A framework for the assessment of top-down vs. bottom-up control of heterotrophic nanoflagellate abundance

Abstract

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 bacteria 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 specif~c growth rates.

Morphology-based classification of functional groups for potamoplankton

“…The Gasol model (Gasol, 1994) that was developed for nanoflagellates, allows assessment of the importance of top-down vs. bottom-up mechanisms. It helped in identifying the controlling direction when each MBFG R dominates a river.…”

“…Fig. 4 shows the positioning of all river-samples with >95% dominance of a MBFG R within the framework of the Gasol model (Gasol, 1994). According to this model, Groups RIa, RIII, RVIc, and RVII purely dominated in bottom-up system, whereas only Group RVIa dominated Tab.…”

Plankton in the open Mediterranean Sea: a review

“…According to the model by Gasol (1994), the plot of the relationships between log HNF abundance (HNF ml −1 ) and log bacterial abundance (ml −1 ) suggests that HNF are resource, or bottom-up, controlled by bacteria (Fig. 14c).…”

Dissecting the microbial food web: structure and function in the absence of autotrophs

“…If we apply the qualitative model of Gasol (1994) to determine the control of HNF (Fig. 9), we obtain a lower abundance of HNF than theoretically expected based on the bacterial abundance detected, indicating top-down control of the HNF by ciliates or, more likely, other predators such as crustaceans.…”

“…Although these cave lagoons are oligotrophic systems, there is indication that predators control HNF abundance, a situation commonly reported for eutrophic systems. This apparent discrepancy might indicate that Gasol's (1994) model is not applicable to these particular environments as data sets from such environments were not included in the original model. Another explanation of the observed pattern might be that a large fraction of the DAPI-stained particles was inactive or dead cells, not grazed or grazed only at a lower efficiency by HNF than highly active cells (Sherr et al 1992, López-Amorós et al 1998).…”