δ15NBase isoscapes in
the NP
Our isoscape shows that the eastern Bering Sea continental shelf has
higher δ 15NBase
(δ 15NBase > 7.0‰;
Fig. 1) than any other potential habitat range of chum salmon. This highδ 15NBase is likely to be a
result of coupled partial nitrification–denitrification in this region
(Brown et al . 2015). High rates of sedimentary denitrification
fueled by active nitrification preferentially remove
14N from the system, thereby enriching
15N in the remaining nitrate (Granger et al .
2011). Spatial patterns ofδ 15NBase in the pelagic
subarctic NP are largely determined by nitrate utilization, which is the
ratio of nitrate assimilation by phytoplankton to nitrate supply in the
euphotic layer, and is inversely correlated with surface-water nitrate
concentrations because of isotope fractionation during nitrate
assimilation by phytoplankton (Fig. S3). In the western subarctic NP,
which is a nutrient-rich, low-chlorophyll region, nitrate is not
depleted throughout the year and phytoplankton growth is instead limited
by the availability of other micronutrients, such as iron (Nishioka &
Obata 2017). This results in less fractionation associated with nitrate
utilization, which in turn accounts for the lowδ 15NBase in the western
subarctic NP (Yoshikawa et al . 2018). Nitrate concentrations
decrease with distance from the nutrient-rich, low-chlorophyll region
and nitrate δ 15N increases with nitrate
depletion (Yoshikawa et al . 2018), which may account for the
increased δ 15NBase around the
eastern coast of Japan and the Gulf of Alaska. Unfortunately, the
isoscape compiled in this study suffers from a paucity of data in the
Okhotsk Sea and the western Bering Sea Shelf. Although previous studies
have suggested that nitrate δ 15N in the Okhotsk
Sea is similar to that of the eastern coast of Japan and not as high as
that of the Bering Sea Shelf (Yoshikawa et al . 2006), it remains
possible that δ 15NBase in the
western Bering Sea Shelf resembles that of the eastern Bering Sea Shelf.
Thus, we cannot discount the possibility that the salmon analyzed in
this study migrated to the western Bering Sea Shelf, although this would
not affect the substance of our conclusions.
Our results also suggest thatδ 15NBase can be used as a
direct indicator of the origin of marine organisms as suggested by
previous studies (Hetherington et al . 2017; McMahon & Newsome
2019). This is supported byδ 15NBase values in the first
vertebral section (Section 1) of two salmon (OK2: 4.4‰ and OK8: 3.6‰)
similar to that of the isoscape in coastal Japan and the Okhotsk Sea
(ca. 4‰; Fig. 1). A previous study also confirmed that Japanese chum
salmon initially move into the Okhotsk Sea just after their seaward
migration (Urawa et al . 2009). Large seasonal variations in the
isotopic composition of primary producers (Bronk et al . 1994;
Rolff 2000; Hannides et al . 2009) often complicate estimations of
the isotopic baseline in the ocean. However, copepods effectively
provide time-averaged δ 15NBase
values in a given region because they have relatively long lifespans
(from 1 month to 2 years) (Paffenhöfer 1993; Tsuda et al . 2001),
low dispersal ability, and low intra-species TP variation (Table S3). In
our study, copepod samples from the pelagic NP were mostly composed of
large Neocalanus species, which have a lifespan of 1–2 years
(Tsuda et al . 2001). In coastal areas, we sampled copepods which
have a shorter lifespan across different seasons to take any seasonal
variation into account, and to capture the annual meanδ 15NBase of each region.