Apparent homeostasis in the surface salinity of a turbid macrotidal estuary.

A simple, one-dimensional tide-ignoring model has been fitted to surface salinity measured at monthly intervals over 3 years on an extensive sampling grid in the Humber and tidal Ouse on the North Sea coast of the UK. With a constant and uniform axial dispersion, estimated at 296+/-7 m2 s(-1) (95% confidence limits), model deviations vary systematically in space and time. Postulating an influx of dispersive tendency with fresh water largely eliminates the systematic variation in space. Deviations from the simple model (observations-predictions) show a significant seasonal cycle, with minima in the summer and maxima in the winter, and decline significantly over the 3 years of the study. A model that incorporates a homeostatic tendency for the estuary to revert to a constant salinity profile, independent of river flow, appears to eliminate both the seasonal cycle and the trend. The half time of the homeostatic process is estimated to be between 76 and 97 days (95% confidence limits). It is concluded that sediment movement might be the basis of such a mechanism.