Extracting signals of predation and competition from paired plankton time series

Abstract

Although competition and predation among plankton in seasonal aquatic systems must be ubiquitous, it remains a challenge to actually identify these interactions and to evaluate their relative strengths. Based on paired time series for zooplankton and phytoplankton in three lakes: Mjosa (12 years), Farris (2 years) and Lake Washington (29 years), we construct phase portraits and quantitatively analyse their trajectories in an attempt to characterize interaction types and strengths. Because we can infer the type of interactions each pair partake in a priori (e.g., a phytoplankton/zooplankton pair is a prey/predator pair), we hypothesize that it is possible to distinguish trajectories resulting from a pair that compete from those of a pair constituting a prey/predator pair. We found that competition and predation can be distinguished based on the time series of the interacting species' biomass. Break points in population trajectories (i.e., population collapse) and the counter clockwise rotation of the trajectory in the phase portrait give the strongest signals of predation and the signals correspond to what we expect from classical predation theory. For competition we can only give plausible biological interpretations of identified signals, since signals from grazing, seasonality and competition are confounded.