A MODEL FOR ACARTIA-TONSA - EFFECT OF TURBULENCE AND CONSEQUENCES FORTHE RELATED PHYSIOLOGICAL PROCESSES

A mathematical model of the energy budget of the copepod Acartia tonsa is proposed. It takes into account the processes of ingestion (encoun ter, capture, ingestion sensu stricto), digestion (assimilation, gut t ransit and egestion), catabolic expenses and biomass production. In or der to represent the potential effects of small-scale turbulence on th e whole physiological processes of a copepod, some process submodels a lready published are combined. A major assumption of the model is a sa tiety effect resulting from midgut filling, which leads to a decrease in the feeding activity. The model permits the simulation of the short -term dynamics of ingestion under different food and turbulence condit ions, as well as an integrated physiological balance over 24 h. The mo del is validated through comparison with data at both scales. Simulati ons show that turbulence increases ingestion rates and gut contents, a nd causes a decrease in gut passage time and assimilation efficiency. As a consequence, the dependent physiological processes are affected d ifferently by turbulence. which preferentially increases egestion and egg-production rates. Simulated daily ingestion rates of A.tonsa, for suspension feeding on Thalassiosira weissflogii and for ambush predati on on Strombidium sulcatum, are in good agreement with the available e xperimental observations. The concurrent direct effect of turbulence o n the copepod's metabolism, due to increased escape reactions, is also simulated. Results of the model show that a switch from suspension fe eding on diatoms in calm conditions, to ambush predation on ciliates i n turbulent conditions, might allow A.tonsa to maintain its gross grow th efficiency at the same level. It is suggested that a dynamic repres entation of processes occurring over a time scale of a few seconds is necessary to obtain, once integrated over 24 h, the correct simulation of the effect of microscale turbulence on ingestion and the related p hysiological processes.