24
1
Introduction
The coastal area of northeast Japan was damaged
by a tsunami after the earthquake on March 11,
2011 (Goto et al. 2011 ; Urabe et al. 2013 ). Facilities
near the coast were damaged by the tsunami. More
than 10,000,000 L of fuel oil fl owed into Kesennuma
Bay from 22 oil tanks (Sasaki 2012 ). In addition, a
heavy oil tank was destroyed in Ofunato Bay
(Sasaki 2012 ). As components of oil are toxic to
aquatic organisms (Okumura et al. 2003 ), there
was concern for marine pollution in the ecosystem.
More than ten sewage disposal plants on the
coast stopped operations temporarily because of
electrical problems caused by the tsunami
(Ministry of Land, Infrastructure, Transport and
Tourism 2011 ). If untreated wastewater fl owed
into the coastal area, the nitrogen and phosphate
concentrations would increase in seawater. The
increase in nutrients could cause eutrophication,
which would deteriorate the coastal environment.
Kesennuma and Ofunato bays are aquaculture
grounds for oyster, scallop, and seaweed (Hayakawa
et al. 2001 ; Ito et al. 2007 ). These shellfi sh grow by
eating suspended materials in seawater such as phytoplankton (Kusui et al. 1983 ). If the phytoplankton
diversity changes because of an environmental disturbance, phytoplankton unsuitable for aquaculture
increase in numbers, and restoration of the marine
product industry may be hindered. For example, if
the amounts of harmful algae increase shellfi sh
toxin concentrations, shellfi sh shipments are
suspended.
After the earthquake, several environmental
pollution (Harino and Yatsuzaka 2012 ) and plankton surveys (Yamada 2012 ) were conducted in
Kesennuma Bay (Tanaka 2012 ; Yokoyama and
Hatakeyama 2012 ; Masuda 2012 ). High concentrations of oil in sediments (Yamamoto et al. 2012 )
and a high density of toxic algae (Kaga et al. 2012 ;
Nishitani et al. 2012 ) were observed after the
earthquake. However, comparisons with preearthquake levels are limited because of a lack of
data. Thus, the infl uence of the earthquake and tsunami on total quantity and diversity of phytoplankton, which is the diet of shellfi sh, is unknown.
We measured the concentration of chlorophyll a (Chl a ) as a fi rst step to environmentally
monitor the aquaculture grounds after the
earthquake to determine the quantity of phytoplankton in the seawater of Kesennuma and
Ofunato bays. Then, we compared the change in
phytoplankton diversity from pigment concentrations before and after the tsunami because
pigment profi les vary with phytoplankton taxa
(Jeffrey and Vesk 1997 ).
2
Materials and Methods
2.1
Investigation Procedure
We took samples at Iwaisaki, at the mouth of
Kesennuma Bay (Fig. 1a ), approximately every
month from May 2010 to February 2011 before
the earthquake and from June 2011 to March
2012 after the disaster. Water temperature and salinity were measured at the sampling sites. These
parameters were measured at each depth using
a conductivity-temperature-depth instrument.
Seawater was also collected from depths of 0, 5,
10, 15, and 20 m. We collected seawater in a
water column from the surface to the sea bottom
in Ofunato Bay (Fig. 1b ) approximately every
week from May 18 to August 17, 2011, before the
earthquake and collected seawater at 2-m intervals from the surface to 22-m depth twice in June
and July 2012.
2.2
Pigment and Phytoplankton
Diversity Analyses
Pigment concentrations in seawater samples
were determined according to a previously
reported method (Okumura et al. 2012 ). Briefl y,
200-ml seawater was fi ltered through Whatman
GF/F glass microfi ber fi lters (GE Healthcare UK
Ltd., Buckinghamshire, UK). The phytoplankton
pigments were extracted from the fi lter with 1-ml
methanol. The pigments were analyzed by highperformance liquid chromatography (HPLC;
Shimadzu, Kyoto, Japan) using the method of
Zapata et al. ( 2000 ).
As phytoplankton have taxa-specifi c pigments
(Jeffrey and Vesk 1997 ), the ratios of each phytoplankton taxon in seawater were calculated by
Y. Okumura et al.
1
Introduction
The coastal area of northeast Japan was damaged
by a tsunami after the earthquake on March 11,
2011 (Goto et al. 2011 ; Urabe et al. 2013 ). Facilities
near the coast were damaged by the tsunami. More
than 10,000,000 L of fuel oil fl owed into Kesennuma
Bay from 22 oil tanks (Sasaki 2012 ). In addition, a
heavy oil tank was destroyed in Ofunato Bay
(Sasaki 2012 ). As components of oil are toxic to
aquatic organisms (Okumura et al. 2003 ), there
was concern for marine pollution in the ecosystem.
More than ten sewage disposal plants on the
coast stopped operations temporarily because of
electrical problems caused by the tsunami
(Ministry of Land, Infrastructure, Transport and
Tourism 2011 ). If untreated wastewater fl owed
into the coastal area, the nitrogen and phosphate
concentrations would increase in seawater. The
increase in nutrients could cause eutrophication,
which would deteriorate the coastal environment.
Kesennuma and Ofunato bays are aquaculture
grounds for oyster, scallop, and seaweed (Hayakawa
et al. 2001 ; Ito et al. 2007 ). These shellfi sh grow by
eating suspended materials in seawater such as phytoplankton (Kusui et al. 1983 ). If the phytoplankton
diversity changes because of an environmental disturbance, phytoplankton unsuitable for aquaculture
increase in numbers, and restoration of the marine
product industry may be hindered. For example, if
the amounts of harmful algae increase shellfi sh
toxin concentrations, shellfi sh shipments are
suspended.
After the earthquake, several environmental
pollution (Harino and Yatsuzaka 2012 ) and plankton surveys (Yamada 2012 ) were conducted in
Kesennuma Bay (Tanaka 2012 ; Yokoyama and
Hatakeyama 2012 ; Masuda 2012 ). High concentrations of oil in sediments (Yamamoto et al. 2012 )
and a high density of toxic algae (Kaga et al. 2012 ;
Nishitani et al. 2012 ) were observed after the
earthquake. However, comparisons with preearthquake levels are limited because of a lack of
data. Thus, the infl uence of the earthquake and tsunami on total quantity and diversity of phytoplankton, which is the diet of shellfi sh, is unknown.
We measured the concentration of chlorophyll a (Chl a ) as a fi rst step to environmentally
monitor the aquaculture grounds after the
earthquake to determine the quantity of phytoplankton in the seawater of Kesennuma and
Ofunato bays. Then, we compared the change in
phytoplankton diversity from pigment concentrations before and after the tsunami because
pigment profi les vary with phytoplankton taxa
(Jeffrey and Vesk 1997 ).
2
Materials and Methods
2.1
Investigation Procedure
We took samples at Iwaisaki, at the mouth of
Kesennuma Bay (Fig. 1a ), approximately every
month from May 2010 to February 2011 before
the earthquake and from June 2011 to March
2012 after the disaster. Water temperature and salinity were measured at the sampling sites. These
parameters were measured at each depth using
a conductivity-temperature-depth instrument.
Seawater was also collected from depths of 0, 5,
10, 15, and 20 m. We collected seawater in a
water column from the surface to the sea bottom
in Ofunato Bay (Fig. 1b ) approximately every
week from May 18 to August 17, 2011, before the
earthquake and collected seawater at 2-m intervals from the surface to 22-m depth twice in June
and July 2012.
2.2
Pigment and Phytoplankton
Diversity Analyses
Pigment concentrations in seawater samples
were determined according to a previously
reported method (Okumura et al. 2012 ). Briefl y,
200-ml seawater was fi ltered through Whatman
GF/F glass microfi ber fi lters (GE Healthcare UK
Ltd., Buckinghamshire, UK). The phytoplankton
pigments were extracted from the fi lter with 1-ml
methanol. The pigments were analyzed by highperformance liquid chromatography (HPLC;
Shimadzu, Kyoto, Japan) using the method of
Zapata et al. ( 2000 ).
As phytoplankton have taxa-specifi c pigments
(Jeffrey and Vesk 1997 ), the ratios of each phytoplankton taxon in seawater were calculated by
Y. Okumura et al.