A hypothesis is presented that the combined impact of dynamics, nutrients, and suspended matter produces enhanced biogeochemical fluxes to the seabed on the stratified side of tidal mixing fronts in tide-stirred shelf seas.
In summer much of the algal production in the euphotic zone takes place in the thermocline and the greatest production occurs near tidal mixing fronts. This probably results from in situ growth of plankton due to optimal combination of light and nutrients: nutrient renewal during the summer, due to mixing by tide and wind, and surface stabilisation and reduction of the mixed layer optical thickness during fairweather and neap tides. A chlorophyll maximum is observed at the thermocline and at fronts. The optimal conditions for rapid algal growth are at fronts since lateral mixing across a front is greater than vertical mixing across the thermocline.
Organic matter generated by algae is incorporated in suspended particulate matter (SPM) with most of the mass in large aggregates which settle to the seabed. Aggregation is facilitated by carbohydrates produced by the algae. In mixed waters, aggregated matter is frequently resuspended by tides so that significant remineralisation takes place in the water column. In stratified waters, low turbulence at the base of the thermocline provides conditions for aggregation and export of organic matter to the seabed where there is only limited resuspension; but remineralisation takes place as SPM sinks through the extended water column. Remineralisation therefore limits biogeochemical supply to the seabed in both mixed and stratified waters. In frontal zones, the potential for remineralisation is reduced due to rapid settling and limited resuspension.
Combination of enhanced algal production, rapid settling of aggregates, and limited resuspension means that fluxes of aggregated biogeochemical components per unit area of seabed are greatest under fronts. However, resuspension in combination with cross-frontal mixing in summer and storms in winter should disperse benthic SPM away from fronts. Net deposition of this material is most likely on the stratified side of the frontal regions. The end result is that there should be spatial gradients in the supply of biogeochemical components to the seabed across mixed, frontal, and stratified zones.
This is important because deposition of aggregates as benthic fluff controls redox conditions at the seabed and determines whether biogeochemical exchanges are oxic or anoxic. Temporary anoxia occurs even in mixed regions but it is particularly marked on the stratified size of frontal zones. High benthic oxygen demand occurs near the Frisian frontal, compared with the mixed, zone of the southern North Sea. Biogeochemical gradients also impact on the biota. Spatial gradients in dinoflagellate cysts and foraminifera in sediments occur across the Celtic Sea front. It is likely that the proposed biogeochemical gradient has significant impacts on benthic and pelagic biota and exchanges in tide-stirred shelf seas. |
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