      ANUMEHA DUBE and GIRIJA JAYARAMAN Centre for Atmospheric Sciences Indian Institute of Technology New Delhi % 110016 INDIA anumeha.dube@gmail.com, girija.jayaraman@gmail.com   In this study, a four%compartment non%linear mathematical model is proposed to understand plankton dynamics in brackish water bodies. Local stability criteria are derived which are also used to estimate the range of values of model parameters. Numerical experiments and sensitivity tests with different parameters show that of all the seventeen parameters in the model the critical parameters affecting the stability of the solution are the growth rate of the phytoplankton which depends on light and nutrients. As a case study, the model is then applied to Chilika Lagoon (19 0 28’N%19 0 54’N and 85 0 06’E%85 0 36’E), the largest brackish water lagoon with estuarine character on the east coast of India. A distinct salinity gradient exists along the lagoon due to the mixing of freshwater and saline water from the adjoining sea. This salinity gradient has a pronounced effect on the seasonal variability of the freshwater and marine phytoplankton species in the lagoon. The model is tested for different sectors of the lagoon which have distinct characteristics in terms of depth, light penetration, nutrients, salinity and hence planktonic growth. A fine tuning of the model parameters was done in order to validate the model results with the available data of 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, major plankton peaks etc. : Plankton, Local Stability, Seasonal variability, Chilika Lagoon   Several difficulties arise while developing an ecological model for an aquatic/marine ecosystem. In addition to the practical difficulties of getting an accurate data for validation modelers are often faced with the formidable task of selecting the right functional representation of the complex ecosystem. The complexity of the model should depend upon the purpose of the model and the availability of data [1]. A simple ecological model can be formulated on the basis of only three primary dependent systems: phytoplankton, herbivorous zooplankton and the nutrient system. Any complexity due to the addition of more state variables in such a simple model will influence the dynamics of the model. A lot of research has been done in order to examine the changes in the dynamics of the ecosystem models due to the incorporation of more variables in such models [1]; [2]; [3]; [4]. An ecosystem model is largely dependent upon the values of the parameters included in the model equations. The model can exhibit different dynamics depending upon the changes in the parameter values. Depending on the response of the model to the changes in the parameter values, the parameters can be classified as critical or non%critical. For analyzing the changes in dynamics in accordance with the changes in the parameter values a local stability analysis of the model equations is essential. A local stability analysis also helps in restricting the degrees of freedom of the parameter values [5]; [6]; [7]; [8]. In the present paper the effect of salinity changes on the ecosystems is analysed which is an important aspect of the brackish water ecosystems. In brackish waters there are no firm salinity boundaries between fresh and brackish waters. In principle, therefore, a complete continuum of concentration must exist between freshwater and sea water. It is for this reason that brackish waters may develop much greater ecological diversity than fully marine water bodies and have communities which comprise representatives from both fresh and marine waters. Changes in salinity in brackish waters serve as indirect switch mechanisms which may cause the lagoon community to change from submerged plant dominance to phytoplankton dominance [9]; [10]. Despite longstanding recognition of the role of salinity as a primary influence on plankton in various types of aquatic ecosystems, there have been very few experimental studies of the effects of salinity on plankton communities [11]; [12]; [13]. Ecological models developed for modelling Proceedings of the 8th WSEAS Int. Conference on Mathematics and Computers in Biology and Chemistry, Vancouver, Canada, June 19-21, 2007 103