Mesoscale variability of physical and biological fields in southeastern Lake Superior

Zhou, Meng, Yiwu Zhu, Shawn Putnam, Jay Peterson

Limnol. Oceanogr., 46(3), 2001, 679-688 | DOI: 10.4319/lo.2001.46.3.0679

ABSTRACT: A cruise was conducted in southeastern Lake Superior east of Marquette, Michigan from 15 to 20 July 1999. A towed, vertically undulating instrument package containing a conductivity-temperature-depth (CTD) profiler, a fluorometer, and an optical plankton counter was used for data collection. Results indicated that the concurrent, high-resolution survey of both physical and biological variables provided the fundamental basis for illustrating the correlation between mesoscale physical and biological fields and for investigating mesoscale physical and biological processes. It was revealed that the high chlorophyll a biomass beneath the thermocline co-occurs with up-tilting of the thermocline. This tilting of the thermocline was associated with the thermally driven coastal current. We hypothesize that the up-tilting of the thermocline lifted deep water into the euphotic zone, leading to high productivity of phytoplankton beneath the thermocline. The presence of both warm water and food provided a favorable physical and biological environment for zooplankton. The abundance of zooplankton reached 10 x 103 individuals m-3 in the warm surface water and was down to 3 x 103 individuals m-3 at a depth of 40 m.

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