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

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.

Persistent Clones and Local Seed Recruitment Contribute to the Resilience of Enhalus acoroides Populations Under Disturbance

“…Water transparency was low at sites with high abundances of epiphytic algae, anemones and jellyfish in the water and on seagrass shoots ( Figure 1D ) which indicate that the sampled lagoons were highly eutrophic ( Purcell et al, 2007 ); however, no relationship between land use and turbidity and no effect of turbidity on the genetic variables of E. acoroides populations could be found. Nevertheless, extreme reductions in seagrass coverage in our study area have been shown and are most likely associated with extensive human activities in lagoons and high levels of turbidity ( Khanh Ni et al, 2020 ; Vo et al, 2020 ). No evidence of inbreeding or loss of genetic diversity was found for E. acoroides and demonstrates the high level of resilience of E. acoroides populations toward environmental perturbations.…”

“…Moreover, it is important to identify the direct local environmental drivers caused by human-induced land use and their relative effect on populations of E. acoroides . The studied meadows are most likely remnants of large, connected multi-species seagrass beds which disappeared with increasing human activities ( Chen et al, 2016 ; Khanh Ni et al, 2020 ) and E. acoroides being the only remaining seagrass species persisting under high levels of human-induced disturbance. We hypothesize that seagrass ecosystems with E. acoroides as key species will eventually shift to systems dominated by macro-algae, epiphytic algae and phytoplankton when environmental conditions continue to diminish, combined with the effects of climate change and potential large disturbance events in the nearby future.…”

“…The study area was located in the provinces Khanh Hoa (Thuy Trieu and Van Phong Bay) and Phu Yen (Xuan Day and Cu Mong) ( Figure 1A ). Studies determined seagrass distribution over time and identified possible drivers for seagrass loss in Khanh Hoa which revealed a total loss of 74.2% of seagrass area between 2008 and 2018 and various human activities in the coastal zone (e.g., tourism, shellfish collection and aquaculture) appeared as main causes of seagrass decline ( Chen et al, 2016 ; Khanh Ni et al, 2020 ). In Van Phong Bay, a total decrease of 35.8% over 30 years (1988–2019) with an approximate loss of 46.5–87.3 Ha per decade has been shown ( Vo et al, 2020 ).…”

“…In Van Phong Bay, a total decrease of 35.8% over 30 years (1988–2019) with an approximate loss of 46.5–87.3 Ha per decade has been shown ( Vo et al, 2020 ). Consequently, seagrass meadows in Thuy Thrieu and Van Phong Bay are fragmented and restricted to shallow waters with a maximum depth of 3.5–5 m near the coast ( Khanh Ni et al, 2020 ; Vo et al, 2020 ). Thieu Thrieu is characterized by high turbidity with spatio-temporal variations driven by rainfall and resuspension of the sediment with maximum values for NTU exceeding 13.1 ( Quang et al, 2017 ).…”

The application of drones for mosquito larval habitat identification in rural environments: a practical approach for malaria control?

“…While the performance metrics indicated very little difference in the accuracy obtained using the two approaches, a more extensive investigation would need to be undertaken over a more environmentally diverse area before any conclusions can be drawn on the benefits to incorporating this additional technology. Further sensors such as shortwave infrared (SWIR) and thermal infrared may further improve the surface water classification accuracy [ 34 ].…”