Abstract: Phytoplankton dynamics in the region of 55-70°S, 170°W were investigated using Sea-viewing Wide Field-of-View Sensor satellite imagery, shipboard sampling and experimental rate assessments during austral spring and summer, 1997-1998. We used image-analysis microscopy to characterize community biomass and composition, and dilution experiments to estimate growth and microzooplankton grazing rates. Iron concentrations were determined by flow injection analysis. The phytoplankton increase began slowly with the onset of stratification at the Polar Front (PF) (60-61°S) in early November. Seasonally enhanced levels of chlorophyll were found as far north as 58°S, but mixed-layer phytoplankton standing stock was highest, approaching 200 mg C m-3, in the region between the receding ice edge and a strong silicate gradient, which migrated from similar to 62°S to 65°S during the study period. The most southern stations sampled on four cruises were characterized by small pennate diatoms and Phaeocystis. From the PF to the Southern Antarctic circumpolar current front (similar to 65°S), this ice margin assemblage was seasonally replaced by a community dominated by large diatoms. The large diatom community developed only in waters where measured iron concentrations were initially high (≥0.2 nM), and crashed when dissolved silicate was depleted to low levels. Phytoplankton growth rates were highest (0.5-0.6 d-1) between the PF and silicate front (60°S and 63°S) in December. In January, growth rates were lowest (0.1 d-1) near the PF, and the highest rates (0.34.4 d-1) were found in experiments between 64.8°S and 67.8°S. Phytoplankton production estimates were highest south of the PF through December and January, averaging 2.2-2.4 mmol C m(-3) d-1 and reaching levels of 5 mmol cm(-3) d-1 (64.8°S and 67.8°S in January). Microzooplankton grazers consumed 54-95% of production for experiments conducted on four AESOPS cruises. They were less efficient in balancing growth rates during the time of highest phytoplankton growth and increase in December, and most efficient in February-March, after the large diatom bloom had collapsed. The diatom bloom region in the present study is in an upwelling zone for Antarctic circumpolar deep water with high iron content. This may explain why this marginal ice zone differs from others where blooms have not been observed. (C) 2002 Published by Elsevier Science Ltd.

Abstract: In order to study the factors controlling the phytoplankton distribution across the Antarctic Polar Frontal Region (PFR), surface pigment samples were collected during austral summer (January/February 1998) near 170°W. Both the Polar Front (PF) and the Southern Antarctic Circumpolar Current Front (SACCF) were regions of enhanced accumulation of phytoplankton pigments. The mesoscale survey across the PF revealed two distinct phytoplankton assemblages on either side of the front. The phytoplankton community was dominated by diatoms south of the PF and by nanoflagellates (primarily by prymnesiophytes) to the north. Surprisingly, chlorophyll a concentrations did not correlate with mixed-layer depths. However, an increase of the dominance of diatoms over prymnesiophytes was observed with decreasing mixed-layer depths. Despite this relationship, we conclude that the average light availability in the mixed layer was not an important factor influencing the shift in phytoplankton composition across the PF. Although no correlation was found between the surface distribution of the major phytoplankton taxa and dissolved iron or silicic acid concentrations, the location of the strongest vertical gradient in silicic acid and iron concentration coincides with the maximum abundance of diatoms. We conclude that the difference in taxonomic composition is a result of increased silicic acid and iron flux to the upper mixed layer as a result of the increased vertical gradient of these key nutrients south of the front.

Abstract: The concentration of dissolved Fe was determined in upper ocean waters along 170°W between 53 and 72°S during four cruises between October 1997 and March 1998 as part of the US-JGOFS Southern Ocean Antarctic Polar Frontal Zone cruises. Fe concentrations were extremely variable in both space and time as a result of advection of meso-scale eddys through the region. Observed values ranged from a maximum of 0.34 nM at 64°S in November next to the retreating ice-edge to 0.075 nM in March at 71°S in the northern Ross Sea gyre. In general, the highest average mixed-layer Fe concentrations and the largest seasonal changes were observed in the two frontal zones at similar to 60°S and similar to 64°S. Formation of deep mixed layers during winter and the entrainment of sub-surface waters enriched in Fe is the primary source of this element to surface waters in this region of the Southern Ocean. Additionally, upwelling of circumpolar deep water in the circumpolar current along its northern boundary at the Polar Front and its southern boundary at the Southern Antarctic Circumpolar Current Front is an important mechanism supplying Fe to surface waters in these areas throughout the summer. Comparison of integrated Fe concentrations and Th-derived POC export and N drawdown all suggest that the maximum changes in these properties are comparable to those that would be predicted by Fe availability. Fe concentrations determined by shipboard Flow Injection Analysis (Measures et al., Mar. Chem. 50 (1995) 3) were significantly higher than those found in replicate samples determined by shored based flameless atomic absorption spectrophotometry after pre-concentration by chelation-solvent extraction (Johnson et al., Mar. Chem. 57 (1997) 137). The concentration differences, which appear to be restricted to the upper 200-300 m, do not appear to arise from different sampling or handling processes but instead are believed to result from the different sensitivity of the methods to organic fractions of Fe in seawater.

Abstract: The subtropical to equatorial Atlantic Ocean provides a unique regime in which one can examine the biogeochemical cycles of antimony and arsenic. In particular, this region is strongly affected by inputs from the Amazon River and dust from North Africa at the surface, and horizontal transport at depth from high-latitude northern (e.g., North Atlantic Deep Water) and southern waters (e.g., Antarctic Bottom and Intermediate Waters). As a part of the 1996 Intergovernmental Oceanographic Commissions Contaminant Baseline Survey, data for dissolved As(III + V), As(III), mono- and dimethyl arsenic, Sb(III + V), Sb(III), and monomethyl antimony were obtained at six vertical profile stations and 44 sites along the 11,000 km transect from Montevideo, Uruguay, to Bridgetown, Barbados. The arsenic results were similar to those in other oceans, with moderate surface depletion, deep-water enrichment, a predominance of arsenate (> 85% As(V)), and methylated arsenic species and As(III) in surface waters that are likely a result of phytoplankton conversions to mitigate arsenate stress (toxicity). Perhaps the most significant discovery in the arsenic results was the extremely low concentrations in the Amazon Plume (as low as 9.8 nmol/l) that appear to extend for considerable distances offshore in the equatorial region. The very low concentration of inorganic arsenic in the Amazon River (2.8 nmol/l; about half those in most rivers) is probably the result of intense iron oxyhydroxide scavenging. Dissolved antimony was also primarily in the pentavalent state (> 95% antimonate), but Sb(III) and monomethyl antimony were only detected in surface waters and displayed no correlations with biotic tracers such as nutrients and chlorophyll a. Unlike As(III + V)s nutrient-type vertical profiles, Sb(III + V) displayed surface maxima and decreased into the deep waters, exhibiting the behavior of a scavenged element with a strong atmospheric input. While surface water Sb had a slight correlation with dissolved Al, it is likely that atmospheric Sb is delivered with combustion byproducts and not from mineral aerosols. In the Amazon Plume, antimony concentrations dropped substantially, and an Amazon River sample had a concentration (0.25 nmol/l) that was Less than one-fourth those found in other major rivers. Using these river data, and estimates of atmospheric fluxes based on shipboard measurements and collections from Barbados, the atmospheric deposition of antimony to the equatorial Atlantic (2°S-8°N) is twice the Amazon flux, while the atmospheric deposition of arsenic is only 10% of the rivers flux.

Abstract: Underway dissolved Al and Fe determinations were made on surface-water samples collected between 34°S and 8°N in the southern and tropical Atlantic Ocean using a towed surface-water sampler. The general distribution of dissolved Al, which ranged from similar to8 nM in the sub-tropical gyre to similar to 61 nM under the Intertropical Convergence Zone (ITCZ) north of the Equator, corresponds clearly with atmospheric dust deposition patterns. A striking correlation between Al and surface-water salinities between 28°S and 8°N further indicates that wet deposition is the predominant mechanism of delivery of this atmospheric source. Iron concentrations, in contrast, were more uniform varying between 0.6 and 0.8 nM south of the equator. Highest Fe values (up to 1.4 nM) also were found associated with the maximum dust inputs at the ITCZ, indicating an atmospheric source for this element whose input appears to be limited by the solubility of Fe in sea water. The lowest Fe values (similar to 0.4 nM) also were observed in the equatorial region and are assumed to result from the biological removal of Fe from nutrient-rich water masses originating in the nearby equatorial and coastal upwelling regions. The high-frequency data acquisition of the towed sampler revealed significant variations (up to 50%) in both Al and Fe concentrations over short distances, most notably along the interface between the South Equatorial and Canary Currents. The high variability of a biologically important trace element such as Fe in these dynamic regions is indicative of the interplay between chemical and biological patchiness in surface waters.

Abstract: Undisturbed sediment samples were collected for chemical analyzes at six sites during winter and summer cruises to the eastern Canadian continental margin. Micro-electrode oxygen profiles were obtained in freshly collected multicorer samples, and replicate cores were incubated at in situ temperature for 48 h to monitor changes in the concentrations of dissolved oxygen and nitrate. In addition, box cores were subsampled vertically for porewater chemistry, porosity, and particulate carbon. The data obtained are combined with estimates of sedimentation rate based on sediment trap measurements, Pb-210 dating and historical data to evaluate the role of benthic processes in the carbon cycle on the eastern Canadian continental margin. With one exception, oxygen uptake rates determined from incubations and calculated from micro-profiles were very similar, indicating that exchange of oxygen across the sediment-water interface was dominated by molecular diffusion. On the basis of this observation, transport by diffusion is assumed for the calculation of the flux rates for other solutes from their respective porewater gradients. The fluxes of oxygen into the sediments were low, but generally comparable to other continental margins at comparable depths. They varied from 1.4 to 1.8 mmol/m2/d in December 1993 and from 2.8 to 4.5 mmol/m2/d in June 1994. Uptake of nitrate by the sediment occurred at all sites except for the continental slope off Nova Scotia. Both oxygen and nitrate uptake were higher in summer than in winter, indicative of a lingering response to the input of organic matter associated with the early spring bloom. At one of the sampling sites, Miscou Channel, the measured oxygen uptake rate far exceeded the flux calculated from the oxygen gradient. The difference suggests biologically enhanced exchange with the overlying waters at this site, consistent with the greater abundance of benthic organisms. The rate of organic carbon mineralization at the seafloor(1.6 - 4.2 mmol/m2/d) was estimated as the sum of the sediment oxygen and nitrate uptake rates. The contribution of other electron-accepters to the mineralization of organic carbon is assumed to be accounted for by the oxidation of most of their reduced by-products by O2 and NO3-. The return fluxes of dissolved inorganic carbon (Sigma CO2 efflux corrected for carbonate dissolution) were generally comparable to the carbon oxidation rates. A detailed carbon budget was established at one of the sampled stations for which a complete set of measurements is available. The vertical flux of total particulate carbon through the water column (measured with a sediment trap at 150 m depth) accounts for approximately 9 % of the annual primary production (PP). In the sediment, about 6% of the PP is remineralized and total carbon equivalent to 4.5% PP is buried. The self-consistency of the flux data at this station is used to justify similar budget calculations at the other stations where a limited data set was gathered. Sequestration of organic carbon via burial with the accumulating sediments ton average 0.5 mol/m2/yr) is partially offset by the release of CO2 associated with carbonate precipitation and burial (about 0.2 mol/m2/yr).

Abstract: Concentrations of dissolved Al and Fe in the surface mixed layer were measured during: five cruises of the 1995 US JGOFS Arabian Sea Process Study, Concentrations of both Al and Fe were relatively uniform between January and April, the NE Monsoon and the Spring Intermonsoon period, ranging from 2 to 11 nM Al (mean 5.3 nM) and 0.5 to 2.4 nM Fe (mean 1.0 nM). In July/August, after the onset of the SW Monsoon, surface water Al and Fe concentrations increased significantly (Al range 4.5-20.1 nM; mean = 10 nM, Fe range 0.57-2.4 nM; mean = 1.3 nM), particularly in the NE part of the Arabian Sea, as the result of the input and partial dissolution of eolian dust. Using the enrichment of Al in the surface waters, we estimate this is the equivalent to the deposition of 2.2-7.4 g m-2 dust, which is comparable to values previously estimated for this region. Approximately one month later (August/September), surface water concentrations of both Al and Fe were found to have decreased significantly (mean Al 7.4 nM, mean Fe 0.90 nM) particularly in the same NE region, as the result of export of particulate material from the euphotic zone. Fe supply to the surface waters is also affected by upwelling of sub-surface waters in the coastal region of the Arabian Sea during the SW Monsoon. Despite the proximity of high concentrations of Fe in the shallow sub-oxic layer, freshly upwelled water is not drawn from this layer and the NO3/Fe ratio in the initially upwelled water is below the value at which Fe limitation is through to occur. Continued deposition of eolian Fe into the upwelled water as it advects offshore provides the Fe required to raise this ratio above the Fe limitation value.

Abstract: We sampled hydrothermal plumes over the N. Gorda Ridge four times between March and August 1996 to document Mn and Fe discharge resulting from a magmatic intrusion/seafloor eruption. Two separate event plumes, EP96A and B, and chronic hydrothermal emissions lasting similar to 6 months were characterized. Shipboard time-series measurements of an event plume sample were used to calibrate an Fe phase clock useful for estimating sample age up to similar to 6 days after fluid discharge. Samples collected from EP96A. and B had Mn/heat (< 0.15 nmol J-1) and Fe/Mn (> 2 mol mol-1) ratios similar to historical event plume observations. We suggest these 'signature' ratio values are generally characteristic of event plumes and hypothesize that Mn and Fe may be supplied to event plumes by different processes: Mn by entrainment of fluids from an extant shallow subseafloor reservoir, and Fe by short-lived, high-temperature water-rock reaction coincident with dike emplacement. Calculations based on the Fe phase clock indicate that the two event plumes were released more than a month apart. The largest event plume, EP96A (similar to 2.3 x 10(6) M Mn and 13 x 10(6) M Fe), formed similar to 7 March soon after seismic activity began. The smaller EP96B (similar to 0.49 x 10(6) M Mn and 3.5 x 10(6) M Fe) was not discharged until similar to 11 April, 3 weeks after the cessation of seismic activity detectable by SOSUS T-phase monitoring. We hypothesize that the subseafloor disturbance that triggered EP96B also resulted in the episodic flushing of a reservoir of chronic-plume-like fluids. Total event plume inventories of Mn and Fe at N. Gorda Ridge are much smaller than those associated with the 1986 event at N. Cleft segment of the Juan de Fuca Ridge, but comparable to event plume inventories at N. Cleft segment in 1987 and CoAxial segment in 1993, Mn/heat values for chronic plumes over the eruption site underlying EP96A evolved from moderate (similar to 0.25 oon-mol J-1, reflecting probable admixture with event plume formation fluids) to high (similar to 0.7 nmol J-1, typical of chronic plumes) to low (similar to 0.1 nmol J-(1,) similar to diffuse vent fluid values), marking a complete episode of intrusion/eruption-induced hydrothermal discharge.