Abstract
Oil spills are a global phenomenon with impacts that cut across socio-economic, health, and environmental dimensions of the coastal ecosystem. However, comprehensive assessment of oil spill impacts and selection of appropriate remediation approaches have been restricted due to reliance on laboratory experiments which offer limited area coverage and classification accuracy. Thus, this study utilizes multispectral Landsat 8-OLI remote sensing imagery and machine learning models to assess the impacts of oil spills on coastal vegetation and wetland and monitor the recovery pattern of polluted vegetation and wetland in a coastal city. The spatial extent of polluted areas was also precisely quantified for effective management of the coastal ecosystem. Using Johor, a coastal city in Malaysia as a case study, a total of 49 oil spill (ground truth) locations, 54 non-oil-spill locations and Landsat 8-OLI data were utilized for the study. The ground truth points were divided into 70% training and 30% validation parts for the classification of polluted vegetation and wetland. Sixteen different indices that have been used to monitor vegetation and wetland stress in literature were adopted for impact and recovery analysis. To eliminate similarities in spectral appearance of oil-spill-affected vegetation, wetland and other elements like burnt and dead vegetation, Support Vector Machine (SVM) and Random Forest (RF) machine learning models were used for the classification of polluted and nonpolluted vegetation and wetlands. Model optimization was performed using a random search method to improve the models’ performance, and accuracy assessments confirmed the effectiveness of the two machine learning models to identify, classify and quantify the area extent of oil pollution on coastal vegetation and wetland. Considering the harmonic mean (F1), overall accuracy (OA), User’s accuracy (UA), and producers’ accuracy (PA), both models have high accuracies. However, the RF outperformed the SVM with F1, OA, PA and UA values of 95.32%, 96.80%, 98.82% and 95.11%, respectively, while the SVM recorded accuracy values of F1 (80.83%), OA (92.87%), PA (95.18%) and UA (93.81%), respectively, highlighting 1205.98 hectares of polluted vegetation and 1205.98 hectares of polluted wetland. Analysis of the vegetation indices revealed that spilled oil had a significant impact on the vegetation and wetland, although steady recovery was observed between 2015-2018. This study concludes that Chlorophyll Vegetation Index, Modified Difference Water Index, Normalized Difference Vegetation Index and Green Chlorophyll Index vegetation indices are more sensitive for impact and recovery assessment of both vegetation and wetland, in addition to Modified Normalized Difference Vegetation Index for wetlands. Thus, remote sensing and Machine Learning models are essential tools capable of providing accurate information for coastal oil spill impact assessment and recovery analysis for appropriate remediation initiatives.
Highlights
Coastal ecosystems are the most densely populated zones [1,2], housing diverse elements like marine mammals, invertebrates and plants
While the former represents the result for the study area alone, the latter shows the performance of similar training and validation data set in classifying larger areas by including Pontian, Johor Baharu and part of Keluang
It can be seen from the producers’ accuracy (PA) that the classification of nonpolluted vegetation has a high accuracy for both Support Vector Machine (SVM) and Random Forest (RF)
Summary
Coastal ecosystems are the most densely populated zones [1,2], housing diverse elements like marine mammals, invertebrates and plants. Oil spills are hazardous because of their long-term environmental impacts. Oil spills affect different elements of the coastal environments at different levels [13], with vegetation and wetlands being the most impacted because of their location at the intertidal zone of the marine ecosystem [14]. Coastal vegetation and wetland cannot survive long-term exposure to oil due to plants smothering and poisoning [15,16]. Over 238 significant marine oil spill incidents have occurred close to coastal vegetation and wetlands worldwide in the past 60 years, with over 5.5 million tons of oil released directly, affecting approximately 1.94 million ha of vegetation and wetland [18]
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