J. Mar. Biol. Ass. India, 59 (1), January-June 2017 First report of Leptocylindrus sp. bloom in the coastal waters of Kerala, southeast Arabian Sea M. Nashad, N. Nandini Menon*, C. Ajith Joseph, Lasse. H. Pettersson 2 and N. R. Menon 1 Nansen Environmental Research Centre India, 6A, Oxford Business Centre, Sreekandath Road, Ravipuram Kochi 682016, Kerala, India. 1 Department of Marine Biology, Microbiology and Biochemistry, Cochin University of Science and TechnologyW Kochi - 682016, Kerala, India. 2 Nansen Environmental and Remote Sensing Center, Bergen, Norway. * Correspondence e-mail: nandinimenon@yahoo.com Original Article Received: 16 Sep 2016, Accepted: 27 Feb 2017, Published: 15 May 2017 Abstract 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 10 5 to 1.8 x10 5 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"- 09˚55'13"; E 076˚10'43"- 076˚03'34") on 11 th 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. Keywords: Leptocylindrus, flocculation, algal sinking, phytoplankton community, southeast Arabian Sea. Available online at: www.mbai.org.in doi: 10.6024/jmbai.2017.59.1.1937-13 Introduction Algal blooms are formed in marine and limnetic ecosystems by phytoplankton belonging to dinoflagellates, diatoms and cyanobacteria, resulting in the discoloration of water from red to green hue depending on the species involved (Sellneret al.,2003).The formation of blooms is influenced by a variety of reasons which include combination of light and upwelling, increased nutrient conditions (eutrophication) (Anderson et al., 2002; Smayda, 2006), and biological processes like competition, grazing and allelopathy (Granéli and Johansson 2003). The majority of the algae blooms reported along the west coast of India are caused by dinoflagellates whereas diatom blooms dominate the east coast (D’Silvaet al., 2012). Excluding the toxic diatom Pseudo-nitzschia whose toxin, domoic acid, causes Amnesic Shellfish Poisoning and mortality of birds and marine mammals, diatoms usually are not included among harmful phytoplankton groups. However, diatom blooms cause mortality and physiological impairment to fish, most often due to their morphology or non-toxicological stressors (Smayda, 2006). Hornell and Nayudu (1923) reported the first diatom bloom from Indian waters along the Malabar coast. Since then several diatom blooms were reported from the South West coast of India. Blooming of Fragillaria oceanica, (Devassy,