Autumnal reduction of stratification in the northern North Sea and its impact.

In the autumn vertical density stratification in the northern North Sea is reduced primarily by atmospheric induced mixing. However, observations from moored instruments show that a sudden enhanced exchange of nutrients from the light-limited, nutrient-rich deep layer into the nutrient-depleted near-surface layer occurred indirectly when inertial shear was largest, across the stratification. An associated response in phytoplankton biomass was not observed, perhaps because the observations were stopped before a proper response could occur. In general, phytoplankton were observed to vary strongly with stratification close to the surface. Different methods for observing stratification are compared-vertical current shears inferred from Acoustic Doppler Current Profiler data are a good indicator of layers of enhanced stratification, whereas the back-scattered amplitude data from the same instrument are not. The latter data are also found not to be a good indicator for suspended sediment, despite extensive reworking of the bottom at 110 m depth due to the action of surface waves. The observed enhancement of the turbulent bottom boundary layer exceeding the tidal frictional layer is discussed, when stratification is still enhanced near mid-depth. (C) 2002 Elsevier Science Ltd. All rights reserved.