Nitrogen stable isotope ratios of lake macrophytes in relation to growth form and nutrient-limitation

Values are presented of natural abundance nitrogen stable isotope ratios (15N ‰) for macrophyte samples from 30 U.K. upland tarns. Values for individual macrophyte species at the different sites varied between –9.9‰ and 10.6‰, a range of 20.5‰. The average value of a particular macrophyte species was slightly less variable with a range of 14.5‰ and the average range per site was 10.4‰. To explore the possible causes of this very high variation, macrophytes were allocated into three categories according to their growth form and hence whether they probably obtained their nitrogen i) from both water and sediment (such as elodeids and isoetids), ii) mainly from the sediment (such as rooted emergent or floating-leaved plants) and iii) mainly from the water (such as filamentous algae, bryophytes and rootless vascular plants). An analysis of variance showed that δ15N differed according to presumed nitrogen source: average values were 0.3, 1.6 and 3.0‰ for macrophytes that obtain their nitrogen respectively from the water, from both sources and from the sediment. Hence part of the overall variation may be related to the lower mean site δ15N of the total nitrogen in water (1.6‰) compared to the sediment (3.0‰). However, much of the observed wide variation in macrophyte δ15N could be related to differences in nutrient limitation. Macrophytes restricted to the water for their nitrogen source showed statistically significant positive correlations between δ15N and nitrogen-limitation at the site and negative correlations with phosphorus-limitation at the site. The results suggest that the large variation in the δ15N value of freshwater macrophytes may be caused by a combination of differences in the δ15N value of the main nitrogen source used and differential isotopic discrimination caused by variable nitrogen-availability relative to demand. Thus analysis of the δ15N value of macrophytes that obtain their nitrogen from the water, such as bryophytes and filamentous algae, might offer a convenient and effective way of assessing nitrogen-limitation at a site