Primary production and chlorophyll a in the northwestern Pacific Ocean in summer

Abstract

Primary production was measured in the Western Subarctic Gyre (WSG) and adjacent regions in July 1993 and 1994. The primary production integrated from the surface to the 2% light depth (16–40 m) ranged from 255 to 1198 mg C m−2 d−1 in the WSG. Mean ± standard error (SE) of the primary production was estimated to be 663±86 mg C m−2 d−1 (n = 11) in both years. Chlorophyll a (chl) concentration in the upper 2% light depth ranged from 0.32 to 6.95 μg L−1, and the mean ± SE was calculated to be 1.03±0.15 μg L−1 (n = 44). The mean ± SE of chl standing stock integrated in the upper 2% light depth was calculated to be 28.6±2.9 mg m−2 (n = 11). The mean primary production in the WSG was roughly equal to the values in the adjacent regions, whereas the mean chl concentration and standing stock were about 1.5 times higher in the WSG than in the adjacent regions. In addition, the primary production integrated from the surface to the 0.2% light depth (26–62 m) was estimated in the WSG. The depth‐integrated primary production in the upper 0.2% light depth ranged from 278 to 1397 mg C m−2 d−1, and the mean ± SE was calculated to be 751±94 mg C m−2 d−1. On the basis of recent observations in the summertime Alaskan Gyre (AG), depth‐integrated primary production in the upper 0.2–1.7% light depth (60–80 m) ranged from 290 to 1550 mg C m−2 d−1, and the mean ± SE was calculated to be 642±55 mg C m−2 d−1 (n = 19). Chl concentration in the surface layer reported previously in the AG was about 0.4 μg L−1. There was no substantial difference between the areal primary production in the WSG and AG, whereas the chl concentration in the surface layer was higher in the WSG than in the AG. In addition, primary productivity (primary production per unit of chl) at the surface tended to be lower in the WSG (mean ± SE of 33.4±2.5 μg C (μ chl)−1;n = 11) than in the AG (51.3±5.6 μg C (μg chl)−1 d−1; n = 19). The difference in phytoplankton species composition between the gyres may be related to the differing primary productivity.