Oxygen dynamics in coastal and lagoon ecosystems

Aquatic systems with a high trophic level (for example some coastal lagoons connected with sea, estuaries, and the neighbourhood) can be affected by some dystrophic phenomena. The instability of these ecosystems greatly depends on the nutrients availability and on the meteorological and climatic conditions. The variation of these factors can produce fast evolution of dystrophic phenomena into anoxis crises: that is the depletion of oxygen in the water column that, Starting from localized areas, can interest all the basin. Due to these modifications, great changes can be observed in the composition of the biotic community. These processes may be described by a reaction-diffusion system based on the following biological hypotheses: (i) the primary production is assumed to be constant (in particular of macrophytes), (ii) the model consists of the balance equations of some chemical species, in the water column and in the sediment at the bottom of the water, (iii) a simplified formalization of reaction kinetics. The model presented can be used to evaluate the influence of environmental factors, such as temperature, wind, and lighting on the level of oxygen, and it is described by a system of semilinear partial differential equations of parabolic type. A numerical study, based on the semidiscrete Galerkin Method with finite elements, is proposed. The present approach uses an iterative process of successive approximations which splits the system in three independent systems of linear equations. Finally, some results of numerical simulation of oxygen consumption, with reference to the lagoon of Orbetello (Tuscany, Italy), are presented and discussed. (C) 2000 Elsevier Science Ltd. All rights reserved.