Cryptic chemoautotrophic and methanotrophic processes complicate the use of carbon stable isotopes in rivers

Amanda Gay DelVecchia 

The use of carbon stable isotopes has contributed to many important discoveries regarding the base of consumer production in freshwater ecosystems. There is increasing recognition for the prevalence of anoxic conditions and the contributions of methane-derived carbon to freshwater food webs, highlighting the potential for methanotrophy and chemoautotrophy to complicate interpretations of δ13C values in consumer biomass. These reduced substrates are sometimes cryptic, or present at such low concentrations that they might not provide an obvious carbon source, especially in rivers which are typically conceptualized as well-oxygenated. At most, chemoautotrophy and methanotrophy might be important, yet overlooked, carbon sources and at least, even a small amount of these isotopically lighter contributions could complicated assessments of basal carbon sources to river food webs. An improved application of carbon-based stable isotope mixing models to freshwater ecosystems, and subsequent inferences of dietary resources, is limited by three major assumptions reviewed herein. These assumptions include: representation of dietary end members using bulk sampling of organic matter sources, estimation of fractionation rates associated with microbial oxidation reactions, and the decision of whether or not to include proxies for the heavily fractionated biomass of chemoautotrophs and methanotrophs. With improved recognition and continued study of these assumptions, the conceptualization of basal carbon sources can be expanded to include the redox complexity of both lentic and lotic systems, and could expand how we conceptualize the boundaries of river food webs. This has implications for recognition and management of habitat heterogeneity in freshwaters.