Spatio-temporal structure of tidally mixed coastal waters: variability and heterogeneity

In the tidally mixed coastal waters of the eastern English Channel, the vertical properties of temperature, salinity, in vivo fluorescence, turbidity and light were investigated along an inshore-offshore transect, sampled in different tidal conditions. The vertical distribution of these parameters was then characterized in terms of mean value, variability (i.e. standard deviation) and heterogeneity (i.e. fractal dimension), regarded as a quantitative characterization of the structure of the vertical variability of both physical and biological parameters. These different estimates were then subjected to multivariate spatio-temporal analysis and showed a very specific spatio-temporal structure suggesting a differential control of the vertical properties of water masses;associated with both inshore-offshore gradient and tidal advection. In particular, the fractal dimension (i.e. heterogeneity) of in vivo fluorescence is higher during ebb for offshore waters, suggesting a density dependence of the phytoplankton biomass structure. In contrast, the vertical variability of fluorescence is higher during flood for inshore waters, leading to an inverse relationship between variability and heterogeneity of the vertical distribution of phytoplankton biomass.