Biotic interaction as the triggering factor for blooms under favourable conditions in tropical estuarine systems.

The formation of harmful algal blooms (HABs) in the marine environment is detrimental to the ecosystem function affecting the sequence of biological events. Hence, the present study is focused on the seasonal distribution of HAB-forming diatoms and dinoflagellates and their ecological interactions in two tropical estuaries, namely Chapora and Sal in the west coast of India. A total of 17 species of diatoms belonging to nine genera and 13 species of dinoflagellates belonging to eight genera were recorded from the present study (monsoon and non-monsoon season). The redundancy analysis (RDA) revealed that the salinity and phosphate concentration influence the distribution of HAB-forming diatoms and dinoflagellates in both the estuaries. Out of 13 species of dinoflagellates observed, six species are known as potentially toxic such as Alexandrium minutum, A. tamarense, A. pseudogonyaulax, Cochlodinium polykrikoides, Prorocentrum micans and Gonyaulax spinifera. Among these estuaries, Sal was represented with a high diversity of toxic species, associated with high phosphate and nitrate content in the water column. Further, the results indicate that the bloom of A. minutum suppressed the growth of other species of diatoms and dinoflagellates, suggesting its possible allelopathic effect.