Seasonal characteristics of the nitrogen isotope biogeochemistry of settling particles in the western subarctic Pacific: A model study

Abstract We used moored time-series sediment traps to collect settling particles at station KNOT (44°N, 155°E; trap depth 770 m) in the western subarctic Pacific (WSAP) from October 1999 to May 2006. Particulate nitrogen content (PN) and isotope ratios ( δ 15 N PN ) were measured in the samples collected. The general pattern of variation in δ 15 N PN results showed lower values during the spring bloom periods and summer, and higher values during winter. To interpret the processes controlling such variations quantitatively and reveal some implications for paleoceanographic use of δ 15 N PN , we developed an ecosystem model that included nitrogen isotopes. This model was validated with an observed data set and successfully reproduced the seasonal variations of δ 15 N PN . In simulations, the lower δ 15 N PN during the spring bloom period was caused mainly by the highest proportion of dead large phytoplankton (diatom) in PN within a year, the highest f -ratio of the year, and phytoplankton assimilation of nitrate with the lowest δ 15 N of the year. The lower δ 15 N PN in summer was due to the high relative proportion of dead non-diatom small phytoplankton and microzooplankton fecal pellet with the lowest δ 15 N values among all the PN components in our model. The higher δ 15 N PN in winter was mainly caused by the highest proportion of zooplankton components in PN, with higher δ 15 N values than phytoplankton components, and the enhanced δ 15 N values of ammonium induced by nitrification and its subsequent assimilation by phytoplankton. Our identification of nitrification as one cause of higher δ 15 N PN in winter is consistent with previous findings in a proximal marginal sea, the Okhotsk Sea, with an ecosystem model simpler than our model. This might indicate that the cause of higher δ 15 N PN in winter is common in the WSAP. In our model, we optimized the isotope effect of each process using our observational data of δ 15 N PN and δ 15 N of nitrate published elsewhere as constraints, and investigated the sensitivity of the annual flux-weighted mean of δ 15 N PN to the isotopic fractionation effects. As a result, we found that the isotope effects of nitrate assimilation appear to be different for non-diatom small phytoplankton and large diatom, and the annual flux-weighted mean of δ 15 N PN can be influenced to some extent by the isotope effect of nitrification.