Subsurface Chlorophyll Maximum and Associated Cyclotella Pulse in Lake Superior

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Abstract
An investigation of the subsurface chlorophyll layer (SCL) in Lake Superior was conducted from 1 May through 15 October. During spring unstratified conditions (1 May–17 July), chlorophyll a and phytoplankton biomass were vertically and temporally uniform. Phytoflagellates such as Rhodomonas minuta, Cryptomonas spp., and Ochromonas spp. were dominant at this time. The photosynthetic maximum was associated with the depth receiving 10% of surface irradiance, ca. 15–25 m. Once thermal stratification occurred, a SCL developed around 20 m. Although epilimnetic chlorophyll concentrations remained similar to spring mixing levels, concentrations in the SCL increased 3‐fold. In situ production was the principal process associated with the development of the SCL. The upper portion of the SCL was dominated by Cyclotella species; C. stelligera, C. comensis, and C. ocellata. The increased abundance of Cyclotella appeared to be related to stable irradiance fields produced during thermal stratification.