TESTING THE MICROBIAL LOOP CONCEPT BY COMPARING MESOCOSM DATA WITH RESULTS FROM A DYNAMICAL SIMULATION-MODEL

Data from marine enclosures were compared with model results from the European Regional Seas Ecosystem Model (ERSEM). The simulation runs we re performed with the December 1992 (V5.1) version of the pelagic subm odel of ERSEM. The aim of the experiment was to verify the microbial l oop as implemented in ERSEM against direct observations from marine en closures. Initial conditions of the state variables for the model runs were derived from the same data. In general, the model results tracke d the measured data indicating the model concept and implementation to be realistic. In contrast, the simulated concentrations of nitrate an d phosphate in the nutrient-enriched bag stayed higher than observed, indicating the necessity of including mechanisms to allow for luxury u ptake in the model description of the phytoplankton. Normalised energy cycles for carbon were established for the control and the enclosure with added nutrients. In the control enclosure respiratory losses from phytoplankton and bacteria were relatively high compared to the nutri ent-enriched system, leading to significant differences in the routing of energy through the various microbial loop components without affec ting the transfer to mesozooplankton. Although the average levels and changes of several pools and rates were simulated satisfactorily, furt her model development is certainly needed to predict the functioning o f the microbial food web in more detail.