Spatial variation in aquatic food webs in the Amazon River floodplain

摘要

Food webs are spatially variable and temporally dynamic in heterogeneous and species-rich river floodplains. However, empirical evidence that shows how food webs vary across landscapes and scales in river-floodplain ecosystems is limited, especially in the tropics. Here, we evaluate how the flow of energy and matter varies among food webs in aquatic habitats and across scales in the lower Amazon River floodplains. We surveyed 109 habitats across 19 floodplain units (lake systems) and analyzed the isotopic composition of primary production sources and fish tissues to estimate relative contributions of these sources to fish biomass at local and regional scales. Basal production sources and fish species each varied considerably in their carbon and nitrogen isotopic ratios across the floodplain landscape. Aquatic macrophytes and suspended particulate organic material in the water column were inferred to be the principal basal sources contributing to the biomass of most fish species at the regional scale. However, the estimated contribution of different production sources to fish biomass varied, on average, by similar to 40% across lake systems. The sources estimated to contribute most to fish biomass at the regional scale were sometimes unimportant in certain species and lake systems. Conversely, the least important sources at the regional scale were sometimes very important at the local scale. Spatial variation in the isotopic composition of production sources and fishes, and the proportional contributions of sources to fish biomass in the Amazon River floodplain, are probably influenced by multiple factors including variation in the quality and quantity of basal sources. Future stable isotope investigations of aquatic food webs of river-floodplain systems should consider not only suitable replication and appropriate temporal scale, but also spatial scale.