Patterns ofZooplankton Functional Response in Communities with Vertical Heterogeneity: a Model Study

Abstract

Parameterization of zooplankton functional response is crucial for constructing plankton models. Theoreticalstudies predict enhancing of system stability in case the response is of sigmoid type. Experiments on feeding in laboratories tell us infavor of non-sigmoid types for most herbivorous zooplankton species. However, recent field observations show that the overall functionalresponse of zooplankton in the whole euphotic zone can exhibit a sigmoid behavior even when the response for the same species in laboratorymesocosms is non-sigmoid. Here we propose a simple model explaining the observed alterations of functional response. We divide the euphoticzone into a number of layers and take into account the food-dependent migration of zooplankton. In each layer, the functional response (localresponse) is suggested to be non-sigmoid. We show that the overall response of zooplankton exhibits different behavior compared to thepatterns of the local response. In particular, the model predicts emergence of a sigmoid type as a result of zooplankton accumulation andfeeding in layers with high phytoplankton density. We show the importance of light attenuation by phytoplankton on the alteration offunctional response. The modelling results allow us to hypothesize that the sigmoid functional response in real communities should emergemore often than it was suggested earlier based only on experimental studies on zooplankton feeding.