Influencia de la turbulencia y de la Dinámica de Interfases de Densidad sobre Organismos Planctónicos."Aplicación al estuario del Ebro"

Résumé

Physical variables that describe mixing are compared with experimental laboratory results from a large number of studies of mixing dynamics in environmental fluids to find habitat regimes for primary production. The work relates physical and biological variables, and emphasises the utility of laboratory studies. Several laboratory experiments that focused on turbulent mixing dynamics in stratified shear flows are used to describe (a) mixing in the estuary and (b) induced circulation in the river plume. Mixing descriptors as entrainment, Richardson number and Reynolds number and field data were employed, and advanced techniques of laboratory simulations, image processing and numerical modelling were used to match (a) to (b). Four kinds of experiments were used to describe the dynamics in the whole estuary. 1) Mixing turbulence across a density interface generated by an oscillating grid inside a mixing-box. 2) The horizontal advance of a turbulent front in a stratified system with a lateral current, inside a 1 m x 1 m square box. 3) Induced circulation in the delta del Ebro slope and shelf that were performed with an experimental model in a 2 m x 4 m rectangular tank on a 5-m diameter turntable. 4) Dispersion simulations in the river plume with the OCK3D code. Experiments 1 and 2 were developed in the UPC applied physic laboratories in Barcelona, experiments 3 were performed in the SINTEF laboratories in Trondheim and numerical experiments 4 were realized in the LSEET laboratories in Toulon. The estuary under study is the delta of the del Ebro, where a stable saline wedge is present the greater part of the year, driven by the river flow. A field campaign was carried out to observe a complete large stable saline wedge during the month of July of 1997. Additionally, to obtain the physical descriptors, cruises were made on 5 April, 1998, 12 July and 5 October, 1999 and 5 February, 2000 to observe several flow conditions in the river estuary. Particle tracking and optical measurements in the water column were used to study the induced length scale dynamics in the shelf and slope of the delta del Ebro. The dominant structures and scales from SAR images and numerical and laboratory simulations were compared with the field data. With the help of laboratory and field experiments and numerical simulations, qualitative criteria were suggested to determine the extent of the interface ending in a stratified water column, the different mixing characteristic zones within the estuary, and the distance of the estuary head, since elsewhere points in the estuary (including the river mouth) are obtained by means of the minimum density in the water column. Both estuary mixing and plume dynamics are driven by the local flow. In the present analysis of mixing efficiency, the phytoplankton abundance is in accord with the Reynolds number as a mixing descriptor.