Mapping Sargassum on beaches and coastal waters of Bonaire
using Sentinel-2 imagery
William Tjiong
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)
• 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)
• 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)
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)
• 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.)
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"
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
Fig 11. Spectral signature
Methodology objective 2:
mapping seaweed on the sea using unsupervised classification
• 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
Sargassum on the sea preliminary results
Fig 17. Clustering result C
UFO?
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…
Fig 19. Cloud animation
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
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
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