A study was conducted to find out the possible cause of sudden changes in the colour of the Karapad lagoon water and associated variation in the hydrography. The discolouration of lagoon water was noticed after the excavation work to deepen the lagoon area by the Municipal authorities to enable free boating. Microscopic examination of the lagoon water revealed the blooming of micro alga Chlorella marina. The blooming lasted for a month and has not caused any adverse effect. During the bloom days, the algal cell density attained a peak of 3.1x 105 cells l-1 on 5th day. Very high chlorophyll concentration was noticed in the lagoon water during the bloom days than the post bloom days. The studies revealed that the nutrient enrichment due to the re-suspension of sediments from the excavation activities might have triggered the algal outburst in the Karapad lagoon. Along with high nutrients, the surface water temperature and salinity might also have supported the algal proliferation.

Tuticorin Bay, Chlorella marina, discolouration, nutrient enrichment.