Limnological aspects of Loch Logan, an urban impoundment

[摘要] English: At present many water resources in South Africa are polluted by industrial effluents,domestic and commercial sewage, agricultural runoff and litter. Declining waterquality, associated with human activities, has created environmental and public healthproblems. Loch Logan is an urban impoundment near the city centre of Bloemfontein, FreeState, South Africa, which is fed through Bloemspruit, a canal, which collectsstormwater runoff. The catchment area is -16 x 106 m2 and after a rainstorm ±30 %of the rain ends as runoff water in Loch Logan.The study was conducted between January 2000 and May 2001 to determine amongothers the seasonal and diel variation patterns of limnological characteristics, i.e.physical, chemical and biological parameters, to contribute to the limnologicalknowledge for proper management and conservation of the system.The physico-chemical characteristics of Loch Logan showed a distinctive seasonaltrend, which was primarily influenced by climate and rainfall. Discharge has beenproven to be an important variable influencing the water quality of Loch Logan. Aftera rainstorm the nutrient concentrations increased in Loch Logan, as well as theturbidity, but the conductivity decreased. All the data collected showed that LochLogan is an eutrophic system with an average reactive ortho-phosphate (PO4-P)concentration of 78.7 µg/l and a maximum of 648.3 µg/l.The average and maximumnitrogen concentrations were also high: ammonium nitrogen (NH4-N) 181.5 µg/l and1,032.6 µg/l,respectively, and nitrate nitrogen (N03-N) 201.2 µg/l and 1,375.1 µg/lrespectively.The pollution sources, organic and inorganic, are mainly stormwater runoff, gardenand domestic trash and also human sewage. During the decomposition of thesepollutants the oxygen (02) concentration decreased drastically, especially in thebottom water layer where the oxygen concentration was usually low. The averageoxygen concentration in the surface water was high at 6.6 mg/l (72.8 %) compared tothe average of 2.0 mg/l (16.5 %) in the bottom layer.The annual average chlorophyll-a concentration in Loch Logan (average 56.8 µg/landmaximum 487.2 µg/l) was typically that of a eutrophic system (15-75 µg/l).Eutrophication is therefore seen as the single most important threat to the waterquality of the Loch Logan aquatic system. A higher average in Chl-a concentrationwas found during the summer than during the winter. Some of the dominant algalspecies (Chlamydomonas; Chlorella, Euglena, Scenedesmus) are indicators ofpolluted water and the algae also shown seasonal trends. During blooms the algalcomposition mainly consists of cyanobacteria, namely Microcystis sp. (dominating)and Anabaena sp.In the diel variations there was no distinctive patterns in the physico-chemicalcharacteristics, except for temperature, oxygen and pH. The photosynthesis rate perchlorophyll-a was higher in October (3.68 mgC/mgChl-a/h) than in July(1.24 mgC/mgChl-a/h), which indicated that temperature was one of the controllingfactors of photosynthesis.Various management and restoration options for Loch Logan were discussed, forexample diversion of stormwater, diluting, flushing and phosphorus inactivation. Thesolution for the pollution is possibly a combination of the options mentioned above.

[发布机构] University of the Free State