Biogeochemical processes create distinct isotopic fingerprints to track floodplain rearing of juvenile salmon

Abstract

Floodplains represent critical nursery habitats for a variety of fish species due to their highly productive food webs, yet few tools exist to quantify the extent to which these habitats contribute to ecosystem-level production. Here we conducted a large-scale field experiment to characterize differences in food web composition and stable isotopes (δ13C, δ15N, δ34S) for salmon rearing on a large floodplain and adjacent river in the Central Valley, California, USA. The study covered variable hydrologic conditions including flooding (1999, 2017), average (2016), and drought (2012–2015). In addition, we determined incorporation rates and tissue fractionation between prey and muscle from fish held in enclosed locations (experimental fields, cages) at weekly intervals. Finally, we measured δ34S in otoliths to test if these archival biominerals could be used to reconstruct floodplain use. Floodplain-reared salmon had a different diet composition and lower δ13C and δ34S (δ13C = -33.02±2.66‰, δ34S = -3.47±2.28‰; mean±1SD) compared to fish in the adjacent river (δ13C = -28.37±1.84‰, δ34S = +2.23±2.25‰). These isotopic differences between habitats persisted across years of extreme droughts and floods. Despite the different diet composition, δ15N values from prey items on the floodplain (δ15N = 7.19±1.22‰) and river (δ15N = 7.25±1.46‰) were similar, suggesting similar trophic levels. The food web differences in δ13C and δ34S between habitats were also reflected in salmon muscle tissue, reaching equilibrium between 24–30 days (2014, δ13C = -30.74±0.73‰, δ34S = -4.6±0.68‰; 2016, δ13C = -34.74 ±0.49‰, δ34S = -5.18 ±0.46‰). δ34S measured in sequential growth bands in otoliths recorded a weekly time-series of shifting diet inputs, with the outermost layers recording time spent on the floodplain (δ34S = -5.60±0.16‰) and river (δ34S = 3.73±0.98‰). Our results suggest that δ13C and δ34S can be used to differentiate floodplain and river rearing habitats used by native fishes, such as Chinook Salmon, across different hydrologic conditions and tissues. Together these stable isotope analyses provide a toolset to quantify the role of floodplains as fish habitats. Show more