Abstract
We monitored the stable nitrogen isotopic composition (δ15N) of suspended matter and ammonium in the freshwater stretch of the Scheldt estuary (Belgium) over a full year to investigate for seasonal evolution and possible co-variation between isotopic signatures. The δ15N value of ammonium remained rather constant during winter (average = +11.4‰) but increased significantly with the spring and summer bloom, reaching values as high as +70‰. This enrichment of the ammonium pool in 15N coincided with significant ammonium depletion during summer period, suggesting a close causal relationship. Based on a semi-closed system approach we deduced an apparent fractionation factor associated with NH 4 + utilization (i.e. combining effects of uptake and nitrification) of 18.4‰ (SE = 2.0‰), which is similar to values reported in literature. Observed variations of ammonium δ15N could account for about 69% of δ15N variation in suspended matter.
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