Algal bloom, hypoxia, and mass fish kill events in the backwaters of Puducherry, Southeast coast of India

The Chunnambar backwater of Puducherry experienced changes in water quality over a period. The most significant impact was the sudden mass fish kill event coincided with the Pseudo-nitzschia bloom. On 25^th September 2019, a mass fish kill event was reported, i.e. about a 0.25 metric ton (MT) floating on the water surface. On 29^th September 2019, a much larger (∼1 MT) than the earlier incident had occurred. Sampling was carried out to assess the causes thereof. The results indicate that high organic matter and bacterial loads accumulated in the water and sediment due to the closure of the river mouth for an extended period. High ammonia (61.4 µM) and phosphate (6.2 µM) levels attributed to eutrophic conditions in the water column and hypoxemia due to low dissolved oxygen (1.62 mg/L at St.1 and 2.4 mg/L at St.5) and algal bloom were the proximate cause of these sudden fish kills. We recommend periodic dredging to facilitate proper water exchange between the backwater and the sea.