Surface-water quality and flow Modeling Interest Group

Does the Sverdrup Critical Depth Model Explain Bloom Dynamics in Estuaries?

¹USGS, 345 Middlefield Rd, MS 496, Menlo Park, CA 94025
²Environmental Fluid Mechanics Laboratory, Stanford University, Stanford, CA 94305-4020

Please direct correspondence to:
  Lisa Vidergar Lucas
  Postdoctoral Research Associate, U.S. Geological Survey
  345 Middlefield Road, MS 496, Menlo Park, CA 94025
  Internet: llucas@usgs.gov
  Phone: (650) 329-4588 or (650) 723-1825
  FAX: (650) 329-4327

Citation:
Lucas, L.V., Cloern, J.E., Koseff, J.R., Monismith, S.G., and Thompson, J.K., 1998, Does the Sverdrup critical depth model explain bloom dynamics in estuaries?: J. Marine Research, 56(2), 375-415.

Only the contents and abstract for this article are shown below. The full article is available in two versions: one with all of the figures inline, and one with the figures converted to thumbnail images with links to the full figures. The full inline version of the article will take longer to load, but the inline figures may be more convenient for viewing and printing.

Contents

Abstract
I. INTRODUCTION
II. THE PHYSICAL SYSTEM

a. Stratification in Estuaries
b. Modeling Stratified Estuarine Turbulence
c. Results: Turbulent Mixing Coefficients and Stratification
d. Summary and Caveats

III. THE BIOLOGICAL SYSTEM

a. Phytoplankton Dynamics
b. The Phytoplankton Model

IV. PHYSICAL/BIOLOGICAL COUPLING

a. The General Effects of Periodic and Persistent Stratification on Phytoplankton Dynamics
b. Applicability of the Critical Depth Model to Shallow Estuaries

i. Background
ii. Approach
iii. Relationship of Surface Layer Depth to Bloom Inception and Magnitude

c. Origin of the Non-monotonic Relation Between Population Growth and Mixed Layer Depth

i. Turbulent Mixing
ii. Sinking in the Presence of Turbulent Mixing
iii. Average Net Growth Rate: Surface Layer and Below Pycnocline

d. Leakage Due to Mixing
e. The Effects of Leakage on Phytoplankton Blooms

CONCLUSIONS
APPENDIX: DETAILS OF THE PHYTOPLANKTON MODEL

a. The Biological Model
b. The Finite Volume Discretization
c. Boundary Conditions
d. Accuracy and Stability

REFERENCES

Abstract

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 stratification 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 stratification greatly increases the probability of a bloom, semidiurnal periodic stratification does 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, 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 stratification 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 non-monotonic 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 refined 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 refinement 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 diffusivities, a significant percentage of the phytoplankton biomass produced in the surface layer can be lost by these processes.

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