Biomass, size structure, and composition of phototrophic and heterotrophic nanoflagellate communities in Lakes Huron and Michigan

The abundance and biomass of surface (5 m) and deep (20-45 m) nanoflagellate communities in Lakes Huron and Michigan were determined during 1987. Abundances (10²-10³ cells.mL ^-1) were comparable between lakes and similar to those reported from other oligotrophic environments. Community composition was skewed towards the small end of the size spectrum due to the prevalence of small chrysomonads. In general, heterotrophic flagellates (Hnano) were more abundant that phototrophic flagellates (Pnano), while standing stocks of Pnano carbon (average 24.7 μg C.L^-1) were greater than Hnano carbon (9.6 μg C.L^-1) on nearly all sample dates. Abundance of nanoflagellates in both Lakes Huron and Michigan peaked in July, perhaps indicating increased growth at higher temperatures and/or a response to higher abundance of prey. Nanoflagellate communities in deep waters during thermal stratification were more abundant (50-70% higher carbon) than surface communities and were dominated by Pnano. High carbon standing stocks of deep communities did not correspond with high prey abundances. Thus, deep communities seem to be influenced by factors (eg light and nutrients) that maintain deep phytoplankton communities in the upper Great Lakes. While Hnano are quantitatively important in Lakes Huron and Michigan, representing nearly 20% of phytoplankton biomass, their trophic role is largely unknown. -Authors