NUTRIENTS IN AFRICAN LAKE ECOSYSTEMS: DO WE KNOW ALL?

SUMMARY

It is nearly sixty years since the first studies were undertaken on the nutrient chemistry of African lakes. There have been numerous studies on the chemical composition of African waters in the intervening years. Yet as recently as five years ago it was stated that little was known about nutrient cycling in lakes. Nutrient ‘species’ simply formed an additional list compiled along with lists of species of algae, zooplankton, aquatic plants and fish. A spate of monographs, papers and reports in recent years, however, has begun to fill some of the gaps in our knowledge of nutrient cycling in African lake ecosystems. This paper reviews the recent literature of nutrients in African lakes from the point of view of nutrient sources, in-lake nutrient kinetics and nutrient sinks, with particular reference to nitrogen and phosphorus, and their cycling rates into and out of various biotic and abiotic compartments. The principle conclusions that can be drawn from the review are as follows:

1. Allochthonous inputs, particularly in terms of external nutrient loading, have been relatively well-studied in recent years, but little is known of autochthonous nutrient inputs, despite numerous observations that nutrient regeneration is likely to be substantial in African lakes.

2. The rôle of microbial processes in nutrient cycling in African lakes is almost totally unknown, except in relation to nitrification and denitrification.

3. Recent studies have begun to examine the kinetics of the uptake of phosphorus by algae in African lakes; nitrogen uptake, other than nitrification, and nutrient release have only rarely been examined.

4. Nutrient uptake and release by aquatic macrophytes is reasonably well known, especially in the ‘nuisance’ weed species.

5. The role of zooplankton, zoobenthos and fish in nutrient cycling in African lakes has largely been ignored.

6. A great deal of study has been devoted to the rôle of abiotic process, particularly at the sediment-water interface, in the nutrient dynamics of African lakes.

As yet, little emphasis has been placed on inter-compartmental exchanges in studies on nutrient cycles but it would appear that these processes are now beginning to receive attention and, as nutrient cycling in African lake ecosystems becomes better known, research will tend toward a more numerical approach.

KEYWORDS:

• Nutrients
• nitrogen
• phosphorus
• African lakes
• nutrient cycling
• inputs
• outputs
• biotic interactions
• abiotic interactions