Benthic nutrient cycling, the role of fish in nitrogen and phosphorus regeneration in the rocky littoral zone of Lake Malawi/Nyasa/Niassa, Africa

Abstract

The interaction between the cichlid fishes of the nearshore rocky littoral zone of Lake Malawi with the pilithic periphyton community is addressed in this study, looking specifically at fish grazing rates and importance of the fish community in recycling of nitrogen and phosphorus through excretion, defecation and mortality.

To determine rates of nitrogen and phosphorous regeneration through excretion and defecation, a series of in situ fish incubations in 13L plexiglass chambers were performed at two islands of Lake Malawi. Fish grazing rates were assessed by determination of fish consumption rates using Kraft's (1992) bioenergetics model where defecation and excretion rates were derived from the in situ experiments and growth rates were determined in the lab by rearing 30 fish specimens for 22 days under controlled conditions. Mortality was assessed by the assumption that it is equal to growth as fishing is negligible and there is no long-term fish biomass change in the nearshore rocky littoral zone of Lake Malawi.

This study provides evidence that rock dwelling cichlids, through excretion and defecation, recycle substantial amounts of nitrogen and phosphorous that can support measured rates periphyton photosynthesis. However the uncertainty about the N and P composition of the living algae hamper assessment of whether N and P recycled by the rocky dwelling cichlids meet periphyton demand for these nutrients. This study also provides evidence that fish may be important in structuring the epilithic periphyton by recycling N and P at low ratios, and therefore favouring dominance of nitrogen-fixing blue-green algae species in the periphyton community. Estimates of consumption rates indicate that the fish community is a major grazer of the periphyton community as about 70% and 103% respectively of nitrogen and phosphorus uptake by periphyton is consumed by fish. Results also suggest that periphyton production can meet the nutritional requirements of the fish community. This observation supports the hypothesis put forward by Fryer (1959) that co-existence of many species in high densities is due to high availability of food (periphyton).