Fragilariopsis sp. bloom causes yellowish-brown waters off Alappuzha, south-central Kerala coast, India, during the mud bank-upwelling phase

Abstract

Mud banks (Chakara) of Kerala are calm coastal waters that form in several isolated stretches along the coast usually during the southwest monsoon (SWM) period (June-September). They are characterized by the damping of incident waves, generating localized calm sea environment conducive for fishing activities, while the high monsoon waves create hostile environment in the rest of the region. Here, we present the scientific basis of the yellowish-brown discoloration of water column that occurs off Alappuzha, Kerala annually during the peak and late SWM associated with coastal upwelling-mud bank event. The discoloured waters that occur off Alappuzha associated with these events are locally known as 'pola vellam', which is nothing but diatom blooms. In 2014, pola vellam was actually caused by the bloom of Fragilariopsis (= Fragilaria) sp.; hereafter Fragilariopsis, which was widespread in the study region, even beyond the mud bank domain. This bloom feature is attributable to the nutrient enrichment associated with intense coastal upwelling that was dominant over a larger spatial extent in the study domain, including the mud bank. FlowCAM-based plankton data strongly support the above view, as the abundance, biovolume and biomass of Fragilariopsis had similar temporal trend both in the mud bank and non-mud bank regions. The general ecology and importance of Fragilariopsis bloom in the study domain, from the point of view of commercial fisheries, is also elaborated in this communication