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

Abstract

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 mu g chl a l-1 resulted in an uptake of 10 muM nitrate, 0.6 muM phosphate and 17 muM silicate. Phytoplankton showed high growth rates with an average value of 1.36 mu g chl l-1 d-1. This phytoplankton community was largely dominated by diatoms (greater than 96 percent), particularly chain forming species. Relative preference index (RPI) value for nitrate was greater than 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.