Leptocylindrus spp. are chain-forming diatoms found in estuaries and  coastal waters. Although commonly encountered in the coastal phytoplankton community, massive blooming of this centric diatom is very rare. This is the first report of a bloom of Leptocylindrus sp. in the coastal waters of Kerala, Southwest coast of India. The cell densities varied
from1.5 x 105 to 1.8 x105 no. l-1 from shallower to deeper waters (10 m to 30 m depth). Unlike the common blooming phytoplankters, Leptocylindrus filaments formed closely interwoven tufts to gain the appearance of woolen ball like aggregates in the surface waters off Kollam (N 09˚56'50"- 9˚55'13"; E 076˚10'43"- 076˚03'34") on 11th November, 2014. contributed 99.9% of the total phytoplankton standing crop consisting of species belonging to 18 genera. High concentration of silicate triggered the bloom, and low N:P ratio probably acted as a major stress factor forcing the algal cells to flocculate and sink into the water column, resulting in dissipation of the bloom. Formation of aggregates and their sinking can be viewed as the culmination of the blooming process probably controlled by inherent growth characteristics of the diatom (Alldredge and Gotschalk, 1989). Sinking of the clumps of this species reduced the turbidity of the blooming area and helped in retaining the Redfield ratio. No toxic effects were recorded during the bloom.

Leptocylindrus, flocculation, algal sinking, phytoplankton community, southeast Arabian Sea.