Dominance of Trichodesmium and associated biological and physico-chemical parameters in coastal water of Port Blair, South Andaman Island

Dominance of the cyanobacteria Trichodesmium was observed in the plankton samples collected from the coastal water of Port Blair during May, 2011. No discernible change in the colour of the surface water was observed. However the cumulative samples collected using plankton net were red in colour. The number of Trichodesmium filaments and colonies were found increasing towards the open sea region. Two species of cyanobacteria viz. Trichodesmium thiebautii and Trichodesmium erythraeum were identified. The number of filaments of T. thiebautii were 10 to 15 times more than that of T. erythraeum. The inorganic phosphate and ammonia concentrations were higher during the time of Trichodesmium dominance. The diatom Navicula membranaceae was the second dominating species followed by Nitzschia seriata, N. closterium and Leptocylindrus danicus. In case of zooplankton, the harpacticoid copepods such as Macrosetella spp., Microsetella spp. and cyclopoid copepods like Copilia spp., Sapphirina spp. were found abundant. Reason for the Trichodesmium dominance in the nearshore water of Port Blair could be due to its stable coastal water conditions during summer.