Deep Sea Research Part II: Topical Studies in Oceanography
Interannual variability in the magnitude and timing of the spring bloom in the Oyashio region
Introduction
The Oyashio is a western boundary current that flows southwest along the southern Kuril Islands and southeast coast of Hokkaido and then circulates counterclockwise in the western North Pacific (Fig. 1). It carries cold water of reduced salinity to the western subarctic Pacific off Hokkaido (Kono, 1997). The Oyashio region has a nutrient-rich water mass, and it supports consumption of nitrate by phytoplankton with high efficiency (Taniguchi, 1999). The primary production is effectively transferred to higher trophic levels such as planktivorous pelagic fishes. The region supports a wide range of commercially important marine fishes (Sakurai, 2007).
In recent decades, the Oyashio ecosystem has shown significant changes in abundance and distribution of numerous species. Therefore, many studies about the ecosystem, from lower to higher trophic levels (phytoplankton, zooplankton, fishes), have been conducted (e.g., Kasai et al., 1997, Chiba et al., 2004, Sakurai, 2007). Spring phytoplankton blooms (e.g., Kasai et al., 1997, Saito et al., 2002) and their associated high primary productivity result in high zooplankton biomass, providing a rich food environment for fishes (Taniguchi, 1999, Sakurai, 2007). Abundant nutrients supplied to the surface layer by vertical mixing in wintertime have been suggested as one of the factors supporting the intense spring blooms (Kasai et al., 1997, Taniguchi, 1999). It has been suggested recently that dissolved iron supplied from iron-rich intermediate waters to the surface also enables the spring phytoplankton bloom in the Oyashio region (Nishioka et al., 2007). Finally, stratification of the water column promotes the nutrient utilization by phytoplankton in the euphotic layer, and stratification is important for initiation of the spring bloom (Yoshimori et al., 1995, Kasai et al., 1997). The magnitude and duration of the spring bloom are different between the coastal and offshore regions because they can have different strengths of stratification and nutrient concentrations (Kasai et al., 1997). After 1998 when sea surface temperature was higher than before 1997, the spring bloom has been larger in magnitude and initiated earlier (Kasai and Ono, 2007). The Oyashio spring bloom is not always terminated by macronutrient depletion, and the magnitude and duration of the spring bloom in the region are in part controlled by the grazing pressure of zooplankton (Saito et al., 2002). Thus, many studies about the magnitude, timing and duration of the spring bloom have been conducted. However, as the sampling interval in these studies was about monthly and sampling stations were spatially limited (e.g., A-line in Fig. 1), it has yet to be understood whether the patterns observed in past studies extend over a larger area.
In this study, we present the spatio-temporal variability of Chl-a concentration during spring using satellite ocean color data from 1998 to 2007. Our objective is to clarify the inter-annual variability in the magnitude and timing of the bloom in the Oyashio region. We also discuss the effect on the spring phytoplankton distribution of the horizontal advection of the Oyashio water mass into the offshore region by geostrophic currents.
Section snippets
Satellite data validation
The Sea-viewing Wide Field-of-view Sensor (SeaWiFS) data processed using the Ocean Color 4 version 4 (OC4v4) algorithm (O’Reilly et al., 1998, O’Reilly et al., 2000) was used in this study. In the coastal region, satellite Chl-a concentration data using ocean color algorithms can be falsely interpreted due to light scattering by suspended particulates or absorbance by colored dissolved organic matter (e.g., Lee et al., 1994, Carder et al., 1989). Before analyzing Chl-a concentration data from
Validation of satellite Chl-a concentration data
SeaWiFS Chl-a concentrations were compared with in-situ values along the A-line from A1 to A13 (Fig. 2). Some SeaWiFS data were observed outside of the ±35% line; however, the correlation between SeaWiFS and in-situ values was statistically significant with a high coefficient of determination (n=46, r2=0.90, p<0.0001). This relationship indicates that Chl-a concentration data from SeaWiFS are useful in this study area.
Chl-a seasonality for 10 years from 1998 to 2007
Monthly mean Chl-a concentrations during the 10 years from 1998 to 2007 were
Discussion
The spring bloom occurs in the Oyashio region every year; however, its magnitude, duration and timing are substantially different among areas and years. Usually the termination of the spring bloom corresponds with the depletion of macronutrients (Kudo et al., 2000). On the other hand, Saito et al. (2002) have suggested that its termination is not always dependent on macronutrient depletion, that the magnitude and duration of bloom in the Oyashio region may be controlled by grazing pressure from
Acknowledgments
This study was conducted as part of Oceanic Ecodynamics Comparison in the Subarctic Pacific (OECOS) project. We thank Mrs. Amane Fujiwara and Shintaro Takao, students in Hokkaido University, for their valuable discussions. We also appreciate three anonymous reviewers for their constructive comments. Ocean color data used in this study were produced by the SeaWiFS Project at the NASA Goddard Space Flight Center. Sea surface height anomaly data were produced by the Archiving, Validation and
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Community composition and photophysiology of phytoplankton assemblages in coastal Oyashio waters of the western North Pacific during early spring
2018, Estuarine, Coastal and Shelf ScienceCitation Excerpt :Yoshimori et al. (1995) noted that the blooms in COY waters can exist for a longer time than those in OY waters due to continuous nutrient supply with its weaker vertical stability. The phytoplankton bloom in COY waters would significantly affect primary production in surrounding waters including the OY by physical processes such as advection and eddy diffusion (Shinada et al., 1999; Okamoto et al., 2010). These pilot studies on the spring blooms in COY waters investigated relationships among physical and biological parameters for estimating the bloom dynamics, but no study has been conducted to investigate the photophysiology and community composition of phytoplankton in COY waters.
Reappraisal of meridional differences of factors controlling phytoplankton biomass and initial increase preceding seasonal bloom in the northwestern Pacific Ocean
2015, Remote Sensing of EnvironmentCitation Excerpt :However, the rapid and remarkable increase of Chl-a in the Oyashio area also occurred earlier in April (Fig. 7d) than in the other northern areas. Therefore, although remarkable initial increase of Chl-a took place earlier in the Oyashio area than in most of the other areas to the north of 42.5°N, the onset of the spring bloom in the Oyashio area likely adhered to Sverdrup’s (1953) CD hypothesis, an observation consistent with the conclusions of Okamoto, Hirawake, and Saitoh (2010) and Shiozaki et al. (2014). On the basis of the approximate timing of seasonal Chl-a initial increases and their mechanisms identified above, we classified our data/images pixel by pixel based on MLD and CD differences, MLD deepening/shoaling, and percentage of phytoplankton growth (i.e., 20% to capture Chl-a initial increase) to identify spatial patterns in the mechanisms responsible for the Chl-a initial increase (a proxy for the bloom onset) preceding the seasonal or spring bloom in the NWPO (Fig. 8b).
Biogeography and phenology of satellite-measured phytoplankton seasonality in the California current
2014, Deep-Sea Research Part I: Oceanographic Research PapersPopulation dynamics of phytoplankton, heterotrophic bacteria, and viruses during the spring bloom in the western subarctic Pacific
2011, Deep-Sea Research Part I: Oceanographic Research PapersCitation Excerpt :debilis and the pennate diatom Fragilariopsis kergelensis bloomed, respectively. Recently, Okamoto et al. (2010) found that, in spring, the Coastal Oyashio Water (COW) with low seawater density and high chlorophyll a concentrations was sometimes propagated into the oceanic region by mesoscale eddies and high velocity currents. The seaward transfer of COW can strengthen the stratification of the water-column and determine the magnitude and species composition of spring diatom blooms in the Oyashio region.
Development and growth of ontogenetically migrating copepods during the spring phytoplankton bloom in the Oyashio region
2010, Deep-Sea Research Part II: Topical Studies in OceanographyCitation Excerpt :During the experiments in 2008 to evaluate response of the copepod molting rate to environmental factors, stage durations showed negative but nonsignificant correlations with both the ambient temperature (r=−0.466) and chlorophyll a concentration (r=−0.212). During our 2007 study period, satellite images of sea surface temperature and chlorophyll reveal that hydrographic structures in surface layers were very complex in the Oyashio region (Okamoto et al., 2010). Kono and Sato (2010) identified three different water masses at our sampling station: Oyashio water (OYW), Modified Kuroshio Water (MKW), and Coastal Oyashio Water (COW).