Iron and manganese in surface waters of the Australian subantarctic region

Iron and manganese concentrations were measured in seawater from the upper water column (less than or equal to 350 m water depth) in the subantarctic region to the south and west of Tasmania, Australia, during January 1995. Dissolved and total-dissolvable Fe and dissolved Mn were determined in samples from four stations along 140 degrees E ranging from polar to subtropical waters, and from one station over the continental slope off north-west Tasmania. Dissolved Fe concentrations were generally low (<1 nM) for all stations, ranging from minimum concentrations between 0.1 and 0.2 nM at the southernmost stations to maximum values near 0.8 nM in subtropical waters and over the continental slope. Total-dissolvable Fe concentrations ranged from 0.17 to 1.3 nM, and were more than twice the dissolved Fe concentrations in some samples, implying a significant proportion of particulate Fe in the water column. Dissolved Mn concentrations were generally low (<0.5 nM) along the 140 degrees E transect, with lowest values of around 0.1 nM measured at the southernmost stations, and much higher concentrations (> 2 nM) observed in shallower waters over the continental slope. Vertical concentration profiles suggest biological removal of dissolved Fe from the photic zone at the deep-ocean stations, except in the subantarctic front. The higher mixed-layer concentrations of Fe and Mn observed over the continental slope and in the deep ocean at 40 degrees S are thought to reflect inputs from the continental shelves or atmospheric deposition. Water-column Fe and Mn profiles from 45 degrees S, 50 degrees S and 53 degrees S show no evidence for significant inputs of these metals via atmospheric deposition or upwelling of metal-rich deep waters, although there was evidence of shelf-derived particulate Fe at 45 degrees S. The low concentrations of dissolved Fe measured at 45 degrees S, 50 degrees S and 53 degrees S indicate that iron deficiency could limit summertime primary production in these high-nitrate low-chlorophyll waters. (C) 1997 Elsevier Science Ltd.