Mathematical models of plankton dynamics are sensitive to the choice of type of zooplankton functional
response, i.e., to how the rate of intake of food varies with the food density. Conventionally, the
conclusion on the actual type of functional response for a given zooplankton species is made based upon
laboratory analysis on experimental feeding. In this paper, we show that such an approach can be too
simplistic and misleading. Based on real ocean data obtained from three expeditions of R/V Jan Mayen in
the Barents Sea in 2003–2005, we demonstrate that vertical heterogeneity in algal distribution as well
as active vertical movement of herbivorous zooplankton can modify the type of trophic response
completely. In particular, we found that the rate of average intake of algae by Calanus glacialis exhibits a
Holling type III response, instead of Holling type I or II found previously in laboratory experiments. We
argue that this conceptual discrepancy is due to the ability of the zooplankton to feed in layers with
high algal density and to avoid depths with lower algal density. Since theoretical studies would predict
enhancing in system stability in the case of Holling type III, our results may be of importance for
understanding the main factors controlling plankton dynamics.
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