Interdisciplinary Studies on Environmental Chemistry—Marine Environmental Modeling & Analysis,
Eds., K. Omori, X. Guo, N. Yoshie, N. Fujii, I. C. Handoh, A. Isobe and S. Tanabe, pp. 39–49.
© by TERRAPUB, 2011.

Ecosystem and Nutrient Dynamics in the Seto Inland Sea, Japan
Naoki YOSHIE1, Xinyu GUO 1, Naoki FUJII 1 and Tomohiro KOMORITA2
1

Center for Marine Environmental Studies, Ehime University,
2-5, Bunkyo-cho, Matsuyama 790-8577, Japan
2
Faculty of Environmental and Symbiotic Sciences, Prefectural University of Kumamoto,3-1-100, Tsukide, Kumamoto 862-8502, Japan(Received 24 December 2010; accepted 12 January 2011)

Abstract—We studied the ecosystem and nutrient dynamics in the Seto Inland
Sea from both field observations and numerical modeling. We investigatedspatiotemporal variations in the group composition of phytoplankton and in the
nutrient concentrations in Iyo-Nada, Hoyo Strait and Bungo Channel (parts of
the Seto Inland Sea) with monthly field observations in 2009. From spring toearly summer, nano- and pico-phytoplankton dominated in all three regions.
From late summer to autumn, micro-phytoplankton (diatoms) bloomed in IyoNada and Hoyo Strait, while a bloom was not observed in Bungo Channel. This
autumn diatom bloom was probably caused by nutrient supply associated witha breakdown of the bottom cold water dome. For understanding the mechanismof the nutrient cycle and plankton dynamics, we developed a plankton functional
types model eNEMURO (4 Nutrient, 4 Phytoplankton, 4 Zooplankton, 3
Detritus compartments), which was an extended version of NEMURO (astandard lower-trophic-level marine ecosystem model of PICES) by introducing
the microbial food web and phosphorous cycle and dividing diatoms into two
compartments according to temperature dependency. The model successfully
reproduced the nutrient and phytoplankton dynamics observed in both IyoNada and Bungo Channel.
Keywords: ecosystem dynamics and model, group composition ofphytoplankton, Seto Inland Sea

INTRODUCTION

The Seto Inland Sea is a semi-enclosed coastal sea located in the western part ofJapan. It is divided by about 600 small islands and peninsulas into several widebasins (called “Nada” in Japanese), and these basins are connected by narrowstraits. This complex geometry creates various marine environments in the Seto
Inland Sea. The efficiency of the primary production (nitrogen cycling rate) in the
Seto Inland Sea is much higher than other enclosed seas (Seto Inland Sea: 4–5
yr–1, Baltic Sea: 0.27 yr–1, Mediterranean Sea: 0.18 yr–1, North Sea: 1.5 yr–1,
Takeoka, 1997), and fish catch per unit area (21 t km–2yr–1) in this sea is one of
the world’s most productive (Hashimoto et al., 1997). The Seto Inland Sea is also
one of the most industrialized regions in Japan, and about 35 million people live39

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around this sea. The marine environment and ecosystem in the Seto Inland Sea has
been significantly affected by anthropogenic impacts over the last 40 years(Takeoka, 2002). For sustainable fisheries and recovery of the marine environment
and ecosystem, it is now more important than ever to carry out interdisciplinary
and comprehensive research. In recent years, numerical simulations have been
dramatically improved. A high resolution three dimensional physical ocean
model for the Seto Inland Sea was developed by Chang et al. (2009), and themodel successfully reproduced the physical field in the Seto Inland Sea. As thenext step, development of a realistic marine ecosystem model for the Seto Inland
Sea is expected. Development of the marine ecosystem model needs
biogeochemical data in the Seto Inland Sea for the model validation. However
there is only field data for some specific locations (Koizumi, 1999; Katano et al.,2007; Hirose et al., 2008), and no comprehensive data set is available in thewestern part of the Seto Inland Sea. In this study, we observed the spatiotemporal
distribution of nutrients and phytoplankton in Iyo-Nada, Hoyo Strait, and Bungo
Channel (western parts of the Seto Inland Sea) in 2009. We also developed alower-trophic-level marine ecosystem model for the Seto Inland Sea forunderstanding the mechanism of the ecosystem and nutrient dynamics. We show
the nutrient and phytoplankton dynamics and the preliminary results of ourecosystem model.
METHODS

Field observations were performed in the following three regions, Iyo-Nada
with 7 stations, Hoyo Strait with 4 stations, and Bungo Channel with 16 stations(Fig. 1). We surveyed these regions 10 times from April to November in 2009.
Water samples were collected onboard RV Isana using Niskin samplers on an
Automatic Rosette sampler with a CTD system (JFE Advantech), and temperature,salinity, chlorophyll a (hereafter as Chl. a), and turbidity, were also measured bythis CTD system. Nutrient samples were filtered with disposable filter unit
(DISMIC-25cs 0.45 µm, Advantec), frozen in a deep freezer (–40°C), andmeasured in the onshore laboratory using a nutrient analyzer (Auto Analyzer III,
Bran+Luebbe). Concentration of size-fractionated Chl. a was determined by thefollowing procedures. Water samples (300 ml) were filtered onto three pore size
filters (10 µm, 2 µm, and 0.2 µm, Whatman Nuclepore membrane filter) withcascaded 3 stage filter holder system (Sartorius) under low vacuum (<100 mm
Hg). The filters were soaked in 5.0 ml DMF (N,N-dimethylformamide) and frozen
in the deep-freezer (–40°C) until analysis. Concentration of Chl. a in the extract
was measured by a fluorometer (TD-700, Turner Designs) with the method of
Welschmeyer (1994). We regarded phytoplankton with cell size > 10 µm as
micro-phytoplankton (mainly diatoms), with cell size 2–10 µ m as nanophytoplankton, and with cell size 0.2–2 µm as pico-phytoplankton.
We developed a lower-trophic-level marine ecosystem model eNEMURO(Fig. 2). eNEMURO including 4 nutrient compartments (NO3, NH4, Si(OH) 4,
PO4), 4 phytoplankton compartments (2 types of micro-phyt., nano-phyt., pico-Ecosystem and Nutrient Dynamics in the Seto Inland Sea

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Fig. 1. Sampling locations in the western part of the Seto Inland Sea.

phyt.), 4 zooplankton compartments (macro-zoo, meso-zoo, micro-zoo, nanozoo), 3 detritus compartments (POM, DOM, Opal). eNEMURO was an extended
version of NEMURO (North Pacific Ecosystem Model for Understanding Regional
Oceanography, Yoshie et al., 2003, 2005, 2007; Yoshie and Yamanaka, 2005;
Kishi et al., 2007), which is a standard lower-trophic-level marine ecosystemmodel of PICES (North Pacific Marine Science Organization). We extended

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Fig. 2. Schematic view of the marine ecosystem model “eNEMURO”.

NEMURO by introducing the microbial food web and the phosphorous cycles anddividing micro-phytoplankton into two compartments according to temperaturedependency. We simulated the ecosystem dynamics in Iyo-Nada and Bungo
Channel using box type eNEMURO. For the driving force of the model, we usedseasonal changes in the sea surface temperature, sea surface light condition,
nutrient concentrations of bottom water, and nutrient concentrations in Hoyo
Strait observed in 2009 as a horizontal boundary.
RESULTS AND DISCUSSION

Seasonal changes in the Iyo-Nada
From spring to early summer, concentration of nitrate was low at the surface,and concentration of Chl. a was also low with the dominance of small size
phytoplankton groups such as pico- and nano-phytoplankton (Fig. 3). In summer,
a bottom cold water dome (hereafter, CWD) formed and concentrations of nitrateand silicic acid in the CWD were remarkably high compared with those in thesurface water. From the early summer, concentration of Chl. a over the CWD
increased associated with the dominance of large size phytoplankton such as
diatoms, and the fraction of diatoms in the CWD was also high. From late summer
to autumn, a noticeable diatom bloom occurred with the collapse of the CWD, andconcentrations of silicic acid and nitrate at the surface were depleted by theFig. 3. Seasonal changes of nutrient and phytoplankton in Iyo-Nada. Temp. is water temperature (°C). [NO3] and[Si(OH)4] are concentrations of nitrate and silicic acid (µ M). [SS] is concentration of suspended solids (FTU).
[Chl. a] is the concentration of chlorophyll a ( µg l–1). Diatom%, Nano-phyt.%, and Pico-phyt.% are fractionsof each size group in the phytoplankton assemblage (%). CWD is the bottom cold water dome.

Ecosystem and Nutrient Dynamics in the Seto Inland Sea
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diatom bloom. After autumn, concentration of Chl. a was kept relatively highvalue with the dominance of diatoms. Seasonal change in the silicic acid wassignificantly different from that in the nitrate, while it was similar to that insuspended solids (hereafter, SS). The source of silicic acid might be related withterrigenous suspended matter supplied by rivers in the central and eastern partsof the Seto Inland Sea.
Seasonal changes in Hoyo Strait
The sea water in Hoyo Strait was vertically well mixed by tidal mixing
throughout the year (Fig. 4). From spring to early summer, concentrations of
nitrate and Chl. a were low, and nano- and pico-phytoplankton were dominant in
the phytoplankton assemblage. From the early summer, concentration of Chl. a
and fraction of diatoms increased with the increase in nitrate. From late summer
to autumn, diatoms bloomed with the rapid decrease of silicic acid. Seasonalchanges of phytoplankton in Hoyo Strait after autumn were different from thosein Iyo-Nada. Although nutrient concentrations increased after autumn,
concentration of Chl. a and the fraction of diatoms quickly decreased, becauselight conditions for diatoms might degenerate with the decrease in the solar
radiation and deep tidal mixing. Seasonal change in the silicic acid in Hoyo Strait
was different from that for nitrate as in Iyo-Nada, however it was also different
from that in the SS.
Seasonal changes in Bungo Channel
The ecosystem changes in Bungo Channel before early summer were similar
to the other two regions, however a remarkable diatom bloom did not occur in thisregion (Fig. 5). Though the fraction of diatoms increased after early summer, theother small phytoplankton groups also maintained a relatively high fraction in thephytoplankton assemblage. This is because the concentration of silicic acid in
Bungo Channel was lower than values in the other two regions, and growth of
diatoms in Bungo Channel was limited by insufficient silicic acid. After autumn,
the concentration of Chl. a remained at a relatively high value, and the 3 groupsof phytoplankton coexisted with each other.
Model simulation in Iyo-Nada and Bungo Channel
Our ecosystem model successfully reproduced the nutrient and phytoplankton
dynamics in both Iyo-Nada and Bungo Channel (Figs. 6 and 7). The timing ofchange and variation ranges of nutrients were similar to those observed in 2009.
The different seasonal changes between silicic acid and the other two nutrientswere also well reproduced. Seasonal changes not only in the total phytoplankton
but also in the 3 groups of phytoplankton were also well reproduced in bothregions. Fractions of diatoms simulated in Bungo Channel were lower than thosein Iyo-Nada. Analysis of the nutrient limitation terms in the model showed thatdiatoms in Bungo Channel were under relatively severe stress by insufficientsilicic acid compared with silicic acid concentrations in Iyo-Nada.

Fig. 4. Seasonal changes of nutrient and phytoplankton in Hoyo Strait. Temp. is water temperature (°C). [NO3]and [Si(OH) 4] are concentrations of nitrate and silicic acid (µ M). [SS] is concentration of suspended solid(FTU). [Chl. a] is the concentration of chlorophyll a (µ g l–1). Diatom%, Nano-phyt.%, and Pico-phyt.% are
fractions of each size group in the phytoplankton assemblage (%).

Ecosystem and Nutrient Dynamics in the Seto Inland Sea
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Fig. 5. Seasonal changes of nutrient and phytoplankton in Bungo Channel. Temp. is water temperature (°C). [NO3]and [Si(OH) 4] are concentrations of nitrate and silicic acid (µ M). [SS] is concentration of suspended solid(FTU). [Chl. a] is the concentration of chlorophyll a (µ g l–1). Diatom%, Nano-phyt.%, and Pico-phyt.% are
fractions of each size group in the phytoplankton assemblage (%).

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Fig. 6. Seasonal changes of nutrient and phytoplankton in Iyo-Nada simulated by eNEMURO andobserved in 2009. Upper figures show the concentrations of silicic acid, nitrate, and phosphate( µM). Lower figures show the concentrations of chlorophyll a of pico-, nano-, micro- (diatoms),
and total-phytoplankton. Lines are model results. Circles and triangles are observed data
averaged from 0 to 40 m depth.

Fig. 7. Seasonal changes of nutrient and phytoplankton in Bungo Channel simulated by eNEMURO
and observed in 2009. Upper figures show the concentrations of silicic acid, nitrate, andphosphate ( µM). Lower figures show the concentrations of chlorophyll a of pico-, nano-, micro(diatoms), and total-phytoplankton. Lines are model results. Circles and triangles are observed
data averaged from 0 to 40 m depth.

CONCLUSION

We clarified spatiotemporal distribution of nutrient and phytoplankton in thewestern Seto Inland Sea, Japan by monthly field observations. Between springand early summer, nano- and pico-phytoplankton were dominant under low
nutrient conditions. In late summer, typical diatom blooms occurred with thecollapse of the bottom cold water dome in Iyo-Nada. On the other hand, a

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remarkable diatom bloom was not observed in Bungo Channel due to the low
concentration of silicic acid. After autumn, diatoms maintained a relatively highbiomass in Iyo-Nada, while the three size groups of phytoplankton coexisted in
Bungo Channel. Using a lower-trophic-level marine ecosystem model, we
successfully reproduced the characteristic ecosystem dynamics in both Iyo-Nada
and Bungo Channel. The mechanism of the low diatom fraction in Bungo Channel(i.e., insufficient silicic acid) was confirmed by the model.
Acknowledgments—We thank H. Onishi as the captain of the RV “Isana” and theparticipants in the field observations. The study was supported by a grant-in-aid for the
Global COE Program funded by Ministry of Education, Culture, Sports, and Technologyof Japan (MEXT) and Japan Society for the Promotion of Science (JSPS), and JSPS
KAKENHI (21310012).
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N. Yoshie (e-mail: nyoshie@sci.ehime-u.ac.jp)