Carbon sources supporting large river food webs: a review of ecological theories and evidence from stable isotopes

Quantifying the contributions of carbon sources that support food webs in large rivers is an important and growing field of ecological research with implications for future management and rehabilitation. Here we review theoretical concepts and recent empirical evidence that address carbon flow through aquatic food webs in large rivers.  The literature reviewed focuses on studies using stable isotope analysis, which is a tested framework for identifying the origin of carbon sources that are assimilated by primary consumers and subsequently transferred through the food web to support higher consumers. Theoretical concepts addressing carbon flow in large river food webs have tended to stress the importance of organic matter originating from different sources, such as floodplains (Flood Pulse Concept), local riparian and aquatic primary producers (Riverine Productivity Model), or leakage from upstream processing of terrestrial organic matter (River Continuum Concept).  Recent empirical evidence from a range of studies has highlighted the importance of autochthonous carbon, especially in the form of benthic algae and phytoplankton, to food webs in a variety of large rivers.  However, some flexibility is apparent within food webs and several studies have identified a range of secondary carbon sources that can also be consistently important, depending on the temporal and spatial patterns of hydrogeomorphic conditions.  The geographic spread of studies addressing carbon flow in large river food webs is steadily increasing, although long term data sets remain sparse.  Despite this, opportunities exist to improve our understanding of historical changes in river food webs and to develop predictive models of future responses to environmental change through the use of museum collections and rehabilitation case-studies