Present study is about the impact of macronutrient enrichment on phytoplankton biomass and community structure in a tropical monsoonal estuary. In situ experiments carried out during the pre-monsoon period (February-March 2006), showed that the response time of phytoplankton to nutrient enhancement was 24-32 h. Phytoplankton biomass increased sizably, indicating nitrate and silicate limitation for phytoplankton growth. An increase in 23 Mew g chl a l sup (-1) resulted in an uptake of 10 Mew M nitrate, 0.6 Mew M phosphate and 17 Mew M silicate. Phytoplankton showed high growth rates with an average value of 1.36 Mew g chl l sup (-1) d sup (-1).This phytoplankton community was largely dominated by diatoms (>96%), particularly chain forming species. Relative preference index (RPI) value for nitrate was >1, suggesting that, irrespective of the ambient ammonium concentration, estuarine autotrophs preferred nitrate. Few species like Skeletonema costatum and Thalassionema nitzschioides exhibited the ability to withstand hypoxic condition