Abstract
Recurrent red tide events caused by Noctiluca scintillans (Macartney) Kofoid and Swezy, 1921, were recorded along the Edakkazhiyur (Lat: 10.6090°N, Long: 75.9890°E) and Panchavadi (Lat: 10.6190°N, Long: 75.9840°E) beaches of the southwest coast of Kerala, India, during consecutive years in July 2024 and July 2025. Both events occurred during the peak southwest monsoon season and were accompanied by striking nocturnal bioluminescence, locally referred to as pooneer or karavellam. Microscopic analysis confirmed the blooms were caused by the red heterotrophic variant of N. scintillans, a non-toxic but ecologically disruptive harmful algal bloom (HAB) species. Environmental conditions recorded at the time of bloom, water temperature 27°C, salinity 32 ppt, pH 7.1, and total dissolved solids (TDS) 22.4 ppm, were consistent across both events, suggesting stable environmental drivers. The recurrence of these blooms at the same locations in successive years is interpreted as evidence of progressive eutrophication driven by persistent anthropogenic nutrient inputs from agricultural runoff, domestic sewage, and riverine discharge. The ecological implications include hypoxic stress, displacement of diatoms from the phytoplankton community, disruption of the coastal food web, and potential long-term impacts on commercially important fisheries. The findings underscore the urgent need for continuous monitoring, nutrient management, and integrated coastal zone management strategies along the Kerala coast.