49
Weather Association 2014 ); (4) since the
Otsuchi Bay has a U-shape horizontal profi le,
the wave height in the bay head was higher than
that in the bay mouth (Japan Weather Association
2014 ); and (5) the tsunami waves did not break,
and the speed of water movement was lower
than that required to remove small seaweeds
from the substrates.
4.2
Impact of the Tsunami
on Seagrass Beds and Their
Responses
Elevated waves from the tsunami struck the bay
head where the water was very shallow (Japan
Weather Association 2014 ). The resulting powerful spilling waves and undertows transported seagrasses growing on the sandy seafl oor. We
discovered solitary seagrass seedlings without
rhizomes linking to other fl owering or vegetative
shoots when we removed seedling samples in
October 2011, which indicated that the seedlings
had germinated from seeds. Shabaka and Komatsu
( 2010 ), using bimonthly sampling, found that Z.
caulescens fl owered between June and August in
Funakoshi Bay, which lies near Otsuchi Bay. It is
hypothesised that Z. marina also fl owers in June
and August, similar to Z. caulescens . Hence, we
assume that the seeds had been produced before
2011 and had remained buried until the tsunami.
Yamaki et al. ( 2006 ) found that 90 % of seagrass
seeds that had been preserved for 1 year germinated in a fresh water medium. Thus, it is possible
that seeds produced in 2010, and even before
2010, were capable of germination in 2011. Seeds
resuspended by the tsunami that then fell on the
sand beds would have been able to germinate.
4.3
Germination
of Seagrass Seeds
Field and laboratory studies of seagrass germination have focused primarily on salinity, temperature, light, scarifi cation and, more recently, the
sediments in which the seeds germinate (e.g.
oxygen [oxygen-reduction profi les or Eh]), as
critical factors infl uencing germination processes
(Orth et al. 2000 ). A massive tsunami does not
change the salinity or water temperature of the
seeds’ environment, but does alter the light, sediments and scarifi cation. Some studies have
reported that light is not important for the germination of Z. marina (e.g. Moore et al. 1993 ).
While the latter study demonstrated more rapid
germination of Z. marina seeds in sediments with
no oxygen, mixing by a tsunami oxygenates sediments. Harrison ( 1991 ) reported that scarifi cation
of the seed coat resulted in increased germination
rates for Z. marina , and it is possible that collision of seagrass seeds with sediments during the
tsunami caused scarifi cation of the seed coat and
promoted the seeds’ germination.
4.4
Infl uences of Environments
on Seed Germination
and Seedling Growth
of Seagrasses
It was reported that the lowest water temperature
in which Z. marina seeds germinate was <5 °C,
and the optimal water temperature was between 5
and 10 °C in the Chesapeake Bay, USA (Orth and
Moore 1983 ) and Odawa Bay, Japan (Kawasaki
et al. 1986 ). In Otsuchi Bay, mean water temperatures in March, April, May and June at a depth of
1 m from 2000 to 2009 were 7.1, 9.1, 12.3 and
14.8 °C, respectively (Michida et al. 2010 ). Abe
et al. ( 2008 ) examined germination rates using
culture experiments at water temperatures from 5
to 25 °C at intervals of 5 °C over a period of 30 d
following seed storage for 0–60 d at 0 °C at a
depth of 1.5 cm in sand. They revealed that the
optimal water temperature for seed germination
was in the range of 10–15 °C (the highest germination rate occurred at 10 °C) with a PAR [photosynthetic active radiation] of 50 µmol regardless
of storage duration. 50 % of the seeds were germinated at 10 °C within 2 months. The germinated seeds grew to seedlings approximately
10 cm long within 2–3 weeks in water temperatures of 5–20 °C. Thus, water temperatures in
Otsuchi Bay might be favourable for seeds to germinate and grow to longer than 10 cm, which
Impact of the 2011 Tsunami on Seagrass and Seaweed Beds in Otsuchi Bay, Sanriku Coast, Japan
Weather Association 2014 ); (4) since the
Otsuchi Bay has a U-shape horizontal profi le,
the wave height in the bay head was higher than
that in the bay mouth (Japan Weather Association
2014 ); and (5) the tsunami waves did not break,
and the speed of water movement was lower
than that required to remove small seaweeds
from the substrates.
4.2
Impact of the Tsunami
on Seagrass Beds and Their
Responses
Elevated waves from the tsunami struck the bay
head where the water was very shallow (Japan
Weather Association 2014 ). The resulting powerful spilling waves and undertows transported seagrasses growing on the sandy seafl oor. We
discovered solitary seagrass seedlings without
rhizomes linking to other fl owering or vegetative
shoots when we removed seedling samples in
October 2011, which indicated that the seedlings
had germinated from seeds. Shabaka and Komatsu
( 2010 ), using bimonthly sampling, found that Z.
caulescens fl owered between June and August in
Funakoshi Bay, which lies near Otsuchi Bay. It is
hypothesised that Z. marina also fl owers in June
and August, similar to Z. caulescens . Hence, we
assume that the seeds had been produced before
2011 and had remained buried until the tsunami.
Yamaki et al. ( 2006 ) found that 90 % of seagrass
seeds that had been preserved for 1 year germinated in a fresh water medium. Thus, it is possible
that seeds produced in 2010, and even before
2010, were capable of germination in 2011. Seeds
resuspended by the tsunami that then fell on the
sand beds would have been able to germinate.
4.3
Germination
of Seagrass Seeds
Field and laboratory studies of seagrass germination have focused primarily on salinity, temperature, light, scarifi cation and, more recently, the
sediments in which the seeds germinate (e.g.
oxygen [oxygen-reduction profi les or Eh]), as
critical factors infl uencing germination processes
(Orth et al. 2000 ). A massive tsunami does not
change the salinity or water temperature of the
seeds’ environment, but does alter the light, sediments and scarifi cation. Some studies have
reported that light is not important for the germination of Z. marina (e.g. Moore et al. 1993 ).
While the latter study demonstrated more rapid
germination of Z. marina seeds in sediments with
no oxygen, mixing by a tsunami oxygenates sediments. Harrison ( 1991 ) reported that scarifi cation
of the seed coat resulted in increased germination
rates for Z. marina , and it is possible that collision of seagrass seeds with sediments during the
tsunami caused scarifi cation of the seed coat and
promoted the seeds’ germination.
4.4
Infl uences of Environments
on Seed Germination
and Seedling Growth
of Seagrasses
It was reported that the lowest water temperature
in which Z. marina seeds germinate was <5 °C,
and the optimal water temperature was between 5
and 10 °C in the Chesapeake Bay, USA (Orth and
Moore 1983 ) and Odawa Bay, Japan (Kawasaki
et al. 1986 ). In Otsuchi Bay, mean water temperatures in March, April, May and June at a depth of
1 m from 2000 to 2009 were 7.1, 9.1, 12.3 and
14.8 °C, respectively (Michida et al. 2010 ). Abe
et al. ( 2008 ) examined germination rates using
culture experiments at water temperatures from 5
to 25 °C at intervals of 5 °C over a period of 30 d
following seed storage for 0–60 d at 0 °C at a
depth of 1.5 cm in sand. They revealed that the
optimal water temperature for seed germination
was in the range of 10–15 °C (the highest germination rate occurred at 10 °C) with a PAR [photosynthetic active radiation] of 50 µmol regardless
of storage duration. 50 % of the seeds were germinated at 10 °C within 2 months. The germinated seeds grew to seedlings approximately
10 cm long within 2–3 weeks in water temperatures of 5–20 °C. Thus, water temperatures in
Otsuchi Bay might be favourable for seeds to germinate and grow to longer than 10 cm, which
Impact of the 2011 Tsunami on Seagrass and Seaweed Beds in Otsuchi Bay, Sanriku Coast, Japan