Sources and Ages of Carbon and Organic Matter Supporting Macroinvertebrate Production in Temperate Stream

Stable isotopes (delta13C, delta15N, and delta2H) have been employed frequently in aquatic food web and ecosystem studies to quantitatively assess contributions of allochthonous (i.e., derived from outside of aquatic systems) and autochthonous (i.e., derived from within aquatic systems) organic matter (OM) to the biomass and secondary production of aquatic consumers. However, there has been far less research to date on assessing the ages of carbon (C) and OM from different sources that are utilized by aquatic consumers and incorporated into aquatic food webs. Natural abundance radiocarbon (14C) has been employed less frequently in food web and ecosystem studies, even though it has a far greater dynamic range, and allows for potentially greater differentiation between potential nutritional resources than stable isotopes alone. Natural 14C also uniquely allows for determination of the ages of the nutritional resources that contribute to aquatic consumer biomass. In the present study, natural stable and radiocarbon isotopes were measured to assess contributions of allochthonous and autochthonous C and OM of varying age to the biomass of aquatic macroinvertebrates in a number of temperate streams in sub-watersheds of the Mohawk-Hudson River, New York (USA); Susquehanna River, Pennsylvania (USA); and the Ohio River, Ohio (USA). The overarching goals of the study were to i) estimate allochthonous and autochthonous nutritional contributions to aquatic macroinvertebrate consumers in these systems, ii) determine relative contributions of C and OM resources of varying ages to macroinvertebrate nutrition, and iii) assess how nutritional resource utilization varied as a function of stream physical characteristics, watershed land use and lithology, and temporal variability.;Aquatic macroinvertebrates collected from the majority of streams studied were found to be primarily reliant on autochthonous OM. However, allochthonous OM in the forms of terrestrial vegetation and soil/sediment OM also made significant contributions to macroinvertebrate biomass. Agricultural activity in the study watersheds also appeared to increase the utilization of soil/sediment-derived OM by macroinvertebrates, as well as elevate the delta15N of algal biomass and macroinvertebrate consumers. The delta 14C values and ages of macroinvertebrates varied widely across sites and sampling times (-499 per mil to 23 per mil; 5,550 years B.P. equivalent age to modern-aged, respectively). The apparent 14C ages observed for many of the macroinvertebrates measured were attributable to some combination of their consumption and utilization of i) truly aged soil/sediment OM and ii) apparently aged algae that fixed aged dissolved inorganic carbon (DIC).;Findings from this study as well as a comprehensive review and synthesis of studies that have used natural abundance 14C as a tracer in inland aquatic food webs found that in the majority of systems one or more forms of aged C and/or OM contributed to the biomass of aquatic consumers. This suggests that geologically aged forms of C and OM contribute to modern-day aquatic food webs. Improved knowledge of the ages of C, OM, and consumers in inland water and other aquatic food webs could have implications for our understanding of terrestrial-aquatic linkages and the role(s) that the ages of nutritional resources may play in aquatic food web and ecosystem structure and function.