A model for Acartia tonsa: Effect of turbulence and consequences for the 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 (encounter, capture, ingestion sensu stricto), digestion (assimilation, gut transit and egestion), catabolic expenses and biomass production. In order to represent the potential effects of small-scale turbulence on the whole physiological processes of a copepod, some process submodels already published are combined. A major assumption of the model is a satiety 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 conditions, as well as an integrated physiological balance over 24 h. The model is validated through comparison with data at both scales. Simulations show that turbulence increases ingestion rates and gut contents, and causes a decrease in gut passage time and assimilation efficiency. As a consequence, the dependent physiological processes are affected differently 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 predation on Strombidium sulcatum, are in good agreement with the available experimental observations. The concurrent direct effect of turbulence on the copepod's metabolism, due to increased escape reactions, is also simulated. Results of the model show that a switch from suspension feeding on diatoms in calm conditions, to ambush predation on ciliates in turbulent conditions, might allow A.tonsa to maintain its gross growth efficiency at the same level. It is suggested that a dynamic representation 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 physiological processes.