35

Tsunami-induced Changes in a Shallow Brackish Lagoon Ecosystem(Gamo Lagoon) in Sendai Bay, JapanGen KANAYA1*, Hideaki MAKI1, Takao SUZUKI2, Waka SATO-OKOSHI3 andEisuke KIKUCHI4*1

National Institute for Environmental Studies,
16-2 Onogawa, Tsukuba, Ibaraki, 305-8506 Japan
2
Graduate School of Life Science, Tohoku University,
Aramaki-Aoba, Aoba-ku, Sendai, 980-8578 Japan
3
Graduate School of Agricultural Science, Tohoku University,
1-1 Amamiyamachi, Tsutsumidori, Aoba-ku, Sendai, 981-8555 Japan
4
EEC, Miyagi University of Education,
149 Aramaki-Aoba, Aoba-ku, Sendai, 980-0845 Japan*email: gen@nies.go.jp

Abstract
Impacts of the 2011 tsunami disaster were investigated in a shallow brackish lagoon (Gamo Lagoon) in
Sendai City, Japan. The tsunami caused drastic changes in lagoon topography, vegetation and sedimentcharacteristics. Muddy sediment was flushed away and the sediment became much sandier. Changes innutrients and chlorophyll a (chl. a) budgets implied that the lagoon became less eutrophic. Forty-sevenmacrozoobenthos species and two macroalgal species were nearly extirpated in 2011, while six species
including opportunistic polychaetes and amphipods rapidly recovered their population within five months
after the tsunami. In 2012 and 2013, some of the species that disappeared, including bivalves, amphipods,
decapods and macroalgae, recruited and increased their population sizes. Shorebird populations in Gamo
Lagoon and nine other sites in eastern Japan were less affected by the tsunami, since the population size andnumber of species did not change notably. Compositions of polycyclic aromatic hydrocarbons (PAHs) andnormal alkanes (n-alkanes) in lagoon sediment implied petroleum contamination, which was discharged froman oil refinery in Sendai Port although the levels were quite low. A sharp increase in lagoon salinity caused aninvasion of stenohaline marine species and limited the growth of the marsh reed Phragmites australis after thetsunami. Therefore lagoon salinity should be regulated to restore the reed-dominated brackish marsh andassociated biota of this lagoon. The ongoing construction of huge sea walls is another potential threat to thelagoon ecosystem. Care should be taken to conserve coastal habitats such as tidal flats, salt marshes, sanddunes, as well as their backside landward areas during restoration.
Key words: biotic community, brackish lagoon, macrozoonthos, oil spill, tidal flat, tsunami

1. Introduction
1.1 Background
The Pacific coast of eastern Japan was struck by ahuge tsunami on March 11, 2011, following the Great
East Japan Earthquake (M 9.0). The tsunami caused
extreme physical disturbances of tidal flats and salt
marshes in the Tohoku District (e. g., Suzuki, 2011). As aresult, biotic communities in coastal soft-bottom habitats
were modified drastically (e. g., Kanaya et al., 2012;
Seike et al., 2013; Urabe et al., 2013). At present, however, few studies have reported tsunami-induced changes
in environmental characteristics of tidal flats (but see
Kanaya et al., 2012; Sera et al., 2012). Therefore it is
Global Environmental Research
18/2014: 35-46
printed in Japan

necessary to know how the tsunami modified the structure and functioning of tidal-flat ecosystems to assess theecological consequences of the tsunami disaster.
The coastal area of Sendai City, Miyagi Prefecture,
was struck by a huge tsunami with a height of 7.2 m,
which caused intensive physical disturbances in the area
(Sendai City, 2013). In this paper, we report thetsunami-induced changes in a shallow brackish lagoon
ecosystem, Gamo Lagoon, in Sendai City, based on aseries of surveys conducted during 2011 to 2013. We
discuss the direct and indirect (i.e., subsequential) effectsof the tsunami disaster and potential threats for conservation and management of Gamo Lagoon.
©2014 AIRIES

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G. KANAYA et al.

1.2 Gamo Lagoon
Gamo Lagoon is a shallow hypertrophic lagoon(mean water depth: 0.8 m; area: 0.13 km2) located at thenorth side of the Nanakita River estuary in Sendai City
(Fig. 1). Detailed and long-term data sets on this lagoon
before the tsunami disaster helped us assess thetsunami-induced changes in the ecosystem with highreliability (see Gamo Higata Shizen Saisei Kyogikai,
2008; Kanaya et al., 2012 and references therein).
Salinity fluctuated tidally from near 0 to over 30 and thespring tidal range was about 50 cm before the tsunami(Gamo Higata Shizen Saisei Kyogikai, 2008). Sandy
sediment was distributed near the levee, while organically polluted mud predominated in the inner portion(Kanaya & Kikuchi, 2011). A stone levee with threewater gates restricted water exchange. Adjacent fish
farming ponds continuously discharged freshwater into
the inner portion of the lagoon. The lagoon maintained
high phytoplankton biomass in the water column (chl. a;> 100 μg L-1) due to water stagnation and sufficient supply of regenerated nutrients from the highly reduced mud(Kikuchi et al., 1992).
The lagoon was nominated a National Wildlife Protection Area by the Environmental Agency (currently the
Ministry of the Environment, MOE) in 1987. The intertidal flats were important habitat for migratory shorebirds
and macrozoobenthos, including polychaetes, amphipods,
bivalves and decapods (Gamo Higata Shizen Saisei
Kyogikai, 2008). In muddy areas, by contrast, macro-zoobenthos had low species richness and density due tobottom water hypoxia and sulfide accumulation in thesediment (Kanaya & Kikuchi, 2011). Reeds, Phragmites
australis, vegetated densely along the lagoon edge,
which was a resting place for migratory ducks and geese.
Sand dune vegetation was well developed on sand bars.
The red alga Gracilaria vermiculophylla and green alga
Ulva intestinalis (formerly reported as Enteromorphaprolifera) grew densely in the inner subtidal zone,
especially in warmer months (Aikins & Kikuchi, 2002).

2. Topography, Vegetation, and SedimentCharacteristics
The tsunami caused distinctive changes in the topography and vegetation of Gamo Lagoon. The sand bars
almost disappeared after the tsunami, when sea water
directly entered the lagoon (satellite image on 14 March
2011, Google TM Earth). Within three months, however,the sand bar had mostly recovered due to deposition ofdrift sand, and the lagoon recovered its semi-enclosed
shape (Fig. 2). A GPS-based vegetation map (see Kanaya
et al., 2012 for methods) showed that the pine forest
decreased from 4.2 to 2.0 ha, sand dune vegetation from
8.6 to 0.1 ha, and reed marshes from 7.8 to 1.2 ha. In
contrast, the area of bare tidal flat increased from 4.7 to
5.3 ha due to the dissapearance of reed marshes (Kanaya
et al., 2012). Marsh and sand dune vegetation have notrecovered yet as of 2013.

Fig. 1 Location of Gamo Lagoon on the northern side of the Nanakita River Estuary in Sendai City, Japan.

Tsunami-induced Changes in a Shallow Brackish Lagoon Ecosystem

37

Fig. 2 Lagoon topography and vegetation. Areas of tidal flats, pine forest, sand dune vegetation
and reed marshes are shown. The survey was conducted in June 2011 using a handy GPS(see Kanaya et al., 2012 for methods). Data before the tsunami were obtained from Gamo
Higata Shizen Saisei Kyogikai (2008).

Before the tsunami, macroalgae, including red algae,
Gracilaria vermiculophylla, and green algae, Ulva
intestinalis, grew densely in the inner portion of thelagoon (Fig. 3). The macroalgae were a habitat foralgae-associated macrozoobenthos such as free-living
amphipods (Aikins & Kikuchi, 2002). Almost all
macroalgae, however, were washed away by the tsunami.

In 2011, only small patches of G. vermiculophylla were
found in the inner portion (coverage < 30%) and U.
intestinalis had completely disappeared. In 2012, theirbiomass gradually recovered in the inner portion. In 2013,
their patches became more dense and expanded to thecentral lagoon. In 2013, the stenohaline marine species U.
pertusa (Yamochi, 2013) grew densely, though it was

38

G. KANAYA et al.

Fig. 3 Changes in coverage of the red algae Gracilaria vermiculophylla (a), green algae Ulva intestinalis (formerlyclassified as Enteromorpha prolifera) (b), and stenohaline marine species Ulva pertusa following the tsunami (c).
Coverage (%) was measured at 62, 48, and 45 stations in 2011, 2012 and 2013, respectively, and are indicated withdifferent colors. Data before the tsunami were obtained from Gamo Higata Shizen Saisei Kyogikai (2008).

rarely found before the tsunami.
The sediment characteristics also changed distinctively. Before the tsunami, organically polluted mud was
widely distributed in the inner lagoon (Fig. 4). In thatarea, the biodiversity and abundance of macrozoobenthos
were very low due to the toxicity of sedimental sulfide(Kanaya & Kikuchi, 2011). However, the tsunami
removed the mud from the inner lagoon, resulting in a
sharp decline in the silt-clay content of the sediments(particles < 0.063-mm mesh sieve). We found that sediment in Gamo Lagoon remained sandy for three years at
most. This implies that the tsunami-induced changes in
sediment granulometry significantly improved the habitat quality for macrozoobenthos in the inner lagoon.

3. Changes in Nutrient Budget and Trophic
StatesGamo Lagoon was hypertrophic with a maximum chl.
a concentration of >100 μg L-1 which was supported by asufficient supply of regenerated nutrients from muddy
reduced sediment in the inner lagoon (Kikuchi et al.,
1992; Kanaya et al., 2007). Accordingly the lagoon acted
as a source of nutrients and phytoplankton, since it discharged water containing high levels of NH4+ , PO43-, andchl. a into adjacent waters during ebb tides (Kikuchi et al.,1992).
In August 2011, we measured the tidal flux of dissolved nitrogen, dissolved phosphorus, and particulatematerials including chl. a to assess changes in lagoonal
trophic states. Figure 5 shows the temporal changes inTsunami-induced Changes in a Shallow Brackish Lagoon Ecosystem

39

Fig. 5 Temporal changes in water quality at the lagoon mouth.
Measurements were conducted on August 4 to 5, 2011,
for one tidal cycle. Closed circles, inflowing water; open
circles, outflowing water. Abbreviations: DON, total
dissolved organic nitrogen; DOP, total dissolved organic
phosphorus; SS, suspended solid; POC, particulateorganic carbon; PN, particulate nitrogen. Nutrients, DON,
and DOP were quantified by Traacs-800 (Bran + LuebbeGmbH). All particulate materials were analyzed the same
way as in Kanaya et al. (2008).
Fig. 4 Silt-clay content (particles < 0.063-mm mesh; %)at a 1 to 3 cm deep layer of sediment. n = 45, 62,
48, and 45 for 1997, 2011, 2012, and 2013,
respectively. Methods were the same as in
Kanaya and Kikuchi (2008).

quality of inflowing / outflowing waters at the lagoon
mouth. The inflowing water was rich with NH4+, NO3-,
dissolved organic nitrogen (DON), and dissolved organic
phosphorus (DOP), while the outflowing water was richwith NO2- and PO43-. Increasing NO2- with decreasing
NH4+ during the tidal cycle implies further nitrification in
the lagoon (Dähnke et al., 2012). Increasing PO43- withdecreasing DOP during the tidal cycle may possibly havebeen due to phosphate regeneration from DOP and/or
particulate organic phosphorus in the lagoon (e.g.,
Nowicki & Nixon, 1985; Fang, 2000). A declining concentration of chl. a, suspended solids (SS), particulateorganic carbon (POC), and particulate nitrogen (PN)
during the tidal cycle indicates deposition of theseparticulate materials onto the lagoon sediment.
The diurnal budget of each component was calculated
from the concentration and flow rate data (Table 1, see
Kikuchi et al., 1992 for methods). We compared themwith those measured in 1985 by Kikuchi et al. (1992). In
1985, the lagoon was a significant source of NH4+ (netexport; 21.7 kg N day-1), PO43- (8.7 kg P day-1), and chl. a(0.6 kg day-1) to adjacent waters. In 2011, by contrast, thelagoon became a sink for NH4+ (net import: 0.7 kg N
day-1) and chl. a (net import: 0.7 kg day-1). Though PO43was still exported to adjacent waters, the amount (netexport; 1.5 kg P day-1) was a magnitude lower than that in1985. Such changes would mainly be due to the disappearance of muddy sediment in the inner lagoon.
Before the tsunami, chl. a concentration in innerlagoon water (> 100 μg L-1, Kikuchi et al., 1992; Kanaya
et al., 2007) was comparable with or much higher thanthose reported from other eutrophic estuaries of the world(1.6 to 100 μg L-1; Heip et al., 1995). It became much
lower, however, after the tsunami (11.5 to 25.9 μg L-1,
measurements in July, August, and October 2011). Thisimplies decreasing phytoplanktonic productivity in thewater column. We measured the chl. a sedimentation rate
in August and September 2011 using sediment traps (seeKanaya et al., 2008 for methods) and found that in 2011
it was less than half that of 2005 (Fig. 6). This indicates
reduced organic loading from the water column to thesediment surface in Gamo Lagoon after the tsunami.
This suggests that the essential ecosystem functionsof the lagoon, e.g., decomposition of organic matter,
nutrient cycling and primary productivity (see the review
by Levin et al., 2001) had largely been modified by thetsunami. Reduction in phytoplanktonic productivity

40

G. KANAYA et al.

Table 1 Diurnal budgets (kg day −1) of dissolved and particulatematerials in Gamo Lagoon. See Kikuchi et al. (1992) for
methods.

NO3−+NO2−−N
NO2−−N
NO3−−N
NH4+−NPO43−−P
DON
DOPSS
Chlorophyll a
POC
PN

1985
In
23.6
22.0
7.7
0.5
-

Flux (kg day −1) a
Budget (Δ)
20111985 2011
Out
In
Out
18.5
2.3
7.4 −5.1 +5.1
1.0
7.3
- +6.21.3
0.1- −1.2
43.7
48.3
47.6 +21.7 −0.7
16.5
2.3
3.8 +8.7 +1.5
36
25
−11
1.9
0.8
- −1.0- 1844 1297
- −5471.1
1.0
0.3 +0.6 −0.7
16858
- −11031
11−20aData were obtained in July to October 1985 (4 days) and in August in 2011
(1 day). Data in 1985 were from Kikuchi et al. (1992). Abbreviations: -, nodata; DON, total dissolved organic nitrogen; DOP, total dissolved organic
phosphorus; SS, suspended solids; POC, particulate organic carbon; PN,
particulate nitrogen.

Fig. 6 Chlorophyll a sedimentation rates (mg m-2 d-1) measured
by sediment traps (3.3 cm Ø, 7.8 cm high, n = 3) set in field
for two tidal cycles. This was conducted on July 21 andAugust 21, 2005, and August 4 and September 1, 2011. The
methods were the same as in Kanaya et al. (2008). Bars
indicate SD.

would prevent the sediment from being organicallyenriched (i.e., increased organic pollution), since theexcess deposition of phytoplanktonic detritus was the
major cause of sediment deterioration in this lagoon(Kanaya & Kikuchi, 2011).

4. Macrozoobenthos and Shorebirds
In a 2011 survey, 47 of 79 macrozoobenthic species
identified before the tsunami were found to have nearly
disappeared (Kanaya et al., 2012). For example, algaeassociated amphipods (Eogammarus possjeticus
and Melita sp.), sessile fauna (e.g., Fistulobalanus
albicostatus and Crassostrea gigas), marsh-associatedfauna (Chiromantes dehaani, Assiminea hiradoensis, andCerithidea rhizophorarum), motile epifauna (Pagurusminutus and Reticunassa festiva), and infaunal decapods
and bivalves (Macrophthalmus japonicus, Upogebia

yokoyai, Ruditapes philippinarum, Macoma contabulata,
Laternula marilina, and Mya arenaria) were not foundshortly after the tsunami (Fig. 7).
In April to June 2011, marine species were found in
the lagoon possibly due to stranding from the adjacentsea. These species rarely occurred there before thetsunami since the salinity in Gamo Lagoon was too low
for stenohaline marine species to survive (often less than20; Kurihara, 1990; Kurihara et al., 2000). Most of them,
however, were absent until August 2011 as the sand bar
recovered and lagoonal salinity gradually declined(Kanaya et al., 2012).
In Gamo Lagoon, seven species, including opportunistic polychaetes and amphipods, rapidly recovered
their density within five months after the tsunami. In
general, opportunistic species can rapidly colonize empty
habitats created by pulsed disturbances (e.g., Grassle &Grassle, 1974). In Gamo Lagoon, therefore, newly
created sandy bottoms would provide the opportunists
with a preferable “empty habitat” to colonize.
Several species recruited and recovered their population during 2011 to 2013 (see Fig. 7). In the fall of 2011,
juveniles of A. hiradoensis were found in dense concentrations around the reed marshes in the inner lagoon.
In 2012, the amphipods E. possjeticus and Melita sp.
occurred with high densities in macroalgal patches. In
2012, small individuals of F. albicostatus and C. gigas
were found on hard substrates in the intertidal zone.
Recruitments of ghost shrimp N. japonica and thebivalves R. philippinarum, M. contabulata, and L.
marilina were also found in 2012 and/or 2013. Althoughseveral species such as U. yokoyai, P. minutus, C.
rhizophorarum, and R. festiva are still absent or rarely
found, the macrozoobenthic community gradually recovered after the tsunami.
Tidal flats along the Japanese coast are important
habitat for migratory shorebirds or waders (Ministry of
the Environment, 2013). To assess the effects of thetsunami disaster on the shorebird community, we compiled data from a five-year survey at Gamo Lagoon andnine monitoring sites along the Pacific coast of Japan(data were provided by the Monitoring Sites 1000 Project,
MOE). Figure 8 shows the individual number and species
diversity of shorebirds in 2008 to 2012 at Takase River(Aomori Pref.), Gamo and Torinoumi Lagoons (Miyagi
Pref.), Matsukawaura Lagoon and Natsui River(Fukushima Pref.), Kashimanada Shore (Ibaraki Pref.),and Ichinomiya River, Banzu Tidal flat, Yatsu Lagoon,and Sanbanze Tidal flat (Chiba Pref.). The results
showed that the effects of the tsunami disaster wereinconspicuous at these sites. At the sites, habitat substrates (i.e., tidal flats and sandy shores) were retained
after the tsunami (judging from satellite images on March
18 and 31 and April 6, 2011; Google TM Earth). Rapid
recovery of opportunistic macrozoobenthos, as found in
Gamo Lagoon, may also assure a supply of potential preyfor shorebirds. This suggests that the population of
migratory shorebirds was less affected by the tsunami
disaster.

Tsunami-induced Changes in a Shallow Brackish Lagoon Ecosystem

Fig. 7 Macrozoobenthos in Gamo Lagoon. Representative taxa are shown for three groups, i.e., species that havedisappeared or nearly disappeared (47 taxa) (a), temporal immigrants from the sea, April to July 2011 (12 taxa) (b),and rapidly recovered species (7 taxa) (c). For methods and original data, see Kanaya et al. (2012).

41

42

G. KANAYA et al.

Fig. 8 Individual numbers and species richness of shorebirds at ten sites along the Pacific Coast ofeastern Japan. (a) Takase Riv., (b) Gamo, (c) Torinoumi (d) Matsukawaura, (e) Natsui Riv.,(f) Kashimanada, (g) Ichinomiya Riv., (h) Banzu, (i) Yatsu, (j) Sanbanze. Data are from the MOE
Monitoring Sites 1000 Project (Shorebirds Datapackage 2012.zip, http://www.biodic.go.jp/
moni1000/findings/data/index.html). Maximum densities in three seasons, spring (April–May),
fall (August–September), and winter (December–February), were summed for each year.
For 2012, the data were multiplied by 1.5 since they were collected only in spring and fall.

5. An Oil Spill
An oil refinery near Sendai Port was destroyed on
March 11, 2011 and burned for several days afterwards(Sendai City, 2013). As a result, crude oil and its
pyrolytic products spilled into the sea. In the summer tofall of 2011, we collected sediment samples for analysis
of normal alkanes (n-alkanes) and polycyclic aromatichydrocarbons (PAHs) from several locations in Gamo
Lagoon. These components were analyzed by gas
chromatograph / mass spectrometer (GC-MS) according
to the methods suggested by the Ministry of the Environment (2003) with some modifications. Each sediment sample was subjected to subsequential batch
extraction with acetone and dichloromethane, and theextracts were cleaned up and fractionated by silica gelcolumn chromatography. Fractions for aliphatic oraromatic hydrocarbons with internal standards wereseparately injected into GC-MS with a fused silica capillary column in the splitless mode. Each peak of n-alkanes
and PAHs was qualified and quantified in the selectedions monitoring mode.
Typical chromatograms and contents (μg kg-1) of
n-alkanes, pristane, and phytane are shown in Fig. 9. The
composition differed among sediments and oils measured in this study (Table 2). The carbon preference index(CPI C24-34), i.e., ratio of n-alkanes with even carbon
numbers (C24 to C34) to those with odd carbon numbers(C25 to C33), is used as a source indicator of n-alkanes inmarine sediments (e.g., Bray &Evans, 1961). This is due

Tsunami-induced Changes in a Shallow Brackish Lagoon Ecosystem

to the fact that natural uncontaminated sediments withterrigenous sources are dominated by heavy n-alkanes
with odd carbon numbers (e.g., Bray & Evans, 1961;
Gao & Chen, 2008). Accordingly, a CPI close to 1
implies that the n-alkanes were derived mainly frompetroleum (1.0 to 1.1, see Table 2), while 3 to 6 indicate
the dominance of biogenic sources. In Gamo Lagoon, the
CPI of the sediment was 1.6 ± 1.0 (mean ± SD, n = 7),
which is intermediate between those of biogenic sources
and petrol, suggesting a mixture of these sources. The
pristane / phytane ratio is generally higher than 1 (3 to 5)
in natural uncontaminated sediments, while it is close to
1.0 under petroleum contamination (e.g., Gao & Chen,
2008). In Gamo Lagoon, the ratio was 0.55 ± 0.30 and
was closer to that of the stranded crude oil (0.60). This
indicates that the n-alkanes in the sediment were derived
mainly from stranded crude oil.
The composition of PAHs also differs among sources,

43

e.g., petroleum, sewage, biomass combustion, and petroleum combustion (e.g., Gao & Chen, 2008). In Gamo
Lagoon, the high fractional contribution of Σ alkylated
PAHs to total PAHs implies the contribution of
petrogenic sources (Table 2). We also calculated theratios of phenanthrene / anthracene (PH / AN) andfluoranthene / pyrene (FLU / PY) as source indicators. In
general, sediment polluted by petrol exhibits high PH /AN (>25), while pyrolytic products show high FLU / PY(>1) (Baumard et al., 1998). In Gamo Lagoon, the ratios
were 76 ± 83 for PH / AN and 2.2± 3.3 for FLU / PY,
suggesting a mixture of petrogenic input and its pyrolytic
products in the PAH pool.
Our data indicated that the sediment in Gamo Lagoon
was polluted with petrol and its pyrolytic products. The
total n-alkane content (875 ± 476 μg kg-1) and total PAHs
(102 ± 86 μg kg-1, sum of all PAHs in Table 2) , however,were not considerably higher than those in other estuarine coastal sediments (see Colombo et al., 1989;
Table 2 Normal alkane (n-alkane) and polycyclic aromatichydrocarbon (PAH) content in Gamo lagoon sedimentin 2011 (Gamo), stranded crude oil at Sendai Port in2011 (Strand.), heavy oil from the Fukushima thermal
power station (Heavy), and thin fuel oil A (A-fuel). The
carbon preference index (CPI C24-34) was calculated
according to Bray and Evans (1961). Data are the mean
± SD (n = 7).
Sediment (μg kg-1)
Gamo (SD)Fig. 9 Typical mass chromatograms and content of n-alkanes(C12-38), pristane (Pri), and phytane (Phy) in lagoon
sediment (a) and in stranded crude oil at Sendai Port (b) in
2011. Bars in (a) indicate SD (n = 7). The peak for eachn-alkane is indicated with the carbon number.

Oil (μg g-1)
Strand. Heavy

A-fuelAlkanes
Total n-alkanes (C12-38)
CPI C24-34 1
Pristane
PhytanePristane / Phytane

875
1.6
13
30
0.55

(476)
(1.0)
(9.3)
(26)
(0.30)

29641
1.0
576
963
0.60

22194
1.1
448
285
1.6

101589
1.0
2715
2750
0.99

PAHs
Acenaphthylene
Acenaphthene
NaphthaleneΣC1-4 Naphthalenes
Dibenzothiophene (DBT)ΣC1-3 DBTs
Anthracene (AN)Phenanthrene (PH)ΣC1-3 Phenanthrenes
FluoreneΣC1-2 Fluorenes
Fluoranthene (FLU)
Pyrene (PY)
Benzanthracene
Chrysene
Benzo[b,k]fluoranthene
Benzo[a]pyrene
Benzo[e]pyrene
Indeno[1,2,3-cd]pyrene
Dibenz[a,h]anthracene
Benzo[ghi]perylene
Total PAHs
Σ alkylated PAHs
PH/AN
FLU/PY

0.02
0.14
0.26
8.5
0.65
15
0.23
5.4
27
0.27
6.8
3.8
5.9
2.6
6.2
1.9
3.2
8.7
0.66
0.16
4.0
102
58
76
2.2

(0.05)(0.11)(0.21)(6.32)(0.43)(13.6)(0.33)(2.9)(30)(0.22)
(5.1)
(3.2)
(4.9)
(2.5)
(6.0)
(2.1)
(4.2)
(9.1)
(0.77)(0.22)
(4.4)
(86)
(53)
(83)
(3.3)

1.8
14
4.4
910
33
328
3.6
362
1764
68
361
25
3.5
36
231
29
0.51
2.5
0.38
0.8
n.d.
4178
3363
100
7.1

0.29
12
n.d.
434
11
131
3.9
161
1281
27
397
32
2.9
12
262
20
0.69
3.4
0.52
1.0
n.d.
2791
2243
42
11

174
943
2083
102458
1180
11647
228
2291
10955
832
7839
73
451
9.5
18
0.41
1.5
7.2
1.1
2.2
n.d.
14119213289810
0.16

1

CPI = 1/2[(C25+C27+C29+C31+C33)/(C24+C26+C28+C30+C32)+(C25+C27+
C29+C31+C33)/(C26+C28+C30+C32+C34)].
Sediment samples in Gamo Lagoon were collected in June (n = 1),
July (n = 4), and October of 2011 (n = 2).

44

G. KANAYA et al.

Baumard et al., 1998; Chen & Chen, 2011). Page et al.
(2002) reported that the lower toxicity threshold of total
PAHs for the amphipod Rhepoxynius abronius was 2600
μg kg-1. In Gamo Lagoon, the level of total PAHs was amagnitude lower than that of the lower toxicity threshold.
Rapid recovery of the amphipod and polychaete populations, as discussed above, also indicated that the petrol
pollution hardly affected the macrozoobenthic community in Gamo lagoon.

6. Threats to the Conservation of the Gamo
Lagoon Ecosystem
Several problems arose after the tsunami disaster,which could potentially modify the structure, functioning
and biodiversity of the lagoon ecosystem. The first is
subsequent changes in the saline regime after the tsunami.
Figure 10 shows the changes in diurnal maximum andminimum salinity in 1989-1990, 1999-2000, and2012-2013 measured by a data logger set in field (datafrom Kurihara, 1990; Kurihara et al., 2000). Diurnal
minimum salinity was less than 15 in 1989-1990 and lessthan 20 in 1999-2000 on most days. By contrast, it wasover 20 in 2012-2013. Diurnal maximum salinity was
also constantly high after the tsunami. These suggest thatthe freshwater input significantly declined after thetsunami, possibly due to topographical changes near thelagoon mouth and cessation of the inflow of freshwaterfrom the fish farming ponds. High salinity should havenegative effects on the growth and survival of marsh
reeds (e.g., Lissner & Schierup, 1997). Furthermore,increasing salinity would result in structural changes
in the biotic community in Gamo Lagoon. In fact,stenohaline marine species such as the crab Portunus
trituberculatus and the green algae U. pertusa were
found in the summer of 2013.
The second is anthropogenic disturbances arisingthrough the reconstruction of a huge sea wall. The administration plans to construct huge sea walls with a heightof 7.2 m T.P. along the coastline of Sendai City (e.g.,
http://www.thr.mlit.go.jp/Sendai/kasen_kaigan/fukkou/kouzou.html). Since salt-marsh plants, sand-dune vegetation and macrozoobenthos often show patchy distribution (e.g., Gamo Higata Shizen Saisei Kyogikai, 2008;
Japanese Association of Benthology, 2012), constructionof sea walls can easily destroy habitats at a local scale.
Huge sea walls can also cause habitat quality to deteriorate by breaking connectivity among habitats. For example, semi-terrestrial crabs Chiromantes dehaani and C.
haematocheir use sand dunes, grasslands, and forests astheir main habitats, while they must migrate to the intertidal zone to breed (Kobayashi, 2000). Therefore, it isFig. 10 Diurnal maximum (red solid line) and minimum (blue broken line) salinity in Gamo Lagoon in
1989-1990 (a), 1999-2000 (b), and 2012-2013 (c). Data were obtained by a logger set at 115 m(before the tsunami) and 525 m (after the tsunami) inward from the levee. The data before thetsunami were from Kurihara (1990) and Kurihara et al. (2000). The grey areas indicate no data.

Tsunami-induced Changes in a Shallow Brackish Lagoon Ecosystem

necessary to maintain connectivity among habitats formaintaining populations of such cross-habitat taxa.
During reconstruction of sea walls, destruction ofreed marshes and tidal flats should be carefully minimized. Creation of passages across the sea wall (i.e., likefishways in dams) is one possible solution for keeping
the connectivity between the lagoon and back-land habitats for migrating species. Setting the sea wall back fromthe lagoon area would, at present, be one of the mosteffective and ideal resolutions to conserve the lagoon
ecosystem. Proper assessment and countermeasures arestrongly needed before sea wall restoration to avoid
degradation of habitat quality in estuarine wetlands, i.e.,endangered ecosystems along the Japanese Coast.

Acknowledgements
We would like to thank Y. Nakamura, Y. Takagi, T.
Kondo, and the Alumni of Kikuchi Laboratory for their
help in field work. K. Kinoshita, A. Oishi, J. Nakano, Y.
Okada and the staff of the Marine Biological ResearchInstitute of Japan Co., Ltd., are acknowledged for their
help in laboratory work. The editors and two anonymousreferees are acknowledged for their reviews and criticalcomments. This research was partly supported by
TEAMS (Tohoku Ecosystem-Associated Marine Sciences) from MEXT. Data sets on shorebirds were
provided by the Monitoring Sites 1000 Project, MOE.
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Gen KANAYAGen KANAYA is a researcher at the National
Institute for Environmental Studies (NIES). He
received his Doctor of Science degree in 2005
from the Biological Institute, Graduate School of
Science, Tohoku University. His main research
field is tidal flat ecology, focusing on the ecological functioning of macrozoobenthos in softbottom habitats. He currently aims to reveal ecosystem succession patterns
in this lagoon after the huge disturbance event.

Hideaki MAKI
Hideaki MAKI is a senior researcher at the
National Institute for Environmental Studies(NIES). He got his Ph.D. in engineering from
Osaka University and spent post-doc time at the
Marine Biotechnology Institute. He has studied
microbial degradation of crude oil in marine
environments and conducted some field experiments on crude oil bioremediation. Recently, he has been conducting
monitoring work on water and sediment environments relevant to hypoxia
in Tokyo Bay. Since the time of the Great East Japan Earthquake, he has
been involved in monitoring work on PAH contamination in sediments in
the Tohoku coastal sea.

Takao SUZUKI
Takao SUZUKI is an assistant professor at theGraduate School of Life Sciences, Tohoku University. He studies community ecology of benthic
invertebrate animals inhabiting tidal flats, salt
marshes and seagrass beds. After the Great East
Japan Earthquake and massive tsunami in 2011,
he has continued to monitor the recovery processes of benthic communities along the coastal area of the Tohoku districtof Japan. Now he is promoting citizen participation research methods forstudying macro-benthic animals in tidal flats, which would be useful for
long-term monitoring by the public.

Waka SATO-OKOSHI
Waka SATO-OKOSHI is an associate professor at
the Graduate School of Agricultural Science,
Tohoku University. After she got her Ph.D. fromTohoku University, she worked at the Natural
History Museum of Chiba as chief curator of
invertebrate zoology before returning to theuniversity as an academic staff member. Since
then, she has focused on biological and ecological studies of not only
polychaetes but also various other marine benthic animals which arestrongly correlated with human activities. She has had opportunities tovisit international institutes such as the University of Alberta (Canada),
Murdock University (Australia), Chile JICA (Chile), Universidad la
República (Uruguay), and Kasetsart University (Thailand) and study as avisiting scientist. She also took part in the Japanese Antarctic Research
Expedition. Her interests focus mainly on biology and ecology ofpolychaetes but also focus on the effect of global warming on marine
ecosystems. She is currently involved in the “Environmental and faunalchanges before and after the Great East Japan Earthquake” project, whichstarted in 2011 just after the tsunami-related damage.

Eisuke KIKUCHI
Eisuke KIKUCHI is a professor emeritus at Tohoku
University. He has studied wetland ecosystems
such as rice fields, estuarine tidal flats and small
shallow lakes, focusing on nutrient cycling andfood chain dynamics of these ecosystems,
especially with regard to the functional roles ofbenthic invertebrates. His main research areas
have included Gamo Lagoon, the strongly acidic Lake Katanuma and theRamsar site Lake Izunuma.. After his retirement from Tohoku University
in 2009, he has mainly studied the successional changes in aquaticcommunity structure in a rice field ecosystem as a cooperative researcherof the EEC, Miyagi University of Education, and has spent considerable
time on the management of the Miyagi Prefectural Izunuma-Utinuma
Environmental Foundation as Director General.

(Received 17 January 2014, Accepted 26 May 2014)