| Summary: | M.Sc. (Zoology) === Ari investigation was made into the biological control of the submerged aquatic weeds Potamogeton pectinatus, Lagarosiphon major and L. muscoides in the Florida Lake, Transvaal using the Chinese grass carp Ctenopharyngodon idella (Val.) as biological control agent. The physical, chemical (including selected heavy metals), faunal and bacteriological conditions were also investigated over a period of one year. The total standing crop of P.pectinatusand Lagarosiphon spp, were reduced from an initial 0,193 kg m" 2 (wet mass) in March 1990 to 0,0338kg m-2 (wet mass) in March 1991. Reductions in the densities of the macrophytes coincided with a corresponding increase in chIorophyll-a values in the water columm of the lake. Abnormally high concentrations of the heavy metals copper, manganese, iron, nickel chromium, zinc and lead were found to occur in the lake sediments, aquatic macrophytes, ftlamentous algae, macroinvertebrate fauna as well as in organs and tissues of the perch Perea fluviatilis. These metals could be related to gold mining activities which occurred in the vicinity of the lake some years ago. Faecal bacterial counts of the water in the lake point towards the possible contamination of the lake water with domestic sewage. Ari analysis of the faeces of the numerically dominant birds which occur at the lake i.e. the coot Fulica aistata and the egyptian goose Alopochen aegyptiacus suggest that the main cause of faecal contamination of the lake's water possibly originates from these and other water birds frequenting the lake. By using the results on the physical-chemical conditions in the lake, conditions which presently prevail can be summarized as being largely that of a mesotrophic impoundment with strong tendencies towards eutrophic conditions. Secondary effects of the grass carp on the lake ecosystem was the progressive decline of weed eating birds and a slight increase in the numbers of piscivorous birds, both of which could be linked to the drastic reduction in the standing crop of the submerged weeds in the Lake. The present survey confirms the value of the Chinese grass carp as biological control agent in the management of excessive growths of submerged aquatic weeds in lakes and reservoirs in South Africa...
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