A method to represent seasonal vertical migration of Zooplankton in 3D-Eulerian models

Abstract

To understand the vertical distribution of zooplankton, the swimming behavior of the animals in response to physical processes, food conditions etc., has to be taken into account. The active, vertical migration of copepods can be considered as a reaction to external and internal stimuli. The reaction patterns can be covered by a set of behavioral rules. In this paper, we introduce a method based on so-called evaluation functions that assess the local habitats of the copepods at each grid point of the model, and to quantify the probability of their possible reactions in terms of up- or downward migration. The method is tested for the case of the Baltic Sea and considers seasonal vertical migration. Known observational facts of the main mesozooplankton groups are used to define the evaluation functions for two aggregated groups of model copepods, which occupy different habitats. The copepod groups are divided into five developmental stages. Several simulations were carried out with the aid of one- and three-dimensional Eulerian models to constrain the involved set of parameters. It can be shown that a plausible characterization of the behavioral features of copepods, as generally known from observations, mimics the migration patterns of the model copepods in an excellent agreement with the typical behavioral pattern of the main mesozooplankton in the Baltic Sea. The method provides a relatively easy way to introduce aspects of zooplankton behavior in ecosystem models. It can be applied to Eulerian models and allows numerical optimization of the code in a straightforward manner.