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Ecological studies on Chaoborus (Diptera, Chaoboridae) in Lake George, Uganda
Author(s) -
McGOWAN LESLEY M.
Publication year - 1974
Publication title -
freshwater biology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.297
H-Index - 156
eISSN - 1365-2427
pISSN - 0046-5070
DOI - 10.1111/j.1365-2427.1974.tb00110.x
Subject(s) - water column , plankton , instar , benthic zone , benthos , larva , biology , ecology , shore , upwelling , oceanography , geology , fishery
Summary (1) Chaoborus anomalus and Chaoborus ceratopogones were found together throughout the year in Lake George, Uganda, the larvae forming 65‐85% by number of the macro‐benthos of the central lake area. (2) First and second instar larvae were wholly planktonic and third and fourth instar larvae mainly benthic during the day, migrating into the water column at night. A proportion of the third and fourth instar larvae were found in the water column during the day in the centre of the lake where light attenuation was greatest. C. anomalus showed a greater tendency to be benthic than C. ceratopogones. (3) Planktonic larvae were distributed in dense patches thought to be cohorts resulting from oviposition by swarms of adult females. The maximum density of planktonic larvae recorded was 31.7 × 10 2 /m 2 column. Third and fourth instar larvae were more evenly distributed in the benthos of the central lake area, the maximum density recorded was 79.9×10 2 /m 2 . (4) First and second instar larvae occurred mainly in the inshore regions, very few were found in the centre of the lake. Oviposition is likely to take place inshore. The patches of planktonic larvae were not stationary. Dispersal of larvae away from the shore may be active or may result from complex oscillations of the water mass produced by storms. In C. ceratopogones the floating egg batches could he carried away from the north shore by currents produced by inflows and prevailing winds. (5) Adults of C. ceratopogones were more numerous during the rainy seasons than during the dry seasons. A similar, though less marked seasonality was found in the occurrence of C. anomalus adults. The biomass of benthic larvae of C. ceratopo‐gones was estimated at 387.0 mg C/m 2 in the dry season, June 1970, and 130 0 mg C/m 2 in the rainy season, October 1970. That of C. anomalus was constant at 241.7 and 254.3 mg C/m 2 in June and October, 1970 respectively. (6) No difference was found in the horizontal distribution of the larvae, or in the size and weight attained by the various stages. (7) Interspecific differences were found in the mean number of eggs per adult female (309, C. anomalus and 224, C. ceratopogones); in the nature ofthe egg batches; n i the morphology of the larval mouthparts and pharyngeal sphincters, as well as i n the vertical distribution of the larvae and seasonal occurrence of the adults. On the basis of these differences the co‐existing species are thought not to be competing for the same resources.