Abstract
In order to investigate the possibility of seasonal succession between a grazing food web and
a microbe food web, and the seasonal variation of the dominant species of phytoplankton, an
ecosystem model is developed. The prcy-prcdator interactions and the interspecies competition
are incorporated into this new model. This improved model is applied to analysis of the Lakes
Shinji and Nakaumi ecosystem for 1998.
In the model results of physical processes, the tendency of water mass change and the density
gradient between the upper layer and bottom layer are reproduced well. The simulated results
of the biological processes using the ecological model show good agreement with field data.
Especially the release of P04-P from the bottom sediment and the formation of oxygen-depleted
water in bottom layer of Nakaumi in summer are reproduced well. Sensitivity analysis suggests
that the filtration rate of shellfish and the horizontal diffusivity play important role on
phytoplankton blooming in winter. The model also suggests the possibility of the succession of
the dominant plankton species.
