The nitrogen isotopic composition of nitrate in the Kuroshio Water northeast of Taiwan: evidence for nitrogen fixation as a source of isotopically light nitrate

Abstract

The Kuroshio Water, which provides nutrients to the shelf water of the East China Sea through upwelling at the shelf break northeast of Taiwan, is abundant with nitrogen fixing cyanobacteria. Accordingly, the δ15N values of nitrate at three stations in the sea northeast of Taiwan were analyzed to estimate the flux of biologically fixed nitrogen in the Kuroshio Water by characterization of the isotopic signature of the upwelled nitrate. Prior to sample analysis, we verified using standard KNO3 solution the validity of the sample preparation procedure, which has been modified by reducing the amount of Devarda's alloy to lower the reagent blank. However, the concentrations of nitrate in seawaters estimated from the amount of extracted nitrogen were consistently higher than those measured colorimetrically, suggesting interference from dissolved organic nitrogen of which the δ15N value was estimated to be −3.0 ± 0.2%., Correction was made on each sample based on the estimated δ15N for DON. The δ15N values of waters from 500 to 780 m in the sea northeast of Taiwan fell in the range of + 5.5 to + 6.1%c, which was consistent with those observed in deep waters elsewhere. The δ15N values decreased in the overlying Kuroshio Water by 1–3%. in general. In order to achieve isotope balance in the water column, the input of isotopically light nitrogen, presumably from nitrogen fixation, is required. The nitrogen flux from this source was estimated to be 0.5 mg m−2 day−1 N which is only 8% of the estimated new nitrogen to the Kuroshio surface water. On the other hand, this nitrogen may account for 20–40% of the nitrate in the subsurface water (100–400 m) as a result of accumulation and recycling. The nitrate in the source upwelling water had a δ15N value of +4.7 ± 0.6%., implying 20 ± 9% of it originating from nitrogen fixation.