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Marine litter is a significant threat to the marine environment, human health, and the economy. In this study, beach litter surveys along Vietnamese coasts were conducted in a local context to quantify and characterize marine litter using the modified GESAMP marine litter monitoring guideline. A total of 21,754 items weighing 136,820.2 g was recorded across 14 surveys from September 2020 to January 2021. Plastic was the most abundant type of litter by both quantity (20,744 items) and weight (100,371.2 g). Fishing gear 1 (fishing plastic rope, net pieces, fishing lures and lines, hard plastic floats) and soft plastic fragments were the most frequently observed items (17.65% and 17.24%, respectively). This study not only demonstrates the abundance and composition of marine litter in Vietnam, it also provides valuable information for the implementation of appropriate preventive measures, such as the redesign of collection, reuse, and recycling programs, and informs policy and priorities, with a focus on action and investment in Vietnam. Moreover, insights from this study indicate that citizen science is a useful approach for collecting data on marine litter in Vietnam.

<p>Water hyacinths appear to play an important role in gathering and transporting macroplastic litter in riverine ecosystems. These fast-growing and free-floating invasive freshwater plants tend to form large patches at the water surface, which makes it possible to detect and map them in freely available imagery collected by the European Space Agency (ESA) satellites. In polluted rivers, hyacinth may thus serve as a viable proxy for macroplastics. However, at the ~10m spatial resolution offered by the Sentinel-1 and Sentinel-2 satellites, it’s not possible to discriminate smaller items of plastic caught up within large plant patches.</p><p>We present a case study from the Saigon River around Ho Chi Minh City, Vietnam. Here, we were able to successfully discriminate plastic debris within hyacinth patches using MAXAR Worldview-3 multispectral optical data (1.24m) and panchromatic imagery (0.31m). For the optical data, we selected the ACOLITE atmospheric correction and applied a novel detection index that leveraged the panchromatic band and the red band (band 5) to highlight differences between vegetation and debris. This approach allowed for the detection of riverine plastics within hyacinth patches floating downstream from Ho Chi Minh City towards the coast. Initial results from the Han River and coastal waters of Da Nang in Vietnam suggest that our plastic litter discrimination method is transferable to other aquatic environments. This research is preparatory for further remote sensing monitoring of 'plastic plants' in riverine ecosystems, and will be supporting clean-up operations being trialled in 2022.</p>

Water hyacinth is an aquatic free-floating plant that is highly invasive, to the extent that it is now present in most freshwater bodies in sub-tropical and tropical regions worldwide. Due to the ecological and socio-economic damages these plants can cause, monitoring their spatial coverage and seasonality is key for development of timely and efficient mitigation measures. Hyacinth patches are sufficiently large to be detectable in high-resolution satellite imagery, allowing for monitoring using freely available remote sensing data collected by platforms such as Sentinel-2. In this study, we estimated water hyacinth coverage and seasonal dynamics over three years (2018-2020) for the Saigon river, Vietnam. Using a a Naïve Bayes classifier, hyacinth coverage was mapped in Sentinel-2 imagery with an accuracy of 91%. We show that the dry season (December-May) corresponds to highest water hyacinth abundance, with maximum coverage in February. Lower rainfall and relative humidity were found to be highly correlated (r = -0.56 and r = -0.64, respectively) with higher hyacinth cover. We also detected substantial interannual variability: annual means in hyacinth coverage varied by a factor of five between the 2018/2019 and 2020 yearly averages, with peak cover occuring in February 2020. The percentage of Saigon river covered by hyacinths over the entire study area peaked at 14% and reached as much as 24% for the upstream section. This confirms the prevalence of these invasive plants in the region, and the growing threat to river navigability and biodiversity. Our study provides an openly available automated workflow for long-term monitoring of hyacinth coverage, which can be scaled-up and extended to other freshwater systems. As such, it provides a step for building a large-scale monitoring tool of this highly invasive species, which may also be used for designing mitigation and reduction strategies of hyacinth and the pollutants it carries along.

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