Structural differences between temperate and tropical freshwater food webs

Food web structure (FWS) has been a fundamental pillar of ecology for at least a century. Despite its importance, FWS continues to be cited as a critical knowledge gap hindering our understanding of freshwater ecosystem function. This is primarily because environmental determinants of FWS remain poorly understood, which prevents general conclusions about unstudied communities. We empirically investigated broadscale environmental patterns of FWS in oxbow lakes and wadeable streams in western Canada and Amazonia (Ecuador and Brazil). FWS was measured via stable carbon and nitrogen isotopes and included metrics of maximum trophic position (δ 15 N range and maximum food chain length), primary resource breadth (δ 13 C range), community niche area (CNA), and fish community size structure (mean change in δ 15 N or trophic position per increase in 100 mm total body length). Whether and how food webs differed from each other depended on biogeographic region and habitat type. In western Canada, streams had higher resource breadth and greater CNA than lakes, but in Amazonia, lakes had higher resource breadth, longer food chains, and greater CNA than streams. Primary productivity correlated negatively with all isotopic FWS metrics in streams, suggesting that stream FWS depends strongly and inversely on resource density at a transcontinental scale. However, no such relationships between FWS and primary productivity were observed in lakes. Furthermore, all FWS metrics showed no clear relationships with ecosystem size, total resource availability, or disturbance regime. Although FWS differed greatly among sites, specific environmental factors that drive these differences remain mostly elusive.