Project: Are Feeding and Growth Rates of Planktonic Ciliates Overestimated from Experiments in the Absence of Copepod Predators?

(Extracted from the NSF award abstract)

Predation is a major shaping force for all organisms, including microbes and plankton that have high intrinsic growth rates and can therefore rapidly overwhelm their resources in the absence of population controls. The diversity of organisms, life histories, feeding modes, and trophic interactions in the plankton, coupled with short time scales for metabolic processes and strong driving forces associated with the physics of living in a small Reynolds number world, brings with it a level of complexity that models attempt to mimic but also need to simplify. A good example is the common use of experiments involving two adjacent trophic levels in order to derive functional equations that can be inserted into ecosystem models, which attempt to simulate or predict a more holistic community interaction. The basic assumption of such a reductionist approach is that two-trophic experiments can be accurately extrapolated to a three- or multi-trophic world. This proposal addresses such a fundamental assumption.

Microzooplankton are now universally recognized as being a dominant group of herbivores in most aquatic ecosystems. Among the microzooplankton, ciliates are a pivotal group in terms of both grazing phytoplankton and also passing that reconstituted primary production along to higher trophic levels by being predated. In marine systems, copepods are considered to be major predators of ciliates. Equations that are used to model interactions between ciliates and their phytoplankton prey have invariably been based on laboratory experiments in which only those two groups are present. The resulting rates of herbivory are assumed to be accurate in a world other than that tested, i.e. in situ with copepod predators present while the ciliates are feeding. Extant literature and preliminary data in this proposal provide strong evidence that in fact ciliates spend a significant fraction of time, otherwise spent feeding, on avoiding or responding to predatory copepods. This project will quantify this cost in terms of food energy loss and growth rate reduction of ciliates independent of mortality caused by predators. Several relevant ecological factors will be examined, including type of ciliate behavioral reaction to copepods (jumping vs. cruising); abundance of ciliate prey (saturating versus subsaturating); concentration of copepod predators; and effects of chemical signals from copepods compared to their physical presence.

Thus there are two complementary themes in this project: (1) improved appreciation for the selection processes that determine ecology, and (2) expression of those evolutionary tradeoffs as carbon flows that can help us simplify the complexity of such ecosystems. This research is transformative in nature because it will quantify how individual interactions among plankton direct biogeochemically salient processes, and the resulting information will provide the means to re-evaluate bulk flows in aquatic ecosystems.