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Primary production in the Ross Sea, one of the most productive areas in the Southern Ocean, has previously been shown to be seasonally limited by iron. In two of three bottle incubation experiments conducted in the austral summer, significantly higher chlorophyll a (Chl a) concentrations were measured upon the addition of iron and B 12 , relative to iron additions alone. Initial bacterial abundances were significantly lower in the two experiments that showed phytoplankton stimulation upon addition of B 12 and iron relative to the experiment that did not show this stimulation. This is consistent with the hypothesis that the bacteria and archaea in the upper water column are an important source of B 12 to marine phytoplankton. The addition of iron alone increased the growth of Phaeocystis antarctica relative to diatoms, whereas in an experiment where iron and B 12 stimulated total phytoplankton growth, the diatom Pseudonitzschia subcurvata went from comprising approximately 70% of the phytoplankton community to over 90%. Cobalt additions, with and without iron, did not alter Chl a biomass relative to controls and iron additions alone in the Ross Sea. Iron and vitamin B 12 plus iron treatments caused reductions in the DMSP (dimethyl sulfoniopropionate) : Chl a ratio relative to the control and B 12 treatments, consistent with the notion of an antioxidant function for DMSP. These results demonstrate the importance of a vitamin to phytoplankton growth and community composition in the marine environment.The nutritional controls on marine phytoplankton growth have important implications for the regulation of the global carbon cycle. Nitrogen and iron are thought to be the dominant controllers of phytoplankton growth in the oceans, and hence the discovery of a vitamin such as B 12 having an influence on marine primary productivity would be a finding of significance. The limited information about the biogeochemical cycle of this vitamin suggests that it may be in limiting quantities in seawater. B 12 is a biologically produced cobalt-containing organometallic molecule, and only select bacteria and archaea possess the capability for B 12 biosynthesis. As a result, all eukaryotic organisms, from eukaryotic phytoplankton to humans, must either acquire B 12 from the environment or possess an alternate biochemistry that does not require the vitamin. Removal of B 12 from the water column has never been directly quantified but likely includes photodegradation 1 Corresponding author (mak@whoi.edu). 2 Coauthors. AcknowledgmentsWe thank Peter Sedwick for allowing us to utilize his tracemetal-clean fish sampling system and David Hutchins for allowing us to work in his laboratory van and for helpful discussions. We also thank Bettina Sohst and Carol Pollard for nutrient analyses and Tyler Goepfert for help in Phaeocystis antarctica culture studies, and Sheila Clifford for comments on the manuscript. Special thanks to the captain, crew, and Raytheon marine and scientific technical staff of the RV N. B. Palmer. Thanks also to ...

We present full-depth zonal sections of total dissolved cobalt, iron, manganese, and labile cobalt from the South Atlantic Ocean. A basin-scale plume from the African coast appeared to be a major source of dissolved metals to this region, with high cobalt concentrations in the oxygen minimum zone of the Angola Dome and extending 2500 km into the subtropical gyre. Metal concentrations were elevated along the coastal shelf, likely due to reductive dissolution and resuspension of particulate matter. Linear relationships between cobalt, N 2 O, and O 2 , as well as low surface aluminum supported a coastal rather than atmospheric cobalt source. Lateral advection coupled with upwelling, biological uptake, and remineralization delivered these metals to the basin, as evident in two zonal transects with distinct physical processes that exhibited different metal distributions. Scavenging rates within the coastal plume differed for the three metals; iron was removed fastest, manganese removal was 2.5 times slower, and cobalt scavenging could not be discerned from water mass mixing. Because scavenging, biological utilization, and export constantly deplete the oceanic inventories of these three hybrid-type metals, point sources of the scale observed here likely serve as vital drivers of their oceanic cycles. Manganese concentrations were elevated in surface waters across the basin, likely due to coupled redox processes acting to concentrate the dissolved species there. These observations of basin-scale hybrid metal plumes combined with the recent projections of expanding oxygen minimum zones suggest a potential mechanism for effects on ocean primary production and nitrogen fixation via increases in trace metal source inputs.

Abstract. We report the distribution of cobalt (Co) showing a significant correlation throughout the water column (r 2 = 0.87, 164 samples). A strong seasonal signal for dCo was observed, with most spring samples having concentrations ranging from ∼45-85 pM, whereas summer dCo values were depleted below these levels by biological activity. Surface transect data from the summer cruise revealed concentrations at the low range of this seasonal variability (∼30 pM dCo), with concentrations as low as 20 pM observed in some regions where PO 3− 4 was depleted to ∼0

The vertical distributions of cobalt, iron, and manganese in the water column were studied during the E-Flux Program (E-Flux II and III), which focused on the biogeochemistry of cold-core cyclonic eddies that form in the lee of the Hawaiian Islands. During E-Flux II (January 2005) and E-Flux III (March 2005), 17 stations were sampled for cobalt (n =147), all of which demonstrated nutrient-like depletion in surface waters. During E-Flux III, two depth profiles collected from within a mesoscale coldcore eddy, Cyclone Opal, revealed small distinct maxima in cobalt at ~100m depth and a larger inventory of cobalt within the eddy. We hypothesize that this was due to a cobalt concentrating effect within the eddy, where upwelled cobalt was subsequently associated with sinking particulate organic carbon (POC) via biological activity and was released at a depth coincident with nearly complete POC remineralization (Benitez-Nelson et al. 2007). There is also evidence for the formation of a correlation between cobalt and soluble reactive phosphorus during E-Flux III relative to the E-Flux II cruise that we suggest is due to increased productivity, implying a minimum threshold of primary production below which cobalt-phosphate coupling does not occur. Dissolved iron was measured in E-Flux II and found in somewhat elevated concentrations (~0.5nM) in surface waters relative to the iron depleted waters of the surrounding Pacific (Fitzwater et al. 1996), possibly due to island effects associated with the iron-rich volcanic soil from the Hawaiian Islands and/or anthropogenic inputs. Distinct depth maxima in total dissolved cobalt were observed at 400 to 600m depth, suggestive of the release of metals from the shelf area of comparable depth that surrounds these islands.