Contaminant Pathways in UK shelf seas
Recent work by Cefas has demonstrated that strong organised circulations exist in the Irish, Celtic and central North Seas. These arise directly from the density contrast between areas which become thermally stratified (layered) in summer and those where the energy from tidal and wind stirring is sufficient to maintain mixing of the water column.
Baroclinic jets are associated with cold (and/or salty), dense pools of bottom water which remain trapped in deeper low tidal regions from spring to Autumn (May - November), where tidally-generated turbulent energy is insufficient to mix the increased surface heat input (buoyancy) from solar heating throughout the water column. At the edge of the cold pools, the strong horizontal density gradients - bottom fronts - create a force to set up along-frontal flows.
Extensive areas of the continental shelf are now known to be dominated by strong, persistent and stable jet-like currents transporting water and material over many 100's of kilometres across national marine boundaries. Transportation of contaminants (e.g. nutrients, metals and polycyclic aromatic hydrocarbon's), fish eggs, larvae and plankton is much faster and more extensive than previously supposed. In other cases they act as retention mechanisms about topographic depressions. Additionally, the well-defined boundaries between mixed and stratified waters act as important and extensive sites for phytoplankton and fisheries production as well as playing an important role in nutrient and contaminant dynamics.
In summary, over large areas of the shelf seas, flows are predictable and organised, they vary seasonally, but exist for the majority of the year (April - December). However, in many areas these circulation patterns remain to be described as does the potential effects of climate change. The proposed work is intended to place our understanding of contaminant, nutrient and sediment dynamics and fisheries on a sound scientific understanding of the controlling physical processes, allowing management actions/responses to be appropriately tailored.
