Regional variability of factors controlling the ... - Wiley Online Library

JOURNAL OF GEOPHYSICAL RESEARCH: OCEANS, VOL. 119, 253–265, doi:10.1002/2013JC009187, 2014

Regional variability of factors controlling the onset timing and magnitude of spring algal blooms in the northwestern North Pacific Takuhei Shiozaki,1,2 Shin-Ichi Ito,3 Kazutaka Takahashi,1 Hiroaki Saito,3 Toshi Nagata,2 and Ken Furuya1 Received 7 June 2013; revised 8 December 2013; accepted 9 December 2013; published 16 January 2014.

[2] Satellite imagery and oceanographic data collected between 2003 and 2009 were used to examine factors controlling the onset timing and magnitude of spring algal blooms in the northwestern North Pacific. Consistent with the critical depth hypothesis, the spring bloom onsets coincided with the mixed layer depth (MLD) shoaling in the north of the Kuroshio extension and in Oyashio, where complex frontal physical structures and turbulence weakening, respectively, would be responsible for the MLD shoaling. In contrast, in the formation regions of the dense central mode water (D-CMW) and the transition region mode water (TRMW), bloom onsets coincided with possible turbulence weakening but not with MLD shoaling. The peak of chlorophyll a in the formation regions of the D-CMW (0.44 6 0.23 mg m23) and the TRMW (0.58 6 0.34 mg m23) were ca. 5 times lower than that in the Oyashio (2.54 6 0.74 mg m23), despite the fact that nitrate concentration during the prebloom period was high (10 mM) and MLDs became shallow enough at the bloom peak in all the three regions. These observations indicated that light conditions and nitrate concentration did not explain the regional variability in the magnitude of spring blooms. The bloom magnitude west of ca. 150 E and in the north Kuroshio extension was increased relative to that in the eastern region, suggesting a chemical property in the water delivered from the Okhotsk Sea that would influence the western bloom. Our results demonstrated that factors controlling the timing and magnitude of spring algal blooms depend on the physicochemical regime in the northwestern North Pacific. Citation: Shiozaki, T., S.-I. Ito, K. Takahashi, H. Saito, T. Nagata, and K. Furuya (2014), Regional variability of factors controlling the onset timing and magnitude of spring algal blooms in the northwestern North Pacific, J. Geophys. Res. Oceans, 119, 253–265, doi:10.1002/2013JC009187.

1.

Introduction

[3] The onset of the spring bloom has been classically explained by Sverdrup’s [1953] critical depth hypothesis (hereafter, CDH). This hypothesis states that when phytoplankton growth is not limited by nutrients, spring algal blooms are triggered by the alleviation of light limitation Additional supporting information may be found in the online version of this article. 1 Department of Aquatic Bioscience, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Tokyo, Japan. 2 Atmosphere and Ocean Research Institute, The University of Tokyo, Chiba, Japan. 3 Tohoku National Fisheries Research Institute, Fisheries Research Agency, Miyagi, Japan Corresponding author: T. Shiozaki, Atmosphere and Ocean Research Institute, The University of Tokyo, Chiba, 277–8564 Japan. (shiozaki@ aori.u-tokyo.ac.jp) This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.

©2013. The Authors. Journal of Geophysical Research: Oceans published by Wiley on behalf of the American Geophysical Union. 2169-9275/14/10.1002/2013JC009187

due to the mixed layer depth (MLD) shoaling that allows phytoplankton to accumulate in the sunlit layer [Sverdrup, 1953]. In other words, spring blooms are initiated when the MLD becomes shallower than the critical depth (CD). However, several studies have pointed out that this hypothesis does not always explain the timing of the spring bloom onsets [e.g., Townsend et al., 1992]. On the basis of their modeling study, Huisman et al. [1999] concluded that spring bloom onsets are not necessarily triggered by the shoaling of the deep mixed layer but instead is triggered by the weakening of turbulence. They postulated that the shutdown of the turbulence can allow phytoplankton to accumulate in the surface layer where sufficient light is available for algal growth (critical turbulence hypothesis). Taylor and Ferrari [2011] extended this hypothesis (hereafter, TFH) with the idea that the turbulent mixing weakens when the surface cooling fades. They suggested that the surface heat flux could be a good indicator of the spring bloom onset. The TFH has been supported by the results of numerical simulation and satellite-based observations in the subarctic North Atlantic Ocean [Taylor and Ferrari, 2011]. An alternative hypothesis to explain the initiation of the spring bloom in the North Atlantic Ocean has been proposed by Behrenfeld [2010], who found that net phytoplankton growth was initiated when the MLD was the

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deepest in winter. He suggested that the imbalance between phytoplankton growth and zooplankton grazing [cf. Yoshie et al., 2003] was responsible for the bloom initiation in the region examined. Apparently, bloom onset can be controlled by multiple mechanisms, with the predominant mechanism varying among oceanic regions. However, our knowledge is limited regarding the relationship between the operation of particular mechanism(s) of bloom onset and regional oceanographic features. [4] The Kuroshio and Oyashio are western boundary currents in the North Pacific Ocean. The Kuroshio carries warm, saline, low-nutrient water from the subtropical region, and the Oyashio carries cold, less-saline, high-nutrient water from the subarctic region. The confluence region of the two currents is characterized by complex frontal structures and the presence of mesoscale eddies generated by the frontal disturbances. The northwestern North Pacific is recognized to be highly productive and forms a major fishing ground. In fact, the total fish catch of 20.1 million tons in 2008 was the highest in the world, accounting for approximately 25% of the global fish catch [Food and Agriculture Organization (FAO), 2010]. Small migrating fishes, including Pacific saury, Japanese sardines, and chub mackerel, spawn in the subtropical region in the winter and/or spring and are transported by the Kuroshio to the Kuroshio extension region, where the larvae grow during the spring in the interfrontal zone, which is located between the Kuroshio extension and the subarctic front [Ito et al., 2004a; Watanabe, 2007; Okunishi et al., 2012]. After developing into juveniles, they migrate to feeding grounds in the subarctic region in the summer. The juveniles are successfully delivered to the subarctic region with the aid of the quasi-stationary jets, which flow across the interfrontal zone to the subarctic zone [Isoguchi et al., 2006], since their swimming ability is poor [Ito, 2010; Okunishi et al., 2012]. Food availability is considered to be a critical environmental factor for fish survival and recruitment and strongly depends on the timing and magnitude of primary production during the spring algal bloom [Platt et al., 2003; Ito et al., 2004b; Ware and Thomson, 2005; Okunishi et al., 2012]. Thus, knowledge of the processes affecting spring bloom dynamics is important for a better understanding of the mechanisms by which fisheries are sustained in the northwestern North Pacific region. [5] The Oyashio region is characterized by the occurrence of intensive spring blooms, and bloom dynamics have been intensively investigated [Kasai et al., 1997; Saito et al., 2002; Ikeda et al., 2010; Yoshie et al., 2010; Suzuki et al., 2011]. Consistent with the CDH, the spring bloom occurs in April and May and is initiated when the MLD shoals [Yoshie et al., 2010; Okamoto et al., 2010]. Diatoms prevail in the phytoplankton community during the spring bloom, whereas picophytoplankton are predominant during the prespring and postspring bloom seasons [Liu et al., 2004; Suzuki et al., 2011]. This diatom bloom is supported by macronutrient input to the euphotic zone during winter mixing, although iron supply from the Sea of Okhotsk may also contribute to the development of blooms [Nishioka et al., 2007, 2011; Nishioka, 2012]. In addition, it has been suggested that the spatial extent of the bloom is partly determined by mesoscale eddies [Sugimoto and Tameishi, 1992; Okamoto et al., 2010]. Compared to the

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Figure 1. Surface currents, front, and water-mass structure in the northwestern North Pacific described on the basis of Yasuda et al. [2003] with some modifications. Vectors denote an average geostrophic current larger than 0.1 m s21 between January and June. J1 and J2 are quasi-stationary jets identified by Isoguchi et al. [2006]. The mean subarctic front in April during the study period is indicated in black dash line. An anticyclonic eddy was identified in this averaged geostrophic current field (gray circle). Shaded areas indicate the formation regions of transition region mode water (TRMW), dense central mode water (D-CMW), shallow central mode water (SCMW), and subtropical mode water (STMW). Black squares are the representative areas based on the distribution of geostrophic current fields and of chlorophyll a. Oyashio region, studies on bloom dynamics in the interfrontal zone have been limited, with most biological observations being carried out in the region west of ca. 155 E [Yokouchi et al., 2000; Isada et al., 2009; Yoshie et al., 2010; Suzuki et al., 2011]. The available data have indicated that phytoplankton abundance in the interfrontal zone is lower than that in the Oyashio during the spring bloom periods [Isada et al., 2009; Yoshie et al., 2010; Suzuki et al., 2011]. However, the mechanisms of bloom onset and the spatial variation of bloom magnitude in the interfrontal zone are not entirely clear. [6] The present study examined spatial variation in possible factors affecting the onset and magnitude of spring blooms in the northwestern North Pacific. We used satellite data and a physical and chemical database with spatiotemporal resolutions sufficient to resolve subregional variability in bloom initiation and development. Specifically, we examined if processes involved in bloom onset and development differ among different subregions including the Oyashio and deep mode water formation areas. Potential bloom onset mechanisms were assessed by comparing chlorophyll data with physical parameters such as the critical depth, mixed layer depth, heat flux at the surface, and geostrophic current.

2.

Materials and Methods

2.1. Study Area [7] The study region is 25–45 N, 140–180 E in the western North Pacific (Figure 1). The main stream of the Kuroshio extension flows southeastward, whereas the branch generated in the western area of the Shatsky Rise, which is located around 158 E, flows northeastward [Mizuno and White, 1983]. The quasi-stationary jets called J1 and J2 [Isoguchi et al., 2006] are identified by the geostrophic

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current field in the north of the Kuroshio Bifurcation. The Oyashio flows southward along the south side of the Kurile Islands and Hokkaido, encountering the Kuroshio at the offshore of the northeast coast of Honshu Island. Along the Oyashio, anticyclonic mesoscale eddies are generally observed to the southeast of Hokkaido [Sugimoto and Tameishi, 1992; Yasuda, 2003]. The subarctic front is defined by the 4 C isotherm at 100 m [Favorite et al., 1976]. The region between this front and the main stream of the Kuroshio extension is defined as the interfrontal zone [Yasuda, 2003]. In the western North Pacific, several mode waters are formed regionally [Yasuda, 2003; Saito et al., 2007; Oka and Qiu, 2012]. Mode waters originate from the deep mixed layers generated by the winter convective mixing and are formed after being capped by a seasonal pycnocline. Mode waters are classified according to the formation region and their characteristics; they include the transition region mode water (TRMW), the shallow central mode water (S-CMW), the dense central mode water (DCMW), and the subtropical mode water (STMW). 2.2. Satellite Data [8] Satellite-derived chl a concentration, the geostrophic current field, and sea surface temperatures (SSTs) were examined for the period between 2003 and 2009. To examine the timing of the spring bloom, we mainly focused on the period from January to September. [9] Monthly and daily level-3 chl a (mg m23) for case 1 water were obtained from the European Space Agency (ESA) GlobColor project (http://hermes.acri.fr/) with a 25 km and a 4 km resolution, respectively. The chl a data were processed from the weighted average of multiple sensors (Sea-viewing Wide Field-of-view Sensor (SeaWiFS), Medium Resolution Imaging Spectrometer (MERIS), and Moderate Resolution Imaging Spectroradiometer (MODIS)-Aqua) [Maritorena et al., 2010]. Daily chl a data were used to identify short time scale of the spring bloom and were composited into 7 day periods for comparisons with weekly geostrophic current fields obtained from the Archiving, Validation and Interpretation of Satellite Oceanographic (AVISO) data server (ftp. aviso.oceanobs.com) with 1/3 resolution. The geostrophic current was calculated from the absolute dynamic topography which was derived from the merged products of the Jason-1, Jason-2, GFO, and Envisat satellite altimeters. For comparisons with other parameters, the geostrophic current fields composited into approximately 1 month periods were also obtained. Monthly level-3 SST ( C) with 9 km resolution measured by MODIS was obtained from the OceanColor website (http://oceancolor.gsfc.nasa.gov/). 2.3. Argo and Climatological Nitrate Data [10] Monthly vertical profiles of temperature and salinity measured by Argo floats with 2 resolution in the study region from the periods between 2003 and 2009 were obtained from the Japan Argo Delayed-mode Data Base [Hosoda et al., 2010] (http://www.jamstec.go.jp/ARGO/ argo_web/MapQ/Mapdataset_e.html). The MLD was calculated from the density profiles as the depth at which sigma-theta increased by 0.125 relative to that at a depth of 10 m. The subarctic front, defined by the presence of a 4 C isotherm at 100 m depth [Favorite et al., 1976], was determined from the Argo’s vertical temperature profiles. [11] Climatological monthly nitrate concentration at the surface with 1 resolution in the study region was obtained

from World Ocean Atlas 2009 [Garcia et al., ] (http:// www.nodc.noaa.gov/OC5/WOA09/pr_woa09.html). 2.4. Factors Controlling Spring Bloom Onset 2.4.1. Critical Depth [12] The CD is defined as the depth where the integrated net productivity balances with the integrated loss rate [Sverdrup, 1953]. In this study, we calculated the critical depth using satellite data sets following the method presented by Okamoto et al. [2010]. Briefly, the critical depth is expressed using the following equation: CD Ie 5 12exp ð2Kd ðPARÞ CDÞ Kd ðPARÞ Ic

(1)

where Kd(PAR) is the diffuse attenuation coefficient for photosynthetically available radiation (PAR), and Ie and Ic are the PAR at the surface and at the compensation depth, respectively. Kd(PAR) is calculated from the diffuse attenuation coefficient at 490 nm wavelengths (Kd(490)) [Morel et al., 2007]. Kd ðPAR Þ50:066510:874Kd ð490Þ20:00121ðKd ð490ÞÞ21

(2)

[13] The Ie is obtained by multiplying PAR by the weighting factor of 0.5 [Okamoto et al., 2010]. PAR and Kd(490) data were derived from monthly MODIS 9 km data sets for the periods from 2003 to 2009 (http://oceancolor.gsfc.nasa.gov/) and were regridded to 2 resolution. The Ic was assumed to be constant (2.88 W m22) [Okamoto et al., 2010] in the study region. To convert the unit of Ic from irradiance to photon flux (51.14 mol photons m22 d21), we assumed under daylight condition [Thimijan and Heins, 1983]. 2.4.2. Heat Flux [14] According to TFH, turbulence weakening can be estimated by the net heat flux of the surface water. In the present study, we used monthly total downward heat flux at the surface with 1/3 latitude 3 1 longitude resolution calculated by the NCEP (National Centers for Environmental Prediction) Global Ocean Data Assimilation System for the study periods from 2003 to 2009 (http://www.esrl.noaa. gov/psd/data/gridded/data.godas.html). 2.5. Areal Average [15] To examine spring bloom in each region, the present study selected seven 2 3 2 representative areas. Each parameter was regridded to 1 resolution prior to average in each subarea. The initiation date of the spring bloom in each subarea was defined as the month in which the average chl a was significantly (t test, p < 0.05) higher than the minimum value in winter (December-February).

3.

Results

3.1. Spatiotemporal Variability in the Geostrophic Current and SST [16] The monthly geostrophic current field did not change drastically in the study area during the study months (Figure 2). The SST in the study region increased steadily over time. The spatial distribution of SST was related to the geostrophic current field in each month,

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Figure 2. Climatologically (2003–2009) monthly average sea surface temperature between January and September. Arrows denote geostrophic currents >0.1 m s21 ; white dashed lines denote the subarctic front. reflecting the distributions of warm and cold waters transported by currents (Figure 2). The southern end of the subarctic front moved from 42 N to 41 N from January to March and remained at 41 N until May, moved back to 42 N in June, and remained there until September. The SST in the mainstream of the Kuroshio extension was stable, ranging between 16 and 18 C, from January to April, increasing to 18–20 C in May and to 20–22 C in June, and exceeding 22 C after July. With the increase in SST after May, stratification was established over the whole study region. 3.2. Spatiotemporal Variability in MLD and Nitrate [17] Four mode water formation regions (Figure 1) were broadly identified as the regions where the MLDs were relatively deep between February and March (Figure 3a). The STMW formation region was bounded by 30–35 N and to the west of 155 E, where the MLD was already deep in January and progressively deepened toward March [Masuzawa, 1969; Suga and Hanawa, 1990]. The TRMW and the DCMW formation regions were situated in the east of the quasi-stationary jet J1 (TRMW) and J2 (D-CMW), respectively (Figure 1), where the MLD was deep between February and April (Figure 3a) [Nakamura, 1996; Suga et al., 1997; Saito et al., 2007]. The S-CMW formation region was considered to be a subregion in the east of the bifurcation point of the Kuroshio extension where the MLD was deep in February, although the extent of the deepening during this period was not as pronounced as that in the DCMW formation region [Tsujino and Yasuda, 2004]. [18] MLDs were shallow in the north of the mainstream of the Kuroshio extension and near the Japanese coast from February to April. In the interfrontal zone, the MLD became shallow, in May especially, west of 150 E. From

June to September, the MLD was shallower than 30 m in the whole study region. [19] Climatological surface nitrate increased (decreased) in the lower (higher) SST area (Figures 2 and 3b). Nitrate concentration north of 35 N was higher in the western region than in the eastern between January and May. In the region south of 35 N west of 155 E, nitrate concentration in February was enriched relative to that in the eastern region, which appeared to be a reflection of the strong vertical mixing in the STMW formation region. Nitrate concentration exceeded 1 mM north of 35 N between January and May. 3.3. Spatiotemporal Variability in Chl a [20] Throughout the observation period, in most parts of the region where chl a concentration exceeded 0.4 mg m23, nitrate concentration was >1 mM (Figures 3b and 4). In January and February, chl a was low (0.4 mg m23) occurred in the north of the mainstream of the Kuroshio extension and west of ca. 150 E where the MLD was generally shallow. This tendency held throughout May. However, the tendency that chl a concentrations were higher in regions where the MLDs were shallower was not evident between June and September. In June, a relatively high chl a water mass (>0.4 mg m23) displayed a ‘‘band’’ at ca. 40 N. This chl a band moved northward after July. [21] Figure 5 presents weekly chl a distribution during the spring bloom period in and around the western interfrontal zone where high monthly average chl a (>1 mg

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Figure 3. Climatologically monthly average (a) mixed layer depth (2003–2009) and (b) nitrate concentration at the surface [Garcia et al., 2010] between January and September. White dashed lines denote isopleth of 1 mM nitrate ; gray areas denote no data. Chlorophyll a contours (contour interval of 0.2 mg m23 with 1 mg m23 thickened and 2 mg m23 dashed) are superimposed. m23) was observed (Figure 4). High chl a waters occurred in the north of the Kuroshio extension which was identified as a high geostrophic current flux at ca. 34 N. These waters did not spread homogenously but were distributed along the courses of the currents, indicating that the current-

driven lateral advection affected chl a distribution. In the south of the front of the Kuroshio extension, chl a increased near the center of cyclonic eddies, which presumably reflected the nutrient supply mediated by the eddy induced upwelling.

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Figure 4. Climatologically (2003–2009) monthly average chlorophyll a between January and September. Arrows denote geostrophic currents >0.1 m s21 ; white dashed lines denote isopleth of 2 mg m23 chlorophyll a. 3.4. Spatiotemporal Variability in the Ratio of Critical Depth to MLD [22] In the region north of 35 N, nitrate concentration generally exceeded 1 mM, and the ratio of critical depth to MLD was less than 1 during January and February (Figure 6). In this region at those times, primary production was probably limited by light availability according to the CDH. In March, the critical depth exceeded the MLD in the Oyashio and some regions located at the north of the Kuroshio extension, indicative of an improvement in light conditions Apr. 24 - 30, 2005

for algal growth. Later on, the region where the critical depth exceeded the MLD expanded progressively, with a notable delay in the TRMW and D-CMW formation regions where the MLD was deeper than the critical depth in April. 3.5. Spatiotemporal Variability in Heat Flux at the Surface [23] In the northwestern North Pacific, heat flux estimates have indicated that the Kuroshio extension loses large amounts of heat to the atmosphere when averaged

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Figure 5. Snapshots of 7 day average chlorophyll a during spring blooms in 2005, 2007, and 2009. Arrows denote geostrophic currents >0.3 m s21 ; white lines denote isopleth of 2 mg m23 chlorophyll a; gray areas denote no data. 258


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Figure 6. Climatologically (2003–2009) monthly average ratio of the critical depth to the mixed layer depth between January and June. Arrows denote geostrophic currents >0.1 m s21 ; gray areas denote no data. over a year [Talley, 1984]. This reflects the fact that the Kuroshio carries a large volume of warm water from south to north. However, the mode of heat flux in the Kuroshio extension region changes seasonally (Figure 7). The region loses the largest amount of heat in the winter and changes to gaining heat between April and May, followed by warming during the summer [Qiu and Kelly, 1993]. The area where the net heat flux was close to zero was distributed south of ca. 30 N and east of 170 E in February. In March, the area where the net heat flux was not negative was observed broadly in the study region, except for the mainstream of the Kuroshio extension and the STMW formation region. 3.6. Regional Characteristics of Temporal Variation in Chl a [24] In Area 1 (Oyashio), the MLD became shallower than the critical depth in March, followed by a substantial

(>300%) increase in chl a concentration in April (Figure 8a). Subsequently, nitrate concentration decreased rapidly after the increase in chl a (Figure 8b). These results are in agreement with previous findings by Okamoto et al. [2010] who reported that the timing of the chl a rise was consistent with the CDH in this area in 2006 and 2007. Surface salinity in Area 1 decreased from March to May (Figure S1a). During this period, the MLD became shallow and the spring bloom occurred. A salinity decrease at the surface was not observed in other areas (Figure S1a), and the MLD shoaling was accompanied by changes in vertical temperature distributions (Figure S1b). The timing of increases in chl a was nearly similar in all areas, occurring in either February (Areas 4 and 5) or March (Areas 1, 2, 3, 6, and 7). Surface PAR at the bloom onset varied 17.1–30.0 mol photon m22 d21 (Table 1), and there was no evident latitudinal difference. Although the nitrate level at the bloom onset

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Figure 7. Climatologically (2003–2009) monthly average net heat flux (W m22) between January and June. Arrows denote geostrophic currents >0.1 m s21 ; gray areas denote no data. 259


a) Chlorophyll a, Mixed Layer Depth (MLD), and Critical Depth Area 1

Area 2

0

2.5

0

2.5 100

100

100

2.0

2.0

2.0

1.5

200 1.5

200 1.5

200

1.0

300 1.0

300 1.0

300

0.5

0.5

0.5 400 0.0 Dec Jan Feb Mar Apr May Jun Jul Aug Sep

Area 4

400

0.0 Dec Jan Feb Mar Apr May Jun Jul Aug Sep

Area 5

0

2.5

Area 6

0

2.5

400


0

2.5

100

100

100

2.0

2.0

1.5

200 1.5

200 1.5

200

1.0

300 1.0

300 1.0

300

0.5

0.5

0.5

2.0

400 0.0 Dec Jan Feb Mar Apr May Jun Jul Aug Sep

Area 7

400


Depth [m]

Chlorophyll a [mg m -3 ]

Area 3

0

2.5


0

2.5 100 2.0 1.5

200

1.0

300

Chlorophyll a Mixed Layer Depth (MLD) Critical Depth

0.5 400 0.0 Dec Jan Feb Mar Apr May Jun Jul Aug Sep

b) Chlorophyll a, Nitrate, and Net Heat Flux Area 1 2.5 15

2.0 1.5

Area 2

20

10

1.0

2.5

2.5

2.0

15 2.0

1.5

1.5

10

200 0 -100 10

-300

0.5

0.0 0 Dec Jan Feb Mar Apr May Jun Jul Aug Sep



1.5

10

1.0

15

2.0

200

1.5

10

15

2.0 1.5

10

1.0

5

5

5

0.5

0.5

0.5




20

1.5

100 0 -100 -200 -300 -400 -500 -600

300 200

2.5 2.0

300

20

2.5

1.0

Area 7

-600

Nitrate [µM]

15

-500

Area 6

20

-400

Net Heat Flux [W m -2 ]

Area 5

20 2.5

2.0

-200

5

5 0.5

Area 4

100

15

0.5

2.5

300

20

1.0

1.0 5

Chlorophyll a [mg m -3 ]

Area 3

20

15

100

Chlorophyll a Nitrate Net Heat Flux

0 -100 10

1.0

-200 -300

5 0.5 0.0 0 Dec Jan Feb Mar Apr May Jun Jul Aug Sep

-400 -500 -600

Figure 8. Seasonal variations in (a) chlorophyll a (red), mixed layer depth (blue), and critical depth (green) and (b) nitrate (dark yellow) and net heat flux (sky blue) in each representative area. Error bars denote the standard deviation of parameters in each month during the study period. Shaded lines indicate the initiation date of the spring bloom.

260


Table 1. Summary of Initiation Date of Spring Blooma

Subarea

Location

Area 1 Area 2 Area 3 Area 4

41–43 N, 144–146 E 40–42 N, 156–158 E 40–42 N, 168–170 E 36–38 N, 142–144 E

Area 5

36–38 N, 150–152 E

Area 6 Area 7

34–36 N, 166–168 E 31–33 N, 143–145 E

Oceanic Region Oyashio TRMW D-CMW North of KE in the west of 150 E North of KE in the east of 150 E S-CMW STMW

Initiation Timing of Spring Bloom

CD/ MLD

Mode of Surface Heat Flux

Surface PAR (mol photon m22 d21)

Nitrate (mM)

Maximum of Chl a (mg m23)

Maximum of MLD (m)

March March March February

1