PHOTOTROPHIC PICOPLANKTON IN LAKES HURON AND MICHIGAN - ABUNDANCE, DISTRIBUTION, COMPOSITION, AND CONTRIBUTION TO BIOMASS AND PRODUCTION

The phototropic picoplankton communities of Lakes Huron and Michigan were studied from 1986 through 1988. Abundances in the surface-mixed layer ranged from 10 000 to 220 000 cells.mL-1 with a seasonal maximum during the period of thermal stratification. During thermal stratification, maximum abundances were generally found within the metalimnion/hypolimnion at depths corresponding to the 0.6-6.0% isolumes. The picoplankton community was dominated by single phycoerythrin-containing (PE) Synechococcus (59%) with lesser amounts of chlorophyll fluorescing cells (21%), PE colonial Synechococcus-like cells (11%), other PE colonial Chroococcales (6%), and other cells (3%). Single PE Synechococcus was abundant throughout the year whereas chlorophyll-fluorescing and colonial cyanobacteria were more abundant during the periods of spring isothermal mixing and summer stratification, respectively. Picoplankton accounted for an average of 10% (range 0.5-50%) of phototrophic biomass. Phototrophic organisms that passed 1-, 3-, and 10-mu-m screens were responsible for an average of 17% (range 6-43%), 40% (21-65%), and 70% (52-90%) of primary production. Maximum contributions of < 1, < 3, and < 10-mu-m size fractions occurred during the period of thermal stratification. Primary production by phototrophic picoplankton was found to equal production in the < 1-mu-m size fraction.