Biological and hydrodynamic regulation of the microbial food web in a periodically mixed estuary

Abstract

Abundances of chroococcoid cyanobacteria and heterotrophic bacteria in surface waters of the York River subestuary covary with spring‐neap tidally induced changes in the mixed‐layer depth. Abundances of their principal grazers, heterotrophic protists, however, do not oscillate. A simulation model of this system using nonlinear, density‐dependent functions has been developed to replicate cycles observed in the two bacterial abundances and simulate bacterial production and protistan grazing. A Jassby‐Platt equation is used to determine growth rate from the mean mixed‐layer light and empirically derived growth and α parameters. Changes in mixed‐layer depth regulate light availability, thereby controlling cyanobacterial growth rates. The model predicts a close coupling between cyanobacterial growth and grazing during destratified periods when cyanobacterial stocks are low. During stratified periods, when cyanobacteria biomass values are high, the model suggests that grazing is saturated and has little effect on cyanobacterial biomass. Grazing on heterotrophic bacteria is rarely saturated and is only loosely coupled to heterotrophic bacteria production during destratification. The model was tested at several grazer feeding preferences for cyanobacteria or heterotrophic bacteria and reproduced observed microbial biomass values most accurately when there was no initial preference. These model dynamics suggest that the heterotrophic protists fed equally well on both heterotrophic bacteria and cyanobacteria.