between CO₂ in seawater and the organic material produced by algae, _P, is a function of the ambient CO₂ concentration. The two parameters of this function are derived from observations and are not based on an assumption of any specific mechanism. Thus, this parametrization is purely empirical. The second parametrization is based on fractionation models for microalgae. It is supported by several laboratory experiments. Here the fractionation, _P, depends on the CO₂ concentration in seawater and on the (instantaneous) growth rates, μ_i, of the phytoplankton. In the Atlantic Ocean, where most field data are available, both parametrizations reproduce the latitudinal variability of the mean δ¹³C_POC distribution. The interhemispheric asymmetry of δ¹³C_POC can mostly be attributed to the interhemispheric asymmetry of CO₂ concentration in the water. However, the strong seasonal variations of δ¹³C_POC as reported by several authors, can only be explained by a growth rate-dependent fractionation, which reflects variations in the cellular carbon demand.     

Tidal time-scale variation in nutrient flux across the sediment–water interface of an estuarine tidal flat

We determined the range of the tidal variations in nutrient flux across the sediment–water interface and elucidated mechanisms of the flux variation in two estuarine intertidal flats (one sand, one mud) in northeastern Japan. Nutrient flux was measured using in situ light and dark chambers, which were incubated for 2 h, 2–6 times per day. Results showed that nutrient concentration in overlying water varied by tide and was also affected by sewage-treated water inflow. The nutrient fluxes responded quickly to the tidal variation in overlying water chemistry and the range of the variation in flux was as large as the seasonal-scale variation reported in previous studies. In the sand flat, salinity increase likely enhanced benthos respiration and led to increases in both O₂ consumption and PO₄³⁻ regeneration under low illumination, while benthic microalgae were likely to actively generate O₂, uptake PO₄³⁻ and suppress PO₄³⁻ release under high illumination (>900 μmol photons m⁻² s⁻¹). Also in the mud flat, PO₄³⁻ flux was related with O₂ flux, although the range of temporal variation in PO₄³⁻ flux was small. In both the flats, NH₄⁺ flux was always governed by NH₄⁺ concentration in the overlying water; either an increase in NH₄⁺ uptake or a decrease in NH₄⁺ release was observed as the NH₄⁺ concentration rose due to inflow of river water or input of sewage-treated water. Although NO₃⁻ tended to be released in both tidal flats when low NO₃⁻ concentration seawater dominated, their relationship was likely to be weakened under conditions of low oxygen consumption and suppressed denitrification. It is likely that tidal variation in nutrient flux is governed more by the nutrient concentration than other factors, such as benthic biological processes, particularly in the case where nutrient concentration in the overlying water is relatively high and with wide amplitude.     

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.     

Iodine chemistry in deep anoxic basins and overlying waters of the Mediterranean Sea

Iodate (IO₃⁻) is the predominant dissolved species of iodine in the oxygenated waters of the Mediterranean Sea. Iodide (I⁻) is present in significant quantities (up to 65 nM) in oxygenated waters in the photic zone and near the interface above the anoxic and saline Bannock Basin. Lesser quantities of I⁻ (< 10 nM) are found throughout the rest of the oxic water column. An additional unidentified dissolved iodine species is present immediately above the anoxic interface.Total dissolved iodine (ΣI) increases dramatically across the seawater/brine interface. Part of this increase is undoubtedly the result of the dissolution of iodine-rich evaporites during formation of the brine bodies at the Tyro and Bannock Basins. The vertical distribution of ΣI and other dissolved chemical species (particularly PO₄³⁻) in the Bannock Basin brine, however, suggests an additional, present-day, diagenetic source of dissolved iodine to the brine. Based on the increase in the concentration of the most soluble major ions across the seawater/brine interface, 5–7 μM of the 11.5-μM increase in ΣI concentration must be attributed to diagenesis.     

Measurement of manganese, zinc and cadmium complexation in seawater using Chelex ion exchange equilibria

Equilibria between Chelex 100* and manganese, zinc and cadmium ions were used to determine the complexation of these trace metals in 36‰ Gulf Stream seawater at 25°C and pH 8.2. The method utilized radiotracers (⁵⁴Mn, ⁶⁵Zn, and ¹⁰⁹Cd) to quantify trace metal adsorption from trace metal-amended seawater and from seawater containing a series of ethylenediaminetetracetate (EDTA)—metal ion buffers. Results were consistent with Chelex adsorption of both trace metal ions and trace metal—EDTA chelates. Equilibrium models fitted to the data were used to establish conditional stability constants for Chelex adsorption of manganese, zinc and cadmium ions and for adsorption of EDTA-chelates. These models also yielded ratios of free metal ions to total dissolved trace metal concentrations in seawater: 10^−0.1 for manganese, 10^−0.2 for zinc, and 10^−1.5 for cadmium. Independent measurements with a cadmium ion-selective electrode also yielded a free: total cadmium ratio of 10^−1.5.     

Stable carbon isotope ratios of plankton carbon and sinking organic matter from the Atlantic sector of the Southern Ocean

The stable carbon isotope composition of particulate organic carbon (POC) from plankton, sediment trap material and surface sediments from the Atlantic sector of the Southern Ocean were determined. Despite low and constant water temperatures, large variations in the δ¹³C values of plankton were measured. ¹³C enrichments of up to 10‰ coincided with a change in the diatom assemblage and a two-fold increase in primary production. Increased CO₂ consumption as a result of rapid carbon fixation may result in diffusion limitation reducing the magnitude of the isotope fractionation. The δ¹³C values of plankton from sea-ice cores display a relationship with the chlorophyll a content. High ‘ice-algae’ biomass, in combination with a limited exchange with the surrounding seawater, results in values of about − 18 to − 20‰. It is assumed that these values are related to a reduced CO₂ availability in the sea-ice system. In comparison with plankton, sinking krill faeces sampled by traps can be enriched by 2–5‰ in ¹³C (e.g. central Bransfield Strait). In contrast, the transport of particles in other faeces, diatom aggregates or chains results in minor isotope changes (e.g. Drake Passage, Powell Basin, NW Weddell Sea). A comparison between the δ¹³C values of sinking matter and those of surface sediments reveals that ¹³C enrichments of up to 3–4‰ may occur at the sediment-water boundary layer. These isotopic changes are attributed to high benthic respiration rates.     

Interannual variations of the Antarctic Ocean CO2 uptake from 1986 to 1994

The interannual variations of CO₂ sources and sinks in the surface waters of the Antarctic Ocean (south of 50°S) were studied between 1986 and 1994. An existing, slightly modified one-dimensional model describing the mixed-layer carbon cycle was used for this study and forced by available satellite-derived and climatological data. Between 1986 and 1994, the mean Antarctic Ocean CO₂ uptake was 0.53 Pg C year⁻¹ with an interannual variability of 0.15 Pg C year⁻¹.Interannual variation of the Antarctic Ocean CO₂ uptake is related to the Antarctic Circumpolar Wave (ACW), which affects sea surface temperature (SST), wind-speed and sea-ice extent. The CO₂ uptake in the Antarctic Ocean has increased from 1986 to 1994 by 0.32 Pg C. It was found that over the 9 years, the surface ocean carbon dioxide fugacity (fCO₂) increase was half that of the atmospheric CO₂ increase inducing an increase of the air–sea fCO₂ gradient. This effect is responsible for 60% of the Antarctic Ocean CO₂ uptake increase between 1986 and 1994, as the ACW effect cancels out over the 9 years investigated.     

Depth profiles of volatile iodine and bromine-containing halocarbons in coastal Antarctic waters

Measurements of bromoform (CHBr₃), diiodomethane (CH₂I₂), chloroiodomethane (CH₂ICl) and bromoiodomethane (CH₂IBr) were made in the water column (5–100 m depth) of the Southern Ocean within 0–40 km of the Antarctic sea ice during the ANTXX1/2 transect of the German R/V Polarstern, at five locations between 70–72°S and 9–11°W in the Antarctic spring/summer of 2003–2004. Some of the profiles exhibited a very pronounced layer of surface sea-ice meltwater, as evidenced by salinity minima and temperature maxima, along with surface maxima in concentrations of CHBr₃, CH₂I₂, CH₂ICl and CH₂IBr. These results are consistent with in situ surface halocarbon production by ice algae liberated from the sea ice, although production within the sea ice followed by transport cannot be entirely ruled out. Additional sub-surface maxima in halocarbons occurred between 20 and 80 m. At a station further from shore and not affected by surface sea-ice meltwater, surface concentrations of CH₂I₂ were decreased whereas CH₂ICl concentrations were increased compared to the stations influenced by meltwater, consistent with photochemical conversion of CH₂I₂ to CH₂ICl, perhaps during upward mixing from a layer at  70 m enhanced in iodocarbons. Mean surface (5–10 m) water concentrations of halocarbons in these coastal Antarctic waters were 57 pmol l^− 1 CHBr₃ (range 44–78 pmol l^− 1), 4.2 pmol l^− 1 CH₂I₂ (range 1.7–8.2 pmol l^− 1), 0.8 pmol l^− 1 CH₂IBr (range 0.2–1.4 pmol l^− 1), and 0.7 pmol l^− 1 CH₂ICl (range 0.2–2.4 pmol l^− 1). Concurrent measurements in air suggested a sea-air flux of bromoform near the Antarctic coast of between 1 and 100 (mean 32.3, median 10.4) nmol m^− 2 day^− 1 and saturation anomalies of 557–1082% (mean 783%, median 733%), similar in magnitude to global shelf values. In surface samples affected by meltwater, CH₂I₂ fluxes ranged from 0.02 to 6.1 nmol m^− 2 day^− 1, with mean and median values of 1.9 and 1.1 nmol m^− 2 day^− 1, respectively.     

Particle fluxes to the interior of the Southern Ocean in the Western Pacific sector along 170°W

An array of five bottom-tethered moorings with 19 PARFLUX time-series sediment trap at three depths (1 and 2 km below the surface, and 0.7 km above the sea-floor) was deployed in the western Pacific sector of the Southern Ocean, along 170°W. The five stations were selected to sample settling particles in the main hydrological zones of the Southern Ocean. The sampling period spanned 425 days (November 28, 1996–January 23, 1998) and was divided into 13 or 21 synchronized time intervals. A total of 174 sequential samples were recovered and analyzed to estimate fluxes of total mass (TMF), organic carbon, carbonate, biogenic silica, and lithogenic particles. The fluxes of biogenic material were higher than anticipated, challenging the notion that the Southern Ocean is a low-productivity region. Organic carbon fluxes at 1 km depth within the Polar Frontal Zone and the Antarctic Zone were relatively uniform (1.7–2.3 g m⁻² yr⁻¹), and about twice the estimated ocean-wide average (ca. 1 g m⁻² yr⁻¹). Carbonate fluxes were also high and uniform between the Subantarctic Front and ca. 64°S (11–13 g m⁻² yr⁻¹). A large fraction of the carbonate flux in the Antarctic Zone was due to the presence of pteropod shells. Coccoliths were found only to the north of the Polar Front, and calcium carbonate became the dominant phase in the Subantarctic Zone. In contrast, carbonate particles were nearly absent near 64°S. Latitudinal variations in biogenic silica fluxes were substantial. The large opal flux (57 g m⁻² yr⁻¹) measured in the Antarctic Zone suggests that opal productivity in this region has been previously underestimated and helps to explain the high sedimentary opal accumulation often found south of the Polar Front. Unlike biogenic material, fluxes of lithogenic particles were among the lowest measured in the open-ocean (0.12–0.05 g m⁻² yr⁻¹), reflecting a very low dust input.     

A circumpolar modeling study of habitat control of Antarctic krill (Euphausia superba) reproductive success

A one-dimensional, temperature-dependent model is implemented to simulate the descent–ascent cycle of Antarctic krill (Euphausia superba) embryos and larvae. Inputs to the model are monthly mean climatologies of ambient temperature and density fields obtained from the World Ocean Atlas Database for Southern Ocean waters. Simulations are done with a 1° resolution at a circumpolar scale, south of 60°S, and the results are interpolated to a 5′ grid to match the resolution of the bottom bathymetry data. Simulations of the descent–ascent cycle using environmental conditions corresponding to the Antarctic krill spawning season (December–March) resulted in unconstrained success in completion of the cycle in water deeper than 1000 m. Continental shelf regions favorable to successful hatching of Antarctic krill embryos are limited to areas along the west Antarctic Peninsula, large areas in the Bellingshausen and Amundsen Seas, offshore of Wilkes Land, and to the east and west of Prydz Bay. These are regions where the Southern Antarctic Circumpolar Current Front is along the shelf slope, the Antarctic Slope Front is absent, and Circumpolar Deep Water is present. The effect of seasonal variability in temperature on the descent–ascent cycle tends to enhance the probability of success in regions offshore of Wilkes Land, Queen Maud Land, and the eastern shelf of the Antarctic Peninsula later in the spawning season. The simulations show that success of the descent–ascent cycle is sensitive to initial embryo diameter and larval ascent rate. Initial embryo diameter may provide an additional constraint on success of the descent–ascent cycle, especially in continental shelf waters, where small embryos tend to encounter the bottom before hatching. The circumpolar distributions of simulated embryo hatching depth and larval success show that all regions of the Antarctic are not equal in the ability to support successful completion of the Antarctic krill descent–ascent cycle, which has implications for the overall circum-Antarctic krill distribution and for the development of nutrient and material budgets, especially for Antarctic continental shelf areas.     

Nutrients (organic C, P, N, Si) in the eutrophic River Loire (France) and its estuary

The Loire estuary has been surveyed from 1982 to 1985 by 13 isochronous longitudinal profiles realized at low tide. Nutrient (SiO₂, NO₃⁻, NH₄⁺, PO³⁻₄, particulate organic carbon or POC) patterns are very variable depending on the season, the estuarine section [river, upper-inner estuary, upstream of the fresh-water-saline-water interphase FSI, the lower-inner estuary characterized by the high turbidity zone (HTZ), the outer estuary] and the river discharge. Biological processes are dominant. In the eutrophied River Loire (summer pigment > 100 μg l⁻¹), the high algal productivity (algal POC > 3 mg l⁻¹) results in severe depletion of SiO₂, PO₄³⁻, NO₃⁻. The enormous biomass (55 000 ton algal POC/year) is degraded in the HTZ where bacterial activity is intense. As a result, there is generally a regeneration of dissolved SiO₂ and PO₄³⁻, a marked NH₄⁺ maximum, while NO₃⁻ is conservative or depleted when the HTZ is nearly anoxic. Other processes can be considered including pollution from fertilizer plans (PO₄³⁻, NH₄⁺) and from a hydrothermal power plant (NH₄⁺). In the less turbid outer estuary, nutrients are generally conservative. Major variations of concentrations are observed in the lowest chlorinity section (Cl⁻ < 1 g kg⁻) and also upstream the FSI, defined here as a 100% increase in Cl⁻. Nutrient inputs to the ocean are not significantly modified for SiO₂ and NO₂⁻, but are increased by 70% and 180% for PO₄³⁻ and NH₄⁺ and depleted by 60% for POC. Odd hydrological events, especially some floods, may perturbate or even mask the usual seasonal pattern observed in profiles.     

Spatial and temporal distribution of Fe, Ni, Cu and Pb along 140°E in the Southern Ocean during austral summer 2001/02

The distribution of dissolved (D) and acid-dissolvable (AD) Fe, Ni, Cu and Pb in the upper water column (0–300 m depth) was determined in the Australian sector of the Southern Ocean (140°E meridian) during three cruises conducted between November 2001 and March 2002. For Ni and Cu, there was no significant difference in concentration between dissolved and acid-dissolvable species. DNi and DCu showed significant (P = 0.01) positive correlations with silicate, phosphate and nitrate, reflecting their strong nutrient-type behaviour. For Fe and Pb, the acid-dissolvable concentration mostly exceeded the dissolved concentration, reflecting the importance of labile particulate species for these elements. DPb decreased between January and February in the Polar Frontal Zone and in Antarctic continental shelf water. ADPb maxima occurred in the Antarctic Zone, resulting in a maximum AD/D ratio of 7. The mean DFe concentration in the surface mixed layer was 0.3 nM in the sub-Antarctic zone, 0.4 nM in the Polar Frontal Zone, 0.5 nM in the Antarctic Zone and increased southward beyond the Antarctic Divergence and towards the continent. DFe did not show a clear temporal change in its horizontal distribution, which was in contrast to the other nutrients and trace metals. ADFe substantially increased in Antarctic continental shelf water where the AD/D ratio reached 11. The following conclusions can be drawn from these data. (1) Ni and Cu exist exclusively as dissolved species and their distributions are mainly controlled by their biogeochemical cycling, similar to those of the major nutrients. (2) Pb is dominated by particulate species. The distribution of DPb is temporally and spatially variable due to a sporadic source and strong scavenging. (3) DFe is rather a minor fraction of total Fe in Antarctic continental shelf water where shelf sediments and Antarctic sea-ice appear to be strong sources for Fe. There is substantial temporal variation in the supply of Fe to the upper water column. DFe in the mixed layer of the open Southern Ocean is maintained at low concentrations throughout summer due to uptake by phytoplankton and scavenging.     

The distributions of particulate atmospheric trace metals and mineral aerosols over the Indian Ocean

Data are presented for the concentrations of Al, Fe, Mn, Ni, Co, Cr, V, Cu, Zn, Pb and Cd in aerosols collected over two contrasting regions of the Indian Ocean. These are: (1) the northern Arabian Sea (AS), from which samples were collected in the northeast monsoon, during which the region receives an input of crustal material from the surrounding arid land masses; and (2) the Tropical Southern Indian Ocean (TSIO), a remote region from which samples were collected from air masses for which there were no large-scale up-wind continental aerosol sources. The TSIO samples can be divided into two populations: Population I aerosols, collected from air masses which have probably impinged on Madagascar, and Population II aerosols, which have been confined to open-ocean regions to the south of the area.The data indicate that there are strong latitudinal variations in the chemical signatures of aerosols over the Indian Ocean. The input of crustal material to the Arabian Sea gives rise to an average Al concentration of about 1000 ng m⁻³ of air in the northeast monsoon regime. As a result, the concentrations of all trace metals are relatively high, and the values of crustal enrichment factors are less than 10 for most metals, in the AS aerosols. In contrast, TSIO Population II ‘open-ocean southern air’ sampled during the southwest monsoon season, has an average Al concentration of only about 10 ng m⁻³ of air. Trace metal concentrations in the TSIO ‘open-ocean southern air’ during the southwest monsoon season are representative of ‘clean’ remote marine air and are generally similar to those reported over the central North Pacific.Mineral dust concentrations over the Indian Ocean decrease in a north to south direction, from about 15–20 μg m⁻³ of air in the extreme north to about 0.01–0.25 μg m⁻³ of air in the far south. The deposition of mineral dust over the northern Arabian Sea can account for approximately 75% of the non-carbonate material incorporated into the underlying sediments.In the Arabian Sea the dissolved atmospheric inputs of all the trace metals, with the exception of Cu and Co, exceed those from fluvial run-off by factors which range from 9.6 for Pb to 1.6 for Cr.     

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.     

Effect of changes in water salinity on ammonium, calcium, dissolved inorganic carbon and influence on water/sediment dynamics

The effect of a sudden increase in salinity from 10 to 37 in porewater concentration and the benthic fluxes of ammonium, calcium and dissolved inorganic carbon were studied in sediments of a small coastal lagoon, the Albufera d'Es Grau (Minorca Island, Spain). The temporal effects of the changes in salinity were examined over 17 days using a single diffusion-reaction model and a mass-balance approach. After the salinity change, NH₄⁺-flux to the water and Ca-flux toward sediments increased (NH₄⁺-flux: 5000–3000 μmol m⁻² d⁻¹ in seawater and 600/250 μmol m⁻² d⁻¹ in brackish water; Ca-flux: −40/−76 meq m⁻² d⁻¹ at S=37 and −13/−10 meq m⁻² d⁻¹ at S=10); however, later NH₄⁺-flux decreased in seawater, reaching values lower than in brackish water. In contrast, Ca-flux presented similar values in both conditions. The fluxes of dissolved inorganic carbon, which were constant at S=10 (55/45 mmol m⁻² d⁻¹), increased during the experiment at S=37 (from 30 mmol m⁻² d⁻¹ immediately after salinity increase to 60 mmol m⁻² d⁻¹ after 17 days).In brackish conditions, NH₄⁺ and Ca²⁺ fluxes were consistent with a single diffusion-reaction model that assumes a zero-order reaction for NH₄⁺ production and a first-order reaction for Ca²⁺ production. In seawater, this model explained the Ca-flux observed, but did not account for the high initial flux of NH₄⁺.The mass balance for 17 days indicated a higher retention of NH₄⁺ in porewater in the littoral station in seawater conditions (9.5 mmol m⁻² at S=37 and 1.6 mmol m⁻² at S=10) and a significant reduction in the water consumption at both sites (5 mmol m⁻² at S=37; 35/23 mmol m⁻² at S=10). In contrast, accumulation of dissolved inorganic carbon in porewater was lower in seawater incubations (−10/−1 meq m⁻² at S=37; 50/90 meq m⁻² at S=10) and was linked to a higher efflux of CO₂ to the atmosphere, because of calcium carbonate precipitation in water (675/500 meq m⁻²). These results indicate that increased salinity in shallow coastal waters could play a major role in the global carbon cycle.     

Relationship between cadmium and phosphate in the northwest Pacific Ocean

The relationship between Cd and PO₄ in the Kuroshio and Oyashio regions and the Okhotsk Sea was examined. The resultant equations are as follows: Cd (ng l^–1)=37.0 PO₄ (M)+2.6; Cd(ng l^–1)=32.1 PO₄ (M)+1.2 and Cd (ng l^–1)=34.1 PO₄ (M)+7.9, respectively. These results are in good agreement with previously reported studies, and indicate that during removal from surface waters to deeper waters by biological assimilation and regeneration in deeper waters Cd and PO₄ maintain the same ratio in the open ocean. The relationship between Cd and PO₄ in coastal waters, however, differed from that in the open ocean.