Some environmental factors influencing phytoplankton in the Southern Ocean around South Georgia

Data on phytoplankton and zooplankton biomass, and physical and chemical variables, are combined with a published multivariate description of diatom species composition to interpret variation within an area around South Georgia surveyed during an austral summer. Large-scale species distributions could be equated to the different water masses which reflected the interaction of the Antarctic Circumpolar Current with the island and the Scotia Ridge. Small-scale factors were found to act at an interstation scale and imposed local variation on the biogeographic pattern. Nutrient depletion could be related to phytoplankton biomass but no single inorganic nutrient of those measured (NO 3 −N, PO 4 −P and silica) could be identified as important. The ratio Si:P appeared to be more important as an ecological factor. The impact of grazing by krill and other zooplankton could only be resolved as differences in phytoplankton biomass and phaeopigment content. Diatom species composition showed a relation to local krill abundance very different from that suggested by published studies, but could be explained as the effect of earlier grazing outside the study area. The effects of vertical mixing could not account for interstation differences as pycnocline depth was uniformly greater than euphotic depth, and vertical stability very low. Some comparison was made with data collected in 1926–31 by the Discovery Investigations. Significant differences in the distribution of certain taxa such as Chaetoceros criophilum and C. socialis were traced to major differences in hydrology.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/46983/1/300_2004_Article_BF00443379.pd

Influence of the Partial-pressure of Oxygen On the Redox Chemistry of Multivalent Elements in Molten Sodium Tetraborate

Voltammetric determinations based on current measurements have been developed in order to allow an in situ evaluation of the redox ratios of multivalent elements in molten sodium tetraborate. The same technique has been used to study the influence of the partial pressure of oxygen in the gaseous atmosphere on the redox properties of sodium tetraborate melts. Experiments were carried out at 1273 K in solutions of chromium with oxidation states VI and III and of antimony with oxidation states V and III, the P(O2) values ranged from 5 x 10(-5) atm to 1 atm. Log [Ox]/[Red] plotted against log P(O2) shows linear relationships with slopes of 0.72 and 0.54 in the case of chromium and antimony solutions, respectively. These values lie close to the theoretical values of 0.75 and 0.50, respectively, which are deduced assuming conditions of equilibrium between oxygen and the solute. In agreement with these results, voltammetric and potentiometric determinations confirm Nernstian behaviour of the oxygen-oxide redox system