Abstract:
The upper watershed of Dambagastalawa River in Sri Lanka is a precious ecosystem of esthetic
beauty that supplies direct drinking, bathing and agricultural water for the people in Pattipola-Ambewela and
Agara Pathana areas. Nevertheless, is often subjected to multiple threats due to a range of human related
activities. Therefore, a monthly bio monitoring study was carried out in 2007 and 2008 to assess current
ecological conditions at five selected sites. The aim was to evaluate future sustainability of the catchments. The
invertebrate fauna were collected through in situ direct counting, stone lifting, kick, scope and core sampling
methods and evaluation of faunal composition was done preferring to the Biological Monitoring Working Party
(BMWP) scoring system. A parallel study was carried out to assess the water quality parameters following the
APHA standard methods. Any visible change in the chatchment land use patterns was also monitored.
The highest faunal diversity (<10) was recorded at highly oxygenated (<7.33 mg/l) up stream sites with little/or
no disturbance in Pattipola forest area where Ephemeropterans and Sumiliids were dominant. The second most
diverse site (≤ 7) was at the disturbed Agara Pathana site where Hydrosphychids, Chironomids, Plannarians and
Molluscs were dominant. The lowest faunal diversity (≤ 3) was recorded at the Ambewela reservoir which is
largely colonized by poor water quality indicative taxa Hydra and Chironomid (~208 individuals/m2). The
Average Score Per Taxon (ASPT) of the upstream sties (6.8 and 7) were less comparable with reference stream
site (8.2 out of 10) studied at the Piduruthalagala peak. ASPT was 4.57 in the down stream. However,
significantly lower ASPT values (3.5 at sluice gate and 2.5 at uppermost outlet) were recorded at Ambewela
reservoir evidently showing an associated ecological risk. The reservoir was found to received high levels of
nutrients (ammonia 0.20- 0.25 mg/l, nitrite 0.09 -0.121 mg/l, nitrate 0.50 -1.35 mg/l and phosphate 0.050 -.075
mg/l) due to effluent run-off from the adjoined livestock and crop farms. It was found to have a periodic
eutrophic condition especially during low rain and growing crop seasons. No biological/chemical evidence to
prove self-purification in its down stream in Agara Pathana where the river encounters other additional threats.
Converting of forest/tea plantations into annual crops, garbage dumping, loading tea dust/other waste is severe
in this area thereby water become more turbid (TSS 459.90 mg/l) unpleasant and undrinkable. Present case
study showed the agriculture based human activities in middle catchments of the Dambagastalawa river posed
significant negative impacts on river quality. In the long run this might affect the entire aqua-environment in the
Ambewela/Agra valleys. Actions should immediately be taken to bring these valleys into manageable levels
otherwise we may face irreversible loss of sustainability.
