EXPLORING THE ROLE OF VERTICAL HETEROGENEITY IN THE STABILIZATION OF PLANKTONIC ECOSYSTEMS UNDER EUTROPHICATION

Abstract

Understanding plankton dynamics in marine and lake ecosystems under eutrophication is currently a hot topic in the literature. Simple theoretical models predict appearance of large amplitude oscillations of species densities in nutrient-rich waters; however, such predictions do not always correspond to field observations. Recent models taking into account heterogeneity of the growth rate of phytoplankton and active food-searching behavior of zooplankton demonstrated that grazers can efficiently control phytoplankton densities at low values even for a high nutrient stock. In this paper, we extend the previous modeling findings on the role of fast-moving plankton grazers by exploring a more realistic case where the limiting nutrient is a dynamical variable. Thus, the growth of phytoplankton across the water column depends on both light attenuation and dynamical depletion of nutrients. We also consider a more realistic scenario of a depth-dependent vertical turbulent diffusion. Most of the previous results on stabilization of planktonic ecosystems still hold; however, some alternative mechanisms of bloom suppression can also be possible. In particular, we demonstrate that the foraging of zooplankton according to the ideal free distribution (IFD) of food (which was previously considered to be a crucial condition for stabilization) may be less stabilizing than random foraging of zooplankton. We also show that stable top-down control in the ecosystem would be highly dependent on values of vertical diffusion and on the nutrient concentration in deep layers.