Nitrogen stable isotope ratios in surface sediments, epilithon and macrophytes from upland lakes with differing nutrient status

Abstract

Summary 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 δ 15 N values in all measured nitrogen compartments. Some regional variation was evident but was generally weak. Sediment and epilithon δ 15 N were positively correlated with δ 15 N of TDN, suggesting that baseline nitrogen isotope ratios influence those in some organic matter compartments in the lakes. 3. Sediment δ 15 N 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 δ 15 N values were also negatively related to annual nitrogen deposition. 4. Sediment, epilithon and macrophyte δ 15 N values all showed significant relations to nutrient limitation in the lakes as determined by algal bioassays. We suggest that sediment δ 15 N might be developed as a simple integrating measure of the degree of nitrogen limitation in lakes.