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Mapping Sargassum on beaches and coastal waters of Bonaire using Sentinel-2 imagery

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Mapping Sargassum on beaches and coastal waters of Bonaire

using Sentinel-2 imagery

William Tjiong

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Context

• Sargassum: free floating seaweed in the ocean

• Sargasso sea discovered in the 19th century

• Sargassum bloom in the Atlantic is possibly induced by nutrient loadings from the Mississippi River

Fig 2. Sargasso Sea extent (Krümmel, 1891) Fig 1. Floating Sargassum (Maltese, 2014)

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• two major Sargassum species in the North Atlantic: S.fluitans and S.natans

• large Sargassum rafts provide shelter and food source for many marine species

Fig 4. Sargassum as food and shelter (Gross, n.d.) Fig 3. S.fluitans and S.natans (Louime et al., 2017)

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• excessive amounts of Sargassum can damage blue carbon ecosystems

• recurring large-scale Sargassum invasion in the Caribbean Island since 2011

Fig 6. Sargassum invasion in Lagun, Bonaire (BES-REPORTER, 2018) Fig 5. New Sargassum bloom region since 2011 detected with MODIS.

Adapted from (Wang et al., 2019)

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Problem definition

• STINAPA (nature foundation) & Bonaire's local government working on a Sargassum crisis plan

• early warning/ monitoring system plays an important part in this plan

• earth observation sensors ideal for large scale monitoring systems

• past studies utilize MODIS (250-1000m) data and non-commercial satellite sensors to detect seaweed

Fig 7. STINAPA Bonaire (STINAPA, 2018)

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• Problem: mixed signals between seaweed and coastal features

• Greater need for higher temporal-, spatial- and spectral resolution data

• Sentinel-2: up to 10m with 5 days revisit time

Fig 8. Sentinel-2 spatial- and spectral specifications (European Space Agency, n.d.)

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Objective

• Accurate Sargassum maps important for assessing the extent and impact of future invasion

• Investigating the effectiveness and limitations of Sentinel-2 spectral data for mapping coastal ecosystems

Research questions:

1. To what extent can Sargassum patches be classified on the coast of Bonaire using Sentinel-2 imagery?

2. Can floating Sargassum patches be classified on open sea with Sentinel-2?

3. Can the impact of Sargassum on mangrove forest health be measured with Sentinel-2?

4. Can the impact of Sargassum on seagrass health be mapped with Sentinel-2?

"Mapping Sargassum patches on beaches and coastal waters of Bonaire based on their spectral characteristics"

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Methodology objective 1:

mapping seaweed on the coast (and coastal waters) using supervised classification

• Seaweed invasion periods Bonaire: December, March, July 2018 and March, May 2019

• Collecting training samples from S2 Level-2A images: 2, 9,14 and 19 March 2019 (best cloud condition)

Fig 9. Study area Bonaire

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Fig 11. Spectral signature

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Methodology objective 2:

mapping seaweed on the sea using unsupervised classification

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• K- means clustering with NDVI assume that any NIR reflectance is exclusively due to Sargassum

• compare with DTC and MLC models (all bands + NDVI)

• compare with simple histogram thresholding (NDVI from multiple dates)

• accuracy assessment sample reference/ validation pixel visually and compare with K-means?

Fig 16. NDVI taken on 4th March 2018

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Sargassum on the sea preliminary results

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Fig 17. Clustering result C

UFO?

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Fig 18. Inter-band parallax animation Fig 19.Staggered detector setup and Inter-band parallax angles (European Space Agency, n.d.)

No, it’s just systematic error…

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Fig 19. Cloud animation

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What’s next?

Objective 2:

• investigate NDVI histogram threshold for multiple dates

• investigate partially submerged seaweed (negative NDVI)

• compare with DTC and MLC

Objective 3 (measuring mangrove health):

• extract time-series (2017-2019) data with Google Earth Engine

• anomaly detection based on Seasonal Trend Decomposition using Loess (STL)

• NDVI-LAI map from before and after invasion

Objective 4 (mapping seagrass coverage loss):

• using DTC and MLC again

• compare map results from before and after invasion

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Discussion

Objective 2:

• any metrics to evaluate K-means performance?

• maybe turn into supervised case?

o get reference pixels of submerged seaweed then compare with classification results

Objective 3:

• Sentinel-2 time series resolution ≈ 30m

• how to position sample features (polygon/ line) at mangrove fringes to minimize co-registration error?

Fig 20. Sentinel-2 co-registration error (European Space Agency, 2019) Norm: <0.3 pixel !

Fig 21. Mangrove sample locations

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References

BES-REPORTER. (2018). Massive Effort to Rid Bonaire Coast of Sargassum Seaweed - BES Reporter. Retrieved November 15, 2019, from http://bes-reporter.com/index.php/2018/03/07/massive-effort- rid-bonaire-coast-sargassum-seaweed/

Bricaud, A., Claustre, H., Ras, J., & Oubelkheir, K. (2004). Natural variability of phytoplanktonic absorption in oceanic waters: Influence of the size structure of algal populations. Journal of Geophysical Research: Oceans. https://doi.org/10.1029/2004JC002419

Dutch Caribbean Nature Alliance. (2019). Prevention and clean-up of Sargassum in the Dutch Caribbean. Retrieved from https://www.dcnanature.org/wp-content/uploads/2019/02/DCNA-Sargassum- Brief.pdf

European Space Agency. (n.d.). Sentinel-2 - Missions - Resolution and Swath - Sentinel Handbook. Retrieved November 15, 2019, from https://sentinel.esa.int/web/sentinel/missions/sentinel- 2/instrument-payload/resolution-and-swath

European Space Agency. (2019). L1C Data Quality Report Ref. S2-PDGS-MPC-DQR S2 MPC.

Frantz, D., Haß, E., Uhl, A., Stoffels, J., & Hill, J. (2018). Improvement of the Fmask algorithm for Sentinel-2 images: Separating clouds from bright surfaces based on parallax effects. Remote Sensing of Environment, 215, 471–481. https://doi.org/10.1016/j.rse.2018.04.046

GEF. (2018). Communities innovate to address Sargassum seaweed on coasts of Saint Lucia | Global Environment Facility. Retrieved November 15, 2019, from https://www.thegef.org/news/communities-innovate-address-sargassum-seaweed-coasts-saint-lucia

Gross, S. (n.d.). Fish among Sargassum Seaweed in the Sargasso Sea - Greenpeace USA. Retrieved November 15, 2019, from https://www.greenpeace.org/usa/the-ocean-is-full-of-wonder-blackwater- photos/fish-among-sargassum-seaweed-in-the-sargasso-sea/

Krümmel, O. (1891). Die nordatlantische Sargassosee. Petermann’s Geographische Mitteilungen, 37, 129–141.

Louime, C., Fortune, J., & Gervais, G. (2017). Sargassum Invasion of Coastal Environments: A Growing Concern. American Journal of Environmental Sciences, 13(1), 58–64.

https://doi.org/10.3844/ajessp.2017.58.64

Maltese, L. (2014). A Sea like none other - The Sargasso Conquest - Ecology - Magazine’s files - Ocean71. Retrieved November 15, 2019, from http://ocean71.com/chapters/sea-like-none-other/

STINAPA. (2018). Sargassum Bonaire.

Verweij, P. (2018). Raw data of seagrass observations. Retrieved September 25, 2019, from https://www.dcbd.nl/author/engel-s

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