Preliminary study on surface phytoplankton assemblages and physicochemical parameters, off the west and south-west coasts of Sri Lanka

This study evaluates the phytoplankton community structure in relation to physicochemical properties in the western and southwestern coastal waters of Sri Lanka. Phytoplankton and water samples were collected from March to April, 2017 at three transect lines towards offshore in Colombo, Beruwala, and Mirissa, each containing 10 sampling sites. The distance between sampling stations on each transect line was approximately       2 km.  Phytoplankton samples were collected towing a net (10 µm mesh) by vertical hauls from known depth     (2.5 m) and preserved in Lugol’s solution. The phytoplankton were identified to the lowest possible taxonomic level and counted under the Sedgwick rafter cell using a light microscope. Water samples were collected at 0.5 m depth using the Ruttner Sampler and analysed for chlorophyll-a, nutrients, and total suspended solids (TSS). This study identified 57 phytoplankton species comprised of diatoms (33 species), dinoflagellates (23 species), and cyanobacteria (1 species). In general, phytoplankton abundance and species diversity decreased towards offshore in the three transects. Total phytoplankton and diatom abundances varied significantly among the three transect lines, and significantly higher abundance was reported at Mirissa and Colombo than at Beruwala (One-way ANOVA; p < 0.05). Total phytoplankton abundance significantly correlated with dinoflagellates, toxic species of dinoflagellates, chlorophyll-a, and nitrate-N (p < 0.01). Ten toxic species of dinoflagellates were reported in the study. Diatoms dominated (60%) in the study area, followed by cyanobacteria (31%) and dinoflagellates (9%). Diatoms of Cerataulina sp. (31%) and Navicula sp. (50%) were dominant in Colombo and Mirissa respectively, while cyanobacteria of Trichodesmium sp. (85%) dominated in Beruwala, indicating the possibility of blooms of this species at study sites when environmental conditions are favourable.