Seasonal cycles and their spatial variability

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Abstract
Seasonal variations dominate many processes in continental shelf seas. A comprehensive coherent inter-disciplinary data set for one seasonal cycle was obtained by repeating the same cruise track in the southern North Sea at monthly intervals from August 1988 to October 1989. Measurements were made throughout the water column in vertically homogeneous and summer stratified regions and near the major estuaries. 97% of the surface temperature’s variance was in the seasonal cycle, driven by solar forcing; spatial variability was related to stratification and to contrasts between the waters off northeast England and the German Bight. The salinity seasonal cycle was small; spatial variability was governed by fresh water river inputs. Suspended sediment concentrations were largest near river mouths and coasts; material was transported eastward from East Anglia towards the German Bight in a distinct plume whose magnitude varied with seasonal wind patterns. There were large regional differences, with the greatest phytoplankton biomass and oxygen supersaturation developing in the Southern Bight and German Bight, those regions which experience extensive phytoplankton blooms in the spring. Annual primary productivity ranged from 79 gC m -2 a -1 for the English coastal region to 261 gC m -2 a -1 for the German Bight. Low oxygen concentrations were measured in late summer below the thermocline in regions on either side of the Dogger Bank. A budget of nutrient concentrations throughout the region suggests that nutrient supply to the phytoplankton in the winter is dominated by regeneration processes, rather than input from river run-off.