Constructing Isoscapes of δ13C, δ15N and δ34S Baselines Within a River System: A Spatial Stream Network Modelling Approach

Abstract

1. Stable isotopes of carbon (δ¹³C), nitrogen (δ¹⁵N), and sulphur (δ³⁴S) provide insights into freshwater food webs, but spatial variation in values of organisms at lower trophic levels often complicate interpretations. This is especially true in river ecosystems, in which hierarchical networks, directional flows, and aquatic-terrestrial linkages shape the ecosystem processes that determine the isotopic composition of bioavailable molecules at the base of food webs. To advance understanding of spatial variation in these natural tracers at or near the base of river food webs, we fit predictive spatial models relating the δ¹³C, δ¹⁵N and δ³⁴S of epibenthic biofilms and macroinvertebrate taxa to landscape variables within a north-temperate river ecosystem.
2. Stable isotope ratios were determined in biofilms (δ¹⁵N and δ¹³C) and three macroinvertebrate taxa (δ¹⁵N, δ¹³C and δ³⁴S) commonly used as baselines for isotopic food web analyses (heptageniid mayflies, hydrobiid snails, amphipod crustaceans) across 30 sites in the Boardman/Ottaway River (BOR), Michigan, USA, in the spring of 2022. Spatial stream network implementations of linear mixed-effects regression models (SSNLMM) were used to estimate land cover covariation with the isotopic composition of biofilms (δ¹³C and δ¹⁵N) and macroinvertebrates (δ¹³C, δ¹⁵N and δ³⁴S), and to generate isoscapes for each element across the river network.
3. Spatial patterns in δ¹³C and δ³⁴S were similar across macroinvertebrate taxa, whereas biofilm and macroinvertebrates differed substantially in δ¹³C. Patterns in δ¹⁵N were less consistent, associated most often with forest, wetland, and crop land covers. However, spatial and aspatial errors generally accounted for more variance than fixed effects.
4. Variation in δ¹⁵N was 2‰–6‰ in biofilms and 2‰–9‰ in macroinvertebrate taxa, indicating that these macroinvertebrates are primary consumers or detritivores. Isoscapes predicted higher δ¹³C for biofilms (−28‰ to −17‰) than for macroinvertebrates (−39‰ to −25‰), indicating that macroinvertebrates consumed non-biofilm food resources, although the difference between macroinvertebrate and biofilm δ¹³C varied across the river system. Isoscapes predicted variation in Heptageniidae δ³⁴S from −5‰ to 0‰, although predictive performance was weak, possibly due to small sample size. These mayflies were strongly ¹³C-depleted relative to biofilms and to the expected δ¹³C of terrestrial inputs in areas of the BOR that also tended to be less ³⁴S-depleted, suggesting the assimilation of material produced via an alternative energy flow pathway.
5. SSNLMMs provided novel insights into the type and strength of land cover drivers of δ¹³C, δ¹⁵N and δ³⁴S at fine spatial scales for basal food web taxa in temperate rivers. Together, isoscape maps and inferences about landscape influences help explain spatial and taxonomic variation in isotope baselines, facilitating the assessment of river food web responses to environmental change.