Species composition and diversity of marine plankton related to the water quality along the Karainagar coastal waters, Sri Lanka

Abirami Ravitharan*, Sivashanthini Kuganathan

Department of Fisheries, Faculty of Science, University of Jaffna, Sri Lanka

*abirami4064933@gmail.com

Abstract

Bio-indicators have gained prominence due to their useful information for managing water resources. Plankton populations act as early-warning indicators of the 'health' of an aquatic system, and routine monitoring of biological communities is often trustworthy and affordable in comparison to the cost of evaluating toxicant pollution. The present study aims to better understand the planktonic biodiversity of coastal environments in Karainagar coastal waters, Sri Lanka as it is mostly unknown how the bioindicator planktons and macrobenthos, are distributed along the beaches of Sri Lanka's northern, eastern, and southern coasts.

A field study was conducted between December 2021 and May 2022 in order to collect samples of plankton, a horizontal 55μm pore sized plankton net was hauled horizontally at length of 50m intervals. The samples were then immediately treated with Lugol's solution and 5% formalin, respectively, were used to quickly fix the phytoplankton and zooplankton samples. They were identified using standard manuals and guidelines up to the lowest level of taxonomic classification. Each sample was placed on the lab's Sedgewick rafter counting chamber where the organisms were identified and counted using a trinocular microscope. Among the 200 species identified, plankton samples contained 20 species of crustaceans, 5 species of gastropods, and 8 species of bivalves. Crustaceans made up to 15% of the entire planktonic population and were the most prevalent zooplankton group, followed by mollusks (2%). The study of the phytoplankton revealed that its abundance ranged from 3000 to 20000 cells/L with a mean of 8667±2667 No./L. From the study region, 100 different plankton families were recorded. Dinoflagellates made up 13% of the phytoplankton population in the area, while diatoms made up 57% of it. The Diatomaceae family includes the majority of the prominent phytoplankton species. Rhizosoleniaceae made up 11% of the overall population, followed by 21% of the population of diatoms, an excellent indicator of water quality. Planktons in the vicinity ranged in species richness from 8 to 19. In the study area, there were on average 3334±422 individuals L^-1 of zooplankton, with abundance ranging from 2000 to 5000 L^-1. Along with crustacean larvae (nauplii, 6%), copepods made up the most significant group (13%). Cyclopidae, Calanidae, Oithonidae, Euchaetidae, and Oncaeidae were the most prevalent zooplankton families, accounting for 4%, 2%, 2%, and 2%, respectively. Chlorophyll 'a' and phytoplankton density had a very strong positive Pearson correlation that was reported as 0.9059; it peaked in May 2022 since summer is the productive season. The predominant species of zooplankton in this area was the copepod, which prefers greater salinities as indicated by the CCA analysis. Shannon Diversity index H>2 was present during the whole six-month study period, indicating lower levels of pollution in Karainagar's coastal waters.

Keywords- Phytoplankton, Zooplankton, Jaffna