Mathematical modelling of the seasonal variability of plankton in a shallow lagoon Original Research Article

The quality of coastal waters can be markedly influenced by the growth and distribution of phytoplankton. In this study, a non-linear mathematical model is used to explain the seasonal variability of plankton in shallow coastal lagoons. A local stability analysis for dynamical systems is undertaken in order to estimate the range of values of model parameters. Numerical experiments and sensitivity tests with different parameters show that of the thirteen parameters in the model, the main parameter affecting the productivity is the growth rate of the phytoplankton, which depends on light and nutrients. The time of maximum planktonic growth, in particular, is mostly dependent upon the parameter values within the growth rate term. As a case study, the model is then applied to Chilika Lagoon (19∘28′N–19∘54′N and 85∘06′E–85∘36′E), the largest brackish water lagoon with estuarine character on the east coast of India. Interest in detailed analysis of the ecology of the lagoon and the various factors affecting it is due to the opening of the new mouth on 23 September 2000 to resolve the threat to its environment from various factors — eutrophication, weed proliferation, siltation, industrial pollution and depletion of bioresources. The model is tested for different sectors of the lagoon which have distinct characteristics in terms of depth, light penetration, nutrients and hence planktonic growth. A fine-tuning of the model parameters, with the help of the stability analysis discussed for a general model, was performed in order to validate the model results with the data available for the lagoon. Even in its simplest form, the model is able to reproduce most of the significant characteristics of the planktonic distribution in the lagoon — the bimodal oscillations, and major plankton peaks.