Abstract:
Cyanobacteria are a highly diverse group of prokaryotes and some of them release toxins
(cyanotoxins) into their aquatic habitats. Due to high toxicity, tendency to bioaccumulate and
biomagnify, cyanotoxins may risk human and animal health. Chandrika wewa (448 ha) supplies
drinking water for more than 5000 families in the Walawa river basin. However, a systematic
study has not been conducted to detect cyanotoxin contamination in the reservoir. Therefore,
the objectives of the present study were (a) to confirm the presence or absence of cyanotoxins
and (b) to identify the impacts of physicochemical parameters of water on spatial distribution of
cyanotoxins. Sub-surface water samples were collected from 33 sites representing the entire
reservoir and subsequently NO3
--N, PO4
3--P, cyanotoxins, chlorophyll-a and phytoplankton
composition were measured. The concentrations of two cyanotoxins, microcystin (MC) and
nodularin (NOD) were determined by using enzyme-linked immunosorbent assay (ELISA). In situ physiochemical properties were measured using a multi-parameter water quality meter.
Results showed the presence of phytoplankton in the genera of Tribonema, Microspora,
Pediastrum and Microcystis. Microcystis was dominant in all sampling sites with the density of
200-790 cells/mL. The MC and NOD concentrations were between 0.77-1.19 µg/L and 0.69-1.27
µg/L, respectively. Further, absence of filamentous cyanobacteria that is resembles to the
morphology of the members in the genera of Nodularin was confirmed. However, there were
significant positive correlations between MC (r2 = 0.992, p<0.05) and NOD (r2 =0.883, p<0.05)
concentrations with Microcystis density. Therefore, it can be assumed that NOD is unlikely to be
present in this reservoir and the detected NOD is due to the cross reaction of NOD antibodies
with MC in water samples. Further, filamentous cyanobacteria that are known to produce MC
was not found. Hence, Microcystis is likely to be responsible for the detected MC toxin. There was
no correlation between any of the physicochemical parameters with the spatial distribution of
MC toxin. The concentration of MC in this reservoir was slightly above the World Health
Organization (WHO) guidelines for drinking water (1 µg/L). Therefore, continuous monitoring
of toxins and physicochemical parameters of the reservoir is recommended to minimize
cyanotoxin associated health hazards that could happen in the future.
