In March 1998 we measured iron in the upper water column and conducted iron
- and nutrient-enrichment bottle-incubation experiments in the open-ocean S
ubantarctic region southwest of Tasmania, Australia. In the Subtropical Con
vergence Zone (similar to 42 degrees 5, 142 degrees E), silicic acid concen
trations were low (<1.5 mu M) in the upper water column, whereas pronounced
vertical gradients in dissolved iron concentration (0.12-0.84 nM) were obs
erved, presumably reflecting the interleaving of Subtropical and Subantarct
ic waters, and mineral aerosol input. Results of a bottle-incubation experi
ment performed at this location indicate that phytoplankton growth rates we
re limited by iron deficiency within the iron-poor layer of the euphotic zo
ne. In the Subantarctic water mass (similar to 46.8 degrees S, 142 degrees
E), low concentrations of dissolved iron (0.05-0.11 nM) and silicic acid (<
1 mu M) were measured throughout the upper water column, and our experiment
al results indicate that algal growth was limited by iron deficiency. These
observations suggest that availability of dissolved iron is a primary fact
or limiting phytoplankton growth over much of the Subantarctic Southern Oce
an in the late summer and autumn.