Plankton Diversity in Coastal Waters Near Kalu Ganga River Mouth; Sri Lanka

Abstract

The plankton community plays a significant role in the stability of coastal ecosystems. They serve as key players in marine food webs. The present study investigated the plankton diversity in coastal waters near Kalu Ganga river mouth. Six sampling locations were chosen randomly, three along the right side of the coastline (CR1-CR3) and three along the left-side (CL1-CL3) from the river mouth. Zooplankton and phytoplankton at each location were sampled from September 2020 to February 2021 using a 55µm plankton net in surface waters on a monthly basis. Plankton were morphologically identified to the nearest possible taxonomic level. The Shannon-Weiner diversity index (H) and Simpson's Index of Diversity (SID) were calculated to determine plankton diversity. The number of phytoplankton species found during the wet months (September-October) and dry months (January-February) were 62 and 68 respectively, while a similar number of zooplankton species (43) were recorded in both periods. Altogether, 81 phytoplankton species and 53 zooplankton species were identified during the research period. Bacillariophyta (72%) and Copepods (68%) were identified as the dominant phytoplankton and zooplankton groups respectively. Dinoflagellates including, Peridinium sp., Protoperidinium sp., Ceratium sp., Noctiluca sp., Gonyaulax sp., and Alexandrium sp., which are well-known to form harmful algal blooms (HABs) accounted for 17%. Chlorophyta and Cyanophyta were less dominated and found only during wet months at nearest sampling locations (CL1 and CR1) to river mouth. Rotifera (11%), foraminifera (9%), protozoa (10%) and ichthyoplanktons (<1%) were also reported. The H and SID values for plankton were recorded to be between 2.7 to 3.1 and 0.90 to 0.96 respectively. The both values were not significantly different (p>0.05) between wet months and dry months at each location. According to the H and SID values, the study area has a moderate-high level of plankton diversity. Further research should be conducted to determine the temporal and spatial variation of plankton diversity in the study area.