The primary productivity of a coastal lagoon can be inferred from the data on the concentration of the primary photosynthetic pigment, chlorophyll-a in phytoplankton. Being the essential macronutrient limiting the growth of phytoplankton, the concentration of primary nutrients such as phosphorus and nitrate in their dissolved states can be related to the concentration of chlorophyll-a in the water. Thus it is possible to determine the trophic status of a lagoon through predicted values of chlorophyll-a obtained by using the empirical relationship between the phytopigment and its limiting nutrient. In the present study, three empirical models developed by Dillon and Rigler (1974), Rast and Lee (1978) and Bartsch and Gakstatter (1978) relating the concentrations to inorganic phosphate to chlorophyll-a have been investigated for their capability to predict the concentrations of the phytopigment at Pulicat lagoon in the Southeast coast of India. The results obtained from the study have been used to describe the trophic status of a coastal lagoon in the southeast coast of India, namely, Pulicat lagoon ecosystem. The results obtained suggest that there are large discrepancies in the values obtained from these models and those obtained from analysis of the samples. From the present study model performance is influenced by the non-linear relationships between the nutrient concentration and the production of phytopigment, chlorophyll-a varying over different seasons and years.