Indian Journal of Geo Marine Sciences Vol. 49 (06), June 2020, pp. 989-999 Winter monsoon phytoplankton community in the coastal waters of Northeastern Arabian Sea, with emphasis on harmful and non-indigenous species L C Thomas, S Bijoy Nandan & K B Padmakumar* Department of Marine Biology, Microbiology & Biochemistry, School of Marine Sciences, Cochin University of Science and Technology, Kochi, Kerala – 682 016, India *[E-mail: kbpadmakumar@gmail.com] Received 19 March 2019; revised 19 July 2019 Phytoplankton community structure along the coastal waters of the Northeastern Arabian Sea (NEAS) was analysed for three years (2009, 2011 and 2012) during the winter monsoon season. The coastal waters of NEAS, especially Saurashtra coast are a region of high fishery potential. A total of 137 species of phytoplankton were identified. The community structure of phytoplankton showed significant inter-annual variability. The study highlights the persistence of certain non-indigenous phytoplankton species such as Scrippsiella trochoidea, Karenia mikimotoi and potentially harmful dinoflagellates mainly Gonyaulax polygramma, Dinophysis acuminata, D. miles and Tripos furca in the region that can raise probable threats towards the indigenous species and can cause harmful or toxic events. The increased abundance of diatom, Pseudo-nitzschia spp. that can produce toxins at certain threshold levels was also observed. The possible reason for the increased abundance of such groups can be suggestively due to the increased anthropogenic inputs into the coastal waters and intense fishing and maritime activity in the area. [Keywords: Harmful microalgae, Non-indigenous species, Northeastern Arabian sea, Phytoplankton, Winter monsoon] Introduction Coastal marine ecosystems are dynamic, productive regions of World ocean that support a diverse group and wide variety of organisms. These systems are markedly influenced by various factors viz. oceanographic processes such as currents, waves, climate changes, human impacts, including overexploitation of fishery resources, habitat destruction, pollution etc. The physico-chemical characteristics of the coastal systems alter with these factors and are reflected in the assemblage of primary producers. Hence, the primary producers, mainly phytoplankton accounts to be a first line index towards alteration in the trophic structure and the sustainability of the ecosystem by external forcing 1,2 . Along the coastal waters of peninsular India, biannually reversing monsoon patterns modulates coastal currents, surface temperatures, nutrient availability, etc. Community composition and distributional pattern of phytoplankton get altered by these variations. Their assemblages depend on the ecological conditions like trophic (nutrients, food), physical (light, mixing and temperature) or biological factors (competition and predation) 3 . The changes in the community structure of phytoplankton influence the secondary and tertiary producers in an ecosystem. The coastal waters of the Northeastern Arabian Sea (NEAS) ecosystem margined by Saurashtra region of north-west Indian coast (Gujarat coast) are a region of high fishery potential and fish landing 4 . Moreover, the region is one among the potential fishing zone (PFZ) advisories in India. The coastal belt also harbours highly diverse mangrove forests and coral reefs 5 . However, planktonic studies in this region are still fragmentary 5 . Northeast monsoon (Winter monsoon) is considered as the productive period of the Northeastern Arabian Sea. The process of convective mixing associated with winter cooling is the major physical forcing that controls the productivity pattern in the northern Arabian sea 6-8 . This mixing process results in the nutrient enrichment of the upper water column and favours growth and multiplication of primary producers, phytoplankton and subsequently supports secondary and tertiary production. The convective mixing induced biological production accounts much towards the carbon dynamics as well as the export flux in the region. The seasonal overturning of the water column and nutrient entrainment during the winter monsoon positively influence the phytoplankton community of the Northern Arabian Sea that remains oligotrophic during other seasons 9 .