Mapping Submerged Aquatic Vegetation along the Central Vietnamese Coast Using Multi-Source Remote Sensing

Tran Ngoc Khanh Ni,Hoang Cong Tin,Izuru Saizen,Vo Trong Thach,Cédric Jamet

doi:10.3390/ijgi9060395

Abstract

Submerged aquatic vegetation (SAV) in the Khanh Hoa (Vietnam) coastal area plays an important role in coastal communities and the marine ecosystem. However, SAV distribution varies widely, in terms of depth and substrate types, making it difficult to monitor using in-situ measurement. Remote sensing can help address this issue. High spatial resolution satellites, with more bands and higher radiometric sensitivity, have been launched recently, including the Vietnamese Natural Resources, Environment, and Disaster Monitoring Satellite (VNREDSat-1) (V1) sensor from Vietnam, launched in 2013. The objective of the study described here was to establish SAV distribution maps for South-Central Vietnam, particularly in the Khanh Hoa coastal area, using Sentinel-2 (S2), Landsat-8, and V1 imagery, and then to assess any changes to SAV over the last ten years, using selected historical data. The satellite top-of-atmosphere signals were initially converted to radiance, and then corrected for atmospheric effects. This treated signal was then used to classify Khanh Hoa coastal water substrates, and these classifications were evaluated using 101 in-situ measurements, collected in 2017 and 2018. The results showed that the three satellites could provide high accuracy, with Kappa coefficients above 0.84, with V1 achieving over 0.87. Our results showed that, from 2008 to 2018, SAV acreage in Khanh Hoa was reduced by 74.2%, while gains in new areas compensated for less than half of these losses. This is the first study to show the potential for using V1 and S2 data to assess the distribution status of SAV in Vietnam, and its outcomes will contribute to the conservation of SAV beds, and to the sustainable exploitation of aquatic resources in the Khanh Hoa coastal area.

Highlights

Vietnam is a coastal country located on the western side of the Eastern Sea (Biển Ðông); it has 3260 km of coast, and a highly diverse assemblage of submerged aquatic vegetation (SAV) [1–3], which consists of two main groups-seagrasses and seaweeds
The authors evaluated the performance of the V1 sensor for detection of SAV along the coast of Khanh Hoa province, Vietnam, and compared its results to those achieved using S2 and L8 satellite sensors
This has been the first study to apply V1 data to establish SAV distribution status, and the results showed that all three satellite sensors had relatively high levels of accuracy (Ҡ > 0.83), and could be used to prepare SAV distribution maps

Summary

Introduction

Vietnam is a coastal country located on the western side of the Eastern Sea (Biển Ðông); it has 3260 km of coast, and a highly diverse assemblage of submerged aquatic vegetation (SAV) [1–3], which consists of two main groups-seagrasses and seaweeds. Fifteen seagrass species have been identified in Vietnamese coastal waters, and while the number of Vietnamese species is low, their root systems hold on to the ground, protecting benthic organisms and reducing coastal erosion. They absorb carbon from the ocean, help purify the aquatic environment, and provide raw materials for several industries [2,6]

Results

Discussion

Conclusion