Freshwater Umbrella Research 2004-7

WP 2: Nitrogen as a nutrient

Task 2.3.1 — Palaeolimnological analysis for evidence of biological response to N deposition independent of any acidification response

Palaeoecological analysis of sediment records from many lakes in the boreal and sub-arctic zones reveal significant ecological changes over the past 200 years, following several hundred years of relative stability. The onset of change often coincides with the first appearance of spheroidal carbonaceous particles (SCPs), thereby implicating atmospheric contamination as a likely trigger. Recent work on a core from the UK Acid Waters Monitoring Network site, Loch Coire Fionnaraich, in north-west Scotland, provides a particularly striking example of this behaviour. While the ecological response is not indicative of acidification the mechanism is not yet understood. However, Wolfe et al., (2001), working on cores from remote lake ecosystems in the Rocky Mountains, Colorado, have demonstrated a link between apparent eutrophication and a change in δ15N which is indicative of an increased supply of N from the atmosphere. While N deposition in this region is relatively high, it has been proposed that a similar process may be operating more widely. Recently reported work of Jones et al. (2004) demonstrates that the analysis of δ15N in lake surface sediments might be used as a measure of the degree of N limitation in lakes.

We propose to apply stratigraphic δ15N analysis on new and existing sediment cores to test the hypothesis that the timing of ecological change corresponds to a change in the supply of atmospheric pollutant N to these sites and a change in N limitation. Further cores from high N deposition areas will be analysed for δ15N to allow comparison of the strength of the N deposition signal in low and high deposition areas.

Task 2.3.2 — Bioassays for evidence of current biological response to nutrient N deposition independent of any acidification response

For the sites selected in Task 2.3.1, bioassays to determine current nutrient limitation status will be carried out. Sampling will entail three repeat visits to obtain replicated phytoplankton and epilithon samples, plus analysis of nutrient chemistry in lake water samples.

Task 2.3.3 — Comprehensive analysis of palaeolimnological evidence for nutrient N response in up to 6 lakes where . existing catchment / water chemistry data indicate potential N limitation in Task 2.2

Following the successful testing of the palaeolimnological approach Task 2.3.1, the technique will be expanded to a maximum of 3 furthersites across the gradient of N and P limitation suggested by the empirical catchment predictions under Task 2.2. New cores will be used for analysis of diatoms, SCPs and the stable isotope 15N. Analyses will provide cross-validation of empirical and palaeolimnological approaches for the assessment of N limitation in upland waters.