On-Line Supplemental Material

On-line Supplemental Material

Stable isotopes and Digital Elevation Models to study nutrient inputs in high- Arctic lakes

Edoardo Calizza1*, Maria Letizia Costantini1, David Rossi2, Vittorio Pasquali3, Loreto Rossi1

1Laboratory of Trophic Ecology, Department of Environmental Biology, Sapienza University of Rome, Rome (Italy).

2National Research Council, Institute for Water Research (CNR-IRSA), Monterotondo, Rome (Italy).

3Department of Psychology-Neurosciences section, Sapienza University of Rome, Rome (Italy).

*Corresponding Author: Laboratory of Trophic Ecology, Department of Environmental Biology, Sapienza University of Rome, Via dei Sardi 70, 00185 Rome (Italy). e-mail: [email protected] ; Phone: +390649917803.

Appendix 1

When a significant correlation between isotopic signatures (δ13C and δ15N) and C% or N% in sediments was found at the lake level (i.e. considering the replicate samples collected within each lake), the observed linear regression model was used to extrapolate expected C% and N% across a broader range of isotopic values than that observed in samples. This allowed the modeling of expected variations in the C to N ratio (C/N) in sediments following changes in their isotopic composition, the latter being indicative of a relative increase/decrease in the contribution of terrestrial/aquatic production (δ13C) or guano from aquatic birds (δ15N) to nutrient inputs in lakes

(Blais et al. 2005; Rossi et al. 2010). Then, for each lake, the best model of fit for the regression of

C/N vs δ13C (Tvillingvatnet; Storvatnet) or δ15N (Kolhamna) was selected basing on the AIC criterion to describe the central tendency of expected variations in C/N and associated 95% confidence intervals (Costantini et al. 2014) (Fig. S1).

Basing on model outputs, we expect differences between study lakes as regard the direction and magnitude of changes in the C/N in sediments following changes in their isotopic composition.

In Tvillingvatnet, a potential increase in terrestrial-derived inputs is expected to lead to increasing

1 C/N, with implications for stoichiometric imbalance and trophic interactions between basal resources and consumers in the detritus food web (Hladyz et al. 2009, Costantini et al. 2014, Rossi et al. 2015). On the other hand, increasing organic deposition belonging to aquatic production in

Storvatnet is expected to lead to decreasing C/N, suggesting a positive effect on the quality of the resource available to the invertebrate fauna (e.g. Rossi et al. 2015). Lastly, in Kolhamna, while a potential increase in guano inputs from birds (indicated by an increase in the δ15N, Blais et al. 2005) is expected to lead to increased carbon and nitrogen concentrations (see table 2 in the main text), it could have only a marginal effect on their absolute ratio, with an expected change of C/N from a mean (± standard error) observed value of 18.8±0.9, to a minimum modelled value of around 15.5, independently by the simulated sequential increase in δ15N.

Tvillingvatnet Storvatnet Kolhamna N / C

δ13C (‰) δ13C (‰) δ15N (‰)

Fig. 5 Modelled variations in the carbon to nitrogen ratio (C/N) in sediments (central tendency: red line; ±95% confidence interval: blue lines) in three high-Arctic lakes in North Spitzbergen (Svalbard), as calculated basing on observed linear relationships between both C and N relative content and δ13C (Tvillingvatnet and Storvatnet) or δ15N (Kolhamna) in organic deposits in sediments. The red dot indicates the observed mean value as obtained with six replicate samples per each study lake. The red arrows point to the expected change in isotopic composition of sediments following a climate-change related increase in organic inputs of terrestrial (Tvillingvatnet), aquatic (Storvatnet), or animal (i.e. guano input from aquatic birds) origin (Kolhamna) (Vincent et al. 2008; Post et al. 2009).