 1. Climatology of oceanography & biology in Oyashio mixed layer
18-year time series of oceanographic and biological properties in the Oyashio mixed layer, observed by the A-line cruises, show regular seasonal variations in each parameter [Fig.1-1]. Although the signals of small inter-annual variations can be detected in each 18-y time series, climatology of seasonal variation for each parameter is estimated by plotting all observed data in one-year calendar [Fig.1-2].
Macro nutrients such as nitrate and silicate show seasonal maximum on March, when water temperature is the lowest and mixed layer depth is the largest, earning 18.3±3.5μM and 30.7±5.5μM for nitrate and silicate, respectively [Table 1-1]. These values are about 1.3 times higher than that observed in Station P at winter, but the winter-summer difference of mixed layer nutrient concentration in Oyashio (17.6±3.4μM and 27.2±5.5μM for nitrate and silicate, respectively [Table 1-1]) is about three times larger than in the OSP. Such large difference mainly originates from the difference of summer nutrient concentrations: In Oyashio mixed layer, nitrate and silicate reaches to its minimum of 0.72±0.62μM and 3.5±1.9μM in August, respectively, while those are about 8.9μM and 12.9μM, respectively, in summer mixed layers at the OSP (50°N, 145°W; Boyd and Harrison, 1999) [Table 1-1]. Observed average concentrations of both nitrate and silicate in summer Oyashio mixed layer are close to (or lower than) the commonly-observed half saturation constant (1μM and 3μM for nitrate and silicate, respectively, Kudela and Chabez, 2000 and Brzezinski et al., 1997), indicating summer Oyashio is not HNLC but nutrient-depleted. This feature is highly distinct from other subarctic North Pacific regions such as Station KNOT (45°N, 155°E) and OSP, where macro nutrients are replete even in summer.
Phytoplankton biomass shows its seasonal minimum in January, recording 0.24±0.09 mg/m3 as climatology. Although phytoplankton start their growth even in winter between January and March, an explosive growth occur during April and May making the "spring bloom" in this oceanic region [Fig.1-2]. Seasonal maximum concentration of Chla reaches up to 5.6±3.6mg/m3 in this period, which is over ten times larger than that observed in OSP, or even twice larger than that observed in Station KNOT. This "spring bloom" draws down macro-nutrients significantly, but they still sustain their level above the limiting concentration of diatom when the spring-bloom chla starts decreasing [Fig.1-3]. Small phytoplanktons moderately grow after the spring bloom consuming these remaining nutrients, and this leads rather slow decreasing late of Chla concentrations from May to August [Fig.1-2]. Nutrient consumption after the spring bloom is apparent in nitrate but not in silicate [Fig.1-3], suggesting that phytoplankton community shifted from diatom-dominated to less-diatom after the spring bloom.
Observed seasonal nutrient variation also reveals that the Oyashio region is not the HNLC but is limited by nitrate as a whole, but the "spring bloom" is not limited by macro-nutrients but other factors, such as light and micro-nutrient (iron).
Reference:
Boyd, P. and P. J. Harrison (1999): Phytoplankton dynamics in the NE subarctic Pacific. Deep-Sea Res. II, 11-12, 2405-2432.
Brzezinski, M. A., D. R. Phillips, F. P. Chavez, G. E. Freedrich and R. C. Dugdale (1997): Silica production in the Monterey, California, upwelling system. Limnol. Oceanogr., 42, 1694-1705.
Kudera, R. M. and F. P. Chavez (2000): Modeling the impact of the 1992 El Nino on new production in Monterey Bay, California. Deep-Sea Res. II, 47, 1055-1076.
Saito, H., A. Tsuda and H. Kasai (2002): Nutrient and plankton dynamics in the Oyashio region of the western subarctic Pacific Ocean. Deep-Sea Res. II, 49, 5463-5486.
Whitney, F. A. and H. J. Freeland (1999): Variability in upper-ocean water properties in the NE Pacific Ocean. Deep-Sea Res. II, 11-12, 2351-2370.
Table 1-1: Seasonal maximum and minimum values in Oyashio mixed layer, A-line |
Fig.1-1: Monthly -averaged time series of oceanographic variables observed in the Oyashio mixed layer along the A-line |
Fig.1-2: Climatology of seasonal variation in each oceanographic variable in the Oyashio mixed layer (1988-1998). Figures cited from Saito et al. (2002). |
Fig.1-3: Nitrate-silicate plot observed in the Oyashio mixed layer for each season. For nitrate, 0.1μM was added for each data and then plotted with log-axis. Figures cited from Saito et al. (2002). |
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