PATTERNS IN THE TOP-DOWN VERSUS BOTTOM-UP REGULATION OF HETEROTROPHIC NANOFLAGELLATES IN TEMPERATE LAKES

The importance of resources versus that of predators in determining the abundance of organisms is modulated by several factors which vary over time and space. Here, we evaluate the abundance of heterotrophic nanoflagellates (HNF), the main predators of planktonic bacteria, in the plankton of 16 Quebec lakes as a function of the abundance of resources and zooplankton (predators). We analyze the data at two different scares: among lakes varying in resource supply and within lakes. Resource factors (total phosphorus, chlorophyll, bacterial concentration) best explained the abundances of HNF among lakes differing in resource availability. When the effect of resource availability was removed, the biomass of cladocerans became an important determinant of HNF abundance, with abundance declining and the ratio of bacteria per flagellate increasing, as the cladocerans increased. Within lakes, HNF abundance was not a direct function of resource concentration. Multiple regression models based on data for many lakes showed that both bottom-up (food) and top-down (predation) variables predicted HNF abundance quite well. An examination of the seasonal pattern among individual lakes showed each group of variables differing in importance: bottom-up variables were more important in spring and top-down variables in midsummer. An examination of the effects of different assemblages of zooplankton on HNF showed cladocerans to exert the highest negative impact on HNF and to be responsible for keeping low HNF numbers during most of the summer with that probably affecting the pathways of energy flux in those lakes dominated by cladocerans Versus those dominated by other zooplankton. The absolute, as well as the relative, importance of the different biotic and abiotic factors examined in predicting the abundance of HNF is a function of the scale of the investigation. Among lakes varying greatly in resource supply, resources are found to better correlate with HNF abundance. When the analysis is performed at another scale, within lakes, the relative importance of resources and predation in determining HNF abundances changes depending on the variability superimposed by other factors (seasonality and lake food web structure).