Response of phytoplankton to nutrient enrichment with high growth rates in a tropical monsoonal estuary - Zuari estuary, India

314-325Present 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 23µg chl a l-1 resulted in an uptake of 10µM nitrate, 0.6µM phosphate and 17µM silicate. Phytoplankton showed high growth rates with an average value of 1.36µg chl l-1 d-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.