Processes influencing distributions and concentrations of Cd, Cu, Mn and Ni at the North West European shelf break

Concentrations of total dissolved Cd, Cu, Mn and Ni were measured at the shelf edge in the Celtic and Hebridean Seas during the Ocean Margin Exchange (OMEX) and the Land Ocean Interaction Study (LOIS) programmes, respectively, in order to assess the relative importance of the processes which control the distributions of these elements. These processes include those related to hydrographic features specific to the shelf edge such as fronts, slope currents and cascading, as well as the more ubiquitous processes such as particle exchange, atmospheric and benthic inputs, remineralisation and mixing between water masses. In near-surface waters at both study sites, mixing between shelf and oceanic waters was the predominant process influencing the distributions of Cu, Ni and Mn. The concentrations of Mn decreased from shelf to oceanic waters, but a chemical front could not be characterised at the study sites. Comparison between atmospheric inputs and concentrations in surface waters of the Celtic Sea suggests that the atmosphere may be a source of Mn near the shelf edge. The distributions of Cd were little affected by lateral mixing. Cd concentrations were controlled by inputs from deep water masses, biological uptake and regeneration. The influence of biological activity and possibly of wet atmospheric inputs may explain the differences in concentrations of Cu and Cd observed at different seasons. At depth, off the shelf, the effect of mixing with shelf water decreased and processes, such as mixing between deep water masses and remineralisation, were dominant.