Dissolved zinc and its speciation in the coastal hydrothermal area of Japan

Zinc (Zn) is an essential micronutrient for marine bacteria and phytoplankton. Previous studies indicated that organic complexes of dissolved zinc in surface waters account for about 98% of the total dissolved zinc in the open ocean (e.g., Bruland, 1989; Ellwood and van den Berg, 2000). However, distributions of dissolved Zn and its speciation were not clarified yet in the coastal areas. In this study, we have investigated dissolved Zn distribution and its speciation in the Tachibana Bay (Japan), where an active hydrothermal activity was reported at the coastal areas.

Seawater samples were collected from surface to near bottom (D=36 m) in the Tachibana Bay, Nagasaki Prefecture, by using acid-cleaned samplers in May 2012 and 2013. Total dissolved Zn was determined by using CSV after UV-digestion. For the Zn speciation, ligand concentrations and conditional stability constants were obtained from a titration using competitive ligand equilibrium / adsorptive cathodic stripping voltammetry (van den Berg, 1985).

In the Tachibana Bay, total dissolved Zn (D-Zn) ranged from 0.33 to 3.1 nM, which were similar concentration levels to those in the open ocean. D-Zn was sharply decreased toward the bottom. High sulfide concentration (94 nM) in coastal bottom waters was observed, reflecting active hydrothermal system near the bottom of Tachibana Bay coast. Zn is seemingly removed as Zn-sulfides from the water column. Also, relatively high total ligand concentration was calculated near the bottom. The biogeochemical cycling of Zn in the Tachibana Bay is considerably affected by hydrothermal system.