Nitrogen isotope variations in the subarctic northeast Pacific: relationships to nitrate utilization and trophic structure

The isotopic composition of dissolved nitrate, size-fractionated suspended particulate organic matter (SPOM) and zooplankton was determined on a transect (Line P) between a coastal upwwelling domain and Station Papa in the subarctic northeast Pacific. Station Papa lies in one of the three extensive high-nutrient low-chlorophyll (HNLC) domains of the open ocean, where plankton standing stocks are seasonally uniform and production is lower than the potential production based on available nutrients. In spring 1993, surface water δ15NO3− decreased from east to west along the transect, from ca 12%. in the coastal domain to 8%. at Station Papa, while nitrate concentration ([N03−) increased from 3 to 12 μM in the same direction. Concurrently, δ15NO3− at 400 m depth showed a much smaller change, from 5.3 to 3.1%., indicating a larger δ15N difference between deep and surface waters at the coastal end of the transect. The isotopic trend for SPOM was similar, from approximately 11 to 3%. for bulk SPOM, 8.5 to 3%. for the < 5 μm fraction, 10 to 4%. for the 50–253 μm fraction, and 12 to 5%. for the < 253 μm fraction. Zooplankton had δ15N values ranging from 12%. off the coast to 8%. at Station Papa. The fractionation factor for the nitrogen isotopes from these field observations was estimated to be 5%. from the change in δ15NO3− between surface and deep water and from the difference between δ15NSPOM and δ15NO3− and assuming the SPOM represents an instantaneous product. Seasonal transects of δ15NSPOM and [N03−] show that there are different relationships between these two parameters depending on the availability and relative utilization of N03−; δ15NSPOM is more or less constant in the open ocean where [NO3−] > 7 μM, it increases rapidly where nitrate utilization is high (that is, where biological uptake is greater than physical supply), and it decreases during upwelling events when physical supply of nutrients overwhelms the biological uptake rate. Finally, the nitrogen isotopes show the expected trophic enrichment 15N, with bulk zooplankton being isotopically heavier than SPOM by 3.9%. at Station Papa and 2.2%. in the coastal domain. This difference possibly reflects the existence of a relatively short food chain in the coastal domain and a longer food chain, involving microzooplanktonic grazers, at Station Papa. The range of δ15N among seven zooplankton groups was 3.7%., the data suggesting an increasing trophic hierarchy: pteropods, salps, euphausiids, copepods-medusaeamphipods, chaetognaths.