Abstract:
Lunugamwehera is a large man-made reservoir located in the South Eastern dry zone
of Sri Lanka. It is the main source of drinking water, irrigation, and fisheries for more
than 5000 families in the area. The occurrence of cyanobacterial blooms or high
population densities in water bodies results in various environmental and health issues.
Population density and composition of cyanobacteria in a waterbody may vary with the
trophic status of water. There is a high potential of contaminating the Lunugamwehera
reservoir with mineral nutrients that come from agricultural lands. However,
considerable attention has not been paid to study trophic status and cyanobacterial
diversity of the Lunugamwehera reservoir. Therefore, our objective was to determine
the spatial variation of cyanobacterial diversity in the Lunugamwehera reservoir along
with trophic status. Sub-surface water samples collected from 28 sampling sites
representing the whole waterbody were microscopically examined to identify
cyanobacterial species. Total phosphorous concentration, total chlorophyll-a
concentration, and Secchi depth were determined to assess trophic status using Carlson
Trophic State Index. Also, water quality parameters were measured using a multiparameter water quality meter. Microscopic analysis revealed the presence of only two
cyanobacterial species, Microcystis and Oscillatoria implying less cyanobacterial
diversity in the reservoir. Microcystis spp. (~3.257 x 107
cells mL-1
) dominated all
sampling sites with significant difference (p<0.05) in cell densities among sites
while Oscillatoria spp. (5 - 10 cells mL-1
) occurred rarely. Apart from
cyanobacteria, Pediastrum spp., Tribonema spp. and Nitzschia spp. (1 - 2 cells mL-1
)
were also identified scarcely. Water quality parameters were approximately similar in
all sampling sites except increased turbidity near the inlet. According to the Carlson
Trophic State Index (37.70), the water body was ‘slightly oligotrophic’. Therefore, the
overall results infer the presence of less cyanobacterial diversity and significant spatial
variation in Microcystis spp. as the dominant species in the slightly oligotrophic water
of the Lunugamwehera reservoir.
Keywords: Carlson trophic state index, Cyanobacteria, Oligotrophic, Spatial variation
