Are the energy fluxes through fish food webs in light-limited lagoons dependent on detritus?

It has been proposed that light-limited lakes are characterized by the constraint of the autotrophic pelagic flux (the green pathway) and by the dominance of the detritus-based heterotrophic flux (the brown pathway). However, studies explicitly testing this assumption are scarce in natural food webs. Here, we estimated energy flux in a dystrophic light-limited shallow lagoon to investigate the importance of green vs. brown energy pathways to fuel the freshwater fish food web. Over the course of 1 year, we estimated the energy flux intake for five fish trophic guilds (detritivores, top carnivores, meso-carnivores, herbivores, and omnivores) monthly. We found a high concentration of energy flux in the detritivore compartment, accounting for > 93% of the total energy flux through the food web. A single detritivore species (Cyphocharax voga) dominated the fish community in terms of biomass stock and number of individuals. There was a substantial increase in the detritivore energy flux during warm months. These results illustrate a remarkable dominance of brown pathways, which support the fish trophic energy flux in this kind of system. The flux magnitude was related to temperature, highlighting the synergy between warming and brown pathways.