Exploring nonlinear functional responses of zooplankton grazers in dilution experiments via optimization techniques

Abstract Understanding complex dynamics of planktonic ecosystems requires good knowledge of predator functional responses, especially for microzooplankton grazers. Although natural microzooplankton communities are highly diverse, we often need to quantify microzooplankton grazing at the macrolevel, considering them as a single “box.” Here, we consider community microzooplankton predation rates as a function of their prey abundance. Estimating the functional response of microzooplankton as a function of bulk prey often exploits the dilution approach: joint incubation experiments of grazers and their prey where samples are subjected to a range of dilution factors. However, the classic methodology appears reliable only for a simple linear functional response of grazers and it requires appropriate adjustments for nonlinearity and saturation of food consumption at high prey abundance. We explore how recently suggested methods applying nonlinear functional responses of bulk microzooplankton grazing can be enhanced to provide more robust results. We argue that different formulations of the functional response should be fitted to the data and that an appropriate choice of dilution factors in experiments can substantially improve the accuracy of feeding rate estimation. Pragmatically, our work shows that for oligotrophic ecosystems, it is best to run incubations where the majority of dilution factors are close to D  = 1 (undiluted), whereas, for eutrophic waters, dilution factors should be run close to D  = 0 as well as D  = 1 . Furthermore, running equidistant dilution factors represents a waste of effort; it is more important to run at fewer values of D with greater replication.