Recurrent dinoflagellate blooms in the South-eastern Arabian Sea - A preliminary assessment with focus on Ornithocercus magnificus blooms along the nearshore waters off Dakshina Kannada, South-west coast of India

Abstract

Frequent bloom incidences of the dinoflagellate Ornithocercus magnificus Stein, 1883 along the surface waters off South-eastern Arabian Seas (SEAS) have been reported since 2015 and specifically during the years 2015, 2019 and 2020. All these blooming incidences coincided with the strong El Nino phases in the Indian Ocean. The present study addressed the bloom (1.65 x 106 cells l-1) of this species in the nearshore waters (7 m depth) off Dakshina Kannada (Surathkal and Chitrapur) on 11.02.2016 which occurred during pre-monsoon period and corresponding to the super El Nino year 2015/2016. A detailed evaluation of the physico-chemical characteristics and phytoplankton abundance as well as community structure was made, concurrent to the three phases viz., pre-bloom, bloom and post-bloom periods in this region. The study revealed that the bloom occurred in high saline (35.0±0.91 PSU), well oxygenated (7.414±0.823 mg l-1) and nitrogen limited waters (oligotrophic conditions) and had a positive correlation with oceanic nino index (ONI; rs= 0.790, p<0.001). Some of the O. magnificus (3%) harboured ectosymbionts probably cyanobacteria in their cingulum in response to the beginning of a stratified oligotrophic condition in this region. The water quality was fair during the bloom period with no conspicuous discolouration of the surface waters. The present study also attempted to evaluate the influence of the increasing frequency of the O. magnificus blooms on the oilsardine fishery along the SEAS. Keywords: Cyanobacteria, Dinoflagellate, Ectosymbionts, Nitrogen limited waters, Pre-monsoon season, SEAS, Sea surface temperature (SST), Oilsardine