Assemblage of Bloom-Forming Dinoflagellates in Kalu Ganga River Mouth, Sri Lanka

Abstract

Dinoflagellates are one of the major phytoplankton groups that pose serious threats to ecosystems, fisheries, and both animal and human health by forming harmful algal blooms (HABs). The current study focused on investigating the diversity of bloomforming dinoflagellates in the Kalu Ganga river mouth during wet months (September- October, 2020) and dry months (January-February, 2021) at randomly selected six sampling locations in the river mouth. Plankton sampling was carried out using a 55µm plankton net at each location (20L, n = 2) on a monthly basis. Samples were preserved on site using acidified Lugol’s solution while identification and enumeration were carried out in the laboratory. Standard keys were used to identify the dinoflagellates to the nearest possible taxonomic level. In addition, temperature, pH, salinity, total dissolved solids (TDS), and electrical conductivity (EC) were measured using portable meters onsite, while nitrate and orthophosphate concentrations were determined by standard spectrometric methods. Results showed that dinoflagellates made up 9.4% of the entire phytoplankton community. Altogether, seven marine dinoflagellate species belonging to five genera were reported. Reported toxic genera included Dinophysis, Gonyaulax, Protoperidinium, and Noctiluca. Protoperidinium sp. was the most dominant. Three different species were reported under the genus Ceratium, which is bloom-forming but not toxic. Dinoflagellate density was significantly different (P < 0.05) between dry months (58 individuals/L) and wet months (26 individuals/L). Similarly, the pH, salinity, TDS, and EC were significantly different between dry months and wet months (P < 0.05). Dinoflagellate density had a strong positive correlation (P < 0.05, r>0.7 for each) with pH, salinity, TDS, and EC during the study period. During the dry months, saltwater intrusion into the Kalu Ganga river mouth provides suitable habitat for marine dinoflagellates. Toxic algal blooms are less likely to occur in the study area due to low levels of nutrients, a small number of toxic taxa, and the low density of dinoflagellates.