Akihiro Shiomoto

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

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.

Primary productivity in the offshore Oyashio in the spring and summer 1990

Primary productivity was measured byin situ method using13C in the offshore Oyashio region in the spring (May) and summer (September) of 1990. Most of the values were within the range of 0.1 to 4 μgC 1−1 h−1 although a very large value, 7.96 μgC l−1 h−1, was observed in summer. Most daily primary production fell within the range of 372 to 633 mgC m−2 d−1 although a very large value, 2,109 mgC m−2 d−1, was observed around the frontal area in summer. Chlorophylla (Chl.a) exceeded 1 μg l−1 in many cases, and the maximum was 4.61 μg l−1 in spring and 7.53 μg l−1 in summer. Most primary productivity per unit Chl.a (photosynthetic assimilation ratio) was within the range of 0.1 to 3 μgC μgChl.a−1 h−1 although higher values, 3–6 μgC μgChl.a−1 h−1, were observed where small-size phytoplanktons (<2 μm) were dominant. These results were compared with results obtained until now in the Oyashio region. The values beyond the range obtained so far in the offshore region were also observed in this study. Furthermore, it was pointed out that the size composition of phytoplankton community has significant influence on the results of Chl.a and photosynthetic assimilation ratio in the Oyashio region.

Kinetics of Nitrate and Ammonium Uptake by the Natural Populations of Marine Phytoplankton in the Surface Water of the Oyashio Region during Spring and Summer

The maximum uptake rate (ρmax) and affinity constant (Ks) for nitrate and ammonium were estimated in the surface water of offshore Oyashio in May (spring) and September (summer), 1990. The average ρmax/Chl.a for ammonium was 2.1 times larger than that of nitrate in both seasons. The average ρmax/Chl.a for both nitrogens were 3.5 times larger in summer than in spring. Water temperature and size composition of phytoplankton population were related to the seasonal difference in the ρmax/Chl.a. Phytoplankton population showed high affinity for both nitrogens in the spring and summer. In addition, the contribution of “new” production to total production was estimated by ρmax[ρmax−No3/(ρmax−NO3+ρmax−NH4)]. The spring value was in the range of 0.26 to 0.45 (mean±SD=0.35±0.092), and the values in spring bloom were especially a little over 0.4. The summer value was in the range of 0.30 to 0.37 (0.34±0.04).

Possible mechanisms of decadal‐scale variation in PO4 concentration in the western North Pacific

[1] We suggest possible mechanisms of decadal-scale variation in PO 4 concentration in the Oyashio and Kuroshio-Oyashio transition waters, western North Pacific. Significant decreasing and increasing trends in PO 4 have been observed in the surface and mid-layers of the ocean in this region, respectively. Synchronous bidecadal-scale oscillations in PO 4 were also found between the two layers. Differences in the relationship of the trends and the oscillation in these layers suggest that they are driven by separate processes. The trend component may be induced by attenuation of water exchange between the two layers. In contrast, the influence of the 18.6-yr period nodal tidal cycle on the formation rate of intermediate water in the Okhotsk Sea may cause the bidecadal-scale oscillation. The PO 4 concentration showed a significant positive correlation with Neocalanus plumchrus biomass, the dominant mesozooplankton species in the Oyashio and Kuroshio― Oyashio Transition waters. These relationships suggest that the variation in PO 4 affected production of N. plumchrus due to changes in primary production.

High-West and Low-East Distribution Patterns of Chlorophyll a, Primary Productivity and Diatoms in the Subarctic North Pacific Surface Waters, Midwinter 1996

We have determined chlorophyll a (Chla) concentration, primary productivity, cell density and species composition of diatoms, and the number of microzooplankton at the surface in the subarctic North Pacific in January 1996. The wet weight of copepods obtained by vertical tows from 150 m to the surface was also measured during the cruise. Chla concentration and primary productivity tended to be higher in the region west of 180°, the western subarctic North Pacific (WSNP), than east of 180°, the eastern subarctic North Pacific (ESNP). The same results were observed for the total diatom cell densities and for the genera Thalassiosira and Denticulopsis. Significant linear relationships were observed between the Thalassiosira cell density and Chla concentration and primary productivity, indicating that Thalassiosira contributes to the high-WSNP and low-ESNP distribution patterns of Chla concentration and primary productivity. Moreover, naked ciliate abundance tended to be lower in the WSNP than in the ESNP, whereas copepod biomass showed an inverse trend. Significantly negative Spearman rank correlations were found between the Thalassiosira cell density and the number of naked ciliates and between the number of naked ciliates and the wet weight of copepods. These results indicate that copepod grazing indirectly controls Thalassiosira cell density via predation on the naked ciliates. We conclude that the high copepod biomass in the WSNP is a factor controlling the high-WSNP and low-ESNP Thalassiosira abundance and hence Chla concentration and primary productivity patterns.

Chlorophyll- a and primary production during spring in the oceanic region of the Oyashio Water, the north-western Pacific

The concentration of phytoplankton chlorophyll- a (chl- a ) and primary production within the euphotic zone (at depths corresponding to 100, 30, 10 and 2% light) were measured in the oceanic region of the Oyashio Water, the north-western Pacific, in spring of 1993, 1994 and 1995. The chl- a concentrations ranged from 0.15 to 6.98 mg m −3 and the range of chl- a standing stock integrated from the surface to the 2% light depth was between 4 and 43 mg m −2 . The daily primary production integrated from the surface to the 2 and 0.2% light depths was estimated to be between 94 and 1695 mg C m −2  d −1 and between 114 and 2046 mg C m −2  d −1 , respectively. Phytoplankton bloom was noticed when the depths of the euphotic zone and the upper mixed layer were similar. The maximum chl- a concentration and daily primary production in the upper 0.2% light depth were comparable to those reported previously in the spring bloom in the coastal, offshore and oceanic regions of the Oyashio Water, and in the summer bloom of the Western Subarctic Gyre in the North Pacific. The magnitude of daily primary production in the phytoplankton bloom in the western subarctic North Pacific (the Oyashio Water and the Western Subarctic Gyre) was comparable with those in the eastern subarctic North Pacific and the subarctic North Atlantic. The magnitude of chl- a in the phytoplankton bloom in the western subarctic North Pacific was higher than those in the other subarctic regions. Varied contribution of large diatoms to the phytoplankton biomass, caused by different iron and silicate conditions was hypothesized to be a leading factor for variation in magnitude of the phytoplankton biomass.

Geographical variations in prosome length and body weight of Neocalanus copepods in the North Pacific

Geographical variations in prosome length and body weight of Neocalanus copepods (N. cristatus, N. plumchrus and N. flemingeri) were investigated on samples from North-South and East-West transects in the North Pacific during spring to early summer in 1998 and 1999. Southward and eastward increasing patterns were pronounced for water temperature, although no significant pattern was observed for chlorophyll a concentrations. All Neocalanus species showed large geographical variations in prosome length and body weight, being smaller in the southern and eastern waters. Comparing the relationship between prosome length and body weight, large deviations (lower body weight at a given prosome length) were evident for the eastern specimens of N. cristatus and N. plumchrus. In stepwise regression analysis, the geographical variations of prosome length and body weight revealed a significantly negative correlation with temperature variations. These results suggest that temperature is a more important environmental factor than chlorophyll a concentration in its effect on geographical variations in prosome length and body weight of Neocalanus copepods in the North Pacific.

Primary productivity and solar radiation off Sanriku in May 1997

We measured the in situ primary production at four stations from the surface to 80 m off Sanriku in late May 1997. The depth-integrated daily primary production in the upper 80 m was estimated to be 391, 468, and 855 mgC m−2d−1 in water from the Oyashio, and 336 mgC m−2d−1 in the warm-core ring. The variation in the primary production was primarily due to the variation in phytoplankton activity (chlorophyll α-specific primary production). A combination of previous and present studies in water from the Oyashio and the warm-core ring suggested that phytoplankton activity is proportional to light intensity between 12 and 50 Ein m−2d−1 which is close to the usual light condition (61–75 Ein m−2d−1) off Sanriku in May and June. Light may be a limiting factor for phytoplankton off Sanriku in late spring and early summer.

Photosynthetic responses to the temperature of size-fractionated phytoplankton in the wintertime north-western Pacific

This study shows that the smaller phytoplankton are better able to acclimate to rapid fluctuations in temperature in the open ocean. In contrast, phytoplankton of all sizes have possibly the same ability along the coast.

An early spring bloom of large diatoms in the ice-covered Saroma-ko Lagoon, Hokkaido, Japan—ERRATUM

Saroma-ko Lagoon, the largest body of water that has complete ice coverage during winter in Japan, was not completely covered by ice in the winter of 2009. This condition is considered to be a result of the progression of global warming. A bloom of large diatoms was observed in the ice-free area between February and April. This early spring bloom seemed to have started in the latter part of January, and lasted for about three months. The maximum chlorophyll- a (Chl a ) concentration of about 10 mg m −3 was observed in March, and was similar to the level of 5–20 mg m −3 previously reported for the ordinary spring bloom in Saroma-ko Lagoon. The maximum primary production of 786 mgC m −2 day −1 and the maximum Chl a -specific primary production, an index of the phytoplankton growth rate, were also found in March. Species changes from Thalassiosira spp. to Chaetoceros spp. were observed during the bloom. This early spring bloom could extend into the ordinary spring bloom period. Its duration was obviously longer than that of the spring bloom, which is typically about one month. These results show the phytoplankton condition that could be expected during winter and spring as global warming progresses.