Diffusion in Irish coastal waters

Results are presented from dye-diffusion experiments performed at a selection of near-shore and estuarine sites around the coastline of Ireland. The data from discrete dye releases have been analysed to derive along- and across-patch horizontal diffusivities, and estimates are presented of both Fickian and non-Fickian diffusion coefficients. Most of the sites were characterized by strong tidal currents so that the dye became vertically well-mixed soon after release. Consequently, the mixing was found to be tidally dominated and, to a reasonable approximation, the magnitude of the one-dimensional (radial spreading) horizontal diffusivity (m2s−1) was equal to the tidal current speed (ms−1). Wind-induced mixing was only of secondary importance, but this may be a consequence of the experiments having taken place during conditions of generally light winds. In general, the observed mixing rates agree with those predicted by oceanic diffusion diagrams