NITROGEN ISOTOPE VARIATIONS IN THE SUB-ARCTIC NORTHEAST PACIFIC - RELATIONSHIPS TO NITRATE UTILIZATION AND TROPHIC STRUCTURE

The isotopic composition of dissolved nitrate, size-fractionated suspe nded particulate organic matter (SPOM) and zooplankton was determined on a transect (Line P) between a coastal upwelling domain and Station Papa in the subarctic northeast Pacific. Station Papa lies in one of t he 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 delta(15)NO(3)(-) decreased f rom east to west along the transect, from ca 12 parts per thousand in the coastal domain to 8 parts per thousand at Station Papa, while nitr ate concentration ([NO3-]) increased from 3 to 12 mu M in the same dir ection. Concurrently, delta(15)NO(3)(-) at 400 m depth showed a much s maller change, from 5.3 to 3.1 parts per thousand, indicating a larger delta(15)N difference between deep and surface waters at the coastal end of the transect. The isotopic trend for SPOM was similar, from app roximately 11 to 3 parts per thousand for bulk SPOM, 8.5 to 3 parts pe r thousand for the <5 mu m fraction, 10 to 4 parts per thousand for th r 50-253 mu m fraction, and 12 to 5 parts per thousand for the <253 mu m fraction. Zooplankton had delta(15)N values ranging from 12 parts p er thousand off the coast to 8 parts per thousand at Station Papa. The fractionation factor for the nitrogen isotopes from these field obser vations was estimated to be 5 parts per thousand from the change in de lta(15)NO(3)(-) between surface and deep water and from the difference between delta(15)N(SPOM) and delta(15)NO(3)(-) and assuming the SP,OM represents an instantaneous product. Seasonal transects of delta(15)N (SPOM) and [NO3-] show that there are different relationships between these two parameters depending on the availability and relative utiliz ation of NO3-; delta(15)N(SPOM) is more or less constant in the open o cean where [NO3-] > 7 mu M, it increases rapidly where nitrate utiliza tion is high (that is, where biological uptake is greater than physica l supply), and it decreases during upwelling events when physical supp ly of nutrients overwhelms the biological uptake rate. Finally, the ni trogen isotopes show the expected trophic enrichment of N-15, with bul k zooplankton being isotopically heavier than SPOM by 3.9 parts per th ousand at Station Papa and 2.2 parts per thousand in the coastal domai n. This difference possibly reflects the existence of a relatively sho rt food chain in the coastal domain and a longer food chain, involving microzooplanktonic grazers, at Station Papa. The range of delta(15)N among seven zooplankton groups was 3.7 parts per thousand, the data su ggesting an increasing trophic hierarchy: pteropods, salps, euphausiid s, copepods-medusae-amphipods, chaetognaths. (C) 1997 Elsevier Science Ltd.