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.
