An Analysis of a Persistent Isotherm Tilt During Early - Spring 2003 and 2004 and Its Effect on the Diatom Bloom

About this Report

A persistent isotherm tilt was observed during the early spring diatom bloom, the isotherms had an upward tilt from north to south. Concurrently, phytoplankton biovolume concentrations showed different concentrations near the northern, central, and southern parts of the lake, phytoplankton concentrations were higher at the central part.

The persistent tilt was caused by frequent northward wind events with wind period being less than one-fourth the first mode internal wave period. The Wedderburn Number was generally less than two and often times less than one, which indicated a strong relation between the isotherm tilt and wind stress. Isotherm response was functionally described by the Wedderburn Number. This condition indicated the isotherm response behaved as a forced system. The northward winds maintain the observed isotherm tilt and transported surface water and phytoplankton downwind.

The isotherm tilt created a variable mixed layer depth with deeper mixing at the north end (downwind) and shallower mixing at the south end (upwind). At the north end, the deeper mixing reduced the depth integrated photosynthesis compared to the south end of the lake. At the south end of the lake, hydrodynamics that maintained the tilt also caused bottom water to upwell; the upwelled water was transported downwind. The upwelled water diluted the phytoplankton biomass and the downwind currents transport them out of the area faster than they could grow. These conditions reduced the effective phytoplankton growth rate at the north and south ends compared to the central region. The bloom occurred when vertical mixing relaxed and the phytoplankton growth rate exceeded vertical diffusive fluxes out of the euphotic zone, but vertical diffusion balanced sinking.

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Due to the large size of the document, it is provided here for download in sections:

• Cover, Abstract, Table of Contents (247 KB)

Section 1: Influence of Wind on Isotherm Tilt

• Introduction, Methods, Results (3.2 MB)
• Results cont'd (4 MB)
• Results cont'd (3.5 MB)
• Results cont'd (3.3 MB)
• Results cont'd, Discussion (3.4 MB)

Section 2: Effect of Isotherm Tilt and Hydrodynamics on Early Spring Diatom Bloom

• Introduction, Methods, Results (3.4 MB)
• Results cont'd (3.3 MB)
• Results cont'd (4 MB)
• Results cont'd (2.8 MB)
• Results, Discussion, Relevance, Conclusions, Future Studies (3.1 MB)

References and Appendices.

• References (111 KB)
• Appendix A Eddy Diffusion Profiles (4.3 MB)
• Appendix A cont'd (3.1 MB)
• Appendix A cont'd (2.4 MB)

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Updated: Feb. 20, 2008

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