Variation in the abundance of pico, nano, and microplankton in Lake Michigan: Historic and basin-wide comparisons

The Lake Michigan ecosystem has undergone numerous, systemic changes (reduced nutrient, changing climate, invasive mussels) that have altered portions of the food web and thus, appear to have changed the lake's trophic state. That said, little is known about the components of the microbial food web (MFW, heterotrophic and phototrophic pico, nano, and micro-plankton), which we hypothesized have compensated as a food source for crustacean zooplankton given the recent declines in the biomass of large phytoplankton (mainly diatoms). Therefore, we measured the abundance of the entire MFW using complementary microscopic techniques, flow cytometry, and size fractionated chlorophyll concentrations at sites in northern and southern Lake Michigan, and one site in Lake Superior; the latter site served as a benchmark for oligotrophic conditions. In addition, a historic comparison was made between 1987 and 2013 for the southern Lake Michigan site. Ppico numbers (i.e., picocyanobacteria) in 2013 were lower compared with those in the 1980s; however, the percent contribution of the <2μm fraction increased 2-fold (>50% of total chlorophyll). The abundance of small, pigmented chrysomonads and cryptomonads (Pnano size category) was not significantly different between 1987 and 2013 at the same time Pmicro did decline; this shift towards Ppico and Pnano dominance may be related to the recent oligotrophication of Lake Michigan. The abundance of ciliated protists (Hmicro size class) was 3-fold lower in 2013 compared with levels in 1987, while the abundance of both Hpico (eubacteria, range 0.24-1.36×10⁶cellsmL^-1) and Hnano (mainly colorless chrysomonads; range 0.11-6.4×10³cellsmL^-1) remained stable and reflected the resilience of bacteria-flagellate trophic linkage.