Continental Shelf Research 112 (2016) 68–77

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Continental Shelf Research
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Research papers

Temporal variation in water intrusion of a tidal frontal system anddistribution of chlorophyll in the Seto Inland Sea, Japan
Tomohiro Komorita n, Xinyu Guo, Naoki Yoshie, Naoki Fujii 1, Hidetaka Takeoka
Center for Marine Environmental Studies, Ehime University, 2-5 Bunkyo-Cho, Matsuyama 790-8577, Japan

art ic l e i nf o

a b s t r a c t

Article history:
Received 20 January 2015
Received in revised form
14 October 2015
Accepted 23 October 2015
Available online 30 October 2015

Monthly ﬁeld surveys conducted from April to November 2009 in a tidal front in the Seto Inland Sea,
Japan provided a spatial and temporal dataset for investigating seasonal variations in nutrient supply andthe formation of a chlorophyll a (Chl-a) maximum. The upward diffusive ﬂux of nutrients is estimatedfrom observational data but it accounted for less than 5% of the nutrients needed to support the primary
production of phytoplankton in the front area of the stratiﬁcation region when the density difference
between the surface layer and bottom layer is greater than 0.5 kg m  3. Instead of vertical diffusion, thelateral intrusion of water with high nutrient concentration from the mixed area represented the major
nutrient supply in the front area. The depth of the lateral intrusion changed with the month: the surface
layer in July became the middle layer in August. According to the calculation of numerical model, an anticlockwise circulation is intensiﬁed by removing river runoff (i.e., low precipitation) in this study area, andthe change of lateral intrusion is likely caused by the change of anti-clockwise circulation along with thetemporal variation in river runoff. Consequently, the Chl-a peak appeared in the vicinity of the surface
front (up to 3 mg L  1) in July, but was in the subsurface (up to 9 mg L  1) in August. Diatom species were arelatively minor taxa of the phytoplankton community up to July, although a relatively high Si(OH)4-Si
concentration (up to 20 mmol L  1) was conﬁrmed. In contrast, the subsurface Chl-a maximum (SCM) in
August was mainly comprised of diatoms as evidenced by the reduction of both Si(OH)4-Si and Si/N fromthe surface to subsurface layer (0–20 m depth). Therefore, the supply of both nutrients and the seedpopulation necessary for the formation of the SCM results from the tidal frontal system and phytoplankton assemblages within the tidal front system should be varied on a monthly basis.
& 2015 Elsevier Ltd. All rights reserved.

Keywords:
Tidal front
Nutrient cycles
Thermal stratiﬁcation
Phytoplankton
Coastal zone
Seto Inland Sea

1. Introduction
Tidal fronts develop seasonally in many coastal waters (Simpson and Hunter, 1974), forming a transition zone between stratiﬁed regions developed as a result of solar heating and regions of
weak tidal currents and mixed regions due to strong tidal currents.
Traditionally, the stratiﬁed region has been considered to be a
two-layered system in which the transfer of nutrients from thebottom layer to the surface layer is suppressed by strong thermal
stratiﬁcation, allowing phytoplankton to remain in the euphotic
layer for extended periods of time causing the development of
nutrient depletion. In the mixed region, phytoplankton remain innCorresponding author. Present address: Faculty of Environmental and Symbiotic
Sciences, Prefectural University of Kumamoto, 3-1-100, Tsukide, Kumamoto 8628502, Japan.
E-mail address: komorita@pu-kumamoto.ac.jp (T. Komorita).
1
Present address: Institute of Lowland and Marine Research, Saga University, 1,
Honjyo-Cho, Saga 840-8502, Japan.

http://dx.doi.org/10.1016/j.csr.2015.10.015
0278-4343/& 2015 Elsevier Ltd. All rights reserved.

the euphotic layer for only a short period of time due to strong
vertical mixing and growth is likely to be limited by light availability; consequently, depletion of inorganic nutrients does notoccur. Optimal nutrient and light conditions are therefore achieved
only in the frontal zone between the mixed and stratiﬁed regions(Le Fevre, 1986; Mann, 2000). As a result, an area of high chlorophyll concentration forms in the surface layer or in the subsurface layer of the frontal zone (Franks and Chen, 1996; Holligan,n.d.; Martínez and Ortega, 2007; O'Boyle and Silke, 2009).
Several researchers have proposed an alternate model to the
two-layered system. In the proposed three-layered system of thestratiﬁed region at the tidal front, stratiﬁcation is separated into
surface, bottom, and middle layers, and nutrients from the bottom
layer of the stratiﬁed region are carried to the subsurface layer
through the adjacent mixed region by newly described mechanisms such as “nutrient bypass” (Takeoka et al., 1993), “tidally-fed
pump” (Pedersen, 1994), and “tidal pumping” (Richardson et al.,2000). In such three-layered systems, a subsurface chlorophyllmaximum (SCM) may develop in response to the horizontal

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69

Fig. 1. Study area and sampling station locations. On the left panel, the open circles represent sampling stations and the numbers in the white boxes represent water depth
in meters.

intrusion of nutrient-rich water into the photic zone (Richardson
et al., 2000) and the introduction of a seed population of phytoplankton species by the horizontal intrusion from the mixed
region (Kaas et al., 1991; Lund-Hansen and Vang, 2004; Nielsen
et al., 1990).
Tidal fronts are generated in early spring and disappear in early
autumn mainly because of seasonal variations in sea surface heat
ﬂux, wind forcing, and river discharge (Yanagi and Tamaru, 1990).
However, most observations of tidal fronts are carried out only inthe summer when the tidal front is fully developed (Gowen andBloomﬁeld, 1996; Holligan, n.d.; Townsend and Thomas, 2002). By
making observations only on established tidal fronts, seasonalvariations in biogeochemical dynamics and biogeochemical cycles
related to the tidal fronts remain largely unknown.
The Seto Inland Sea is a large, shallow, semi-enclosed sea inwestern Japan with an area of 17,000 km2 and mean water depthof 37 m (Fig. 1). This area has high biological productivity withannual primary production of 218 g C m  2 yr  1 in the water column (Tada et al., 1998). The annual ﬁsh catch in the sea is 3.2–28
times larger than that in other typical semi-enclosed seas such as
the Chesapeake Bay, the Baltic Sea, the North Sea, and the Mediterranean Sea (Takeoka, 2002). The deep but narrow straits in the
Seto Inland Sea produce strong tidal currents, leading to thoroughvertical mixing of the water column in the straits, which remain
mixed throughout the year (Takeoka, 2002). In contrast, the wideand shallow areas between the straits, known as nada in Japanese,
have weak tidal currents and strong stratiﬁcation develops in thesummer. Consequently, several tidal fronts form between the nadas and straits (Takeoka, 2002). The strait-nada system, which is a
tidal front system, along with the efﬁcient transport of nutrientsfrom the bottom layer of the nada ( ¼stratiﬁed region) to the euphotic layer near tidal fronts is considered to be one of the mostimportant mechanisms for sustaining the high biological productivity in the Seto Inland Sea.
In the eastern part of the Seto Inland Sea, tidal fronts develop in
spring and are sustained until early autumn (Yanagi and Koike,1987) between the Hoyo Strait and the Iyo-Nada (Fig. 1). In summer, intrusion of water carrying nutrients from the Hoyo Strait
into the middle layer of Iyo-Nada (Takeoka et al., 1993) is suggested to stimulate the photosynthetic activity of phytoplankton
assemblages (Yamamoto et al., 2000). In the central part of IyoNada (i.e., stratiﬁed region), residual currents show an anticlockwise circulation from spring to summer (Chang et al., 2009).

The anti-clockwise circulation should be affected by several
meteorological forcings such as solar heating, river discharge(i.e., rainfall), and wind forcing (Yu et al., in press). Thus, it is easily
expected that the biogeochemical cycle related to the middle layer
intrusion also varied with those seasonal meteorological forcings.
However, ﬁeld studies to examine seasonal variation in phytoplankton and nutrient distributions near the front have not been
documented.
In this study, we conducted monthly ﬁeld surveys from April to
November 2009 to span the development and disruption of stratiﬁcation in the Seto Inland Sea. We investigated spatio-temporalvariation in physical (water stratiﬁcation), chemical (nutrient), andbiological (phytoplankton assemblages) processes. We used thevertical distribution patterns of water temperature, salinity, density, dissolved inorganic nutrients [NO3 þNO2-N and Si(OH)4-Si],and chlorophyll a (Chl-a) (both total and size-fractionated) in the
tidal front system to examine the formation and disruption of thestratiﬁed region. Based on ﬁeld data, we evaluated the horizontal
intrusion of mixed region water into the middle layer of thestratiﬁed region, and we demonstrate the seasonal variation ofhorizontal intrusion, dominant phytoplankton taxa and nutrient
composition in the tidal front system. Finally, we discuss the timeseries variation of the phytoplankton assemblages within a tidal
frontal system based on the examination of the relationship between the Chl-a peak and the depth of the horizontal intrusion
and the supply of seed species to the phytoplankton community.

2. Materials and methods
Ten ﬁeld surveys were conducted from April to November 2009
(Table 1) in the three regions of Iyo-Nada with eight stations (Stns.
I1–I8: approximately 50–80 m in depth), Hoyo Strait with threestations (Stns. SA–SC: over 150 m in depth) and Bungo Channel
with one station (Stn. B4: approximately 80 m in depth) (Fig. 1).
The daily tidal range recorded at the Matsuyama Observatory(Fig. 1) was obtained for survey days. The M2 constituent was
dominant over the entire Seto Inland Sea, and the amplitude of theM2 tidal current constituent reached approximately 1.5 m s  1 in
the Hoyo Strait (Takeoka, 2002).
At each station, a rosette sampling unit equipped with conductivity, temperature and depth probes, a chlorophyll ﬂuorometer (JFE Advantech Co., Ltd.), and ten 3 l Niskin bottles (JFE

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T. Komorita et al. / Continental Shelf Research 112 (2016) 68–77

Table 1
Cruise information. Tidal difference (cm), and station and depth of size fraction Chla collection on each sampling occasion.
Dates in 2009

Tidal range (cm)23-Apr3-Juna25-Jun

278
271
361

22-Jul23-Aug29-Aug17-Sep1-Oct
14-Oct20-Nov

385
300
149
330
239
248
273

a

Size fraction of Chl-a
Stn.

Depth

I5, I8, B4
I5, I8, B4
I4
I8, B4I4, I8, B4
I4, I8, B4
I5, I8, B4
I4, I8I4, I8, B4
I4, I8, B4
I4
I8, B40, 10, 20, 30, 50 m
0, 10, 20, 30, 50 m
20, 25, 30, 50 m
0, 10, 20, 30, 50 m
0, 10, 20, 30, 50 m
0, 10, 20, 30, 50, 60 m
0, 10, 20, 25, 30, 50 m
0, 10, 20, 30, 50 m
0, 10, 20, 30, 50 m
0, 10, 20, 30, 50 m
0, 15, 30, 50, 60 m
0, 20, 40, 60, 80 m

Stns. SB and SC were observed on 5 June 2009.

Advantech Co., Ltd.) was deployed for sampling from the sea surface to 1 m above the sea bottom. Water sampling for nutrients[NO3 þNO2-N and Si(OH)4-Si] was conducted at 10 m intervals at
all stations. In addition, water sampling for total and size-fractionated Chl-a was conducted from 5 to 20 m interval at Stns. I4 or I5,
and I8 and B4 (Table 1).
After collection, samples for nutrient analysis were immediately ﬁltered through disposable ﬁlter units (25CS DISMIC
0.45 mm, Advantec), and the ﬁltrate was stored at  40 °C until
analysis on a BranþLuebbe Auto Analyzer III using the method ofStrickland and Parsons (1972).
For Chl-a analysis, water samples (300 ml) were ﬁltered through a cascaded 3 stage ﬁlter holder system (Sartorius) under low
vacuum (o 100 mm Hg) with ﬁlters of three pore sizes (10, 2 and
0.2 mm, Whatman Nuclepore membrane ﬁlter). The ﬁlters were
soaked in 5.0 ml DMF (N, N-dimethylformamide) and frozen in a
deep freezer ( 40 °C) until analysis. The Chl-a concentration in
the extract was measured with a ﬂuorometer (TD-700, Turner
Designs) using the method of Welschmeyer (1994). The resultsof total Chl-a concentration were used to construct a calibration
curve for converting measured in vivo ﬂuorescence from the CTD
cast values by the ﬂuorometer to Chl-a concentration (r2 ranged
between 0.595 and 0.918, varying for different cruises).
Daily mean temperature and precipitation from April to November were obtained at three meteorological stations (Nagahama, Seto, and Musashi; see Fig. 1 for locations). The 10 min averagewind velocity data collected at the meteorological station of Seto
were obtained for the periods of 19–22 July and 19–22 August,which were both 3 days prior to our survey cruises.

3. Calculation
Water in the middle layer of a three-layered stratiﬁed region
formed by mixing water from the two (i.e., surface and bottom
waters) or three (i.e., surface, bottom, and mixed area water) zones
is represented by straight and curved lines, respectively, in atemperature–salinity (T–S) diagram (Takeoka et al., 1993) (Fig. 2a).
Based on this concept, the deviation of data from two-end member mixing line on the T–S diagram reﬂects the presence of waterfrom other regions in the middle layer of stratiﬁed region (Fig. 2a).
To quantify the contribution of water from regions other than thesurface and bottom layers, we plotted the temperature and salinityof the simple mixing line, a straight line from the surface water(surface end member: mean from depth of 5 m to the surface) tothe bottom water (bottom end member: mean from depth of 5 m

Fig. 2. (a) Schematic diagram of the two and three end-member mixing models,and (b) temperature–salinity (T–S) diagram showing the schematic for estimating
intrusion strength (RT and RS).

above the bottom to the bottom) along with the actual data, andthen calculated the temperature and salinity difference between
the actual data and the simple mixing line, RT and RS, respectively,at each depth and at each station (Fig. 2b). The formula for R is

RTd = Td –(a*Sd + b),(1)

RSd = Sd – ( Td –b)/a,(2)

where subscript d is depth, S is salinity measured at depth d, T iswater temperature at depth d.
A positive RT and RS indicate an intrusion of comparatively
warmer and saline water, respectively, while a negative RT and RS
indicate an intrusion of comparatively colder and less saline water,
respectively.

4. Results
4.1. Meteorological data
The tidal range was between 239 and 385 cm, except for 29
August (Table 1). Mean daily air temperature and precipitation

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71

Fig. 3. Seasonal variation in air temperature and precipitation. Arrows indicate
sampling surveys.

data collected at three meteorological stations (Nagahama, Seto,and Musashi meteorological stations) were used to assess seasonalvariation of air temperature and precipitation in our samplingregion (Fig. 3). The air temperature increased from 15.8 °C in April
to 24.3 °C in July, and was within the range of 21.8–27.4 °C during
the summer (June–August) and decreased gradually starting in
September. Daily precipitation was 110 mm on 30 June, and themean daily precipitation was higher from June to July (7.0 mm)
than in August (2.8 mm).
4.2. Seasonal variation
Fig. 4a shows an isopleth depicting the time series of thedensity difference between the surface (2 m depth) and bottom(deepest available data) layers. We deﬁned the stratiﬁed period ashaving a density difference greater than 0.5 kg m  3. Using thiscriterion, the stratiﬁed region was shown to extend from approximately 40 km (Stns. I3–I6) to approximately 80 km (Stns. I1–
SA) in distance during the period from 3 June to 29 August. Density differences were mainly attributable to water temperature(Fig. 4b), but salinity played a contributing role in July (Fig. 4c).
According to Fig. 4, stratiﬁcation was strengthening from June toJuly and weakening from July to August.
Fig. 5(a) and (b) shows the isopleth at a depth of 20 m (between surface and bottom layer) of residual RT and RS as deﬁned by
Eq. (1), respectively. The RT showed a larger range during period ofstratiﬁcation (ca.  1 to 2 °C) than after the stratiﬁed period (lessthan 0.5 °C). Over the entire season, the RT was positive, indicating
the intrusion of relatively warm water from May to July, indicating
the intrusion of relatively cold water from July to August. The RS
was positive on July and slightly negative on August in the central
part of Iyo-Nada, indicating the intrusion of relatively saline andless saline water, respectively (Fig. 5b).
The temporal variation in physical and biogeochemical variables at the center of the stratiﬁed region (Stns. I4 and I5) is shownin Fig. 6. In the surface water, the temperature reached 26.3 °C inlate August (Fig. 6a), and the salinity dropped to 32.8 in July(Fig. 6b) due to high precipitation (Fig. 3). Although salinity was
the lowest in the surface water in July, water from the subsurface
to the bottom (depths of approximately 10–60 m) showed thelowest salinity in August (Fig. 6b). Chl-a concentration showed apeak of 2.4 mg L  1 in the subsurface layer (approximately 20–30 m
in depth) before July but reached a higher value after August(approx. 4 mg L  1 at a depth of 24 m on 29 August; Fig. 6d). A large
fraction ( 410 mm) Chl-a ranged between 0.07 and 0.34 in thesurface layer (depth of 0–20 m) from April to June and increased
over 0.5 after August (Fig. 6e). NO3 þNO2-N concentration was
below 1 mmol L  1 in the surface layer (depth of 0–20 m) from
April to October. A clear nitracline could be identiﬁed in theFig. 4. Seasonal variation for differences in density between the surface layer at a
depth of 2 m and bottom water at all sampling stations based on (a) sigma-t,(b) temperature, and (c) salinity. Distance is from Hoyo Strait at station SB along thesampling transect.

middle layer between depths of 20–30 m during the period ofstratiﬁcation. Si(OH)4-Si concentration was relatively high (above
10 mmol L  1) in the surface layer before July and decreased toapproximately 1.0 mmol L  1 after August. Since the reduction of
Si(OH)4-Si concentration after August coincided with the increasein overall Chl-a concentration and large-size fraction Chl-a (Fig. 6dand e), the dominant phytoplankton taxa likely changed from nondiatom species before July to diatom species after August.
4.3. Difference between pre-July and post-August
As shown in Fig. 6, the physical and biogeochemical variables in
the center of the stratiﬁed region showed a dramatic change between the July and August surveys. Here, we make a comprehensive
analysis of the possible differences in the physical environment.
To analyze the existence of differences in the pattern of winds
between July and August, wind rose diagrams were made based on
10 min average wind velocity and wind direction at the meteorological observatory of Seto (Fig. 1) during the 3 days preceding
each sampling survey (Fig. 7). There was little difference in thewind ﬁelds. The prevailing winds in both July and August were arelatively strong southerly winds (8–12 m s  1) that accounted fora small ratio (o5.0%) of the winds experienced in the area in these
2 months. Therefore, it is difﬁcult to expect that winds caused thedifferences in the data for the two months presented in Fig. 6.
Mean precipitation at the three meteorological stations exceeded 10 mm per day in the 6 days during the last two weeks ofJuly, and reached 40.5 mm per day before the sampling day of 21

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and showed low values in the subsurface layer (depth approximately 10–20 m) of the stratiﬁed region (Stn. I5) (Fig. 9e, f). Chl-a
concentration was higher in the subsurface layer at Stns. I1 and I5(Fig. 9g) where the residual is relatively high (Fig. 9e). The T–S plotof subsurface layer (approx. 10–20 m) on the stratiﬁed region (i.e.,
Stn. I5) curves in the opposite direction as that in the mixed andfrontal regions (i.e., both temperature and salinity were lower thanthat of mixed water) (Fig. 9h). Thus, the opposite curve indicates that the intrusion of water has less saline and low watertemperature.
4.4. Nutrient upward ﬂux on the stratiﬁed region in August
The highest Chl-a concentration was observed in the subsurface layer in the stratiﬁed region in August (Fig. 9g). To determine
whether the vertical ﬂux of nutrients could sustain the SCM, wecalculated the vertical nitrogen diffusive ﬂux across the nitracline
using the following equation,

Nitrogen flux

( mmol m−2 d−1) = − KZ (ΔN/ΔZ ),(3)

where KZ (m2 d  1) is the vertical eddy diffusion coefﬁcient, ΔN
is the difference in nitrogen concentration (mmol m  3) above andbelow the nitracline, and ΔZ is the nitracline thickness. Taking ΔN
as the difference between the mean NO3 þNO2-N concentration in
the surface layer and the middle layer in the central area of thestratiﬁed region on 22 August (Fig. 6f), ΔZ value of 20 m, or the
distance between the center of surface layer (10 m depth) and thedepth of 30 m (Fig. 6), and using the KZ value of Takeoka et al.
(1986) (0.9–3.5 m2 d  1) that was the same order as at the top of
bottom cold water in this study area was calculated by the 3D
numerical model (Yu et al., in press), the diffusive ﬂux forNO3 þNO2-N became 0.1–0.4 mmol N m  2 d  1.

5. Discussion

Fig. 5. Seasonal variation in the residuals from the simple mixing line at subsurface
layer (depth of 20 m) based on (a) temperature (RT) and (b) salinity (RS).

July (Fig. 8a). The vertical distribution of water temperature(Fig. 8b) showed the presence of a cold water mass in the bottom
layer of the stratiﬁed region (Stns. I4–I6). The lowest sigma-t (21.7)
was found in the surface layer of the stratiﬁed region (Fig. 8d)
where low salinity (Fig. 8c) was found along with high watertemperature (Fig. 8b). RT and RS indicating deviation from thesimple mixing line increased to above 1 °C and 0.2, respectively,around the subsurface layer (depth of 10–20 m) of the stratiﬁed
region (Stns. I2–I6). The T–S diagram for the center of the stratiﬁed
region (Stn. I5) shows a gentle curve to the mixed region (Stn. SC)
and partially overlaps with data for the frontal region (Stns. I7 andSA) (Fig. 8g). Chl-a concentration was high (up to 3 mg L  1) in thesurface layer (approximate depth of 10 m) of the frontal region(Stns. I7–SA) where a large horizontal gradient of water temperature and salinity exists (Fig. 8b and c).
In August, there was little rainfall (0.2–8.2 mm per day)(Fig. 9a). The survey conducted on 22 August suggests that thermal
stratiﬁcation was still in place in the stratiﬁed region (Stns. I3–I6)(Fig. 9b). However, the salinity gradient from the surface layer tothe bottom layer showed a difference of less than 0.4 and wassmaller than the gradient observed in July. Salinity in the surface
layer was higher in August than in July but that in the bottom layer
was lower in August than in July (Fig. 9c). The difference in density
between the surface layer and bottom layer was also smaller inAugust than in July (Fig. 9d). The RT and RS in August were negative

Typical nutrient sources driving the Chl-a peak in frontal regions are suggested as (1) horizontal intrusion from the mixed
region in the surface layer (Holligan et al., 1984), (2) horizontal
intrusion from the mixed region in the middle layer (Lund-Hansen
and Vang, 2004; Pedersen, 1994; Takeoka et al., 1993), and(3) vertical upward ﬂux from the bottom layer to the surface layer(Lips et al., 2011; Lund-Hansen, 2011). In the case of horizontal
intrusion in the surface layer, the peak Chl-a concentration is
generally found in the vicinity of the surface front; in the case ofhorizontal intrusion in the subsurface layer, the peak Chl-a concentration is in the subsurface layer of a stratiﬁed region (Le Fevre,1986). In this study, the peak Chl-a concentration was in the vicinity of the front in July (Fig. 8f), but the peak was in the subsurface in August (Fig. 9f). Thus, the nutrient source driving thesurface Chl-a peak in July was probably surface intrusion whilethat for the subsurface Chl-a peak in August was subsurface
intrusion.
Previous ﬁeld studies suggest that under certain conditions,
upward nutrient diffusive ﬂux is one of the important nutrient
sources for the development of the SCM (Lund-Hansen, 2011). The
primary production in our study area was found to be approximately 700 mg C m  2 d  1 in July and September (Tada et al.,1998), and the nitrogen-based production of phytoplankton was
estimated to be approximately 9 mmol N m  2 d  1, assuming C/
N¼ 6.6 (Redﬁeld et al., 1963). Since the upward diffusive ﬂux ofNO3 þNO2-N (0.1–0.4 mmol N m  2 d  1, Section 4.4) was less than
5% of the nutrients required to support the primary production ofphytoplankton, it must represent a minor contribution to theavailable nutrient pool. Therefore, the middle layer intrusion of

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Fig. 6. Seasonal variation in vertical distribution of (a) temperature, (b) salinity, (c) sigma-t, (d) Chl-a, (e) large fraction of Chl-a, (f) NO3 þ NO2-N concentration, (g) Si(OH)4-Si
concentration, and (h) Si/N recorded at Stns. I4 and I5. All the data are the average of data from two stations (Stns. I4 and I5), except for large Chl-a fraction which wasmeasured at only one station (Table 1).

Fig. 7. 96-h Wind rose diagrams for the periods of (a) 19–22 July and (b) 19–22 August.

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Fig. 8. (a) Precipitation in the vicinity of the 22 July sampling day and vertical distribution of (b) temperature, (c) salinity, (d) sigma-t, (e) residuals of temperature (RT),(f) residuals of salinity (RS), (g) Chl-a concentration, and (h) T–S diagram.

water from the mixed layer naturally becomes the most likely
candidate for supplying nutrients to the SCM that was observed inAugust.
The horizontal intrusion of the mixed region water to the
middle layer of the stratiﬁed region brings not only nutrients but
also other materials, including seed populations of diatoms. As
shown in Fig. 6, diatom species had a relatively minor representation among the taxa of the phytoplankton community
before July, although Si(OH)4-Si was relatively abundant. Only inAugust did diatom species become a major taxa in the phytoplankton community, and this shift was corroborated by the reduction of Si(OH)4-Si (and Si/N) (Fig. 6g, h). Without a seed population of diatom species, this shift would not have been possible.
Therefore, supply of a seed population of diatoms through the

middle layer intrusion presumably occurred sometime in July andcaused the subsequent uptake of dissolved inorganic nutrients,
especially Si(OH)4-Si. A similar case has been reported for the
haptophycean ﬂagellate Chrysochromulina polylepis which produces a subsurface bloom that is sustained by a middle layer intrusion of nutrients and seed populations from neighboring regions (Kaas et al., 1991).
According to a three-dimensional numerical ocean model for
the Seto Inland Sea (Chang et al., 2009), anti-clockwise circulationin the summer is reproduced at the central part of Iyo-Nada, whichis the stratiﬁed region in this study (Stns. I3–I6). The numericalmodeling using high resolution Princeton Ocean Model (POM) for
the Iyo-Nada (Yu et al., in press) shows that the anti-clockwise
circulation is weakened by the addition of river discharges, which

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Fig. 9. (a) Precipitation in the vicinity of the 22 August sampling day and vertical distribution of (b) temperature, (c) salinity, (d) sigma-t, (e) residuals of temperature (RT),(f) residuals of salinity (RS), (g) Chl-a concentration, and (h) T–S diagram.

is related to the precipitation (Fig. 10). A direct interpretation ofthis result to our observation is that the anti-clockwise circulation
was intensiﬁed between the July and August surveys due to thelower precipitation in August than in July (Figs. 3, 8a and 9a). With
the intensiﬁcation of anti-clockwise circulation, cold and less saline water originating from other regions around the sampling area
could be introduced to the stratiﬁed region.
The differences of residual currents ﬁeld between the case withriver runoff (Fig. 10b) and the case without river runoff (Fig. 10a)do highlight the importance of intrusion from north-western partof Iyo-Nada (i.e., Suo-Nada, see Fig. 1 for its place) (Fig. 10c). The surface
and bottom water of Suo-Nada are relatively less saline and colder,
respectively, than the water in the middle layer of Iyo-Nada (Kobayashi
et al., 2006). The anti-clockwise eddy across the Iyo-Nada and
Suo-Nada should carry the cool and less saline Suo-Nada water to thestudy area. Moreover, 12 planktonic diatom species are detected in thesurface sediment (including species have a resting stage such as Thalassiosira sp., Coscinodiscus sp., Chaetoceros sp., and so on) of Suo-Nada
during summer (Sarker et al., 2009). Thus, it is likely that neighboring
region (i.e., Suo-Nada) is one of the important sources of the cold andless saline water with seed population of diatom species.

6. Conclusion
Seasonal variation in the supply of nutrients in addition to an infusion of a seed population of diatoms in a tidal frontal system was
examined based on the results of water mass structure, and nutrientand Chl-a distributions.
The source water of the middle layer intrusion to the stratiﬁed

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T. Komorita et al. / Continental Shelf Research 112 (2016) 68–77

region changed from warm water to cold water. Such a drasticchange was likely caused by an increase (decrease) of freshwaterthat weakens (strengthens) the anti-clockwise circulation in July
(August) in the stratiﬁed region of Iyo-Nada. Moreover, formation of
the SCM with diatoms requires not only a sufﬁciently high supply ofnutrients but also the introduction of a seed population. Thus,
phytoplankton assemblages within the tidal front system should bevaried on a monthly basis.
To further clarify both the supply process of nutrients and thesource of the seed population in other similar systems, possible
sources and supply processes should be predicted by the numericalocean model and veriﬁed by the ﬁeld observations of phytoplankton
composition. Moreover, bottle incubation experiments are also required, that is conducted by mixing possible source water of seedpopulation and bottom water of stratiﬁed region (as nutrient source)
under the medium light irradiance to reproduce the phytoplankton
assemblages in the SCM.

Acknowledgment
We thank the captain of R/V Isana, Hidejiro Oonishi and Dr.
Morrow J. Stewart for his critical reading of the English text. We
also thank ﬁnancial support from the JSPS KAKENHI (21310012)
and a special project funded by Ehime University. The sampling
surveys were conducted with the cooperation of members of theCenter for Marine Environmental Studies.

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speed (m s  1).

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