The behaviour of dissolved Mn, Ni and Zn in the Forth an industrialised, partially mixed estuary

The distributions of dissolved Mn, Ni and Zn concentrations in the Forth estuary are described, interpreted and discussed. The Forth estuary has a long flushing time and, in summer, is characterised by a pronounced turbidity maximum and oxygen minimum in the upper estuary. Sampling throughout an annual cycle enabled the effects of riverine flow and varying hydrodynamic regimes to be assessed. Dissolved Mn profiles for the estuary show broad mid-estuarine maxima, indicating that reduced Mn is being added to the water column from the bottom sediments. Peak dissolved Mn concentrations are higher in summer (e.g. 2800 nM in June) than in winter (e.g. 660 nM in January) when high freshwater flows and shorter flushing times tend to mask the benthic source. Dissolved Ni and Zn profiles in the Forth, like those for Mn, show broad mid-estuarine maxima. Significant correlations between dissolved Ni and Zn concentrations in summer suggest common sources for the elements. In summer Ni and Zn appear to be remobilised with Mn in the oxygen minimum zone of the upper estuary. Possible additional sources of Ni and Zn in May include release from disturbed bottom sediments or anthropogenic emissions.