Trophic Diversity in Aquatic Food Webs Along a Tropical Coastal River Continuum in West-Central Africa

In the wet Afrotropics, information on trophic diversity and resource use in freshwaters is extremely limited. Here we used stable isotope ratios (δ13C, δ15N) from food webs along the continuum of a coastal river in the Republic of the Congo to examine seasonal and spatial patterns of trophic diversity. We estimated assemblage-wide isotopic metrics and compared relative proportions of fish trophic positions along a fluvial gradient including a range of stream orders in the dry and wet seasons. Headwater fish assemblages exhibited relatively high trophic niche breadths comparable to those in larger downstream sites, which deviates from the pattern expected based on widely accepted lotic models. Benthic algae was not visibly available at study sites across seasons, and fish, macroinvertebrate, and particulate organic matter δ13C values were consistently depleted (< 25‰), suggesting predominance of forest C3 plants as a basal resource. However, we also observed δ13C values in some fish and macroinvertebrate consumers which were well below riparian or particulate organic matter values, perhaps reflecting methane-derived sources from anoxic detritus. Tropical riverine food webs are more complex than expected, with forested headwaters and swamp forests contributing unique energy pathways for aquatic species. This highlights the critical importance of intact riparian forests in the maintenance of freshwater biodiversity and ecosystem services in the region.