An under-appreciated component of biodiversity in plankton communities: the role of protozoa in Lake Michigan (a case study) Hunter J. Carrick School of Forest Resources, Pennsylvania State University, University Park, PA 16802, USA E-mail: hjc11@psu.edu Key words: biodiversity, plankton, protozoa Abstract 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 suc- cession 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 lm in size) ranged in abundance from 10 2 to 10 3 cells ml )1 , while micro-protozoa (Hmicro and Pmico, >20 and <200 lm 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 phy- toplankton 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. Introduction Biodiversity is perhaps the most basic tenet in ecology, where species are the units, through which evolution is expressed and meaningful environ- mental change is manifested (Ricklefs, 1990). Without a basic knowledge of natural biodiversity it is difficult to truly understand and appreciate nature and the factors that shape it (Lugo, 1988; Wilson, 1988). While much progress has been made to measure the biodiversity of natural com- munities, knowledge is not equivalent among taxonomic or ecologic groups (see Wilson, 1984) and some groups are more prone to extinction at the hand of anthropogenic change (Terborgh, 1972). Interestingly, recent data for some of the most under-studied groups of organisms and fea- tures of biodiversity have promoted paradigm- level shifts in our understanding of ecosystems (see Naeem, 2002), because this knowledge can provide critical blocks of information that simply (by def- inition) did not previously exist (Kuhn, 1962). Our understanding of ecosystem food web structure has undergone substantial revision, from a traditional view where energy is transferred in succession from one feeding guild to the next originating with plants (Elton, 1927; Lindeman, 1942). In contrast, recent technological advances (e.g., fluorescent staining, flow-cytometry) now allow more accurate census of natural microbial Hydrobiologia (2005) 551:17–32 Ó Springer 2005 J.N. Beisel, L. Hoffmann, L. Triest & P. Usseglio-Polatera (eds), Ecology and Disturbances of Aquatic Systems DOI 10.1007/s10750-005-4447-0