Optimization of exergy and implications of body sizes of phytoplankton andzooplankton in an aquatic ecosystem model

Size appears to be an important parameter in ecological processes. All phys iological processes vary with body size ranging from small microorganisms t o higher mammals. In this model, five state variables - phosphorus, detritu s, phytoplankton, zooplankton and fish are considered. We study the implica tions of body sizes of phytoplankton and zooplankton for total system dynam ics by optimizing exergy as a goal function for system performance indicato r. The rates of different sub-processes of phytoplankton and zooplankton ar e calculated, by means of allometric relationships of their body sizes. We run the model with different combinations of body sizes of phytoplankton an d zooplankton and observe the overall biomass of phytoplankton, zooplankton and fish. The highest exergy values in different combinations of phytoplan kton acid zooplankton size indicate the maximum biomass of fish with relati ve proportions of phytoplankton and zooplankton. We also test the effect of phosphorus input conditions corresponding to oligotrophic, mesotrophic, eu trophic system on its dynamics. The average exergy to be maximized over phy toplankton and zooplankton size was computed when the system reached a stea dy state. Since this state is often a limit cycle, and the exergy copies th is behaviour, we averaged the exergy computed for 365 days (duration of 1 y ear) in the stable period of the run. In mesotrophic condition, maximum fis h biomass with relative proportional ratio of phytoplankton, zooplankton is recorded for phytoplankton size class 3.12 (log V mum(3) volume) and zoopl ankton size 4 (log V mum(3) volume). In oligotrophic condition the highest average exergy is obtained in between phytoplankton size 1.48 (log V mum(3) volume) and zooplankton size 4 (log V mum(3) volume), whereas in eutrophic condition the result shows the highest exergy in the combination of phytop lankton size 5.25 (log V mum(3) volume) and zooplankton size 4 (log V mum(3 ) volume). (C) 2001 Elsevier Science B.V. All rights reserved.