ABSTRACT
ABSTRACT Lake Chivero is a eutrophic manmade lake situated near Harare, Zimbabwe, which receives sewage effluent discharged into its tributaries. The first evidence of eutrophication appeared in 1960 and the water quality deteriorated steadily until 1970, when a temporary improvement was brought about by the use of sewage effluent for irrigation. From 1980 onwards, water quality began to decline again because of the growth of Harare's population and a corresponding increase in sewage effluent. This decline was masked for a period by a major infestation of water hyacinth, which evidently removed nutrients from the water. This plant has been a problem on the lake because of the several major outbreaks in the fifty years, since its creation although biological control has now been effective in reducing its impact. Permanent blooms of blue-green algae have led to very high levels of primary productivity, now limited by reduced light penetration, while increased levels of organic matter in the hypolimnion have led to a strong oxycline, which would probably be
absent if the lake was oligotrophic, owing to its weakly-developed thermocline. This has allowed ammonia-which has been implicated in fish kills-to accumulate in the hypolimnion. Zooplankton abundance is considerably greater than it is in oligotrophic Zimbabwean waters. The effect of eutrophication on the benthos is less clear since the impact of water level fluctuations is marked, but there does appear to have been an increase in molluscs and a change in the species composition of the oligochaetes over the last thirty years. Fish production is high and dominated by cichlid fishes that are able to utilize blue-green algae, but fish kills caused by deoxygenated water do occur from time to time. A major fish kill in March 1996 affected only one species, Oreochromis macrochir, and may have been caused by a combination of high ammonia levels, low dissolved oxygen at night and, possibly, algal toxins. The early control measures, which involved the use of sewage effluent for irrigation, have failed to cope with the increased quantity of sewage effluent and the lake continues to be eutrophic. An economic collapse in Zimbabwe has left the autorities without resources to invest in the treatment of sewage or control of nutrients and there is little prospect of the situation improving in the near future.
