Measuring food web indices
We measured allochthony using amino acid specific Carbon-13 profiles (δ13C) (Liew et al., 2019) while TP was measured using amino acid specific Nitrogen-15 profiles (δ15N) (Chikaraishi et al., 2009). Importantly for our study, amino-acid δ13C and δ15N profiles are both unaffected by ethanol or formalin preservation (Hetherington et al., 2019, Chua et al., 2020). Fin clip samples from all 96 suitable specimens (57 from the before period and 39 from the afterperiod) were rinsed in distilled water, dried over 48 hours at 70 °C, homogenised, and packed in 12 mL borosilicate tubes for laboratory analyses at the Stable Isotope Facility at the University of California, Davis. There, amino acid δ13C and δ15N profiles were measured using established protocols (Walsh et al., 2014).
We estimated allochthony using the methods described in Liew et al. (2019). Briefly, essential amino acid (i.e., isoleucine, leucine, phenylalanine, and valine) δ13C profiles of our specimens were compared against a global dataset of carbon source amino acid δ13C values reported in the study (Liew et al., 2019). Given that the essential amino acid δ13C profiles of aquatic and terrestrial carbon sources (Liew et al., 2019) are distinct, we estimated their relative contributions to consumer tissue using Bayesian stable isotope mixing models (Parnell et al., 2013) on the simmr statistical package (Parnell, 2016).
We estimated TP using δ15N primarily, although allochthony(estimated with δ13C) were important for improving the accuracy of our calculations (Choi et al., 2017). We calculate TP using the following formula (after Chikaraishi et al., 2009):
\(TP=\ \frac{\delta 15N_{\text{Glu}}-\delta 15N_{\text{Phe}}+\beta_{\text{mix}}}{\text{ΔD}_{\text{Glu}}-\text{ΔD}_{\text{Phe}}}+1\)…..(eq. 1),
where δ15NGlu and δ15NPherepresent the δ15N profiles of glutamic acid and phenylalanine of the fish specimen, respectively, and ΔDGlu andΔDPhe represent their respective trophic discrimination values, while βmix represents the difference in the δ15NGlu andδ15NPhe in carbon sources (i.e., primary producers) at the bottom of the food web. We usedΔDGlu and ΔDPhe values of +8.0‰ and +0.4‰, respectively (Liew et al., 2019). However, asβmix differs between aquatic (−3.4 ± 0.9‰) and terrestrial (+8.4 ± 1.6‰) carbon sources, we adjusted the value used in our calculations using allochthony estimates on a per-individual basis (after Chua et al. (2020)).