Contrasting complexation of Cu and Co by organic ligands in the East China Sea determined by forward and reverse titration CLE-AdCSV

Both Cu and Co are essential trace metals for marine phytoplankton to grow. While dissolved Co exists in low pico-molar concentrations and can sometimes limit phytoplankton growth, natural dissolved Cu is at relatively high concentrations and tend to be toxic. Organic complexation therefore controls the toxicity and bioavailability of Cu and Co for marine phytoplankton. In this study, we determined the concentrations and characteristics of organic ligands bound to Cu and Co in the East China Sea using two different analytical methods.

Samples were collected during the GEOTRACES-Japan cruise KH-15-3 in the East China Sea in October 2015. Concentrations of dissolved Cu and Co were determined using CSV, while the ligands were determined using CLE-AdCSV. The forward titration method was used to determine Cu-binding ligands while the reverse titration method was applied to determine the Co-binding ligands due to excess of unbound Co.

Total dissolved Cu concentrations ranged from 0.55 nM to 3.86 nM. Profiles of dissolved Cu are nutrient type with elevated concentrations of Cu found near to the Yangtze Estuary. Two classes of Cu-binding organic ligands were determined in this study. The concentrations of the stronger ligand, L₁ (log K > 13.5) ranged from 0.92 nM to 4.03 nM, while those of the weaker ligands, L₂ (log K < 13.5) were found to be between 1.39 nM and 29 nM. Distributions of L₁ followed those of dissolved Cu but L₁ were absent in deep waters. On the other hand, L₂ did not exhibit a clear distribution pattern and were found throughout the water column. Total dissolved Co concentrations ranged from 11 pM to 64 pM, with maximum concentrations in the intermediate depths. The concentrations of Co-binding ligands ranged from 19 pM to 46 pM (log K = 17) and were generally less than those of total dissolved Co, resulting in the presence of free unbound Co.