Carbon and nitrogen stable isotope ratios in surface sediments from lakes of western Ireland: implications for inferring past lake productivity and nitrogen loading

We used statistical analyses to determine which subset of 36 environmental variables best explained variations in surface sediment δ¹³C and δ¹⁵N from 50 lakes in western Ireland that span a human-impact gradient. The factors controlling lake sediment δ¹³C and δ¹⁵N depended on whether organics in the lake sediment were mostly derived from the lake catchment (allochthonous) or from productivity within the lake (autochthonous). Lake sediments with a dominantly allochthonous organic source (high C:N ratio sediments) produced δ¹³C and δ¹⁵N measurements similar to values from catchment vegetation. δ¹³C and δ¹⁵N measurements from lake sediments with a dominantly autochthonous organic source (low C:N ratio sediments) were influenced by fractionation in the lake and catchment leading up to assimilation of carbon and nitrogen by lacustrine biota. δ¹³C values from lake sediment samples in agricultural catchments were more negative than δ¹³C values from lake sediment samples in non-impacted, bogland catchments. Hypolimnetic oxygen concentrations and methane production had a greater influence on δ¹³C values than fractionation due to algal productivity. δ¹⁵N from lake sediment samples in agricultural catchments were more positive than δ¹⁵N in non-impacted bogland catchments. Lower δ¹⁵N values from non-impacted lake catchments reflected δ¹⁵N values of catchment vegetation, while higher δ¹⁵N values in agricultural catchments reflected the high δ¹⁵N values of cattle manure and inorganic fertilisers. The influence of changing nitrogen sources and lake/catchment fractionation processes were more important than early diagenesis for lake sediment δ¹⁵N values in this dataset. The results from this study suggest a possible influence of bound inorganic nitrogen on the bulk sediment δ¹⁵N values. We recommend using a suitable method to control for bound inorganic nitrogen in lake sediments, especially when working with clay-rich sediments. This study confirms the usefulness of δ¹³C and δ¹⁵N from bulk lake sediments, as long as we are mindful of the multiple factors that can influence these values. This study also highlights how stable isotope datasets from lake surface sediments can complement site-specific isotope source/process studies and help identify key processes controlling lake sediment δ¹³C and δ¹⁵N in a study area.