Seasonal variations in nitrate isotope composition of three rivers draining into the North Sea

Abstract. Nitrate loading of coastal ecosystems by rivers that drain industrialised catchments continues to be a problem in the South Eastern North Sea, in spite of significant mitigation efforts over the last 2 decades. To identify nitrate sources, sinks, and turnover in three German rivers that discharge into the German Bight, we determined δ ¹⁵N-NO₃^- and δ¹⁸O- NO₃^- in nitrate and δ ¹⁵N of particulate nitrogen for the period 2006–2009 (biweekly samples). The nitrate loads of Rhine, Weser and Ems varied seasonally in magnitude and δ ¹⁵N-NO₃^- (6.5–21‰), whereas the δ ¹⁸O-NO₃^- (-0.3–5.9‰) and δ ¹⁵N-PN (4–14‰) were less variable. Overall temporal patterns in nitrate mass fluxes and isotopic composition suggest that a combination of nitrate delivery from nitrification of soil ammonia in the catchment and assimilation of nitrate in the rivers control the isotopic composition of nitrate. Nitrification in soils as a source is indicated by low δ ¹⁸O-NO₃^- in winter, which traces the δ ¹⁸O of river water. Mean values of δ ¹⁸O-H₂O were between –9.4‰ and –7.3‰; combined in a ratio of 2:1 with the atmospheric oxygen δ ¹⁸O of 23.5‰ agrees with the found δ ¹⁸O of nitrate in the rivers.

Parallel variations of δ ¹⁵N-NO₃^- and δ ¹⁸O-NO₃^- within each individual river are caused by isotope effects associated with nitrate assimilation in the water column, the extent of which is determined by residence time in the river. Assimilation is furthermore to some extent mirrored both by the δ ¹⁵N of nitrate and particulate N. Although δ ¹⁵-NO₃^- observed in Rhine, Weser and Ems are reflected in high average δ ¹⁵N-PN (between 6‰ and 9‰, both are uncorrelated in the time series due to lateral and temporal mixing of PN. That a larger enrichment was consistently seen in δ ¹⁵N-NO₃^- relative to δ ¹⁸O-NO₃^- is attributed to constant additional diffuse nitrate inputs deriving from soil nitrification in the catchment area. A statistically significant inverse correlation exists between increasing δ ¹⁵N-NO₃^- values and decreasing NO₃^- concentrations. This inverse relationship – observed in each seasonal cycle – together with a robust relationship between human dominated land use and δ ¹⁵N-NO₃^- values demonstrates a strong influence of human activities and riverine nitrate consumption efficiency on the isotopic composition of riverine nitrate.