First Report on the Bloom of Diatom Haslea Gretharum Simonsen, 1974 from the Arabian Sea

Abstract

A bloom of solitary pennate diatom Haslea was observed for the first time along the open ocean waters of the Northern Arabian sea during the late phase of winter monsoon (first week of March 2011). Mucilaginous macroscopic aggregations of the diatoms were observed on the sea surface consisting of two species Haslea gretharum and Haslea gigantea of which Haslea gretharum Simonsen, 1974 dominated (3 × 105 cells L−1) and the species was not reported from the Indian Ocean after its first report in 1974. The weakly silicified lanceolate pennate raphid diatom H. gretharum in aggregations were observed to span over nearly 6km2 in the open sea surface. Along with the genus Haslea, the dinoflagellate Noctiluca scintillans was observed in fewer cell densities (16 × 102 cells L−1). The diatom bloom was observed to follow an intense bloom of N. scintillans in the region and the decaying cells of this dinoflagellate were observed to contribute towards the macroscopic mucilaginous aggregations of the Haslea bloom by providing a substratum for aggregation. Even though pennate diatoms are known to produce mucilaginous extracellular substances that leads to cell aggregation, the massive dinoflagellate blooms in the North eastern Arabian sea tentatively provides supplementary up keepings to the open ocean bloom of the diatoms.