We used statistical analyses to determine which subset of 36 environmental variables best explained variations in surface sediment delta C-13 and delta N-15 from 50 lakes in western Ireland that span a human-impact gradient. The factors controlling lake sediment delta C-13 and delta N-15 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 delta C-13 and delta N-15 measurements similar to values from catchment vegetation. delta C-13 and delta N-15 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. delta C-13 values from lake sediment samples in agricultural catchments were more negative than delta C-13 values from lake sediment samples in non-impacted, bogland catchments. Hypolimnetic oxygen concentrations and methane production had a greater influence on delta C-13 values than fractionation due to algal productivity. delta N-15 from lake sediment samples in agricultural catchments were more positive than delta N-15 in non-impacted bogland catchments. Lower delta N-15 values from non-impacted lake catchments reflected delta N-15 values of catchment vegetation, while higher delta N-15 values in agricultural catchments reflected the high delta N-15 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 delta N-15 values in this dataset. The results from this study suggest a possible influence of bound inorganic nitrogen on the bulk sediment delta N-15 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 delta C-13 and delta N-15 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 delta C-13 and delta N-15 in a study area.