In order better to understand brackish ecosystem functioning, this study deals with zooplankton spatial organization in a North-Mediterranean lagoon (etang de Thau). A statistical treatment - spatial contiguity constrained clustering - is used to divide the lagoon into several homogeneous zones with regard to zooplankton communities. Confrontation between biological and physical patterns (hydroclimatic context data together with results of a numerical simulation of hydrodynamic circulation in the lagoon) tends to show that: 1) Zooplankton community spatial patterns have a short-term significance (few days) and do not reveal any seasonal characteristic; 2) Inversion of the water flows between the lagoon and the sea plays a major role in structuring and destructuring the biological system: the zooplankton compartment appears to be particularly sensitive to this hydrodynamic constraint even when it is a very smooth one (e.g. each event affecting only a small percentage of the total volume of the lagoon); 3) Independently of these short-term hydroclimatic events, the general pattern of zooplankton distribution appears very well correlated to the hydrodynamic energy dispersion deduced from a numerical model simulating the water circulation into the lagoon. In agreement with other plankton studies, these results encourage the introduction of physical descriptors into the modelling of biological processes in the aquatic environment. It seems also true that - especially in these kinds of ecosystem (lagoons) - the biology is strongly controlled by the physical system.