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-offshoretransect, sampled in different tidal conditions. The verticaldistribution of these parameters was then characterized in termsof mean value, variability (i.e. standard deviation) and heterogeneity(i.e. fractal dimension), regarded as a quantitative characterizationof the structure of the vertical variability of both physicaland biological parameters. These different estimates were thensubjected to multivariate spatio-temporal analysis and showeda very specific spatio-temporal structure suggesting a differentialcontrol of the vertical properties of water masses associatedwith both inshore-offshore gradient and tidal advection. Inparticular, the fractal dimension (i.e. heterogeneity) of invivo fluorescence is higher during ebb for offshore waters,suggesting a density dependence of the phytoplankton biomassstructure. In contrast, the vertical variability of fluorescenceis higher during flood for inshore waters, leading to an inverserelationship between variability and heterogeneity of the verticaldistribution of phytoplankton biomass.