OCEAN LIFE Volume 3, Number 1, June 2019 E-ISSN: 2580-4529 Pages: 1-12 DOI: 10.13057/oceanlife/o030101 Micro-phytoplankton density and diversity at a pilot oyster culture barachois site of Mauritius Island MARIE ESTREIA ANGELLIA ARMANCE 1 , SUSHMA MATTAN-MOORGAWA 1 , RANJEET BHAGOOLI 1,2,♥ 1 Department of Biosciences and Ocean Sciences, Faculty of Science and Pole of Research Excellence, Sustainable Marine Biodiversity, University of Mauritius, Réduit 80837, Republic of Mauritius. 2 Institute of Oceanography and Environment (INOS), Universiti Malaysia Terengganu, 21030 Kuala Terengganu, Terengganu, Malaysia Tel: +230-4037916  email: r.bhagooli@uom.ac.mu. Manuscript received: 16 February 2018. Revision accepted: 12 May 2019. Abstract. Armance MEA, Mattan-Moorgawa S, Bhagooli R. 2019. Micro-phytoplankton density and diversity at a pilot oyster culture barachois site of Mauritius Island. Ocean Life 3: 1-12. The density and diversity of marine micro-phytoplankton were studied at 4-6 stations from October to December 2014 within and around a pilot culture site of the oyster Crassostrea cuculata in the north-east of Mauritius Island to evaluate the water quality of the marine ecosystem suitable for oyster culture. Three micro-phytoplankton samples were taken at each station and the physicochemical parameters such as temperature, pH, and salinity were measured in situ while turbidity was measured ex-situ. No bloom was observed but a gradual increase in micro-phytoplankton density occurred from October to December 2014. The micro-phytoplankton density was positively correlated with temperature. A total of 20 genera of micro- phytoplankton were observed belonging to the classes of Baccillariophyta, Dinophyta, and Cyanophyta. The genera observed were Bacteriastrum, Chaetoceros, Coscinodiscus, Leptocylindrus, Cylindrotheca, Nitzschia, Pseudo-Nitzchia, Asterionellopsis, Licmophora, Striatella, Flagilariopsis, Navicula, Pleurosigma, Thalassionema, Dinophysis, Gymnodinium, Alexandrium, Peridinium, Nitzschia, and Pleurosigma in the Bacillariophyta and in Dynophyta, Gymnodinium and Prorocentrum were the dominant genera, while only Lyngbya was observed as Cyanophyta. Simpson's diversity index revealed an increase in species diversity from October to December 2014 at studied stations. These findings indicate spatiotemporal variations in micro-phytoplankton density and diversity at the oyster culture site and further long-term studies are warranted to identify the optimal stations for oyster culture. Keywords: Baccillariophyta, Crassostrea cuculata, Cyanophyta, Dinophyta, micro-phytoplankton, physicochemical parameters, species diversity INTRODUCTION Phytoplankton is a large group of tiny drifting photosynthesizing organisms that comprises different subgroups according to their size ranges. Micro- phytoplankton size ranges between 20 μm to 200 μm (Reynolds 2006). There is a wide range of micro- phytoplankton classes around the globe but the main ones are diatoms, dinoflagellates, and cyanobacteria. Being at the base of the marine food web, these microorganisms play a key ecological role in the marine environment. Moreover, the abundance of the micro-phytoplankton can be used as a toolkit to evaluate the water quality of the marine ecosystem (Sagert 2008). These photosynthesizing microorganisms depend on several factors for their growth, survival, and reproduction and have a direct effect on their density and diversity. These factors can either be physical, chemical or biological. The physical factors affecting the abundance of micro-phytoplankton are notably temperature, salinity, turbidity, currents, wind, light radiation as well as freshwater input from nearby rivers or precipitation (Gilbes et al. 1996). The chemical factors are dissolved oxygen, pH, as well as nutrient availability, mainly of phosphate, silicate, and nitrate (Gilbes et al. 1996). Biological factors are their interaction with other marine organisms, such as the grazing activities of zooplankton and oysters (Chung et al. 2012). Furthermore, it is known that micro- phytoplankton density differs spatially and temporally (Chandy et al. 1991; López-Flores et al. 2011; Sadally et al. 2012; 2014b). Oysters are filter-feeding organisms that feed on phytoplankton, including the micro-phytoplankton. Evidence has shown that oysters reduce the abundance of phytoplankton in the sea and thus increase water quality (Newell et al. 2007). Algal blooms are the rapid proliferation of phytoplankton on the sea surface resulting in the decrease of the penetration of light radiation to the hydrosphere. In the coastal waters of Poudre d'Or and Anse La Raie, located in the north of Mauritius, microalgal blooms have been reported along with death of corals (AFRC, 2009). The main impact that an algal bloom has on marine environment is that it depletes oxygen to a level that is insufficient to maintain the biodiversity of the marine ecosystem and may pose a threat to aquaculture activities and the surrounding ecological system (Kibria 2014). Very few scientific studies have so far been published on the density and diversity of micro-phytoplankton at oyster culture sites in Mauritius, though a few studies have been undertaken on micro-phytoplankton abundance and distribution within the coastal waters of Mauritius Island and at Poudre d'Or Barachois. These studies have shown