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 15N PN) were measured in the samples collected. The general pattern of variation in δ 15N 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 15N 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 15N PN. In simulations, the lower δ 15N 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 15N of the year. The lower δ 15N PN in summer was due to the high relative proportion of dead non-diatom small phytoplankton and microzooplankton fecal pellet with the lowest δ 15N values among all the PN components in our model. The higher δ 15N PN in winter was mainly caused by the highest proportion of zooplankton components in PN, with higher δ 15N values than phytoplankton components, and the enhanced δ 15N values of ammonium induced by nitrification and its subsequent assimilation by phytoplankton. Our identification of nitrification as one cause of higher δ 15N 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 15N 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 15N PN and δ 15N of nitrate published elsewhere as constraints, and investigated the sensitivity of the annual flux-weighted mean of δ 15N 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 15N PN can be influenced to some extent by the isotope effect of nitrification.