Volume 07 October 1998

Vibrational Modes of a Rabana (Drum)

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Vibrational Modes of a Rabana (Drum) Wijeratne, Agra T.; Peiris, M.G.C.; Dharmapriya, G.G. The vibrational modes of a "Virindu" Rabana membrane (of diameter 13 inches), which was driven by a loudspeaker, were studied by using a microphone.The vibrational modes were observed at frequencies below 500Hz. The nodal patterns seen here were slightly different from theoretically expected patterns for an ideal membrane. It was observed that most of the vibrational modes were degenerate. These degeneracies could be due to the non-uniformity of the membrane and the imperfect membrane boundary. However, the observed frequencies were close to that obtained from theory.

Trophic Status of Beira Lake

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Trophic Status of Beira Lake Nahallage, Charmalie; Piyasiri, Swarna Beira Lake is located in the heart of the city of Colombo.and in an highly urbanized catchment of 432 ha. Beira Lake covers 65.4 ha and comprises four main basins the East Lake, the Galle Face Lake, the West Lake and the South West Lake. The objective of the study was to determine the trophic status of the lake using the, physical, chemical and biological properties of the water. The mean water transparency depth of the lake was 15.2±9.2 ern. The euphotic zone was approximately 41.8 cm. Water temperature ranged from 26°C to 32°C. Conductivity ranged from 212 to 500 IlS. pH value ranged from 7.2 to 9.8 Dissolved Oxygen, BOD and Orthophosphate values of the lake ranged from 7.82 to 14.41 mg/I, 33.45 to 68.35 mg/1 andO.014 to 0.08 mg/l respectively. The highest and lowest Sulphate values recorded were 4.04 ppm and 2.33 ppm. The Chloride content ranged from 29.02 to 53.09 mg!. The phytoplankton community of the Biera Lake composed of the dominant blue green algae and the green algae. Blue green algae represented 99 percent ofthe total phytoplankon population. The major zooplankton groups observed werecladocerans, Copepods and Rofiters. Copepods were the most dominant zooplankton group. According to the physical chemical and biological properties obtained for Beira Lake, it is at the advanced stage of eutrophication process.

Studies on Diel Feeding and Digestibility of Cyanobacteria

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Studies on Diel Feeding and Digestibility of Cyanobacteria Kariyawasam, L.M.; Piyasiri, Swarna Beira Lake is eutrophic and covered with a bloom of Cyanobacteria. Hybrids of Oreochromis niloticus and Oreochromis mossambicus dominate the lake. Present study deals with feeding cycle and digestibility of the bloom by Oreochromis species. Volume method and the point method were used to determine feeding intensity of the hybrids. Stomach pH and Chloropityll-a content in stomach and hindgut were determined at different hours of the day. Four to five diumals were conducted in the field and in the laboratory. Results indicated that, oreochromis hybrids commenced their feeding at the 6th hour and stomachs were filled up to 25%. At the 12th hour, majority of fish were fed from 50% to 75% and on the 18th hour up to full stomach level. Feeding decreased from 75% to 50% at the onset of resti ng phase at the 24th hour. Results were similar under laboratory conditions. Al the onset of feeding pH was 7-8 in the stomach, Ch-a concentration in the hindgut was high. and food passed undigested. Atthe 12th hour. pH decreased from 3-4 and reached 1-2 at the 18th hour indicating onset of digestion. From 18th to 24th hour, active digestion occurred.

Seasonal Trends of Anopheles culicifacies Population and Its Sibling Species Status at Gomadiyagala-A Village in the North -western Province of Sri Lanka

2022-02-24T05:34:09Z

Seasonal Trends of Anopheles culicifacies Population and Its Sibling Species Status at Gomadiyagala-A Village in the North -western Province of Sri Lanka de Silva, B.G.D.N.K.; Gunasekera, Maya B.; Abeywickreme, W.; Wickramasinghe, M.B.; Karunanayake, E.H. Anopheles culicifacies s.s and An. subpictus were the most predominant species found in this study area and accounted for 31.2% and 41.4% respectively of the total mosquitoes collected from the cattle - baited hut collections during the 3- yr period. The relative adult densities of An. culicifacies s.s recorded during cooler and rainy conditions were significantly higher than those recorded during hot periods (p < 0.013) indicating a tendency to increase the vector density during the cool, rainy seasons. Screening of 364 mosquitoes collected from four sampling methods namely cattle - baited hut collection (CBHC), human baited night collections (HBNC), larval collections (LC) and hand collections (HC) using Rp2l7 and Rp234 DNA probes showed the absence cf sibling species A.