The Behaviour of Manganee and Cobalt in Hypoxic Fjord (Loch Etive, Scotland)

Abstract

Measurements of dissolved and particulate manganese (Mn) and cobalt (Co) were made on samples collected during two seasons (November 1999 and April 2000) in the oxidising and hypoxic waters of Loch Etive, Scotland. Collection of samples was made using an Aqua Monitor that was mounted on Autosub, an autonomous underwater vehicle. In addition, an in-situ chemical analyser was developed and tested for the determination of dissolved Mn (Mna). Analysis of collected w£ter samples were undertaken in a clean room Class-100 laminar flow hood, and the data subjected to rigorous quality control. Due to the unique sampling technique the study presents high-resolution data not previously obtained. The results give evidence for the presence of localised variations within the general depth trend for Mn and Co. During both seasons the Mna concentrations in the oxidising surface waters (44.5 nM 'in November and 79.8 nM in April, respectively) were significantly lower than those recorded in the hypoxic bottom waters (120 nM in November and 358 nM in April, respectively). Results indicate that the high concentrations in deep-water were produced by a sedimentary source of Mna. Pronounced seasonal variability was observed in the Mny concentration, this being the result of increasing reducing conditions that favour Mn reduction. Particulate manganese concentrations (Mp) followed the same trend as that observed for Mna, i.e. increasing concentrations in deep waters. This trend is suggested to be the result of scavenging of Mnj. The dissolved Co (Coa) concentrations were higher than those recorded in the open ocean. Highest Coj concentrations were recorded in surface waters (81 - 288 pM in November and 71 - 246 pM in April, respectively), values decreasing with depth. In deep-water, low, stable concentrations were observed during both seasons. In surface waters, riverine inputs are expected to influence the concentrations while in deep-water the controlling mechanisms are suggested to be desorption from Mn-oxides and microbial oxidation. The study investigates the correlation between Co and Mn, arguing that in a hypoxic environment biogeochemical processes affect Mn and Co differently, resulting in a geochemical separation of these two elements.

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