THE APPLICATION OF A MODEL WITH DYNAMIC STRUCTURE TO SIMULATE THE EFFECT OF MASS FISH MORTALITY ON ZOOPLANKTON STRUCTURE IN LAGO-DI-ANNONE

A mass mortality of the most abundant zooplanktophagous fish due to an infectious gill disease eliminated almost the entire population of th is species in the Eastern basin of the Lake Annone, Italy in '75-76, w hile no mortality occurred in the Western basin. This event offered th e opportunity to study whether a recently developed structural dynamic modelling approach would be able to describe the changes in the prope rties of zooplankton observed in the Eastern basin as a result of an a bnormal high fish mortality in August '75. As a change in size of zoop lankton was observed (from 1000 mu m to 1600 mu m) it would even be po ssible to see if the modelling approach could describe the actually ob served changes in structure (properties of the organisms, in this case zooplankton). The modelling approach uses the thermodynamic concept e xergy as a goal function which determines the current changes of the p roperties of the species included in the model description. From the s tudy it can be concluded that this modelling approach was able to desc ribe the focal state variables of the model (phytoplankton, zooplankto n and fish) in accordance with the observations from '75 to '76 and al so to assess the actually observed changes in zooplankton size.