New Zealand Journal of Marine and Freshwater Research abstracts

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Mixing processes relevant to phytoplankton dynamics in lakes

ROBERT H. SPIGEL¹

JORG IMBERGER²

¹Department of Civil Engineering University of Canterbury Christchurch, New Zealand

²Centre for Water Research University of Western Australia Nedlands, Western Australia 6009

Abstract Active turbulence in lakes is confined to the surface mixed layer, to boundary layers on the lake sides and bottom, and to turbulent patches in the interior. The density stratification present in most lakes fundamentally alters the pathways connecting external mechanical energy iftputs, for example by wind, with its ultimate fate as dissipation to heat; the density stratificatioi supports internal waves and intrusions that distribute the input energy throughout the lake. Intrusions may be viewed as internal waves with zero horizontal wavenumber and are formed each time localised mixing occurs in a stratified fluid. Intrusions are also formed in the epilimnion by differential heating or cooling and by differential deepening. The fraction of lake volume below the diurnal mixed layer that is subject to active turbulence is very small, probably of the order of 1% or lets in small to medium-sized lakes. By contrast, in t|he surface mixed layer, turbulence is less intermittent and maintains phytoplankton in suspension and controls their exposure to the underwater solar flux. Nutrient transport to individual cells depends not only on the cell Reynolds number but also on the Peclet number, which, if large, implies enhanced mass transfer above purely diffusive values.

Keywords cyanobacteria; eddy difflisiVity; lakes; mixing; phytoplankton; stratification; turbulence

New Zealand Journal of Marine and Freshwater Research, 1987, Vol. 21: 361-377 0028-8330/87/2103-0361$2.50/0 © Crown copyright 1987 Received 19 February 1987; accepted 11 May

PDF file of entire paper: medium quality (1566K); (scanned from paper original: notes about this process)

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