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Ecology and Conservation

Marine Knowledge

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Dongsha Atoll - One of the Rarest Complete Atolls in the World
Dongsha Atoll is located in northern South China Sea. With a diameter of approximately 25 km, it was formed about 10,000 years ago. Comprising different ecological habitats including islands, lagoons, tidal flats, seagrass beds, coral reefs and ocean, Dongsha Atoll is one of the very few complete atolls in the world. Spanning an area of about 350,000 hectares, Dongsha Atoll became Taiwan’s 7th national park in 2007 and first marine national park in the country. The diverse environment nurtures nearly 700 species of fish and over 1,000 species of marine life such as corals, spiral shells, shells, crabs and macrophytic algae, making it a hot spot teeming with valuable marine biodiversity. In addition, Dongsha Atoll is an excellent environment for coral growth, which, together with upwelling cold water brought by South China Sea’s internal waves, offer protection benefits amid the warming and acidifying seawater around the world.

Satellite Imagine Classification and Ground Truth Survey
Due to the remote location of Dongsha Atoll, it is protected from human disturbance and therefore endowed with an independent ecosystem and rich biodiversity. Since the atoll spans a wide area that is relatively inaccessible by boats, optical satellite imagery is suitable for investigating environmental changes at Dongsha Atoll. According to satellite images, Dongsha Atoll can be divided into several habitats, including coral, seagrass, silt and reef. Preliminary classification is conducted through satellite imagery, which is combined with the outcome of ground truth survey to generate spatial distribution information for Dongsha Atoll’s habitats and environment.

After comparing satellite images with the result of ground truth survey, many habitats unique to Dongsha were discovered. For instance, satellite images reveal a high concentration of lobe coral on the north crest of Dongsha Atoll, a coral transition belt on the south side of the atoll, the existing platform on the patch reef and seagrass distribution area to the east of the atoll.

 

2017 03 31 01
Google satellite image (large scale) of lobe coral
(approx. 5 km long)
2017 03 31 02
WorldView-2 satellite image (50 cm/pixel)
 

 

2017 03 31 03
Surface image (medium scale)
2017 03 31 04
Underwater image

 

2017 03 31 05
Surface image (medium scale)
2017 03 31 06
Underwater image

  

2017 03 31 07
Formosat-2 satellite image
2017 03 31 08
SPOT-5 satellite image

 

2017 03 31 09
Underwater image
(Coral coverage rate nearly 100%)
2017 03 31 10
Underwater image
(Coral coverage rate nearly 10%)
2017 03 31 11
Aerial photography
2017 03 31 12
Underwater image

 

2017 03 31 13

Satellite images of Dongsha Atoll over the years 

Formosat-2 satellite images

2017 03 31 14

Dongsha atoll satellite image analysis

Formosat-2 satellite image analysis

 

Advantages of satellite observation  

2017 03 31 15
Applying satellite imagery to seaweed observation
 
2017 03 31 16
Applying satellite imagery to seagrass habitat
observation

 

Application of Remote Sensing to Monitor Habitat Change
According to satellite image classification conducted from 2006 to 2013, coral, seagrass, silt and coral conglomerate distributions are in a cycle of successive changes. The atoll’s north crest and interior have exhibited significantly more changes; after comparing satellite images with ground truth data, it was discovered to consist of mostly coral and seagrass habitat. Variations in coral and seagrass coverage are attributed to changes in the macro environment such as water temperature, lighting and nutrient salt.

2017 03 31 17

 

Founded in 2014, Dongsha Atoll Research Station has explored the ecology, environment and development history of the atoll. Analysis of SPOT and Formosat-2 satellite images reveal that seagrass cover an area of roughly 9,000 hectares, and it expands or diminishes depending on seasonal conditions, with changing period of about one year. Coral coverage is nearly 4,000 hectares, and it has exhibited a diminishing trend due to the acidification and warming of seawaters around the world. From June to September 2014, seagrass on the north and northeast crest have also diminished by about 2,500 hectare, roughly the size of nearly 100 Daan Forest Parks. Similar situations have not been discovered over the past 2 decades. In the future, changes in the overall habitat of Dongsha Atoll will continue to be monitored and studied in order to collect and discuss environmental information related to Dongsha Atoll. This information will become an important reference in devising potential sustainable management strategies.

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