There has been much research to date assessing the contributions of allochthonous and autochthonous carbon (C) and organic matter (OM) to the nutrition of aquatic macroinvertebrates in lotic systems, yet less work that considers the ages of C and OM that are incorporated into lotic food webs. Gaining a better understanding of the ages of C and OM that are assimilated by aquatic consumers is important as it could have implications for the structure and function of aquatic ecosystems. We aimed to determine the relative contributions of allochthonous and autochthonous C and OM to macroinvertebrate biomass in streams of the Susquehanna River watershed, how these contributions varied among study streams, and whether or not aged forms of C and OM were assimilated by macroinvertebrates.
Nutritional resources to macroinvertebrates across seven streams (which varied in stream order and the amounts of watershed agriculture) in a headwater network of the upper and west branches of the Susquehanna and Chemung River basins, Pennsylvania, were assessed using stable isotopes (δ13C, δ15N, and δ2H) and radiocarbon (∆14C). Isotopic signatures of macroinvertebrates and their potential nutritional resources were used to quantify the sources and ages of C and OM contributing to macroinvertebrate biomass.
Results/Conclusions
Autochthonous OM contributed more (44-89%) than allochthonous OM to macroinvertebrate biomass of multiple functional feeding groups across most sites, with the exception of some predators and shredders in lower order streams that were more reliant on allochthonous OM. Allochthonous contributions to macroinvertebrate nutrition, while generally secondary to autochthonous sources, were still significant (up to 85%), particularly to the shredder functional feeding group. Allochthonous OM contained in soils and sediments contributed more (up to 48% combined) than fresh terrestrial vegetation to macroinvertebrate biomass in high-agriculture streams. Macroinvertebrate ∆14C varied widely across sites (-405‰ to -0.7‰; 4,170 years BP to modern in equivalent 14C age), suggesting variable utilization of aged C and OM by macroinvertebrates. Equivalent 14C ages of macroinvertebrates collected from Little Muncy Creek in 2011 when methane seepage was present were the oldest of all organisms measured in this study. Utilization of nutritional resources by macroinvertebrates was influenced in a predictable manner by stream characteristics such as canopy cover, and human activities within the watershed also increased the assimilation of aged C and OM by macroinvertebrate consumers.