1. Thirty small upland lakes in Cumbria, Wales, Scotland and Northern Ireland were each visited once during June and July 2000. From each lake, samples of surface sediment epilithon, macrophytes and total dissolved nitrogen (TDN) were collected for nitrogen stable isotope analysis. As part of a wider programme, samples were also collected for chemical analysis and bioassays. 2. Considerable variation was found in δ15N values in all measured nitrogen compartments. Some regional variation was evident but was generally weak. Sediment and epilithon δ15N were positively correlated with δ15N of TDN, suggesting that baseline nitrogen isotope ratios influence those in some organic matter compartments in the lakes. 3. Sediment δ15N was higher when inorganic nitrogen concentration in the water was low, possibly reflecting reduced isotope fractionation under these conditions. However, this was not the case for epilithon or macrophytes. Sediment δ15N values were also negatively related to annual nitrogen deposition. 4. Sediment, epilithon and macrophyte δ15N values all showed significant relations to nutrient limitation in the lakes as determined by algal bioassays. We suggest that sediment δ15N might be developed as a simple integrating measure of the degree of nitrogen limitation in lakes.
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