Seminar: Thursday January 24, 2008 3:30-4:30pm, Mueller Room 153
Speaker: Dr. Kevin Schock P.E., Civil Engineer, Water Quality Modeling,
King County Wastewater
Treatment Division
Title: "An Analysis of a Persistent Isotherm
Tilt During Early-Spring and It’s Effect
on the Diatom Bloom: Lake Washington,
Abstract
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-forth 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. Overall, the physical aspects of the isotherm tilt sets conditions that determine longitudinal
differences in diatom biomass and the magnitude and vertical distribution of eddy diffusion
determined the size of the diatom bloom.
http://dnrp.metrokc.gov/WLR/Science/Seminar/pdfs/PDF82.pdf
Education:
1985 BS Civil
Engineering,
1992 MS Civil
Engineering,
2007 PhD Civil &
Environmental Engineering, U of
Email: Kevin.Schock@METROKC.GOV
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