Abstract:
Diversity and abundance of zooplankton in relation to nitrate–N and total phosphorus
levels of 20 minor and medium scale reservoirs in the low country intermediate zone
of Sri Lanka were studied in June-October 2013. Total phosphorus content ranged
from 0.029 mg L-1 recorded in Wendaruwewa reservoir to 0.529 mg L-1 recorded in
Karangamuwa reservoir and Nitrate-N content varied from 1.186 mg L-1 recorded in
Adukkane reservoir to 5.625 mg L-1 recorded in Anukkane reservoir indicating that
all reservoirs studied are eutrophic. During the present study, a total of 43 species of
zooplankton, comprising 31 species of rotifers, 9 species of cladocerans and 3 species
of copepods were recorded. Number of zooplankton species in any one reservoir,
which ranged from 4 to 13, was significantly correlated with the nitrate-N content
(p<0.05). The most widely distributed zooplankton species was Diacyclops nanus,
which was present in 15 reservoirs. Abundance of zooplankton ranged from 4
individuals L-1 recorded for Alona monocantha and Chydorus parvus in
Wendaruwewa and Metiyagane reservoirs respectively to 1017 individuals L-1
recorded for Filinia terminalis in Galewela reservoir. Margalef index which ranged
from 0.532 in Adukkane reservoir to 2.096 in Dewahuwa reservoir, was significantly
correlated with nitrate-N content of water (p<0.05). Pielou index ranging from 0.65
in Galewela reservoir to 0.92 in Polpitiya reservoir and Shannon-Weiner index
ranging from 0.91 in Munamaldeniya reservoir to 2.13 in Dewahuwa reservoir were
not significantly correlated with nitrate-N and total phosphorus levels of water
(p>0.05). Keratella quadrata was present only in Anukkane reservoir where the
nitrate-N content was the highest and therefore it may be considered as a bio-indicator
of high levels of nitrate-N. Eutrophic conditions of theses reservoirs may possibly be
due to runoff of nutrients from agricultural fields as these are located in an intense
agricultural area. Therefore, it is necessary to take immediate action to control the
overuse of inorganic fertilizer in this area so that contamination of water bodies with
nutrients can be minimized.
