Primary Production, Extracellular Release, and Heterotrophy in Two Lakes in the ELA, Northwestern Ontario

During short-term experiments with phytoplankton, excretion rates of 14C labeled organic substances may change without concomitant changes in photosynthetic rates. Percentage extracellular release (p.e.r.) values generally decreased with time.In eutrophic Lake 227, p.e.r. after 3-h incubation ranged from about 1 to 4%, depending on light intensity, when phytoplankton biomass was high, but the range was from about 7 to 15% at a time of low biomass. In oligotrophic Lake 239 p.e.r. was about 20%. Mainly large molecules, exceeding 1500 mol wt, were excreted by phytoplankton. A tracer bioassay gave significantly lower estimates of acetate and glucose concentrations than the Michaelis–Menten analysis. Extracellular release rates of dissolved organic compounds by phytoplankton exceeded the total heterotrophic uptake rates of acetate, glucose, and glycollate.

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