D. Poornima, K. Thennarasu, R. Ranith, R. Shanthi, R. K. Sarangi, T. Thangaradjou

In-situ nutrient enrichment experiment was conducted to know the relationship between chlorophyll and phosphate so as to assess the significant role of phosphate in the phytoplankton growth. During the experiment period temperature (27.9-33.7°C), salinity (31-35 ‰) and pH (7.74-8.07) values were not shown dramatic changes as the experimental studies are conducted for the short period only. DO was found to show an oscillating trend (2.77-6.68 mgl^-1) with phytoplankton population variations. Nutrient concentrations (NO₃:1.05-12.63 µM, NO₂:0.29-1.67 µM, PO₄: 0.07-28.32 µM and SiO₃:4.19-23.89 µM) showed maximum concentration at the first and second day of the experiment in enrichment tanks and it gradually decreased (PO₄ and SiO₃) on last day of the experiment period. The pronounced maximum chlorophyll concentration in tank 5 on 5^th day corresponding with addition of highest concentration of phosphorus, clearly pointed out that phosphorus addition had influenced the plankton growth. Increased utilization of PO₄ and support of SiO₃ indicates diatoms prefer silicate and phosphate for growth than the nitrates. Nitrate enrichment in the tank in later part of the experiment indicates nitrogen cycling processes. Increased phytoplankton uptake and growth rate at tank 5 when compared to control, substantially proved the uptake of phosphate by phytoplankton under culture system.