Keywords:Marine nitrogen cycle, Nitrogen isotope, Marine ecosystem model
The nitrogen isotopic composition (δ15N) of nutrients in the surface ocean transfer to the δ15N of phytoplankton, settling particles, and eventually benthic sediment. The δ15N of settling particles and sediment can therefore be used to trace the past nitrogenous nutrient environment. Previous observations have revealed that there is an inverse relationship between seasonal δ15N and flux of settling particles. In winter at high latitudes, the settling particles have high δ15N and low flux as compared with other seasons. In contrast, the surface water nitrate in winter has the lowest δ15N in a year due to convective mixing. The winter settling particles should also have the lowest δ15N, if winter phytoplankton assimilates only nitrate. Previous studies pointed out three reasons why δ15N of settling particles from autumn to winter increases despite the decrease in δ15N of surface nitrate: (1) the increase in contribution of zooplankton, which have a δ15N about 3 ‰ higher than that of phytoplankton; (2) the contribution of old particles most degraded with isotope fractionation in a year; and (3) the winter phytoplankton assimilates not only nitrate but also ammonium, which has higher δ15N than nitrate due to nitrification. In this study, to clarify the reason we applied the compound-specific stable isotope analysis of amino acid and a marine nitrogen isotope model. Sediment trap experiment was conducted at 1000 m depth at station K2 (47°N, 160°E) from June 2014 to July 2015. The bulk δ15N was determined by a sensitivity-improved EA/IRMS. The δ15N of glutamic acid and phenylalanine were determined by GC/C/IRMS. The bulk δ15N show relatively low values around 2‰ from July to August and increases to 5‰ from September to June, which is a typical seasonal variation observed at high latitudes. Surprisingly, the apparent trophic positions of settling particles estimated from the δ15N of glutamic acid and phenylalanine are 2.0±0.1 both in summer and winter. This is the first evidence that the winter high-δ15N of settling particles does not reflect the increase in contribution of zooplankton. Our model result suggested that the winter high-δ15N value of settling particles mainly reflects the winter high-δ15N of ammonium due to nitrification, which was strongly supported by the nitrogen isotopic compositions of amino acids.