PLANKTON DIVERSITY OF SEAGRASS ECOSYSTEM IN MANDAPAM COAST, GULF OF MANNAR

Seagrasses are the rooted marine flowering plants play a key role in the coastal ecosystems by supporting wide variety of life forms from microbes to marine mammals. In Tamil Nadu, seagrasses are found principally along the Gulf of Mannar and Palk bay regions. Plankton are the important source of energy in the marine environment.

The present investigation was carried out to assess the distribution of plankton in seagrass ecosystem in comparison with that of the open waters in Gulf of Mannar. Monthly sampling was carried out to collect the water and plankton samples from the seagrass ecosystem (Station 1) and the control station (station 2) from September 2016 to May 2017. Water quality parameters were analysed using the standard procedures. The plankton samples were subjected to both quantitative and qualitative analysis for biomass and species diversity respectively.

The physico chemical parameters were analysed and the mean values of surface water temperature, salinity, pH and dissolved oxygen were 27.72⁰C, 34.17 ppt, 7.99 and 3.85 ml.l^-1. Average values of nutrients such as nitrite, nitrate, phosphate and silicate were  0.25µg - at NO₂-N/l, 0.01 µg - at NO₃-N/l, 0.63 µg - at PO₄-P/l and 1.03 µg - at Si/l. Gross primary productivity and chlorophyll-a values were 0.22 mg.C.m^-3.h^-1 and 0.24 mg.m^-3. Totally, 60 and 62 species of phytoplankton were recorded with maximum cell density of 1123 and 1081 cells.l^-1 in station 1 and 2 respectively. Regarding the zooplankton 59 species were recorded from each of the two stations with the maximum density of 667400 and 935300 nos.m^-3 in station 1 and 2 respectively. The higher density of both phytoplankton and zooplankton were observed during the summer months. Species richness and diversity indices showed the maximum values in seagrass ecosystem which indicates that the diversity of plankton is more in seagrass ecosystem as that of the open waters.