Energy and nutrient dynamics of rock-dwelling cichlids grazing on epilithic periphyton of the littoral zone of Lake Malawi, Africa

摘要

Periphyton primary production rates of ～1 g C m~(-2) d~(-1) in the rocky littoral zone of Lake Malawi are among the highest rates of benthic primary production reported in fresh water systems (Bootsma 1993, Bootsma Hecky 2001) and are comparable to coral reef ecosystems. Despite high primary productivity and substantial nitrogen (N) and phosphorus (P) uptake by the Lake Malawi periphyton (Andre et al. 2003), there is no evidence for the accumulation of epilithic periphyton biomass. This implies that loss of organic matter from periphyton through respiration, organic compound release and grazing must balance these high periphyton growth rates. Of these loss processes, only grazing can efficiently lead to high rates of fish production. A dense and stable cichlid fish community (up to 10 fish m~(-2)) that feeds directly on periphyton characterizes the littoral zone of Lake Malawi (Bootsma et al. 1996, Ribbink et al. 1983). Consumption by these fish likely accounts for a major portion of organic matter losses from the periphyton. Fryer (1959) proposed that the existence of this unusual diversity of fishes sharing limited space is made possible by a high periphyton productivity that provides an abundant source of nutrition, thus reducing food resource competition. We tested the hypothesis that the fish community, due to its density and stability, mediates the main loss flux of nutrients from the periphyton community by consuming a major proportion of periphyton production. Furthermore, we propose that the fish community is able to derive most of its nutritional requirements from periphyton consumption, thus maintaining its current density.