Rainfall Pattern Drives Green Noctiluca scintillans Bloom in Tropical Estuary Mandovi-India

Green Noctiluca scintillans, a non-photosynthetic, bio-luminescent marine planktonic dinoflagellate species bloom was reported for the first time from the tropical Mandovi river estuary-India, during the south west monsoon season-2017. The bloom subsides and intensifies with respect to rainfall intensity. This was evident from statistically significant inverse relation between bloom intensity and rainfall magnitude (R²=0.44, p<0.05). Highest N. scintillans cell density (4.6 ± 2.7 x 10⁴ cells l^-1) was observed during the lowest rainfall (1.02±1.5 mm), whereas almost negligible cell count during maximum rainfall (39±9.6 mm). Chl-a concentrations were high in the study region (7.44±8.4; 3 to 27 mg m^-3). Existence of average temperature of 28.6±0.65 ºC and low suspended load (55.3±55.8 mg l^-1) which is known to be ideal for the proliferation of N. scintillans could have boosted the bloom. During the bloom, N. scintillans cell size ranged between 500-700 µm with a maximum and minimum abundance of 8.6 x 10⁴ cells l^-1 and 1.5 x 10⁴ cells l^-1, respectively. Post-bloom, N. scintillans population ranged between 2 to 72 cells l^-1 with increasing trend towards mouth of the estuary. The contribution of N. scintillans to total phytoplankton population ranged between 30.8- 57.2 %. Variations in rainfall intensity and associated factors bring significant variation in plankton community leading to intensification of N. scintillans bloom in the estuary within a short period of 5 days. Other dominant phytoplankton species observed during bloom were the diatoms species such as Chaetoceros lascinious, Coscinodiscus centralis, Rhizosolenia fragilissima, Skeletonema costatum, Rhizosolenia setigera, Thalassiosira condensate, Thalassiothrix frauenfeldii and Rhizosolenia imbricate.

The findings of the study suggest the significance of rainfall driven changes in the estuarine environment in influencing the phytoplankton community and bloom formation.