Effect of River Flow on Estuarine Microalgal Biomass and Distribution

Over the period 1968 to 1999, Brixham Environmental laboratory carried out dye tracing experiments to determine local mixing conditions at coastal and estuarine sites around the United Kingdom. Results from 25 different locations, and from five nonU.K. sites, have been analysed to estimate horizontal and vertical mixing coefficients. Analysis of the data from the 19 sites which had the most complete data sets showed that the minimum lateral dispersion coefficient ranged from 0·003 to 0·42 m2 s-1with a median value of 0·05 m2 s-1. Typical vertical diffusion coefficients ranged between 2·0×10-4to 110·0×10-4 m2 s1and had a median value of 20·0×10-4 m2 s-1. Comparisons of the values obtained have been made with mixing coefficients derived from earlier dye tracer studies in the North Sea, the Irish Sea and off the eastern seaboard of the U.S.A. A plot of minimum lateral dispersion coefficients against the corresponding vertical diffusion coefficients showed that the results fell into two distinct bands, broadly represented by shallow or deep water. Within each band, the lateral mixing coefficients appeared to vary in inverse proportion to the vertical coefficients, as would be expected in shear dispersion in which mixing is restricted by the surface and seabed or pycnocline. Curves representing this variation had different constants of proportionality for the shallow and deep waters. This was ascribed to a reduction in the curvature of the velocity profiles in the deep water areas, where stronger currents may have reduced the degree of stratification. Analysis of the full data set has not identified any generally applicable relationship between the mixing coefficients and tidal currents or wind, however, relationships between these parameters could be found within specific survey areas. For example, lateral diffusion coefficients in the Solent appeared to be correlated with the tidal current amplitude.