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In the 2005 count, a total of 22 carcasses were located |
Buoyancy of cyanobacteria - accumulating in high numbers in surface water at night time - producing carbohydrates in day time
Photos courtesy of Dr Roy Bengus, SANPARKS |
Following two incidents of clustered wild animal mortalities in the Kruger National Park in 2005 and again in 2007, CSIR microbiologist Dr Paul Oberholster and four fellow scientists embarked on a study to identify the cause and source of the deaths.
During the autumn and early winter of 2005, rangers of the Kruger National Park (KNP) found several carcasses in the region of the Nhlanganzwane Dam in the extreme south-eastern corner of the Park, south of Lower Sabie rest camp. During the time of the discovery of the carcasses, water levels at all the dams in the region were unusually low and approximately 100 hippopotami inhabited the dam.
"The large density of hippopotamus in the dam may have resulted in water column enrichment with nutrients, which most probably gave rise to the visible cyanobacterial bloom," Oberholster, a well-known expert on cyanobacterial analyses, explains. Some species of cyanobacteria may produce substances that are highly toxic, causing mortalities that included wild and domestic mammals, and waterfowl and fish.
A total of 52 carcasses were discovered from February to July 2005, including seven white rhinoceros, two lion, two cheetah, 23 wildebeest, one hippopotamus and five buffalo.
"Our first task was to identify potentially toxigenic environmental Microcystis strains as well as to establish a causal link with the animal mortalities," Oberholster explains. "We realised that the tragedy was potentially caused by a high urine and faecal load resulting from an unusually high hippopotamus density in the dam. This triggered a chain of events characterised by a massive increase in the growth of primary producers (the toxic Microcystis), followed by bio-intoxication incidents in wild animals."
Mortalities decreased to zero after July 2005 when the overgrazed area surrounding the dam was subjected to a low-intensity accidental veld fire. Two years later, in June 2007, mortalities of white rhinoceros, zebra and wildebeest were again reported from the area of the Nhlanganzwane Dam. Once again there was a visible cyanobacterial bloom as well as a large resident hippopotamus population in the dam.
Oberholster and his team investigated, among other things, the ecological drivers behind the animal mortalities by unravelling limnological conditions in the dam at the time of the event as well as the water association patterns of the megaherbivores with the dam. They found that the contaminated water intake by adult male white rhinoceros during the toxic event was very high.
"In addition, the analyses that we made for cyanobacterial toxicity have contributed important findings, particularly a method that could be used as a proactive hazard identification technique to alert wildlife managers to improve their preparedness and assist in the management of toxic bloom events."
Lakes have a natural ageing process, but this can be greatly accelerated within decades by human intervention and human activities such as intensive agricultural and industrial activities and the development of large sewage systems associated with large metropolitan areas.
"The objective of this study was to use PCR assays to assess the two mortality clusters in wild animals, associated with cyanobacterial intoxication, in the dry seasons of 2005 and 2007," Oberholster elaborates. "To unravel the ecological drivers behind these animal mortalities in the region of this dam, we investigated some physical and chemical conditions of the dam at the time of the intoxication event, as well as the role played by megaherbivores associated with the dam."
When the KNP was established in 1898, animals concentrated during the dry season around five perennial rivers. Until the 1980s more than 300 boreholes were drilled, 50 earth dams were constructed and various seasonal and perennial rivers were dammed as part of the water provision programme to ensure year-round reliable surface water throughout the park. Nhlanganzwane is an artificial dam.
Unlike white rhinoceros, hippopotami have strong temporal and spatial constraints on their foraging behaviour. Hippopotami are temporally contained in that they forage primarily at night, the period when cyanobacterial cell concentrations are highest in the surface water and exposure to high concentrations of cyanobacteria by oral intake is most unlikely.
The objectives of this study were to identify possible toxigenic environmental strains and to establish a causal link with the animal mortalities. "We established, among other things, that the only time there is assessment of surface water in conservation areas like the KNP, is when there are deaths of wild animals. This is also a major problem when it comes to diagnosis of cyanobacterial poisoning in wildlife, since the carcasses of poisoned animals are usually found decomposed or partially consumed by scavengers.
"The present study is a step forward for South African park rangers and conservation managers confronted with visible algal blooms in artificial water points in conservation areas, Oberholster concludes. "Rangers cannot monitor water sources in terms of quality, neither is it always possible to determine the reason for mortalities by logistics."
The outcome of the study was published in the most recent issue of the international journal Ecotoxicology and Environmental Safety.
The project was co-funded by the National Research Foundation.
Enquiries: Dr Paul Oberholster or CSIR Communications
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