BIOLOGICAL AND HYDRODYNAMIC REGULATION OF THE MICROBIAL FOOD-WEB IN APERIODICALLY MIXED ESTUARY

Abundances of chroococcoid cyanobacteria and heterotrophic bacteria in surface waters of the York River subestuary covary with spring-neap t idally induced changes in the mixed-layer depth. Abundances of their p rincipal grazers, heterotrophic protists, however, do not oscillate. A simulation model of this system using nonlinear, density-dependent fu nctions has been developed to replicate cycles observed in the two bac terial abundances and simulate bacterial production and protistan graz ing. A Jassby-Plan equation is used to determine growth rate from the mean mixed-layer light and empirically derived growth and a parameters . Changes in mixed-layer depth regulate light availability, thereby co ntrolling cyanobacterial growth rates. The model predicts a close coup ling between cyanobacterial growth and during destratified periods whe n cyanobacterial stocks are low. During stratified periods when cyanob acteria biomass values are high, the model suggests that grazing is sa turated and has little effect on cyanobacterial biomass. Grazing on he terotrophic bacteria is rarely saturated and is only loosely coupled t o heterotrophic bacteria production during destratification. The model was tested at several grazer feeding preferences for cyanobacteria or heterotrophic bacteria and reproduced observed microbial biomass valu es most accurately when there was no initial preference. These model d ynamics suggest that the heterotrophic protists fed equally well on bo th heterotrophic bacteria and cyanobacteria.