An under-appreciated component of biodiversity in plankton communities: the role of protozoa in Lake Michigan (a case study)

Recent technological advances have led to the discovery that free-living, planktonic protozoa are ubiquitous in nature and appear to be important components of pelagic food webs (e.g., fluorescent straining, flow cytometry). Despite this, limited information exists tying their seasonality to rate processes that drive succession patterns. The abundance, and seasonal growth and grazing loss of an entire protozoan assemblage were evaluated in Lake Michigan. The protozoan assemblage was species-rich (100 taxa) and abundant throughout the year in Lake Michigan. Nano-sized protozoa (Hnano and Pnano, < 20 mu m in size) ranged in abundance from 10(2) to 10(3) cells ml(-1), while micro-protozoa (Hmicro and Pmico, > 20 and < 200 mu m in size) ranged in abundance from 4 to 17 cells ml(-1). The biomass of Hnano and Hmicro by itself represented more than 70-80% of crustacean zooplankton biomass, while Pnano and Pmicro constituted nearly 50% of phytoplankton biomass. Protozoa exhibited growth rates comparable to other components of the plankton in Lake Michigan, and some populations grew at rates similar to maximum rates determined in the laboratory (rates of 1-2 day(-1)). Overall, it appears that macro-zooplankton predation is a major loss factor counter-balancing growth with only small differences between the two rate processes (< 0.1 day(-1)). Discrepancies between growth and grazing loss in the spring were likely attributed to sedimentation losses for larger species of tintinnids and dinoflagellates (Codonella, Tintinnidium, and Gymnodinium) that can account for their occurrence in the deep chlorophyll layer. In the summer, carnivory among similar sized species (Chromulina and small ciliates) may be additional loss factors impinging on the protozoan assemblage.