Feeding adaptations of filter feeders: Daphnia

In spite of the ongoing discussion about the functioning of filtratory apparatus of the filter feeding freshwater pelagic organism of the genus Daphnia, experiments suggest food size selectivity as well as a high degree of adaptability of filtration to food organism size. A model of feeding adaptation of a filter feeder in the framework of a pelagic ecosystem is constructed by modifying the simplified ecosystem model AQUAMOD for the presence of several species of algae of different size. The assumption that filtration adaptability is an optimization type process is incorporated. Two possible strategies were followed: an instantaneous optimality at each time interval and an integral formulation, maximization of the integral biomass over 1-year time period. Numerical simulations as well as theoretical calculations were performed with the model. The results suggest, that the feedback between food selection by Daphnia and algal size composition is very strong. When growth rates of several species of algae of different size is limited by a single nutrient and a value of Daphnia filter density is constant, competitive exclusion principle may hold, only one species survives. Simulations for different constant values of Daphnia filter density show an effect not only on the size of algae surviving, but also on the number of species able to survive. The integral formulation results in higher biomass suggesting that the organisms do better if not reacting only on immediate changes but having developed mechanisms consistent with more long-term considerations