Cobalt speciation and cycling in Linsley Pond, Connecticut, USA

Cobalt distribution and speciation were quantified in space and time in the water column of a small, stratified, eutrophic lake, Linsley Pond, North Branford, CT. While scrupulously employing clean techniques, we used ligand exchange with dimethylglyoxime and cathodic stripping voltammetry to evaluate free and complexed forms of Co. Free aquo Co ion concentrations [Co 2+ ] were found in the range from 0.014 to 0.28 nM from June to October 2018. Despite the orthograde distribution for total dissolved Co (0.42–3.34 nM), free Co 2+ was higher in the epilimnion, decreasing with depth. Natural organic ligand concentrations [ L ] were in the span from 0.7 to 8.1 nM, with conditional stability constants (as log K ) in the range from 9.43 to 11.13. Nearly all of the Co was complexed with highly selective ligands, and patterns suggest three controlling processes: (1) Co release from dissolving Mn (and perhaps Fe) oxides, (2) possible limitation by solubility of CoS(s), and (3) stabilization in solution via complexation by strong ligands. No correlation was observed between dissolved organic carbon and [ L ] in this study, suggesting that the ligands are not a simple subset of total dissolved organic matter, but may be specific compounds, perhaps S based. The hypothesis that the biological activity of plankton in Linsley Pond might be limited by micronutrient Co is only weakly supported. Cobalamin (VB 12 ) measured via enzyme-linked immunosorbent assays ranged from 0.033 to 0.048 nM in this lake and does not follow a simple pattern with either total dissolved Co or Co 2+ , or with biological activity as indicated by chlorophyll levels.