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Marine Ecology Progress Series

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MEPS 265:17-30 (2003)  -  doi:10.3354/meps265017

A Lagrangian photoresponse model coupled with 2nd-order turbulence closure

Takeyoshi Nagai1, Hidekatsu Yamazaki1,*, Daniel Kamykowski2

1Department of Ocean Sciences, Tokyo University of Marine Science and Technology, Konan 4-5-7 Minato-ku, Tokyo108-8477, Japan
2Department of Marine, Earth and Atmospheric Sciences, North Carolina State University, 1125 Jordan Hall, Raleigh, North Carolina 27695-8208, USA
*Corresponding author. Email:

ABSTRACT: A 2nd-order turbulence closure approach was coupled with a Lagrangian phytoplankton model, in order to examine the effect of time-dependent vertical eddy diffusion on the photoresponse of phytoplankton in a wind-driven upper mixing layer. In general, stronger wind mixing in a lower-transparency water column contributes to greater phytoplankton production. According to our study, vertical mixing is insignificant for photoinhibition in relatively clear open ocean water, while it can be more important in relatively turbid coastal water. A simple Ekman layer model provided surprisingly similar production to that observed with the 2nd-order closure scheme when the starting distribution of the phytoplankton cells was normalized. Two factors involved in the process are the change in the background stratification and the time dependence of the diffusivity coefficient. The influences of these 2 factors cancel each other to reduce the apparent difference between the total production estimated by the Ekman model compared to that estimated by the 2nd-order closure scheme.

KEY WORDS: Lagrangian phytoplankton simulation · Random walk model · 2nd-order turbulence closure · Photoinhibition

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