Further information
Previous scientific wisdom held that over the majority of the shelf seas the net long-term circulation is weak, with mean advection of approximately 1 - 2 cm s-1 (<1.7 km day-1). Over large areas of the shelf seas this view is no longer defensible. We now know that extensive areas of the continental shelf are dominated by strong, persistent and stable jet-like currents, with peak speeds exceeding 20 cm s-1 (> 17 km day-1). The systems transport water and material over many 100's of kms, or 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 and play an important role in nutrient and contaminant dynamics.
To date the study of such systems has largely concentrated on the peak of the heating season. However, there is strong evidence that during periods of comparatively high riverine input (winter/spring) the resultant buoyancy input interacts with tidal stirring to produce salinity dominant bottom fronts. Whilst the geographical extent of flows is unlikely to be as pronounced as those associated with thermal stratification, organised circulations will exist in near-coastal waters in the vicinity of major contaminant and nutrient inputs. In addition, these frontal regions act to 'pre-condition' the water column at the beginning of the heating season promoting the rapid development of thermal stratification. The extent and impact of such flows remains to be described, with no account taken of them within management decisions or hydrodynamic models.
Conversely, during the breakdown of thermal stratification (September - December) limited data indicates that the gradual erosion of the summer thermocline and bottom fronts causes a continual alteration in pathways and an associated release of nutrients previously isolated in cold bottom water. As yet these processes are undescribed, and an adequate description by models requires a fuller understanding of the mixing and cooling processes that determine the erosion of the seasonal thermocline, in addition to operating at resolutions appropriate to the physical processes (order 1 km in the horizontal & 1 m in the vertical).
Away from the fronts, in the deeper stratified regions where tidal stirring and the influence of waves are comparatively low, residual circulation is weak (e.g. off the north east coast of England, the Celtic Deep and the western Irish Sea). There is increasing evidence that these sites of high natural deposition receive material from the coastal zone and act as long term sinks for particle-reactive contaminants (organics, metals and radionuclides) and organic carbon. Generally primary and secondary production is high, continuing at the base of the thermocline throughout the summer and fuelled by the high nutrient concentration in the bottom waters. The predominantly fine sediments are particularly effective at sequestering contaminants to areas that generally have valuable fisheries, particularly shellfish.
This project, carried out by Cefas and the University of Wales, Bangor, has been funded by the Department for Environment, Food and Rural Affairs and funding is also received 'in kind' through Dr Kevin Horsburgh at the School of Ocean Sciences, University of Wales, Bangor and from Dr Robin Raine at the University of Galway. Additional collaboration has been received with thanks from the Proudman Oceanographic Laboratories; Department of Agriculture and Rural Development; Netherlands Institute for Sea Research; and ASOF (Arctic/Subarctic Fluxes).
