Energy Channel Coupling by Mid-Trophic Level Fish Challenges the Landscape Theory for Food Web Architecture in a...

Landscape theory for food web architecture (LTFWA) posits that the stability of food webs is supported by the coupling of fast pelagic and slow littoral energy channels mediated by large mobile consumers. Empirical tests of the LTFWA are limited, and support for the LTFWA varies among and within aquatic ecosystems. While the LTFWA is thought to be supported in large lake ecosystems, empirical studies have focused on temperate environments with little emphasis on large tropical lake ecosystems. Lake Turkana, a climatically sensitive East African Great Lake, is a highly resilient desert lake and an ideal model for examining how consumer fish species may mediate the coupling of fast and slow energy channels within food webs in a large tropical lake ecosystem. To empirically test the LTFWA, we explored the ecological niche characteristics of five important Lake Turkana fish species including Alestes baremoze, Brachyalestes ferox, Hydrocynus forskahlii, Lates niloticus, and Oreochromis niloticus across ontogeny and used a Bayesian isotope mixing model approach to identify which species play important roles in coupling the fast pelagic and slow littoral energy channels. We reveal that mid-trophic level fish species like A. baremoze, B. ferox, and O. niloticus play important roles in coupling energy channels in Lake Turkana, subverting expectations that upper trophic level consumers H. forskahlii and L. niloticus would couple energy channels the most. We demonstrate that the energy coupling patterns proposed by the LTFWA are not ubiquitously supported across all large lake ecosystems, and small-bodied mid-trophic level fish species are vital for coupling energy channels in Lake Turkana. Thus, fisheries management in Lake Turkana should aim to sustain populations of mid-trophic level species that couple pelagic and littoral energy channels via a multispecies approach that considers interspecific trophic interactions between targeted fish species.