Seasonal comparisons between dissolved trace metals (Copper, Zinc and Nickel. in the Southern Ocean. Insights into deep winter mixing

Abstract

The re-occupation of 3 stations (46°S, 50°S, 54°S. along the Bonus Goodhope Line, Southern Ocean, during a consecutive summer-winter cycle allowed us to investigate the biogeochemical cycling of three bio-active elements, copper (Cu., zinc (Zn. and nickel (Ni., in summer, when conditions favoured algal growth (e.g. high light levels and a stable, shallow mixed layer., and in winter, when conditions limit algal growth (e.g. low light levels and a deep mixed layer.. Results have implications in constraining the importance of winter deep mixing (entrainment. in supplying essential trace metals to depleted surface waters which then sustains phytoplankton productivity over the subsequent spring and summer seasons. Profiles of dissolved copper (dCu., zinc (dZn. and nickel (dNi. showed a general nutrient type behaviour throughout the study. In summer, dZn observed the greatest concentration range with sub-nanomolar surface concentrations increasing to 8 nmol kg-1 in bottom waters. dCu was typically 1 nmol kg-1 in surface waters and increased to 3 nmol kg-1 at depth. dNi showed a comparatively conservative profile throughout the water column with measured values between 5 and 7 nmol kg-1. Trace metal seasonality was most apparent in the mixed layer where average winter concentrations exceeded summer average concentrations by 0.2 nM for dCu, 0.9 nM for dZn and 0.5 nM for dNi. Summer-winter profiles in the intermediate and deep waters (>500m., were tightly correlated for each metal indicating a loss of seasonal signal in the deeper waters. Subsurface trace metal reservoirs were calculated by depth integrating the metal profiles between surface and mixed layer depth. Results show that the subsurface reservoir, that portion of the water column between the summer and winter mixed layers, contained between 59.18 – 139.64 nmol m-2 dCu, 56.77 – 344.21 nmol m-2 dZn and 243.74 – 675.70 nmol m-2 dNi, which represents an important source of micronutrients, at times larger than the summer mixed layer inventory, during the onset of the growth season. - Abstract as displayed in the - Abstract booklet. The presentation on the day may differ from the - Abstract.