Search

Iron zinc-enrichment experiments were carried out at Ocean Station Papa in the subarctic North Pacific. In iron-enriched treatments, phytoplankton chlorophyll a (Chl a) increased 20-fold (9.7 µg L-1) above the concentration on day zero. No stimulation of Chl a production or nitrate drawdown was observed on addition of zinc alone compared to the control. In the iron-enriched treatment, bioavailable zinc concentration decreased to 0.2 pmol L-1 lower than that which is known in culture experiments to limit some phytoplankton growth. Theoretical analyses suggest that this zinc concentration would cause diffusion-limited growth of large diatom cells present at the end of the incubation. Direct measurements of zinc-binding ligands indicate that the natural microbial planktonic assemblages have the ability to respond rapidly to conditions of high dissolved zinc concentrations. Rapid ligand production may be a mechanism by which certain phytoplankton reduce zinc toxicity or for maintaining zinc concentrations in the upper water column. Zinc-binding ligands were observed to be both produced and removed on the timescale of 1 d. We suggest that these zinc-binding ligands are produced to assist assimilation, particularly under iron-enriched conditions when concentrations of bioavailable zinc were extremely low, thereby alleviating the effects of zinc limitation. [ABSTRACT FROM AUTHOR]

In the northern Gulf of Alaska, mesoscale, anticyclonic eddies have been implicated as a mechanism of the cross-shelf exchange of iron (Fe)-replete coastal waters with Fe-deplete subarctic Alaskan gyre waters. Based on existing hydrography and macronutrient distributions, a Sitka eddy sampled during August 2007 is divided into a surface eddy core, a shallow subsurface eddy core, and a deeper subsurface eddy core. The distributions of aluminum (Al), manganese (Mn), and Fe in the eddy are examined and compared to distributions at shelf stations similar to where the eddy formed as well as basin stations representative of the Fe-limited subarctic Alaskan gyre. Relative to basin stations, dissolved and particulate Al and dissolved Mn were elevated in eddy core waters. Reactive Fe concentrations within the shallow subsurface eddy core were nearly seven times greater than reactive Fe concentrations at similar densities at the basin stations. This shallow subsurface eddy core is likely mixed into the euphotic zone during storm-induced mixing as well as mixing during isopycnal relaxation as an eddy dies out. The flux of reactive Fe to surface waters from the shallow subsurface core of eddies in the Gulf of Alaska is a significant source compared to both remote and local atmospheric dust deposition. It is hypothesized that Fe supply from eddies in the eastern Gulf of Alaska shifts the core of the high-nutrient, lower-than-expected chlorophyll water farther west in conjunction with an "eddy desert" in that same region. [ABSTRACT FROM AUTHOR]

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 B12, relative to iron additions alone. Initial bacterial abundances were significantly lower in the two experiments that showed phytoplankton stimulation upon addition of B12 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 B12 to marine phytoplankton. The addition of iron alone increased the growth of Phaeocystis antarctica relative to diatoms, whereas in an experiment where iron and B12 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 B12 plus iron treatments caused reductions in the DMSP (dimethyl sulfoniopropionate) : Chl a ratio relative to the control and B12 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. [ABSTRACT FROM AUTHOR]