Abstract
A declining crocodile population and fish mortalities attributed to pansteatitis, along with increasing blooms of Microcystis aeruginosa and Ceratium hirundinella, have led to serious concerns about water quality in Loskop Dam, on the Olifants River, South Africa. Major impacts include acid mine drainage and eutrophication associated with sewage effluent. However, the specific causes of pansteatitis remain elusive. In 2011 the water chemistry and limnology of Loskop Dam were studied to determine factors that may be influencing aquatic ecosystem health. Long-term monitoring data collected by the Department of Water Affairs were analysed for trends using a seasonal Mann-Kendall trend test, and were used to determine the trophic state of Loskop Dam using the Carlson index. Multiple sites were sampled which showed the reservoir was heterogeneous with regard to nutrient concentrations, algal biomass and dissolved metals. Specifically, the transitional zone was characterised by frequent algal blooms, resulting in fluctuating dissolved oxygen (range = 2.1–14.5 mg/ℓ) and pH (range = 7.35–10.59) levels. Using total phosphorus, Secchi depth, and chlorophyll-a concentrations, the trophic state of Loskop Dam was classified as meso- to eutrophic. Significant positive trends were observed in total (Tau = 0.422) and dissolved inorganic (Tau = 0.193) phosphorus.The reservoir showed a monomictic pattern of summer stratification (October to April) and holomictic winter circulation (June to July), with an increase in the depth and extent of anoxia in the hypolimnion when compared to previous research. Simultaneous elevated concentrations of manganese (>370 μg/ℓ) and iron in near-bottom water samples coincided with hypolimnetic anoxia. Aluminium concentrations exceeded the target water quality range (>10 μg/ℓ) during summer (December) in both surface and near-bottom water samples. We conclude that fish in Loskop Dam are periodically exposed to several physiological stressors including elevated ammonia, aluminium, iron and manganese and possibly hydrogen sulphide, as well as low dissolved oxygen. While these factors have never individually been linked to pansteatitis, their combined impacts have not been studied. To ensure the sustainability of Loskop Dam, catchment management plans must focus on reducing phosphorus inputs, and continue seeking treatment solutions for mine-water associated with abandoned and working coal mines.Keywords: Loskop Dam, limnology, pansteatitis, eutrophication, Olifants River
Highlights
Loskop Dam was constructed in 1937 to supply irrigation water to downstream agricultural areas (Van Vuuren, 2008)
Time-series trends of physico-chemical data collected by the Department of Water Affairs (DWA) using a seasonal Mann-Kendall trend test; and
Water quality according to the DWAF guidelines for eco system health (DWAF, 1996a) where applicable
Summary
Loskop Dam was constructed in 1937 to supply irrigation water to downstream agricultural areas (Van Vuuren, 2008). The irrigation canal system, measuring 480 km, was completed just over 10 years later, in 1948. The system provides water to the Loskop Irrigation Board, which is the second largest irrigation area in South Africa, supplying 700 properties and covering an area of 16 117 ha (Oberholster and Botha, 2011). The dam is located about 32 km south of the town of Groblersdal in Mpumalanga Province, and is surrounded by a nature reserve. The main inflow to Loskop Dam is the Olifants River, which has the Wilge and Klein Olifants Rivers as its main tributaries. From Loskop Dam, the Olifants River flows north-east through the Drakensberg mountain range, the Kruger National Park, and into Massingir Dam in Mozambique, before continuing to the Indian Ocean
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