An important ongoing debate in ecosystem ecology concerns the origin of the main source of energy for freshwater food webs. Traditionally, allochthonous organic matter (allo-OM) was considered the main energy source for freshwater food webs due to its high input into freshwater ecosystems. More recently, such viewpoint has been questioned, given the low nutritional quality of allo-OM. By contrast, autochthonous organic matter (auto-OM) represents a high quality, but low quantity resource for freshwater consumers, and increasing evidence suggest that auto-OM could be the main source of energy for some freshwater food webs. As a result of this conflicting evidence, the current state of knowledge has made it difficult for ecologists to make general inferences about the main energy pathway in aquatic ecosystems. Here, we conducted a meta-analysis of the relative contribution of allo and auto-OMs to freshwater food webs. We gathered data on the outcome of stable isotope mixing models of published observational studies, which estimated the contribution of allo and auto-OMs to freshwater consumers.
Results/Conclusions
We extracted data from 20 articles, conducted in estuaries, lakes, streams and rivers. Auto and allo contributions were evaluated for 59 aquatic species (vertebrates and invertebrates), from several taxonomic groups and trophic levels. Globally, we found that both OMs sources are equally important food resources for freshwater consumers. Such result highlights that we may not find an overall answer for the allo vs auto debate. Instead, we may consider the regional and local environmental conditions to understand the main energy pathways in freshwater food webs. Recent empirical studies have pointed in the same direction, showing, for example, that canopy openness is an important driver of auto-OM contribution. However, many other important factors, which could explain that pattern (considered as moderators in our meta-analysis) have not been evaluated yet. We predict that factors controlling primary producers biomass (e.g., nutrients concentration and light incidence) could be the key for understanding freshwater consumers’ reliance on auto or allo OMs.