Defining drivers of nitrogen stable isotopes (δ15N) of surface sediments in temperate lakes

The nitrogen stable isotopic signature (delta N-15) of sediment is a powerful tool to understand eutrophication history, but its interpretation remains a challenge. In a large-scale comparative approach, we identified the most important drivers influencing surface sediments delta N-15 of 65 lakes from two regions of Canada using proxies that reflect watershed nitrogen (N) sources, internal lake microbial cycling and productivity. Across regions, we found that water column total nitrogen (TN), %N in the sediments and lake morphometric variables were the best predictors of sedimentary delta N-15, explaining 66 % of its variation. Significant relationships were also found between sediment delta N-15 and human-derived N load ( = 0.23, p < 0.001), the latter being a strong predictor of TN ( = 0.68, p < 0.001). Despite a relatively strong overall relationship, variation partitioning revealed an interesting difference in the dominant variable that influenced regional delta N-15. Alberta lake sedimentary delta N-15 signature was dominated by human derived N load. In contrast, internal processing appeared to be more important in Quebec lakes, where sediment delta N-15 was best explained by %N in the sediments and lake volume. Overall, our findings support the use of delta N-15 in paleolimnological investigations to reconstruct changing N sources to lakes but also highlight that regions may have distinctive drivers. Interpretations of sediment delta N-15 are likely to be strongest when multiple lines of evidence are employed and when placed in a regional context.