Cédric Bacher (France)
In bays and estuaries, the interactions between cultivated species and the environment is most often driven by the hydrodynamics. For extensive systems for instance, the food supply to the cultivated species and the impact of the excretion from these species depend on the water mixing and on the residence time of the water, e.g. the time needed for the natural renewal of the water within the cultivated area.
A new algorithm was proposed to compute water residence time in bays or estuaries. Based on a probabilistic approach of the tidally averaged circulation of the water, the method allowed to generalize other temporal scales definitions as transit time and local residence time in sub-areas of the system. The method required the knowledge of local flows, for instance derived from an hydrodynamical model. In this case, the area concerned with the water circulation was divided in a discrete number of spatial cells. The connection between cells yielded a connection matrix describing existing exchanges. From this, a transition matrix was built to assess the probability for elementary water particles to leave a cell and arrive to another cell connected to the former. Iterating transitions over time provided a way to describe the transport of water particles under the assumption of steady state flows. At each time step the probability of exiting the system was computed, which established the basis for defining different temporal scales. Considering several water particles distributions among the whole area, the residence, transit, or local residence time were represented as the expected value of a random variable which distribution was derived from the probability of exiting the system. These concepts and algorithms were validated in the one dimension case for systems with constant flows and water volumes, for which the transit and residence times are known.
It was also used to compute the residence time of the water in the Marennes-Oléron Bay (France), where the importance of the tidal excursion resulted in the use of a spatial box model. The bay was therefore divided in 15 spatial boxes and the flows between the boxes were estimated from residual flows computed with an hydrodynamical model. An ecosystem model was conceived to assess the phytoplankton consumption by cultivated molluscs and the growth rate of these molluscs due to the phytoplankton abundance. Combining the spatial box and the ecosystem models yielded to assess the impact of the cultivated molluscs on the phytoplankton abundance and production. It was shown that this impact is linked to the water residence time which was computed as described previously.