Historical (1850–1995) nitrogen changes in UK catchments recorded by lake sediment δ15N

Environmental context: Unprecedented modification of the global nitrogen cycle by human activity is strongly imprinted in UK lakes by the changing abundance of nitrogen isotopes found in layers of lake sediment. UK lakes in the last century have responded to this change depending on their location, particularly if they are remote and occur above ~300-m altitude. Historical sediment nitrogen isotope data illustrates the impact of variable N sources, from fossil fuel combustion, synthetic fertilisers, land use change and wastewater on UK lakes. Rationale: The global nitrogen cycle has been fundamentally reconfigured by human activity in the last two centuries. This alteration has played out especially in freshwaters, where changes in nitrogen inputs have transformed whole lake and river ecosystems by boosting primary production in nutrient-limited systems and contributing to eutrophication in others. Global alteration of N is manifest in the nitrogen isotopic signature of organic material that moves through freshwater ecosystems. Lake sediments store organic material and can provide a unique historical, stratigraphic record of changes in nitrogen inputs that can be compared with modelled historical changes and contemporary monitoring. Methodology: Lake sediment nitrogen isotopes (δ15N) were measured in archived lake sediment cores (n = 90) at the well-resolved time intervals of 1850, 1900 and 1980 and ‘surface’ samples with a median date of 1995, determined by radiometric (210Pb; 137Cs) dating. Total C, N and δ13C were also measured to provide co-variables for assessment. Lake and catchment morphometries and environmental parameters were determined from Open-Access databases. Isotopic changes over time and space were investigated using geospatial and multivariate statistical analyses. Results: The difference in δ15N values between 1850 and 1980 in upland lakes (mean altitude of catchment (MAC) > 300 m above sea level) are largely negative (median −0.75‰, s.e. 0.15), whereas lowland lakes with MAC < 300 m show a positive difference (median 0.92‰, s.e. 0.3). Discussion: Regional and local differences in the record of δ15N in lake sediments are identified and investigated. Co-measured C, N and δ13C and known characteristics of the lakes are used to indicate causes of the recorded differences. Limits to the spatial and temporal sampling approach used are identified and discussed, which include issues with bulk sediment analysis, effects of diagenesis and the operation of multiple, potentially conflicting global and local environmental drivers that become integrated to form lake sediment δ15N records.