First report of a rare bloom of Ornithocercus magnificus, Stein 1883 along the coastal waters of Kochi; A possible indicator of increasing sea surface temperature

A bloom of the planktonic dinoflagellate, Ornithocercus magnificus was reported for the first time in the surface waters of the off Kochi coastal region with an evidently higher density of 1.7 x 106 cells L-1 on 18.12.2019. No conspicuous colouration of the surface waters was noticed even with the higher abundance of (1.7 x 106 cells L-1) observed near the barmouth area (6-7 m). The tracking on the occurrence of this genus in this region from 2013 to 2019 revealed their occurrence only during three times over the six years, i.e. the post monsoon season of 2013 and 2014 and during pre-monsoon of 2016. The SST of the three stations of the post monsoon season was compared for the last three years from 2017 to 2019, and an evident variation in the SST distribution was noticed in this region. The SST in 2019 was 2.1°C higher than that observed during 2017. The occurrence of Ornithocercus magnificus showed a positive correlation with SST (rs =0.425, p<0.05). It was also noted that the occurrence of this species was observed during the El Nino years. The Oceanic Nino Index (ONI) also positively correlated with the SST during the period (rs =0.432, p<0.05). The higher abundance of this species in 2019 when warmer SST was observed and their positive correlation with SST indicates the preferences of O.magnificus to warmer waters. The Shannon and Weiner diversity index (H’) ranged from 2.87-3.29 and the water quality index (WQI) remained good during the bloom. As Ornithocercus magnificus did occur during higher SST, this genus can be used as a proxy of climate change and hence has a strong potential to be used as indicator species of higher SST variation if found in novel environments.