Red Fluorescing Phototrophic Picoplankton in the Laurentian Great Lakes: What Are They and What Are They Doing?

Epifluorescence microscopy, flow cytometry, and transmission electron microscopy were used to characterize the community of red fluorescing (emission >665 nm when excited with blue light) phototrophic picoplankton (RFPP) in lakes Huron and Michigan. A population of coccoid to ovate eukaryotic cells with a mean size of 1.2 μm dominated the RFPP community in both surface and deep water samples. Abundant prochlorophyte populations were not found in any samples. Comparisons of counts with epifluorescence microscopy and flow cytometry, revealed that RFPP were adequately enumerated with standard epifluorescence microscopy. These RFPP were significant contributors to total phototrophic picoplankton abundance in both lakes Michigan (24%) and Huron (18%), with maximum seasonal abundance during the May‐June period (surface mixing layer temperatures, 3‐9 °C). During thermal stratification, maximum vertical abundance was found in the metalimnion/hypolimnion at the 1‐5% isolumes. RFPP were only minor contributors (1‐7%) to total primary production. Growth rates of RFPP measured with dilution and small inocula growth experiments ranged from 0.05‐1.0 d⁻¹. Microzooplankton grazing rates on RFPP measured with dilution experiments were similar to estimated growth rates, accounting for 52‐280% of growth on any given date.