Shipboard studies were performed for testing the classical hypothesis that Antarctic phytoplankton suffers from a deficiency of Fe In a suite of 5 experiments over 8 to 12 d periods and encompassing different water masses (Weddell Sea water proper, Weddell-Scotia Confluence water, Scotia Sea water), and various plankton communities, biomass and dynamic spring/summer (ice) conditions, we always observed Fe to stimulate chlorophyll a synthesis and nutrient assimilation. In 3 out of 5 experiments there was an immediate response to added Fe, while in the other 2 expenments an effect was observed after 3 to 6 d. In 4 out of 5 experiments final particulate organic carbon (POC) levels were also higher in Fe-enriched cultures compared to controls. However the controls were also found to outgrow steadily typical chlorophyll a and POC levels found in ambient waters. This strongly suggests that the in situ Fe concentration in itself does not hamper build-up of high biomass levels. Extrapolation to the in situ ecosystem therefore suggests that, despite enhancement of phytoplankton growth, Fe is not the major factor controlling phytoplankton in the Weddell/Scotia Seas. Marginal sediments appear to supply adequate dissolved Fe for supporting at least minimum growth of phytoplankton. More remote sectors of the Southern Ocean might be more likely candidates for occasional limitation by Fe alone.

During the European Polarstern Study (EPOS 1988(EPOS /1989 in the Weddell and Scotia Seas, five series of metal enrichment experiments were carried out with natural plankton communities under ultraclean conditions. Despite a clear stimulation of growth by the addition of Fe, control bottles (no additions) also showed rapid buildup of Chl a and complete utilization of a major nutrient within 2 weeks, indicating nonlimiting ambient Fe levels. Effects of Mn additions were less pronounced or absent, whereas extra additions of Zn and Cu in one experiment showed little or no effect. The species composition of the plankton community, monitored by HPLC pigment analysis and microscopic observations, changed in favor of diatoms when Fe was added. The addition of Fe also caused an increase in microzooplankton densities and concentrations of pigment breakdown products. However, metal-mediated shifts in the plankton community were minor compared to major changes resulting from incubation. Changes were most pronounced in experiments where microzooplankton was strongly developed, presumably as a result of excluding mesozooplankton from the bottles. Fe had an impact on plankton growth and species composition, but other factors seem to be responsible for keeping phytoplankton productivity far from its potential in these Antarctic waters.Over the past few years a lively discussion has come about on the matter of the presumed limiting role of Fe in various oceanic, high nutrient level ecosystems (e.g.

Cadmium (Cd) is one of the best studied trace metals in seawater and at individual stations exhibits a more or less linear relation with phosphate. The compilation of all data from all oceans taken from over 30 different published sources into one global dataset yields only a broad scatterplot of Cd versus phosphate. However, the smaller high-quality dataset obtained by rigorous selection of only those stations with uniform Cd/PO4-ratio in the deep waters, provides a consistent global description of the deep (> 1000 m) waters. The deep Cd/POn-ratio increases from about 0.18 x 10 3 in the subarctic North Atlantic to about 0.33-0.35 × 10 3 in the northern Indian and Pacific Oceans, in accordance with increasing phosphate content, i.e. age, of the deep water. The increasing Cd/PO4-ratio with age (and phosphate) of the deep water masses is a function of the coupling between biogeochemical cycling and deep water circulation. Changes in the latter, for example during a glacial period, inevitably lead to significant shifts in the Cd/PO4 relationship of seawater.There is a statistically significant bimodality of deep Atlantic versus deep Antarctic/Indo/Pacific waters, suggesting that the deep Atlantic is a distinct biogeochemical province for Cd cycling. This distinction is likely caused by the high inventories of both Cd and phosphate in Weddell Sea source waters. For each of both populations, a given concentration of phosphate yields a predicted value of Cd within il00 pM (Atlantic) and ~200 pM (Antarctic/Indo/Pacific), respectively, at the 95% confidence level. If one ignores the bimodality, then for a given phosphate the corresponding Cd might be predicted within i150 pM at the 95% confidence level; the validity of this is currently being verified by studies of South Atlantic waters which may or may not provide the missing link between both populations.Currently, the global distribution of the Cd/PO4-ratio in surface, thermocline and deep waters is consistent with preferential biogeochemical removal of Cd versus phosphate from surface waters. The net result for Cd/PO 4 is not dissimilar to the preferential surface removal of ~2C over 13C driving the deep distribution of the dissolved 12C/13C_ratio, although for Cd/PO 4 the underlying mechanism is obviously very different and not well understood.

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