Browsing by Author "Weerakoon, S.N."

Now showing 1 - 5 of 5

Abundance and diversity of freshwater molluscs in some selected reservoirs of the Kala Oya river basin, Sri Lanka
(Sri Lanka Association for Fisheries and Aquatic Resources, 2015) Weerakoon, S.N.; Chandrasekera, L.W.H.U.; Amarasinghe, U.S.
Benthic macroinvertebrates such as molluscs are known to be possible bioindicators of aquatic ecosystems. We carried out a study to investigate how the abiotic factors affect the abundance and diversity of molluscan fauna at 10 irrigation reservoirs in the Kala Oya river basin during the dry season from August to November 2014. In each reservoir, three sampling sites were selected where at each site, a 50 m long perpendicular line transect was established running from its maximum water supply level towards the center of the reservoir. Three soil samples were collected at 10 m intervals each along each transect using a 0.5 dm3 soil corer. These samples were sieved in situ through a 1 mm sieve and the remains were preserved in 10% formalin solution containing Rose Bengal. Later the molluscs and other associated macrofauna were identified to the nearest possible taxonomic category using standard taxonomic keys and the fauna belonging to each taxon were enumerated separately. In addition, data related to vegetation cover, intensity of human disturbances, intensity of pollution, agrochemical inputs, soil texture and human settlements based on a modified scoring system were also collected. Altogether 4328 molluscs belonging to 11 genera were recorded. Molluscan genera such as Bellamya (45.9%), Melanoides (26.7 %) and Thiara (16.9 %) dominated the samples while the remainder (10.5%) belonged to less dominant genera consisting of Planorbis, Gyraulus, Indoplanorbis, Lamellidans, Planorbella, Lymnaea and Pila. The abundance of the molluscan genera varied significantly between the reservoirs (MANOVA; F (99, 899) = 4.300, P = 0.000; Wilk’s λ = 0.06515) and along the transect distances (MANOVA; F (44, 483) = 0.901, P = 0.654; Wilk’s λ = 0.73988). High diversity measures of molluscs were observed at Angamuwa wewa, Katiyawa wewa, Rajanganaya reservoir and Usgala-siyambalangamuwa wewa. Principal Component Analysis (PCA) for the mollusc abundance and the relationship between PC1 scores and Shannon–Wiener diversity index endorsed that Bellamya, Melanoides and Thiara were the highly contributing genera to the high diversity of the molluscs in the above reservoirs. Results of the PCA for abiotic factors revealed that both the abundance and species diversities were high at moderately sandy bottoms with high vegetation cover and agrochemical inputs. In contrast, the intensity of human disturbances, pollution, human settlement and shadiness showed a negative effect on the abundance and diversity of these molluscs. This study was carried out as part of a comprehensive study which investigates how the aspects related to biotic integrity and multiple uses of reservoirs could be used to develop effective management options for reservoir fisheries in Sri Lanka.
The effect of height from ground level and the age of coconut shell on the oviposition and fecundity of Aedes albopictus mosquitoes
(Institute of Biology, Sri Lanka, 2013) Weerakoon, S.N.; Ganehiarachchi G.A.S.M.
Gape size of wild guppy (Poecilia reticulata): Does it matter in mosquito larval control?
(Faculty of Science, University of Kelaniya, Sri Lanka, 2016) Nayakarathna, N.M.N.G.; Weerakoon, S.N.; Ganehiarachchi, G.A.S.M.
Larvivorous fish is very important tool in controlling disease transmitting vector mosquitoes in many parts of the world. However, there is a lacuna of knowledge about the vector control potency in relation to the gape size of these fish. Since the wild guppy, Poecilia reticulata; is a topmost larvivorous fish, the current study was performed to study the effect of gape size of guppy in controlling mosquito larvae. For the current study, wild guppies were collected from their own natural habitats located around the Kelaniya area. In the laboratory, measurements of total length (TL) and Gape Size (GS) of individual specimens were obtained. These fish were then grouped in to ten length classes of 0.2 cm intervals ranging from 0.8 to 2.8 cm. For each length class of fish, 5L capacity glass tank was assigned. Accordingly, there were 10 experimental tanks for 10 size classes of fish, and for each tank assigned for a particular size class, 10 fishes were introduced. After acclimatization, fish were kept without food for 24 hours. In the meantime, laboratory reared mosquito larvae were collected and separated according to their instar level. Four instar stages of larval and the pupal stage were used for investigating of prey selection by P. reticulata. The rationale used here was that all the immature stages of mosquitoes were separately exposed to each length class of fish to determine the mosquito larval/pupal mortalities. Ten mosquito larvae/pupa form a particular instar were introduced to a fish tank at a time. Subsequently, the mortality rates of mosquito larvae/pupa in each tank were determined after 24 hours. All the fish were kept under 24 hour fasting period prior to each experimental cycle. In the next experimental cycle, ten mosquito larvae/pupa form a particular instar were introduced to another fish tank. This procedure was continued until all the instar/pupal stages expose to all the fish tanks. Relationship between TL and GS was tested with a Pearson correlation and with linear regression analysis. One way – ANOVA was performed to analyse the significance of the mortality of different instars along in different length classes. The results revealed that the relationship between TL and GS is best fitted for liner regression model (R2 = 0.80) with the equation; GS = 0.1 TL – 0.02. The 24 hour percentage mortality of different mosquito instars indicated that the 1st and 2nd mosquito larval instars were vulnerable to all the length classes tested for these instars (GS range = 0.06 – 0.26 cm). Fishes of 0.8 – 1.0 cm length class (GS range = 0.06 – 0.08 cm) were unable to consume 3rd instar mosquito larvae. Fishes of 0.8 – 1.2 cm length range (GS range = 0.06 – 0.1 cm) were unable to consume 4th instar larvae. Pupae were consumed by the fishes whose length range was 1.4 – 2.8 cm (GS range = 0.12 – 0.26 cm). Hence, the gape size of the guppy fish has a significant effect on the mosquito larval controlling and the fishes with GS > 0.12 cm (TL > 1.4 cm) are excellent in controlling the pupae and all the instars of mosquito larvae.
Seasonal water level fluctuation, habitat alteration and colonization of marginal benthic fauna in irrigation reservoirs in the Kala Oya River basin in Sri Lanka
(Faculty of Graduate Studies, University of Kelaniya, 2015) Weerakoon, S.N.; Chandrasekara, W.U.; Amarasinghe, U.S.
The innumerable benthic microhabitats in freshwater ecosystems support a wide variety of fauna. These benthic fauna are sensitive to natural as well as anthropogenic disturbance events such as water level fluctuations, temperature variations, agrochemical inputs etc. The present study was carried out to investigate the effect of seasonal water level fluctuation on the distribution and colonization of benthic fauna at some irrigation reservoirs in Sri Lanka. The study was carried at 10 reservoirs namely Angamuwa wewa, Balalu wewa, Dewahuwa wewa, Ibbankatuwa wewa, Kandalama wewa, Katiyawa wewa, Kala wewa, Siyambalangamuwa wewa, and Usgala-Siyambalangamuwa wewa in the Kala Oya river basin in Sri Lanka. The study period (September, 2014 to March 2015) included a dry season followed by a rainy season. Soil core samples (n = 15, volume = 0.5 dm3 each) from within 3 random locations in the marginal zone of each reservoir at each season were collected using a soil corer. After the samples were wet sieved in situ through a 1 mm sieve, the macrobenthic fauna in each sample were separated, identified to the nearest possible taxonomic level and enumerated separately. Environmental parameters such as aquatic vegetation, shadiness, soil texture, soil pH and the degree of water inundation were also measured using standard methods. The Shannon-Wiener diversity index for each reservoir for the two climatic seasons were determined and were statistically compared by a t-test. The abundance of benthic species at different study sites were analyzed using cluster analysis. The importance of environmental variables on the abundance of macrobenthos for both seasons were tested by the Principal Component Analysis (PCA). Altogether 17 taxa were recorded. Bellamya (Mystery snail) (62.2 ± 17.2) and Melanoides (Trumpet snail) (38.5 ± 11.6) were the dominant taxa during the dry season while Bithynia (Mud snail) (15.77 ± 6.75) and Bellamya (4.83 ± 1.38) dominated during the rainy season. The species richness was higher in rainy season (17 taxa) than in the dry season (11 taxa). This was due to the presence of six additional taxa i.e. Chironomidae, Oligocheata, Ephemeroptera, Trematoda, Zygoptera and Bithynia) during the rainy season. The species diversity of the reservoirs was significantly high during the rainy season (t – test, P = 6.2148, α = 0.05, df = 22). PCA revealed that the degree of water inundation and aquatic vegetation to be the key factors that determine the species distribution of these reservoirs. The elevated species diversity perhaps may be due to the enhanced growth of aquatic vegetation at the shallow marginal habitats when the reservoirs were inundated by water during the rainy season. The change of this habitat alteration may have positively supported the colonization of new insect taxa such as Chironomids, Ephemeropterans and Zygopterans into these shallow marginal habitats of irrigation reservoirs in the Kala Oya River Basin in Sri Lanka.
Seasonal water-level fluctuations and changes in macro-benthic community structure in tropical reservoirs: A Sri Lankan case study
(Lakes & Reserv, 2021) Weerakoon, S.N.; Chandrasekara, W.U.; Amarasinghe, U.S.
Seasonal water-level fluctuations (WLFs) in reservoirs influence ecological processes, in turn affecting their benthic macroinvertebrate communities. The present study was carried out in 10 irrigation reservoirs of Sri Lanka to investigate the effects of seasonal WLFs on the benthic macroinvertebrate fauna. The study period was divided into two distinct phases: the drained period and the inundated period. Benthic macroinvertebrates were sampled during both periods and enumerated. Air temperature and soil quality parameters were also measured simultaneously, and hydrological data related to the Kala Oya River basin were obtained from irrigation authorities. Data analysis using non-metric multi-dimensional scaling revealed two distinct reservoir groups, namely, Hydrological Stress Group 1 (HSG1) and Hydrological Stress Group 2 (HSG2). A low hydrological residence time and low relative reservoir level fluctuations (RRLF) characterized HSG1, with the opposite describing HSG2. Temporal variations of macrobenthos indicated a high taxa richness and diversity in HSG1 reservoirs during the drained period, with scrapers and collector-gatherers being the dominant functional feeding groups. Reservoirs in the lower river basin (HSG1) exhibited a high taxa richness and diversity. During the inundated periods, insect taxa colonized, particularly, chironomids and ephemeropterans. Molluscan taxa were prominent during the drained periods. These observations highlight the influence of seasonal fluctuations of reservoir water levels on benthic macroinvertebrates. Benthic macroinvertebrate communities in reservoirs were significantly influenced by the RRLF, manifesting the importance of hydrological regimes in reservoirs to develop macroinvertebrate metrics for water-quality monitoring.