Influence of land use and lithology on sources and ages of nutritional resources for stream macroinvertebrates: a multi-isotopic approach

Abstract

Terrestrially-derived carbon (C) and organic matter (OM)—often of significant age—dominate in many streams and rivers, yet little is known about their potential nutritional contributions to aquatic macroinvertebrate consumers. Impacts of watershed characteristics (e.g., land use and lithology) on the sources and ages of C and OM utilized by aquatic consumers are also poorly understood. To assess these factors, macroinvertebrates were collected from six headwater streams having different watershed lithologies and land uses in the Hudson-Mohawk River system (New York, USA) and analyzed for natural δ13C, δ15N, δ2H, and ∆14C. A Bayesian stable isotopic mixing model revealed that autochthonous primary production dominated (62–92%) the biomass of all functional feeding groups (FFGs) across all sites, with allochthonous sources being of secondary but still significant (21–31%) importance. Macroinvertebrates collected from streams in watersheds having low vs. high agricultural land use were estimated to assimilate 0–13 and 4–31% soil-derived C and OM, respectively. ∆14C values and apparent ages of macroinvertebrates from shale-rich and shale-poor sites were also significantly different (mean ∆14C = −75 and −34‰; equivalent 14C ages = 630 and 280 years B.P., respectively). Inclusion of ∆14C data in mixing models confirmed the importance of autochthonous primary production, and also demonstrated indirect lithological control of nutritional resource utilization by influencing stream substrate type and potential retention of allochthonous C and OM. Findings from this study further showed that the relative magnitudes of autochthonous vs. allochthonous contributions to macroinvertebrates were dependent on FFG, land use type, and lithology.