Pierre Garreau

Phytoplankton dynamics in the English Channel: a simplified three-dimensional approach

A physical model has been built to account for the main physical features of the English Channel, in both horizontal and vertical dimensions. It combines a long-term residual model and a vertical thermal model, dividing the water column into ten levels. Its simplicity provides relatively rapid simulations of phytoplankton dynamics over a large area and appears to be sufficient to represent the physical structure of the Channel. Thus, the yearly evolution of ecosystem dynamics can be studied. Results from the simulation highlight the influence of thermal stratification occurring in the western part of the area, since the vertical structure of the water column mainly determines the date of the spring bloom. In the eastern English Channel, the main characteristic is the influence of nutrient supply from the river Seine and the corresponding high chlorophyll concentrations. This study also allowed us to compare results from this model and those from a previous box-model. The approaches appear to be complementary, since the former provides more accurate results and the latter enables long-term simulations to be made.

The circlet transform: A robust tool for detecting features with circular shapes

We present a novel method for detecting circles on digital images. This transform is called the circlet transform and can be seen as an extension of classical 1D wavelets to 2D; each basic element is a circle convolved by a 1D oscillating function. In comparison with other circle-detector methods, mainly the Hough transform, the circlet transform takes into account the finite frequency aspect of the data; a circular shape is not restricted to a circle but has a certain width. The transform operates directly on image gradient and does not need further binary segmentation. The implementation is efficient as it consists of a few fast Fourier transforms. The circlet transform is coupled with a soft-thresholding process and applied to a series of real images from different fields: ophthalmology, astronomy and oceanography. The results show the effectiveness of the method to deal with real images with blurry edges.

Multi-resolution eddy detection from ocean color and Sea Surface Temperature images

We propose a multi resolution method for automatic detection of eddies on both simulated and satellite Sea Surface Temperature (SST) or ocean color images. Our approach consists in a curvelet based analysis of image fronts along circles with specified radius values. The main interests of the proposed method are: • It is carried out on image gradient, and is thus not sensitive to the choice of binarization threshold; • With the multi-resolution aspect and the anisotropic spatial support shape of curvelet elements, we can deal with no perfectly circular eddies (curvelet width) and we also take into account the image regularity to distinguish between real eddies and noise (curvelet length). We have tested the method on several modeled and satellite images in the North Western Mediterranean sea and the results prove the relevance of the proposed approach.

Modeling of PCB trophic transfer in the Gulf of Lions; 3D coupled model application

3D coupled modeling approach is used for the PCB dispersion assessment in the Gulf of Lion and its transfer to zooplankton via biogeochemical processes. PCB budgets and fluxes between the different species of PCB: dissolved, particulate, biosorbed on plankton, assimilated by zooplankton, which are governed by different processes: adsorption/desorption, bacteria and plankton mortality, zooplankton excretion, grazing, mineralization, volatilization have been estimated. Model outputs were compared with the available in situ data. It was found that the Rhone River outflows play an important role in the organism contamination in the coastal zone, whereas the atmospheric depositions are rather more important in the offshore zones. The transfer of the available contaminant to bacteria and phytoplankton species is mainly related to the biomass present in the water column. Absorption fluxes (grazing) to zooplankton are rather higher than the passive sorption fluxes, which are themselves also linked to the sorption coefficient.