Food Web Complexity in Freshwater Systems

Abstract

Freshwater food webs -- the networks of trophic interactions linking primary producers to apex predators in lakes, rivers, and wetlands -- determine how energy and nutrients flow through freshwater ecosystems, regulate biodiversity through predation and competition, and provide the mechanistic basis for understanding ecosystem responses to stressors. Despite their ecological centrality, quantitative food web data -- full trophic interaction matrices specifying who eats whom at what rate -- remain scarce for freshwater systems outside a handful of intensively studied benchmark lakes and rivers. This study presents the most comprehensive quantitative freshwater food web analysis yet conducted, combining stable isotope trophic analysis (delta13C and delta15N), gut content analysis, and DNA metabarcoding of stomach contents for 247 freshwater food webs across 84 lake, river, and wetland systems in 18 countries. Food web complexity (connectance C; number of trophic links relative to species richness) was positively correlated with ecosystem productivity (partial R2 = 0.54; p < 0.001) and negatively correlated with human modification (R2 = 0.38; p < 0.001). Food web robustness -- the fraction of primary extinctions before the food web collapses -- was highest in complex webs (r = 0.74; p < 0.001). Stable isotope analysis revealed mean trophic level compression in disturbed systems (mean 3.84 vs. 4.47 trophic levels in undisturbed reference systems), indicating loss of apex predator trophic structure from human impacts