DOES THE SVERDRUP CRITICAL DEPTH MODEL EXPLAIN BLOOM DYNAMICS IN ESTUARIES

In this paper we use numerical models of coupled biological-hydrodynam ic processes to search for general principles of bloom regulation in e stuarine waters. We address three questions: What are the dynamics of stratification in coastal systems as influenced by variable freshwater input and tidal stirring? How does phytoplankton growth respond to th ese dynamics? Can the classical Sverdrup Critical Depth Model (SCDM) b e used to predict the timing of bloom events in shallow coastal domain s such as estuaries? We present results of simulation experiments whic h assume that vertical transport and net phytoplankton growth rates ar e horizontally homogeneous. In the present approach the temporally and spatially varying turbulent diffusivities for various stratification scenarios are calculated using a hydrodynamic code that includes the M ellor-Yamada 2.5 turbulence closure model. These diffusivities are the n used in a time-and depth-dependent advection-diffusion equation, inc orporating sources and sinks, for the phytoplankton biomass. Our model ing results show that, whereas persistent stratification greatly incre ases the probability of a bloom, semidiurnal periodic stratification d oes not increase the Likelihood of a phytoplankton bloom over that of a constantly unstratified water column. Thus, for phytoplankton blooms , the physical regime of periodic stratification is closer to complete mixing than to persistent stratification. Furthermore, the details of persistent stratification are important: surface layer depth, thickne ss of the pycnocline, vertical density difference, and tidal current s peed all weigh heavily in producing conditions which promote the onset of phytoplankton blooms. Our model results for shallow tidal systems do not conform to the classical concepts of stratification and blooms in deep pelagic systems. First, earlier studies (Riley, 1942, for exam ple) suggest a monotonic increase in surface layer production as the s urface layer shallows. Our model results suggest, however, a nonmonoto nic relationship between phytoplankton population growth and surface l ayer depth, which results from a balance between several ''competing'' processes, including the interaction of sinking with turbulent mixing and average net growth occurring within the surface layer. Second, we show that the traditional SCDM must be refined for application to ene rgetic shallow systems or for systems in which surface layer mixing is not strong enough to counteract the sinking loss of phytoplankton. Th is need for refinement arises because of the leakage of phytoplankton from the surface layer by turbulent diffusion and sinking, processes n ot considered in the classical SCDM. Our model shows that, even for lo w sinking rates and small turbulent diffusivities: a significant perce ntage of the phytoplankton biomass produced in the surface layer can b e lost by these processes.