Limnological aspects of Loch Logan, an urban impoundment

English: At present many water resources in South Africa are polluted by industrial effluents, domestic and commercial sewage, agricultural runoff and litter. Declining water quality, associated with human activities, has created environmental and public health problems. Loch Logan is an urban impoundment near the city centre of Bloemfontein, Free State, South Africa, which is fed through Bloemspruit, a canal, which collects stormwater 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 among others the seasonal and diel variation patterns of limnological characteristics, i.e. physical, chemical and biological parameters, to contribute to the limnological knowledge for proper management and conservation of the system. The physico-chemical characteristics of Loch Logan showed a distinctive seasonal trend, which was primarily influenced by climate and rainfall. Discharge has been proven to be an important variable influencing the water quality of Loch Logan. After a rainstorm the nutrient concentrations increased in Loch Logan, as well as the turbidity, but the conductivity decreased. All the data collected showed that Loch Logan 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 maximum nitrogen concentrations were also high: ammonium nitrogen (NH4-N) 181.5 µg/l and 1,032.6 µg/l,respectively, and nitrate nitrogen (N03-N) 201.2 µg/l and 1,375.1 µg/l respectively. The pollution sources, organic and inorganic, are mainly stormwater runoff, garden and domestic trash and also human sewage. During the decomposition of these pollutants the oxygen (02) concentration decreased drastically, especially in the bottom water layer where the oxygen concentration was usually low. The average oxygen concentration in the surface water was high at 6.6 mg/l (72.8 %) compared to the 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/l and maximum 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 water quality of the Loch Logan aquatic system. A higher average in Chl-a concentration was found during the summer than during the winter. Some of the dominant algal species (Chlamydomonas; Chlorella, Euglena, Scenedesmus) are indicators of polluted water and the algae also shown seasonal trends. During blooms the algal composition mainly consists of cyanobacteria, namely Microcystis sp. (dominating) and Anabaena sp. In the diel variations there was no distinctive patterns in the physico-chemical characteristics, except for temperature, oxygen and pH. The photosynthesis rate per chlorophyll-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 controlling factors of photosynthesis. Various management and restoration options for Loch Logan were discussed, for example diversion of stormwater, diluting, flushing and phosphorus inactivation. The solution for the pollution is possibly a combination of the options mentioned above.