Modelling marine ecological dynamics is quite challenging due to the nonlinearities which arise from i) the interactions among various biological state variables and ii) the coupling of the ecological model to the physical model that provides environmental forcing. Moreover, unlike the physical oceanographic models, which have the Navier-Stokes equations as the starting point, there is no such basic equation for formulation of ecological models. The models are evaluated by their ability to predict observations and to increase our knowledge. The upper mixed layer of ocean is one of the important parameters and plays a vital role to understand the several phenomena occurring in the upper ocean. This layer occurs between the ocean surface and up to a depth where the physical properties like temperature, salinity and density are nearly uniform. Consequently, as a simple parameterization, this layer of ocean can be considered as a global representation of all physical processes occurring in the upper ocean which play a significant role in plankton dynamics. This lecture will focus on the need for framing a robust ecosystem model that will explain the distinctly different annual cycles of biological production in the euphotic zone in different basins. To begin with, a four compartment NPZD model (Nutrient, Phytoplankton, Zooplankton and Detritus) with provision for various physical forcings (mixed layer depth, solar radiation, deep nutrients supply, additional nutrients from rivers) will be explained. As a case study, the model will be applied to two specific chosen locations at the same latitude: 1) 180N-880E in Bay of Bengal and 2) 180N-690E in Arabian Sea (AS). The basins provide a study in contrast. Comparison of the seasonal cycles of biological response to the different physical processes in the two basins will be used to understand the efficacy of the model in explaining the plankton dynamics in both the basins.