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

We used moored time-series sediment traps to collect settling particles at station KNOT (44 degrees N, 155 degrees E; trap depth 770 m) in the western subarctic Pacific (WSAP) from October 1999 to May 2006. Particulate nitrogen content (PN) and isotope ratios (delta N-15(PN)) were measured in the samples collected. The general pattern of variation in delta N-15(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 delta N-15(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 delta N-15(PN). In simulations, the lower delta N-15(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 delta N-15 of the year. The lower delta N-15(PN) in summer was due to the high relative proportion of dead non-diatom small phytoplankton and microzooplankton fecal pellet with the lowest delta N-15 values among all the PN components in our model. The higher delta N-15(PN) in winter was mainly caused by the highest proportion of zooplankton components in PN, with higher delta N-15 values than phytoplankton components, and the enhanced delta N-15 values of ammonium induced by nitrification and its subsequent assimilation by phytoplankton. Our identification of nitrification as one cause of higher delta N-15(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 delta N-15(PN) in winter is common in the WSAP. In our model, we optimized the isotope effect of each process using our observational data of delta N-15(PN) and delta N-15 of nitrate published elsewhere as constraints, and investigated the sensitivity of the annual flux-weighted mean of delta N-15(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 delta N-15(PN) can be influenced to some extent by the isotope effect of nitrification. (C) 2010 Elsevier B.V. All rights reserved.