Linking fish diversity to primary productivity: Direct and indirect feedback pathways in Lake Tanganyika

Background/Question/Methods

Animals can affect primary producers through both direct and indirect pathways, potentially giving rise to interesting feedbacks between primary and secondary production.  Our ongoing work in Lake Tanganyika, East Africa, compares how grazing fish density and diversity influences benthic algal productivity directly via consumption and indirectly via nutrient recycling.  Grazers often suppress primary productivity by reducing algal biomass, though they can also stimulate new growth and select for vigorous algae.  When nutrients are scarce, primary productivity can also be enhanced when grazers or higher-level consumers are recycling nutrients in readily-available forms.  The balance between these direct (usually negative) and indirect (usually positive) effects of grazers is poorly understood in natural settings.  If positive effects predominate, then there is the potential for positive feedbacks between primary and secondary productivity that could give rise to highly productive systems despite nutrient scarcity.  We tested the elements of this feedback framework using lab experiments, field experiments, and long-term observations along natural gradients of fish biomass and algal productivity.

Results/Conclusions

Our lab experiments show that grazer density has strong positive effects on algal metabolism by mediating nutrient availability; these positive effects exceeded direct negative effects of grazing on algal productivity.  In the field, exclusion experiments provided strong evidence for grazer suppression of algal biomass, but little impact on algal productivity.  Time series and spatial comparisons of fish assemblage composition show that benthic algal productivity is suppressed by high density and diversity of grazing fishes at some sites.  Although these fish assemblages were remarkably stable through time, we observed both natural and anthropogenic range expansions by individual species.  Our results illustrate the potential for contrasting inferences about grazer effects on algal productivity at different spatial scales and levels of realism.  Overall, we find that shifts in both community composition and overall fish density could affect ecosystem productivity, with strong potential for positive feedbacks in Lake Tanganyika.  We conclude that gain or loss of grazers can have profound repercussions for productivity in this and other ecosystems.