Research Trends and Results

Possibility of restoring eelgrass (Zostera marina)
beds in Miyako Bay damaged by the Great East Japan
Earthquake
OKADA Tomonari, Head (Dr. Eng)
Marine Environment Division, Coastal Marine and Disaster Prevention Department(Key words) Great East Japan Earthquake, tsunami, zostera marina, sediment, particle-size distribution, Miyako Bay
1. Introduction
The tsunami triggered by the Great East Japan
Earthquake of March 11, 2011 destroyed a lot ofeelgrass (Zostera marina) (Photo) growing in sand.
Eelgrass beds, which are called the cradles of the sea,
are one of the foundations of coastal ecosystems, as
they function as spawning grounds for fish andhabitats for fry, and their destruction severely impacts
coastal ecosystems. This research was conducted in
the head of Miyako Bay, where eelgrass beds had
formed before the earthquake, to clarify the state ofeelgrass and sediments, and to learn if it will bepossible to restore the eelgrass beds in the future.
2. Method
A survey was done focusing on grain size, which isa condition for the breeding and growth of eelgrassthat can be greatly changed by a tsunami. In October
2012, the state of distribution of eelgrass was clarified
and sediments were sampled at 120 points in the headof Miyako Bay. And to learn the transport routes ofeelgrass seedlings, a three-dimensional model wasused to calculate the currents inside the bay.

and Odanohama from Takahama, and Kobori. The
direction of the flow is the same in every season, so
similar advection currents can be counted on to occurin every season.
4. Perspectives of future monitoring
This survey reveals that many places with sedimentconditions suitable for use as habitats by eelgrass
remain. We wish to continue monitoring in
anticipation of the small scale eelgrass beds which
remain steadily expanding a little at a time into theirsurroundings and the eelgrass beds confirmed to bedense communities acting as supply sources restoring
eelgrass beds over a wide area.

3500

3000
3. Results
25
There were places behind the breakwater at00
Takahama and at the inlet behind the peninsula on the20
Photo. Eeelgrass in Miyako Bay
east side where eelgrass was found in dense or open00
communities (see Figure). This is assumed to be a15Figure. Residual Current (Autumn) and
00
result of the shadow of port structures or the peninsula
Eelgrass Distribution
preventing their destruction by the tsunami.
1039.605
00The sediment at these locations was evaluated for its
suitability as the habitat of eelgrass based on its 50
Takahama
高浜
小田ノ浜
Odanohamaparticle-size distribution. The sediment between Kobori, 0Hanokihama
葉ノ木浜
and Horiuchi, at Takahama, at Kamagasawa, and0Hachinokihama
八ノ木浜
Akamae, where either dense or open communities of39.595
0Horiuchi
eelgrass was confirmed, had particle-size distribution
堀内
Kanehama
金浜
suitable for the breeding and growth of eelgrass.
Density of eelgrass
10Kobori
小堀
And the figure is an example of a map prepared by00
密生than 50%
more
50– 25%疎生
Kamagasawa
superimposing the distribution of eelgrass whichless
釜ケ沢
点生than 25%
None無しAkamae赤前
survives on the results of calculating the tidal residual
2039.585141.945141.965
141.975
00 141.955
current of the surface layer in the head of the bay in[Sources]the autumn. The eelgrass in Takahama and Kobori are30
T. Okada, et. al. (3 more) (2013):
00 Sediment conditions
upstream of the flow, which means that seeds can beandeelgrass(Zosteramarina)
in Miyako Bay, Octobercounted on to move to Hachinokihama, Hanokihama,
2012, TECHNICAL NOTE of NILIM 752, 11p