Ecotron - Embiez 8 - Modélisation de la production d'organismes planctoniques dans les bassins

Choosing the best conditions for the cultivation of plankton in tanks is a difficult deal, which is generally solved by means of subjective estimations. A better way of looking for these conditions is the use of a mathematical model. _ We have tried to built a model that accounts for the physical and bio­logical characteristics of the tanks. The data concerning the physiological characteristics of the different group of organisms encountered are too scarce to allow all of them to be taken into account. The system studied here is composed of nutrients, phytoplankton, herbivorous copepods and rotifers to which benthic algae have been added. With the hypothesis of 3 elements (phyto - zooplankton - nutrients) it can be showed that when an equilibrium is reached by the system in the tanks, the higher the dilution (from 0,0052 to 0,1 j-1), the more abundant will be the zooplankton biomass. If the various elements are not divided into these 3 groups, the final state of the system is an abundant population of benthic algae. However the former result again appears : an increase by a factor 2 of the water flow (from 833 to 1666 1/h) induce an increase of the maximal copepod biomass in N 2 tank. In the same way, cleaning off the algae from the tanks leads to a definite increase of the biomass of herbivores. Simulations made by means of this model show the importance of limiting the growth of benthic algae and using a strong dilution rate. They show that it is very important to acquire informations concerning the physiology of ben­thic algae, rotifers and the feeding adaptation mechanisms.