Bloom of Trichodesmium erythraeum (Ehr.) and its impact on water quality and plankton community structure in the coastal waters of southeast coast of India

An intense bloom of Trichodesmium erythraeum was observed in the coastal waters (about 600 m away from the shore) of southeast coast of India during the post-northeast monsoon period. The bloom appeared during a relatively high temperature condition with coastal water salinity > 31 psu. A significant reduction in nitrate concentration was noticed during the bloom period, whereas, relatively high concentration of phosphate and total phosphorous was observed. An abrupt increase in ammonia concentration to the tune of 284.36 μmol l -1 was observed which coincided with the highest Trichodesmium density (2.88 × 10 7 cells l -1 ). Contribution of Trichodesmium to the total phytoplankton density ranged from 7.79% to 97.01%. A distinct variation in phytoplankton species number and phytoplankton diversity indices was noticed. The lowest diversity indices coincided with the observed highest Trichodesmium density. Concentrations of chlorophyll-a (maximum 42.15 mg m -3 ) and phaeophytin (maximum 46.23 mg m -3 ) increased abnormally during the bloom.