William R.T. Darwalla,∗, Edward H. Allisonb, George F. Turnerc, Kenneth Irvine d
Ecosystem-focused models have, for the first time, become available for the combined demersal and
pelagic components of a large tropical lake ecosystem, Lake Malawi. These provide the opportunity to
explore continuing controversies over the production efficiencies and ecological functioning of large
tropical lakes. In Lake Malawi these models can provide important insight to the effect of fishing on fish
composition, and the potential competition that the lakefly Chaoborus edulis may have with fisheries
production. A mass-balanced trophic model developed for the demersal fish community of the southern
and western areas of Lake Malawi was integrated with an existing trophic model developed for the openwater
pelagic. Input parameters for the demersal model were obtained from a survey of fish distributions,
fish food consumption studies, and from additional published quantitative and qualitative information
on the various biotic components of the community. The model was constructed using the Ecopath
approach and software. The graphically presented demersal food web spanned four trophic levels and
was based primarily on consumption of detritus, zooplankton and sedimented diatoms. Zooplankton
was imported into the system at trophic levels three and four through fish predation on carnivorous and
herbivorous copepods and Chaoborus larvae. It is proposed that the primary consumption of copepods
was by fish migrating into the pelagic zone. Chaoborus larvae in the demersal were probably consumed
near the lakebed as they conducted a daily migration from the pelagic to seek refuge in the sediments. This
evidence for strong benthic–pelagic coupling provided the opportunity for linking the demersal model to
the existing model for the pelagic community so producing the first model for the complete ecosystem.
Energy fluxes through the resulting combined model demonstrated that the primary import of biomass
to the demersal system was detritus of pelagic origin (72.1%) and pelagic zooplankton (10.6%). Only
15.8% of the biomass consumed within the demersal system was of demersal origin. Lakefly production
is efficiently utilised by the lake fish community, and any attempt to improve fishery production through
introduction of a non-native plantivorous fish species would have a negative impact on the stability and
productivity of the lake ecosystem.
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