Some Effects of Annual Fluctuations in Water Level on the Larval Chironomid Communities of Lake Kariba

Abstract

Lake Kariba, lying in the middle Zambesi Valley between Zambia and Rhodesia, is 280 km (175 miles) long, and has a surface area of 440 000 ha (2000 square miles). Its history falls into two well-defined phases. The first phase covered the period December 1958September 1963 during which the lake was filling. The second phase has been characterized by an annual fluctuation in water level of as much as 8 m/year. At the end of the dry season, October or November, the lake level is lowered to allow for incoming flood waters; the latter produce a rise in water level which usually continues until July. Plant and animal invasions and population explosions were a conspicuous feature of the initial phase (Balinsky & James 1960; Harding 1964, 1966; Mitchell 1965). The water was relatively rich in nutrients (Harding 1964), presumably released from the flooded biota, and to a lesser extent, from soils of the Zambesi Valley. When flooding ceased and no fresh material was inundated, this source of nutrients was no longer available. The remaining sources-the affluent rivers and the rocks of the lake basin (mainly sediments of the Upper Karroo Series)-can be expected to contribute very little (Mitchell & Bond, unpublished), and a lowered nutrient content of the water therefore characterized the second phase. It was during this period that the data presented here were gathered. There are two separate aspects to a fluctuation in water level. There is obviously a change in water depth over any fixed point on the mud's surface. There is also a special set of conditions created by the migration of the shore-line, and attention was focused particularly on this aspect. In order to obtain the maximum possible horizontal movement of the water's edge for any given change in lake level, an area with a gradually shelving bottom was required, if possible with a minimum of wave action (McLachlan 1968). The area selected, Sengwa West A tree-cleared area, provided both these conditions, and was conveniently close to the Nuffield Lake Kariba Research Station (Fig. 1).