Seasonal variation of the nitrogen and oxygen isotopic composition of nitrate in the Scheldt River and Estuary
Andrisoa, A.D. (2013). Seasonal variation of the nitrogen and oxygen isotopic composition of nitrate in the Scheldt River and Estuary. MSc Thesis. Universiteit Antwerpen/Universiteit Gent/VUB: Antwerpen, Gent, Brussel. 12, 39 pp.
The Scheldt estuary is the outlet of the most densely populated watersheds of Europe. Every year, large amounts of nitrate are discharged in the estuary via the tributary rivers Dender, Durme and Rupel and via the Scheldt itself. Despite the efforts conducted to improve the water quality, nitrate concentrations are still high. To identify nitrate sources and sinks, we investigated the longitudinal and seasonal variability of nitrate N and O isotopic compositions and concentrations in the Scheldt. Samples were obtained on a monthly basis and nutrients and isotopic analyses were conducted for the period January 2010 to December 2011 at 16 sampling stations situated in the Belgian part of Scheldt and 7 stations in the Dutch part. The NO3 - concentrations (328.7±52.4 µmol L-1) and mean d15N-NO3 - (11.4±1.1‰) and d18O-NO3 - (3.2±0.9‰) values indicated that the Scheldt can be characterized as an estuary influenced by anthropogenic activity. The NO3 - concentrations showed a seasonal pattern with highest concentrations observed in winter (324.8-487.3 µmol L-1) and lowest concentrations in summer (259.2-288.1 µmol L-1) and increased with discharge. The d15N-NO3 - showed also a clear seasonal pattern with highest signatures observed during late-spring and summer (11.3-12.7‰) whereas the lowest signatures were measured during winter (9.7-10.5‰). High d15N-NO3 - values during summer were attributed to phytoplankton assimilation and the input of nitrate with the same d15N value as ammonium derived from sewage and manure through the nitrification reaction. The d18O-NO3 - values also displayed seasonal variation with highest values observed in winter (3-5.3‰) while lowest values were recorded in summer (1.5‰-4‰). The low d18O-NO3 - observed in summer indicated that nitrification, which introduced nitrate depleted in 18O, was a major source of nitrate and counteracted the increase in d18O of nitrate by nitrate uptake. Water column denitrification was very unlikely to be a major sink of nitrate in the Scheldt Estuary during the study period because the water column was never anoxic.
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