In this paper we use numerical models of coupled biological-hydrody~~arnic processes to search for general principles of bloom regulation in estuarine waters. We address three questions: What are the dynamics of slratificalion in coasial systems a\ influenced by cariable freshwater input and tidal stirring'? How does phytoplankton growth respond to these dynamics? Can the classical Sverdrup Critical Depth Model (SCDM) be used to predict the tinling of bloom events in shallow coastal domains such as estuaries? We present results of simulation experiments which assume that vertical transport and net phytoplankton growth ratcs arc horizontally homogcncous. In the present approach the temporally and spatially varying turbulent diffusivities for various stratification scenarios are calculated using a hydrodynamic code that includes the Mellor-Yarnada 2.5 turbulence closure model. These difl'usivi