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Regional differences in the mechanisms of temporal variation in the lower trophic levels in the western, central, and eastern subarctic North Pacific were studied using the nitrogen stable isotope ratio (δ 15 N) of the major copepod species, Neocalanus cristatus , Neocalanus flemingeri , and Neocalanus plumchrus . We used formalin-preserved specimens collected in the Oyashio region (OY), three sections from north to south along the 180° longitudinal line (180LineSA, TN, and TS), off Vancouver Island (Off-Van), and at Sta. P, during the periods of 1960–2000, 1979–1997, 1981–2007, and 1996–2007, respectively. The regional mean δ 15 N of the three species roughly corresponded to the surface nitrate distribution and the extent of its drawdown from winter to spring; it was higher in regions of larger seasonal drawdown as observed in the coastal regions OY and Off-Van (7–10‰), but lower in regions with less seasonal drawdown, such as in the offshore regions at St. P and stations along the 180Line (3–6‰). Time series analysis revealed possible region-specific mechanisms for temporal variation in Neocalanus δ 15 N. First, δ 15 N indicated shifts in feeding strategies between herbivorous to omnivorous/carnivorous at OY and 180LineSA, where δ 15 N tended to be lower in the years with warmer winters, suggesting that Neocalanus took advantage of enhanced phytoplankton production under favorable light availability due to increased stratification. Conversely, wind-induced latitudinal advection of surface water was considered to be the initial cause of interannual variation in Neocalanus δ 15 N at 180LineTN, 180LineTS, and Off-Van, where δ 15 N was higher in the years with strong southerly or westerly winds at 180LineTN and TS, and the Off-Van site. This suggests that pole-ward transport of relatively oligotrophic, southern water might enhance the uptake of the heavier isotope by phytoplankton, which Neocalanus feed upon. Another possibility at the Off-Van site, where high δ 15 N was observed ( c.a. , 8–10‰), is a switch in the Neocalanus feeding strategy induced by decreased phytoplankton availability. This study demonstrated the usefulness of zooplankton δ 15 N as an indicator of interannual variation in lower trophic level environments and food web structures, which are caused by region-specific mechanisms. |
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