Sampling and onboard analytical methods for determining subnanomolar concentrations of zinc in seawater

Zinc (Zn) is an essential micronutrient for bacteria and phytoplankton in the ocean. However, seawater samples for Zn measurements are highly prone to contamination due to the ubiquity of Zn in laboratories and research vessels. To identify a potential Zn contamination problem during a research cruise to the sub-arctic North Pacific, we compared three operational sampling approaches for trace Zn in seawater. Acid-cleaned Niskin-X samplers were: (1) deployed on a conductivity-temperature-depth carousel multisampling system (CTD-CMS) launched at the vessel's starboard side, (2) attached to a Kevlar wire lowered at the vessel's stern, and (3) attached to a Ti wire lowered from a starboard position. Zn contamination in the case of Kevlar wire hydrocasts was problematic for samples from <= 400 m because Zn is used as a sacrificial anode for the ship's propeller shaft and for the pipes of the seawater cooling system. In deep waters, consistent Zn concentrations were found using the three different sampling methods. Based on the clean seawater sampling with CTD-CMS and the onboard analytical method, vertical profiles of dissolved Zn were then obtained. These profiles were compared with those of Si in the subarctic North Pacific. Both Zn and Si concentrations showed a clear west-east gradient in shallow waters.