Fluorometric 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), and collecting uncontaminated seawater using a trace-metal clean rosette system (Measures et al., 2008). Samples for total dissolved Zn analysis were during the 2009 CLIVAR I5 cruise across the southern Indian Ocean (from Cape Town, South Africa to Fremantle, Australia). Dissolved Zn concentrations have not been previously reported for this region. Extremely low dissolved Zn concentrations (0.02nM) were observed in surface waters of the central Indian Ocean gyre, documenting the extreme biological depletion of Zn typical of the open ocean. Concentrations of Zn and Si both increased with depth. The highest concentrations measured for dissolved Zn (>3.5nM) were collected at 1300m off western Australian. Total dissolved Zn concentrations were observed to be oceanographically consistent, and well correlated with dissolved silicate across the transect. The linear regression of total dissolved Zn vs. Si for all of the data yielded a slope of 0.059±0.003 (nM Zn/μM Si), which is consistent with the values reported for the north Pacific and thus support the previously reported nutrient-type Zn–silicate relationship. The zonal section of the dissolved Zn/Si ratios also exhibit broad maxima and minima, consistent with variable sources for Zn and different recycling rates for Zn vs. Si.