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Loskom Dam in Sekhukhune
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Since 2007, members of the SERA Water Focus Area Team have been trying to unravel the causes of incidences of fish deaths and crocodile die-offs in Lake Loskop and further down the Olifants River in the Kruger National Park. During the past five years, numerous incidents of fish mortality have occurred in the dam.
Even more worrying, has been the crocodile die-offs - the population of Nile crocodiles in Loskop dam has declined from approximately 38 animals to only six in 2008.
At the time, their deaths were ascribed to pansteatitis, which appears to be associated with the intake of rancid fish fat after a fish die-off. And the fish were thought to be dying due to pollutants associated with sporadic incidents of high concentrations of acid mine drainage flowing into the dam.
Since the beginning of 2008, large numbers of dead crocodiles were also reported from Olifants River Gorge downstream of Lake Loskop in the Kruger National Park. Although the upper Olifants river system is affected by many factors, a lake system such as Lake Loskop is usually driven by a few key controlling factors. In this case, it seems clear that heavy metals, reduced pH levels and high concentrations of dissolved salts, derived from acid mine drainage, together with high phosphate concentrations from inflows of untreated or partially treated sewage, are the most important factors that have contributed to the decline in water quality in Lake Loskop over a long period of time.
However, every aquatic system has a relatively limited ability to absorb continual pollutant loads before a dramatic change takes place in the structure and functioning of the ecosystem components. "If an aquatic system consistently moves beyond a critical threshold, it is likely to change dramatically and start to behave in a different way, often with unforeseen or undesirable consequences for people who rely on the system for water supplies. In the case of Lake Loskop, we have seen more frequent occurrences of toxic cyanobacterial blooms and massive fish die-offs in the past few years. This suggests very strongly that adverse changes have already occurred," explains Water Focus Area member, Dr Paul Oberholster.
"This aquatic ecosystem now appears to be in a new state where the basic structure and function of the aquatic ecosystems are different from the original conditions that occurred several years ago. Once this type of change has occurred, it is usually very difficult to return the system to its previous state, and, in most cases, this can only be accomplished through extensive and prolonged remediation measures", he says.
Because Lake Loskop appears to have undergone a change in its chemical and biological state, there are now different interactions within its environment. Oberholster thinks the final answer might lie in the changes that have occurred in the phytoplankton community of the dam. "We think there may be a possible link between two phytoplankton species that we found in the lake, and which interact with the aquatic ecosystem foodweb".
"Both of these species of phytoplankton become toxic under certain conditions and might be responsible for the fish deaths. Mycrocystis is a cyanobacterium (blue-green alga) and is well known for its toxic effects in lakes and reservoirs elsewhere in South
Africa. It is possible that, in combination, these two potentially toxic phytoplankton species could be responsible for the fish deaths. It also seems that when crocodiles eat the dead fish their metabolism is affected in a way that causes them to contract pansteatitis and die," he says.
With funding from the Norwegian Council for Higher Education and South Africa's National Research Foundation, Oberholster has managed to confirm the identity of the second phytoplankton species, which had not previously been recorded from the lake.
"The 'newly arrived' species belongs to the phytoplankton family called Dinophyceae, and several other members of this family are known to cause so-called 'toxic red tides' in the marine environment. However, we have not yet been able to confirm that the species present in Lake Loskop is toxic, and our laboratory tests are continuing," he says.
The Focus Area project team has now received additional funding to monitor Lake Loskop for the next twelve months. Hopefully, this will enable them to confirm the reasons for the changes in Lake Loskop and the cause of the crocodile deaths in the lake.
Source: CSIR Sciencescope
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