Contributions of autochthonous, allochthonous, and aged carbon and organic matter to macroinvertebrate nutrition in the Susquehanna River basin

Previous research suggests that rivers transport large amounts of aged allochthonous material, but it is unclear how much of this material contributes nutritionally to macroinvertebrate biomass. Furthermore, little research has investigated allochthonous vs autochthonous nutritional resource use by macroinvertebrates in the tributaries and low-order streams of major river systems. The primary goal of this study was to quantify both the sources and ages of organic matter (OM) that contributes to the nutrition of stream macroinvertebrates in different functional feeding groups (FFGs). We used natural abundance stable (delta C-13, delta N-15, and delta H-2) and radiogenic (Delta C-14) isotopes to assess resource use in a network of 7 subwatershed streams in the Susquehanna River basin. Isotope mixing models revealed that autochthonous OM contributed as much as 80% of macroinvertebrate biomass for scraper, filtering collector, and collector-gatherer FFGs, whereas allochthonous sources contributed much less (6-44%) to macroinvertebrate biomass of these FFGs. However, allochthonous OM contributed up to 73% of shredder FFG biomass. Contributions of aged (ranging from 75-11,700 y B.P.) OM to macroinvertebrate biomass varied by macroinvertebrate group. Chironomids assimilated the most aged OM (43%) of any macroinvertebrate group, most of which was from soil and sediment sources. We also found that macroinvertebrates in a stream with active methane seepage had the oldest C in their tissues of the streams in this study (5600-4200 y B.P.), which was probably a result of the macroinvertebrates consuming C derived from this methane seep. Our findings show that the sources and ages of OM assimilated by macroinvertebrates largely varied as a function of FFG and that these differences probably resulted from FFG habitats and diet preferences, as well as the presence or absence of ancient forms of C in streams.