Trace metals Cd, Co, Cu, Ni, and Zn in waters of the subantarctic and Polar Frontal Zones south of Tasmania during the ‘SAZ-Sense’ project

Trace metal micronutrients regulate primary production in oceanic surface waters, particularly those characterised as ‘high nutrient, low chlorophyll’, such as the Subantarctic Zone (SAZ). Our goal was to evaluate the distribution and biogeochemistry of Cd, Co, Cu, Ni and Zn in the upper 1000 m of this zone to the south of Australia during the SAZ-Sense Project (Jan–Feb 2007). 13 depth profiles were sampled for dissolved, labile metal measurements through subtropical and SAZ waters – west and east of Tasmania – and southward into the Polar Frontal Zone. We determined Cd from detection limit < 4 pM to 900 pM, Co 5–43 pM, Cu 0.22–1.79 nM, Ni 2.51–7.31 nM and Zn < 0.22–7.9. Throughout the study area, Cd, Cu and Ni correlated closely with the macronutrient phosphorus, but their depletion (relative to winter levels) in surface waters differed: Cd (64–> 96%), Cu (2–35%), Ni (7–20%). The profile south of the Polar Front contrasted markedly with all others, having the lowest depletions in surface waters. Another discordancy was some anomalously high Cu concentrations in surface waters close to Tasmania. Co behaved as a recycled (nutrient) element correlating with P and Chl a in the top 200 m of the water column, but deeper it was different with a distinct subsurface peak. The characteristics of this peak suggested lateral supply of dissolved, labile Co from polar surface waters to lower-latitude ocean depths via Antarctic Intermediate Waters. Zn also typified a recycled micronutrient. It was loosely correlated with the macronutrient silicon—more strongly to the south and in the Polar Frontal Zone (PFZ). In the vicinity of the Subtropical Front to the east of Tasmania, dissolved Zn was decoupled from other micro- and macro-nutrients. Its irregular distribution suggested this region's dynamic eddy field and possibly variable supply and a phytoplankton community dominated by non-diatom species as influential factors. Distributions of Cd, Cu, Ni and Co were much more uniform in the vicinity of the Subtropical Front (only dissolved Fe data from SAZ-Sense exemplified similar patchiness in distribution as Zn), distinguished solely by being either side of the Front, in subtropical or subantarctic waters. These new data for Cd, Co, Cu, Ni and Zn improve scant coverage for them in the Australian sector of the Southern Ocean, and provide a basis for linking their distribution to regional primary productivity and variations in phytoplankton community structure.

► Cd, Ni, Cu and Co (in top 200 m only) were highly correlated to phosphate in SAZ/PFZ
► Deeper profile of Co in SAZ was dominated by intermediate water inputs
► Zn, like Fe, in the dynamic STF region was decoupled from nutrients
► Zn & Fe in STF region respond to change on short timescales, but differently
► Cd, Ni, Cu and Co in STF region respond to change at seasonal or longer timescales