Hippos Cause Lethal Algae Bloom in Man-Made Dams
By Melissa Wray
In the Kruger National Park
A spot of forensic detective work by Kruger National Park (KNP) and state veterinarians has revealed the cause of a number of animal deaths that occurred last winter in the Crocodile Bridge section of the park. Over about six months, at least 52 animals, including two lions, two cheetah and seven white rhino, died mysteriously.
Anthrax and botulism were considered to be possible causes of the deaths, but the first real lead came in May when a fresh zebra carcass was post mortemed. Severe liver damage was spotted, and organs were sent to Onderstepoort's Veterinary Faculty for analysis. The locations of all the dead animals discovered were plotted on a map, revealing two clusters.
The Onderstepoort results showed possible poisoning by algae, and the map pointed the finger at two specific dams in the Crocodile Bridge area - Mpanamana and Nhlanganzwane. Inspection of the dams revealed green scum on the surface of the water, and further tests confirmed that the blue-green algae known as Microcystis was the main reason for the discoloured water. One of the oldest known organisms on earth, blue-green algae or cyanobacteria, were the first green 'plants' on earth and are found in most water bodies in low numbers.
However, they can multiply rapidly under the right conditions to produce algal blooms, great masses of algae that discolour the water. The algae produce strong poisons which can damage the nervous system and the liver of animals.
In the case of an algal bloom, these toxins concentrate in the water at much higher levels than normally occur. The wind plays a major role in concentrating the algae, which blows across the surface of the water, piling up into dense clusters whose location depends on the prevailing wind direction.
In the two Kruger dams, enough algae and poison were present at various times, depending on the weather and the prevailing wind, to kill over 50 animals over a period of months. Although the algae was directly to blame for killing the animals, a variety of factors had combined to create the algal bloom conditions. Both dams are man-made structures, cut off from river systems and depending on rainfall to fill up.
They are also home to large hippo populations, which defecate and urinate in the water, effectively fertilising the water for the benefit of the algae. An uncommonly warm winter, lots of hippos and an ever-decreasing body of stagnant water all combined to give the blue-green algae a chance to multiply frantically and produce their toxic by-products that eventually led to the death of at least ten different species of animals.
Ironically, a hippo was amongst the casualties. Since confirming that the algae were responsible for the mortalities, management options for preventing a repeat of the experience have been investigated.
As similar problems are known to occur in farm dams, existing agricultural solutions were looked at. However, some of these depend on keeping animals from drinking at the dam for six weeks after treatment, not a feasible alternative in Kruger.
Another option would be to manage the size of the hippo population in the dams, or pump out the dams altogether. KNP officials have also considered using floating devices or curtains that stop the algae concentrating on the leeward sides of the dams and reaching poisonous levels.
As the number of animal deaths dropped dramatically after June, following hippo dispersal after fires and over-grazing in the area surrounding the dams reduced the available food, it would appear that the problem is largely confined to the mid-winter months.
Veterinarians have recommended putting into place a monitoring programme that will detect when the algae is approaching toxic levels, and then instituting appropriate management efforts to prevent further toxic tragedies.