A FRAMEWORK FOR THE ASSESSMENT OF TOP-DOWN VS BOTTOM-UP CONTROL OF HETEROTROPHIC NANOFLAGELLATE ABUNDANCE

A model is presented that allows inferences about the main mode of control of planktonic heterotrophic nanoflagellate (HNF) abundance, control by resources or by predation, from the abundances of bacterial and HNF. The model is built upon the observation that field measurements of HNF abundance at most reach concentrations close to those expected for a system in steady-state (with no variation of bacterial abundance). Field and enclosure data from freshwater and marine systems are used to test its utility by regressing D, the distance between the maximal possible concentration of HNF and the actual concentration, to the abundance of micro- or macrozooplankton. It is concluded that the model may help ascertain the impact of microzooplankton on the components of microbial food webs when data on their abundances are not available, and that it can also be used to generate hypotheses on the degree of influence of sources of carbon other than bacteria on flagellates, and the effect of HNF grazing on bacterial specific growth rates.