Research Project: Microzooplankton Ciliates
Among microzooplankton, ciliates are generally considered to be the most significant component which links pico- and nanoplankton to the larger metazoans which cannot feed efficiently on these tiny cells directly. Ciliates are often major herbivores, along with other nano- and microzooplankton. In situ growth rates of planktonic ciliates are typically 1-2 doublings per day in SE USA shelf waters. Fluctuations in abundance, however, are damped in comparison to potential growth curves, which implies substantial predation. Predation is easily documented in the lab as prey disappearance, but it is more difficult in the field due to the presence of multiple predators and prey, and the lack of proper experimental controls (removing all predators).
A further problem is that most numerically abundant ciliates do not have hard morphological features which can be enumerated in the gut contents of predators. We study predation using traditional experiments in which natural plankton communities are incubated with and without added metazooplankton. But instead of using chlorophyll or particle volume as a proxy for grazing, as is typically done, samples are analyzed using imaging systems we have developed. This approach allows us to specify which components (by taxa, size class, and trophic status) of the entire autotrophic and heterotrophic community are receiving unusually strong or weak grazing/predation pressure.
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
Our current research addresses such a fundamental assumption. 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 We are quantifying this cost in terms of food energy loss and growth rate reduction of ciliates independent of mortality caused by predators Several relevant ecological factors are being 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. Future studies can extend these results to other taxonomic components of the microzooplankton, and investigate the effects of different types of zooplankton predation behavior.