Fluorimetric Detection of Total Dissolved Zinc in the Southern Indian Ocean

Zinc acts as a micronutrient in the ocean, capable of influencing and potentially controlling phytoplankton productivity and community structure. Thus, it is important to quantify the distribution of dissolved Zn in the oceans, in addition to understanding the biogeochemical behavior of this important element. Meeting this objective has been elusive since dissolved Zn concentrations in the upper water column can be extremely low, and it is difficult to eliminate contamination during sample collection and analysis. Our approach to this problem was to utilize a Flow Injection Analysis (FIA) method initially described by Nowicki et al. (1994). Shipboard FIA can reveal contamination from sample collection and initial processing. In this Zn-FIA method, dissolved Zn was pre-concentrated using a small-volume column of 8-HQ cation exchange resin. After column rinsing, Zn was eluted into the flowing stream of organic reagent p-Tosyl-8-aminoquinoline (pTAQ), which forms fluorescent complexes with Zn. A flow-through fluorometer was used to record peak heights. Dissolved Zn samples were collected using a rosette system during the 2009 CLIVAR I5 cruise (from Cape Town, South Africa to Fremantle, Australia). Samples were acidified to 0.024M HCl, and then buffered to pH 5.05 with 2 M ammonium acetate buffer prior to analysis. Dissolved Zn concentrations were observed to be oceanographically consistent, and well correlated with dissolved silicate throughout the I5 transect. The slope from a linear regression of Zn vs. Si was 0.059 ' 0.04 (nM Zn/'M Si) for the I5 transect, which is consistent with the values reported for the north Pacific. These FIA-Zn measurements help confirm the previously reported nutrient-type Zn-silicate relationship. Low dissolved Zn concentrations (0.02 nM) were observed in surface waters throughout the I5 transect, demonstrating the extreme biological depletion of Zn typical of the open ocean. Concentrations of Zn and Si both exhibited increased regeneration with depth, and deeper samples contained the highest concentrations. The highest concentrations measured for dissolved Zn (>3.5 nM) were collected at 1300 m off western Australian.