Phytoplankton abundance and diversity in relation to water quality parameters: A study on Beira Lake and Ihalagama Lake in the Western Province in Sri Lanka

Abstract

Phytoplanktons are important components of the aquatic ecosystem. Diversity and abundance of the phytoplankton community of two urban lakes namely; Ihalagama Lake and Beira Lake were assessed in this study. The main aim of the present study is to investigate how the distribution of phytoplankton community is varied among two lakes and their relationship with water quality. Phytoplankton samples were collected from each sampling site from September 2023 to February 2024. Spatial variation of water quality parameters and phytoplankton diversity were assessed by one way ANOVA followed by Tukey’s pairwise comparison using MINITAB 17 software. A total of 20 phytoplankton species were recorded from both Beira Lake and Ihalagama Lake. They belong to six classes namely Class Bacillariophyceae, Class Chlorophyceae, Class Cyanophyceae, Class Zygnematophyceae, Class Euglenophyceae and Class Ulvophyceae. 16 phytoplankton species were identified from Ihalagama Lake, while 12 species were from Beira Lake. Microcystis aeruginosa is the most abundant phytoplankton species having 40% of relative abundance. Based on the results obtained from the Principal Component Analysis (PCA), Beira Lake was characterized by high abundance of Microcystis aeruginosa, Scenedesmus perforates, Pediastrum, Chlamydomonas, Rhizosolenia stolterfothii, Ulothrix, Lyngbya contorta, and Asterionella species. Ihalagama Lake was characterized by Navicula cryptocephala, Scenedesmus obliquus, Trachyneis aspera, and Stephanodiscus species. Total abundance is higher in the both sites of the Beira Lake than that of the Ihalagama Lake. The highest Shannon-Wiener diversity index (H) was recorded from Beira Lake (H = 1.124) while the highest species richness (SR) was recorded from Ihalagama Lake (SR = 1.046). Some water quality parameters including conductivity, secchi depth, NO3- concentration, PO43- concentration, COD, and BOD varied significantly between four sampling sites at 95% level of significance. The highest mean BOD level was recorded from the Beira Lake (7.30 mg/L) and the recorded lowest value was 2.46 mg/L, from the Ihalagama Lake. Similarly, the highest values for COD, nitrate and phosphate were recorded from the Beira Lake (633 mg/L, 1.73 mg/L, 1.07 mg/L respectively) while lowest COD, nitrate, and phosphate levels were recorded from Ihalagama lake (238.8 mg/L, 0.46 mg/L, 0.15 mg/L respectively). In terms of the two lakes combined, Beira Lake had higher than average levels of BOD, COD, and inorganic nutrients like phosphate and nitrate. On the other hand, Ihalagama Lake had lower than average levels of BOD, COD, nitrate, and phosphate, indicating that its water quality conditions are healthier and less contaminated. Therefore, it is suggested to identify the sources of pollutants and carry out continuous monitoring of water quality parameters, particularly in Beira Lake.