Environmental factors affecting incorporation of terrestrial material into large river food webs

Three enduring conceptual models make predictions regarding the basal production sources supporting the upper food web of rivers: the River Continuum Concept, the Flood Pulse Concept, and the Riverine Productivity Model. Sources of primary production supporting metazoan biomass might best be understood by using a pluralistic approach that views basal sources as a dependent variable and key physicochemical and hydrological factors as independent variables. Here, I review studies in which chemical markers (stable-isotope and fatty-acid analyses) were used to estimate dominant primary producers contributing to consumer biomass in large rivers and evaluate associated independent variables (e.g., hydrologic regime, turbidity, concentration of dissolved organic matter, floodplain vegetation, lateral connectivity, and upstream impoundment) that have been hypothesized to contribute to variation in basal production sources. My review shows that C-4 grasses rarely support riverine metazoans and that algae are the predominant source of C supporting upper trophic levels of large rivers worldwide. However, I also found that many consumers assimilate material from C-3 plants in rivers with high sediment loads and low transparency during high-flow pulses. Exceptions to this pattern occur when river reaches are downstream from an impoundment, in which case, algae assume greater importance. Terrestrial C-3 plants also subsidize consumers in rivers with high dissolved organic matter concentrations via the microbial loop and, in other rivers, after periods of high discharge or leaf fall that increase the amount of terrestrial material in the particulate organic matter pool. I highlight the natural causes of differences in turbidity and dissolved organic matter among large rivers and the importance of transported materials as a source of nutrients for ecologically and economically important fish species.