The Kruger National Park (KNP) has implemented a water sampling and monitoring regime to reduce the risk of further animal deaths caused by blue-green algal blooms in man-made dams in the park.
At the 6th Science Network Meeting at Skukuza this week, Dr Roy Bengis, state veterinarian in the Kruger National Park, presented past and recent findings on animals dying from drinking water containing toxic blue green algae, also known as cyanobacteria. He confirmed that the recent deaths of several zebra in the vicinity of the Silolweni Dam were caused by high levels of cyanobacteria in the water and that similar incidents in 2005 and 2007 had occurred in around two dams south east of Lower Sabie Camp. These incidents typically occur in late Summer or Winter during drier years.
The problem appears to be directly related to hippo overcrowding in man-made dams. Hippo dung and urine leads to high level of nitrates and phosphates in the water (a process called eutrophication). Under these conditions the levels of cyanobacteria and their toxic byproducts rocket and they begin to form a floating blue-green scum layer around the edges of the water. This scum is typically concentrated on one side of a dam, depending on the direction of the wind.
Animals that wade into the water beyond the scum line, such as elephants, appear to be less affected, as are buffalo which also wade into the water as a herd and, break the scum layer. Animals that drink on the edge of a dam, such as rhino, wildebeest and zebra, are the more frequent victims.
The current emphasis is to try and understand the causes and risk factors involved in these bio-toxic events. Scientists have started water monitoring projects, aimed at tracking the dynamics of hippos, nutrients and algae for some high risk dams, for which “alert levels” will be developed. Should levels approach those that are considered unsafe for drinking, then management interventions will be considered, which will be tailored to the characteristics of the specific dam.
There are several options for the management of dams with toxic blooms. These include manipulating the water levels of the dams, flushing the dams, breaching the dam wall and draining the dam, followed by scouring out the sediments, burning the veld around the dams to make the area unattractive to herbivores, and manipulating the hippo densities.