Modelling and simulation of the mesoscale mosaic structure of the lower marine trophic levels

Abstract

Marine ecology deals with biological and chemical processes in interaction with their aquatic environment. The possibility of using more of the products of the sea as human food has created at present a keen interest in the study of marine plankton. It is of importance to understand the production of phytoplankton and the predator-prey relationships between phyto- and zooplankton, the path by which the organic matter produced finally reaches the fish. In the sea, plankton populations are almost entirely at the mercy of water movement. In spite of this diffusive process, these populations display a spatio-temporal structure. The spectral analysis of the spatial organization of phytoplankton populations exhibits two main classes of behaviour depending on the range of spatial scale. Below 5 km, the phytoplankton behaves as a passive scalari)from zero to 100 m, the spatial variability of phytoplankton is controlled by turbulence and its spectrum is similar to the spectrum of homogeneous and spatially isotropic turbulence according to Kolmogorov's theory,ii)from 100 m to 5 km, the coherence between chlorophyll and temperature are high. Beyond 5 km and until 100 km, the phytoplankton dynamics in promoting patchiness, i.e. mesoscale spatial heterogeneity, dominates over that of the physical diffusive processes in eroding it. A model is proposed to explain the mechanism of this mosaïc structure. The partial differential equations take into account advection, shear and eddy diffusivity and non-linear ecological interactions. The properties of the solutions of these equations are studied by simulation of simplified sub-models dealing with asymptotic behaviour of the ecological system. The horizontal structure is generated by local spatial instabilities. The most important characteristic is the disposition of the ecosystem to amplify microscopic excitations (fluctuations) to a macroscopic level leading to the emergence of new space and time patterns.