地球環境研究，Vol.21（2019）

Ulva spp. Distribution Observed by Using
High-Spatial Resolution Satellites―Case study: Yatsu tidal flat, Chiba Prefecture―
SHIRAKI Yohei＊

OISHI Yuhei＊＊

LEE Seongwon＊

KITAZAWA Toshiyuki＊

Key words：Ulva spp., High-Spatial Resolution Satellite, NDVI, Water Column Correction Method, Yatsu tidal flat

1．Introduction

been reports of decreases in tidal flats in many regions
in recent years due to the effects of urbanization,

A tidal flat is land formed of clay or mud that iscoastal development and so forth（N. J. Murray et al .

smooth with a gentle gradient and that, as the sea

2014; Z.Y. Wu et al . 2016; Y. Chen et al . 2016）; Japansurface rises and falls with the tides, is below theis no exception to this trend. A decrease of about 40%

water surface at flood tides and exposed above theof the area of tidal flats in Japan has been reported

water surface at ebb tides. Tidal flats are generally

due to land reclamation in the development of coastal

supplied with organic matter, nutrients and the like byregions during the period of high economic growth

rivers. The organic matter and nutrients are consumed

from 1954 to 1973（Nature Conservation Bureau of

by bacteria, shellfish, macroalgae and phytoplankton

the Environment Agency, 1994）. The destruction ofliving in and on the mad. The phytoplankton are

tidal flats from 1945 to 1977 has been reported at

consumed by benthic organisms living in the mud of

89.2% in Tokyo Bay, 54.48% in Ise Bay/Mikawa Bay,the tidal flats, such as protists, polychaetes, crustaceans

and 35.5% in the Seto Inland Sea（T. Kikuchi 2000）.

such as crabs, and shellfish such as Venerupis

In this context, it is important to restore what is left

philippinarum Manila clams. Ultimately, the benthicof our tidal flats to healthy states and to preserve andorganisms are eaten by fish, birds and so forth. This

manage them. This requires that we conduct periodic

characteristic food web of tidal flats strongly influences

monitoring in order to ascertain their conditions.

ecological functions. Tidal flats provide water-cleaning

However, monitoring the conditions of large areas ofeffects, with nitrogen removal by bacteria andtidal flats on foot requires large amounts of time andnitrogen and phosphorus removal by algae preventing

money and is generally not practical.

eutrophication, and organic matter being removed by

Against this background, remote sensing technologies,

filter-feeding organisms; they also provides biologicalbeing capable of acquiring information over large

production facilities and other ecosystem services,

areas and of conducting periodic observations ofsupplying shelter for a wide range of species, nurseries

subject areas, have often been employed to monitor

for their young, and furnishing feeding and rest sites

the conditions of tidal flats in recent years（L. Sochea

for birds. Being a place of easy access to the water, aand Y. Sakuno 2008; T. Takemura et al . 2017; Y. Shiraki

tidal flat also has many benefits for humans, as can be

et al . 2018）. However, previous studies using remoteseen in the popularity of clam-gathering, birdwatching,sensing to monitor the conditions of tidal flats haveother nature watching, environmental studies and so

mostly used satellite images with relatively low spatial

forth（Japan Fisheries Agency）. However, there haveresolutions, such as images from the environmental＊＊＊

Faculty of Geo-Environmental Science, Rissho University
National Land Information Department, PASCO CORPORATION
153

Ulva spp. Distribution Observed by Using High-Spatial Resolution Satellites（Shiraki・Oishi・Lee・Kitazawa）

monitoring satellite Landsat（the resolution of Landsat-

conservation of wetlands. However, excessive growth8’s multi-spectrum Operational Land Imager is 30m）.

of Ulva spp.（green tides）was first confirmed in 1995

The conditions of tidal flats cannot be considered toand has continued at extreme levels to the present day

have been ascertained in detail. Accordingly, in this（Fig.2）. The effects of this excessive growth on the

study we made use of high-spatial resolution satellites

environment—such as hypoxia in bottom mud resulting

capable of monitoring conditions in a tidal flat in morefrom the death and decay of benthic organisms and adetail, with the study subject area being Yatsu tidal flat

decrease in waterfowl—are recognized as a problem

in Narashino, Chiba Prefecture（Fig.1）. We intended（T. Yabe et al . 2009）. Moreover, in recent years Ulva

to assess how effective a role the use of high-spatial

spp. have not been dying out but surviving the winters,

resolution satellites might play in monitoring conditions

leading to various kinds of second-order damage

in a tidal flat.

such as unattractive scenery and strong odors fromdecaying Ulva spp.（T. Yabe et al . 2009）.

Fig.1．Overview of the study subject area（Yatsu
tidal flat） captured by the high-spatial
resolution satellite WorldView-3

2．Outline of the study subject area

Fig.2．Ulva spp. distributed in Yatsu tidal flat

3．Analysis methods
As described above, to restore the environment of
Yatsu tidal flat to healthy conditions and to preserve

Yatsu tidal flat is located in Narashino City, Chiba

and manage it, we must first identify causes ofPrefecture, at the head of Tokyo Bay. Although it

the excessive growth of Ulva spp. Therefore, it isescaped direct land reclamation during the period ofimportant to monitor the distribution of Ulva spp. In

high economic growth, it is encircled by dikes andthis study, we conducted analyses in accordance withhas very particular characteristics: it has an area ofthe following methods with a view to ascertaining

approximately 40 ha, is connected with Tokyo Bay byconditions in Yatsu tidal flat and monitoring thethe Yatsu River and the Takase River, and exchanges

distribution of Ulva spp.

seawater with Tokyo Bay through these rivers.
Yatsu tidal flat is well known as a rest area for birds

3.1．Source data

migrating from cold winter regions to warm regions.

We decided to use the Earth observation satellites

In 1993, it was the first tidal flat in Japan to be listed

GeoEye-1 and WorldView-3 to attempt to ascertain theunder the Ramsar Convention（the Ramsar Convention

distribution of Ulva spp. GeoEye-1 has four bands inon Wetlands of International Importance especiallythe visible and near-infrared regions: 450-510nm（blue）,

as Waterfowl Habitat）, an international treaty for the510-580nm（green）, 655-690nm（red）and 780-920nm

154地球環境研究，Vol.21（2019）（near-IR）. WorldView-3 has eight bands in the visible

vegetation surfaces such as mud or shellfish. We canand near-infrared regions: 400-450nm（coastal）, 450-

make use of NDVI to ascertain the distribution of

510nm（blue）, 510-580nm（green）, 585-625nm（yellow）,

Ulva spp. For the calculations of NDVI in this study,

630-690nm（red）, 705-745nm（red edge）, 770-895nm

from the GeoEye-1 data we used the 780-920nm band（near-IR1）and 860-1040nm（near-IR2）. Both satellites

for NIR and the 655-690nm band for R , and from thehave very high-spatial resolutions: 1.64m for GeoEye-1

WorldView-3 data we used the 770-895nm band forlooking straight down（2.00m at 30°off-nadir）and

NIR and the 630-690nm band for R .

1.24m for WorldView-3 （1.38m at 20° off-nadir）.
Therefore, it seemed likely that we could properly
ascertain the distribution of Ulva spp. in the tidal flat

3.4．Deducing Ulva spp. distributions from abottom index
The water depth in Yatsu tidal flat differs between

by using data from these satellites.

locations. In particular, mud deposited during land
3.2．Selection of study subject dates

reclamation in the western half is known to be

Previous studies have reported that Ulva spp. is most

gradually washed away, as a result of which thepervasive in Yatsu tidal flat in the spring（E. Yauchi

mud thickness is decreasing and the ground level is

et al . 2006）. Accordingly, for this study we chose afalling（Yatsu national wildfowl conservation areasampling date in the spring（using GeoEye-1 data

preservation project）. In this context, although it isfrom 10:20 on April 16, 2015）and analyzed this data

simple to obtain distribution information of Ulva spp.

together with data from the fall six months laterthat is completely exposed at the water surface, it（WorldView-3 data from 10:50 on October 26, 2015）in

is hard to obtain distribution information for Ulva

order to study changes. The tide level at Yatsu tidal

spp. submerged in the water by using NDVI （L.

flat is known to lag 1.5 to 2h behind the tide level at

Sochea. and Y. Sakuno 2008, Y. Shiraki et al . 2018）.

Tokyo（Harumi）. For the tidal ranges at Yatsu tidal

Accordingly, we calculated a bottom index（BI）using

flat at the times of the images, we recorded 60-70cm ina water column correction method that has been usedboth spring and fall.

many times in previous studies（D. R. Lyzenga 1981）
and used the bottom index to assess the distribution

3.3．Deducing Ulva spp. distributions using NDVI

of Ulva spp. This bottom index is an algorithm that

The spectral reflectance of Ulva spp. is low in thecancels the effects of water depth and identifies

visible light range and high in the near-infrared

covering materials structuring the bottom（algae, sand,range, similar to the characteristics of land vegetation

shellfish and the like）calculated with as follows:（Y. Shiraki et al . 2018）. The normalized difference
vegetation index（NDVI）is a typical index for using（2）

such characteristics to estimate vegetation activity
levels. NDVI can be calculated from expression（1）:In this expression, BI ij represents a bottom index
calculated for bands i and j, DN i represents a digital（1）

number（DN value）in band i, DN deepi represents thedeep sea DN value for band i, DN j represents a DN

In this expression, NIR represents a reflectance in thevalue in band j, DN deepj represents the deep sea DN

near-infrared region and R represents a reflectancevalue for band j, and k ij represents the ratio of theof red light in the visible light region. The index

scattering coefficients in water for bands i and j. In thisvalues vary between -1 and +1. The higher the value,

study, we used the blue visible light and green visible

the higher the level of activity. Ulva spp., having

light bands from both GeoEye-1 and WorldView-3 assimilar spectral reflectance to terrestrial plants, canbands i and j. To find k ij , we plotted a scatter graph ofbe expected to produce different values from non-natural logarithms of the DN values of blue light and155

Ulva spp. Distribution Observed by Using High-Spatial Resolution Satellites（Shiraki・Oishi・Lee・Kitazawa）

green light in the satellite images and used the slope ofa regression line of the graph as k ij .

4．Results and Observations
Fig.3 shows the NDVI distribution in Yatsu tidal flat on
April 16, 2015, and Fig.4 shows the NDVI distribution
on October 26, 2015. Although there are differencesin the NDVI values due to seasonal effects, the mostprominent feature is that the values are higher inthe eastern half and lower in the western half. As
mentioned above, the ground level in the western
half of the Yatsu tidal flat is falling and, relatively, theground level in the eastern half is rising. Therefore,

Fig.3．NDVI distribution in Yatsu tidal flat（April 16）

in periods during which the water level is low, suchas at ebb tides, Ulva spp. is more likely to be exposed
above the water surface in the eastern half. Based ona number of site surveys, among materials constituting
the bottom of the tidal flat, there is nothing otherthan Ulva spp. that has similar spectral reflectanceto vegetation. Therefore, areas with higher NDVI
values can be understood to be areas where Ulva spp.
is exposed. Fig.5 shows the bottom index distribution
on April 16, 2015, and Fig.6 shows the bottom index
distribution on October 26, 2015. In these images, it
can be seen that areas with bottom index values close
to +1（brown）on April 16, 2015 match the areas
with high NDVI values in the eastern half; that is, thepattern matches areas at which Ulva spp. is exposed

Fig.4．NDVI distribution in Yatsu tidal flat
（October 26）

above the water surface. A pattern of similar values（close to +1）can also be seen in the western halfwhere the ground level is falling and the water levelis deeper. For October 26, 2015 too, a pattern thatmatches the areas with high NDVI values can be seenin the eastern half, and a pattern of similar values canalso be seen in the western half where the water levelis deeper. If, as described above, we can assume thatareas with high NDVI values show the distribution of

Ulva spp., it seems likely that Ulva spp. is also present
in the areas in the western half with bottom index
values close to +1（brown）.

Fig.5．Bottom index distribution in Yatsu tidalflat（April 16）
156

地球環境研究，Vol.21（2019）

Acknowledgments:
This study is a partial revision of the second author
Yuhei Oishi’s graduation thesis. I have greatly
benefited from Ministry of the Environment andYatsu-higata Nature Observation Center. This study
was supported by JSPS KAKENHI Grant Number JP
15K01174.
References
Chen, Y., Dong, J., Xiao, X., Zhang, M., Tian, B., Zhou, Y., Li,
B. and Ma, Z.（2016）Land claim and loss of tidal flats in
the Yangtze Estuary, Scientific Reports , 6, 24018.

Fig.6．Bottom index distribution in Yatsu tidalflat（October 26）

Japan Fisheries Agency: http://www.jfa.maff.go.jp/j/kikaku/
tamenteki/kaisetu/moba/higata_genjou/ （retrieved
March 17, 2018）.
Kikuchi, T.（2000）Tidal flats now: General remarks, marine

5．Conclusions and Future Challenges
In this study, we selected Yatsu tidal flat located in
Narashino, Chiba Prefecture, as the study subject
area and, using high-spatial resolution data from thesatellites GeoEye-1 and WorldView-3, we sought tomonitor the detailed distribution of Ulva spp. caused

and biological , 22（4）,300-314.
Nature Conservation Bureau of the Environment Agency（ed.）（1994）Fourth basic survey of natural environment
conservation: Survey of marine habitats （tidal flats,
seaweed beds, coral reefs）, Biodiversity Center of Japanand Submarine Park Center , 291 pp.
Murray, N. J., Clemens, R. S., Phinn, S. R., Possingham, H.P.
and Fuller, R.（2014）Tracking the rapid loss of tidal

by the deterioration of the tidal flat environment. We

wetlands in the Yellow Sea, Frontiers in Ecology and thefound that although we could identify signals of Ulva

Environment , 12（5）,267-272.

spp. exposed at the water surface with NDVI, we could
not identify signals of Ulva spp. submerged below thewater surface. However, employing the bottom index
calculated using the water attenuation correction
method allowed us to surmise that it may be possible

Lyzenga, D. R.（1981）Remote sensing of bottom reflectanceand water attenuation parameters in shallow water using
aircraft and Landsat date, International journal of remotesensing , 2（1）,71-82.
Shiraki, Y., Kitazawa, T., Lee, S. and Senga.（2018）A Method
for Determining Batillaria attramentaria Distribution

to obtain detailed information on the distribution ofUsing Aerial Balloon Photography and a Vegetation

Ulva spp. throughout Yatsu tidal flat. By making use

Index Camera: Demonstration at the Yatsu Tidal Flat,of these findings, we think it should be possible tomonitor total amounts of Ulva spp. and learn about
seasonal changes and so forth. However, this study

Chiba Prefecture, Advances in Remote Sensing , 7, 15-24.
Sochea, L. and Sakuno, Y.（2008）Eelgrass bed monitoring
using satellite Terra/ASTER data in Yoshina tidal flat,

Annual Journal of Hydraulic Engineering , 52, 1381-1386.

assumed that areas with high NDVI values are Ulva

Takemura, T., Ochiai, M., and Uchida, Y.（2017）Fundamental

spp. and we can only discuss a comparison with thestudy on Ulva spp. monitoring by satellite data Yatsubottom index. In the future, it will be necessary totidal flat, Journal of Japan Society of Civil Engi-neers B3

obtain detailed ground truth data synchronously with
the imaging times of satellites and to assess whether
the information provided by the bottom index matches
the ground truth. Furthermore, additional analysis will
be carried out about the results acquired in this study.

（Marine Development）, 73（2）,I_833-I_838.
Wu, Z. Y., Saito, Y., Zhao, D. N., Zhou, J. Q., Cao, Z.Y., Li,
S.J., Shang, J. H. and Liang, Y. Y.（2016）Impact of humanactivities on subaqueous topographic change in Lingding
Bay of the Pearl River estuary, China, during 1955-2013,

Scientific Reports , 6, 37742.
Yabe, T., Ishii, Y., Amano, Y., Koga, T., Hayashi, S., Nohara,
S. and Tatsumoto, H.（2009）Green tide formed by free-157Ulva spp. Distribution Observed by Using High-Spatial Resolution Satellites（Shiraki・Oishi・Lee・Kitazawa）
floating Ulva spp. at Yatsu tidal flat, Japan. Limnology , 10

Yauchi, E., Hayami, T., Imoto, T. and Gomyo, M.（2006）（3）,239-245.

Unusual growth of Ulva spp. at Yatsu tidal flat andYatsu national wildfowl conservation area preservationassessment of effects on the tidal flat environment,

project: http://yatsu-hozen.com/entry/geographical.html

Proceedings of Coastal Engineering , 53, 1191-1195.

（retrieved March 17, 2018）.

高空間分解能衛星を用いたアオサ類の分布把握手法に関する一考察―千葉県谷津干潟を例として―

白

木

洋

平＊

大

石

悠

平＊＊

李

盛

源＊

北

沢

俊

幸＊

立正大学地球環境科学部
株式会社パスコ 国土情報部＊＊＊

Abstract:
千葉県習志野市に位置する谷津干潟では，アオサ類の異常繁殖（グリーンタイド）によって底生生物の斃死や腐敗による底泥の嫌気化，水鳥の減少など環境への影響が問題視されている．本研究では高空間分解能衛星のGeoEye-1とWorldView-3を利用して干潟内におけるアオサ類の分布の把握を試みた．その結果，NDVIでは水面上に露出しているアオサ類についてはシグナルを得ることが出来るものの，水面下に沈降しているアオサ類についてはシグナルを得ることが出来なかった．しかしながら，水柱放射量補正手法を用いて算出した底質指標を用いることで，谷津干潟
全体のアオサ類の分布情報を得られる可能性について示唆することが出来た．
キーワード：アオサ類，高空間分解能衛星，NDVI，水柱放射量補正，谷津干潟

158