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The fish community in the shallow Lake Veluwe (3400 ha) has recently changed in composition. The total biomass of fish decreased to ca. 35% of the original biomass. The change in fish biomass followed the introduction of a commercial fishery and was in its turn followed by a rapid expansion of macrophyte beds dominated by Chara aspera Deth. ex Willd. An individual-based fish model was used to evaluate the causation of these events. We estimated natural mortality of the bream population (Abramis brama L.) using data on growth, recruitment and size distribution. The model predicted that without the fishery the biomass of the bream population would hardly change and that the fishery could explain the change in the biomass and size compositionThe fish community in the shallow Lake Veluwe (3400 ha) has recently changed in composition. The total biomass of fish decreased to ca. 35% of the original biomass. The change in fish biomass followed the introduction of a commercial fishery and was in its turn followed by a rapid expansion of macrophyte beds dominated by Chara aspera Deth. ex Willd. An individual-based fish model was used to evaluate the causation of these events. We estimated natural mortality of the bream population (Abramis brama L.) using data on growth, recruitment and size distribution. The model predicted that without the fishery the biomass of the bream population would hardly change and that the fishery could explain the change in the biomass and size composition. The rapid expansion of C. aspera was unlikely without the fishery, as Chara needs transparency down to the bottom and undisturbed sediment. The reduction of the bream population created favourable conditions for zebra mussels Dreissena polymorpha (Pallas) to settle which in turn created the clear water, necessary for the development of the Chara beds. (C) 2003 Published by Elsevier B.V.

Primary production and respiration models were calibrated through parameter estimation methods and applied to an extensive data set on a population dominated by Oscillatoria agardhii. The results demonstrated that inclusion of the frequently used Monod term to represent phosphate limitation deteriorated the model results and caused a negative temperature effect for the primary production rate. This can be attributed to the temperature effect upon the recycling of phosphate and its actual concentration. Inclusion of the production and respiration model in an overall lake do model yielded a good agreement between observed and simulated concentrations.