Does the Sverdrup critical depth model explain bloom dynamics in estuaries?

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ID: 293962
1998
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Abstract
In this paper we use numerical models of coupled biological-hydrodynamic processes to search for general principles of bloom regulation in estuarine waters. We address three questions: What are the dynamics of stratie cation in coastal systems as ine uenced by variable 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 timing 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 rates are horizontally homogeneous. In the present approach the temporally and spatially varying turbulent diffusivities for various stratie cation scenarios are calculated using a hydrodynamic code that includes the Mellor-Yamada 2.5 turbulence closure model. These diffusivities are then used in a time- and depth-dependent advection-diffusion equation, incorporating sources and sinks, for the phytoplankton biomass. Our modeling results show that, whereas persistent stratie cation greatly increases the probability of a bloom, semidiurnal periodic stratie cation does not increase the likelihood of a phytoplankton bloom over that of a constantly unstratie ed water column. Thus, for phytoplankton blooms, the physical regime of periodic stratie cation is closer to complete mixing than to persistent stratie cation. Furthermore, the details of persistent stratie cation are important: surface layer depth, thickness of the pycnocline, vertical density difference, and tidal current speed 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 stratie cation and blooms in deep pelagic systems. First, earlier studies (Riley, 1942, for example) suggest a monotonic increase in surface layer production as the surface layer shallows. Our model results suggest, however, a nonmonotonic relationship between phytoplankton population growth and surface layer 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 ree ned for application to energetic shallow systems or for systems in which surface layer mixing is not strong enough to counteract the sinking loss of phytoplankton. This need for ree nement arises because of the leakage of phytoplankton from the surface layer by turbulent diffusion and sinking, processes not considered in the classical SCDM. Our model shows that, even for low sinking rates and small turbulent
Reference Key
openalex_W1992914374 Use this key to autocite in the manuscript while using SciMatic Manuscript Manager or Thesis Manager
Authors Lisa V. Lucas, James E. Cloern, Jeffrey R. Koseff, Stephen G. Monismith, Janet K. Thompson
Journal journal of marine research
Year 1998
DOI
10.1357/002224098321822357
URL
Keywords Keywords not found

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