Biodiversity of aquatic organisms in the Mekong Delta, Vietnam.

Background: The Mekong River is the 10th largest river in the world. It is recognised as the most productive river in Southeast Asia and economically essential to the region, with an estimated 60-65 million people living in the lower Mekong Basin. The Mekong Delta within Vietnam is considered a highly vulnerable ecosystem under threat from increasing anthropogenic pressure, such as dam construction and, as a consequence, the Delta is sinking and altering the natural hydrological cycle. Dams also lead to eutrophication and pollution of downstream water from regulated water flux and water stagnation. Another threat is climate change coupled with the lower rainfall, which could lead to an increased risk of drought in the Mekong Delta Basin. Thus, these project data represent an important baseline reference. The ecological health of the Mekong Delta's environment, as indicated by the quality and availability of its water and biological resources, largely determines the economic and social development of the region, which produces about half of the agriculture and aquaculture products of Vietnam. New information: This paper reports quantitative data on the biodiversity of six groups of aquatic organisms: bottom and pelagic fish, macrozoobenthos, microorganisms, phyto- and zooplankton in the Mekong Delta within Vietnam, as well as data on the physicochemical parameters of water and bottom sediments. The data were collected during 2018-2022 as part of the Ecolan E-3.4 programme within the framework of the research plan of the Joint Russian-Vietnamese Tropical Research and Technological Center. All presented datasets are published for the first time.

Features of the accumulation of macroplastic on the river bottom in the Mekong delta and the impact on fish and decapods.

For the first time, a quantitative assessment of this pollutant was made at the bottom of the rivers of the Mekong basin, and the features of its accumulation and dynamics in bottom sediments and relationship with abundance of fish and decapods were investigated. Sampling of materials for the research was carried out by bottom trawls in the Mekong delta in Vietnam. The amount of macroplastic caught by the trawl from the bottom averaged 33.4 g/100 m2. The maximum values of the waste content (up to 923.2 g/100 m2) were confined to the districts of large cities. The distribution of macroplastic was characterized by high spatio-temporal variability. Its amount was significantly (p < 0.05) higher in branches with low monthly flow, in shallow-water areas, as well as in the low-water period. During the flood period, which usually lasts from July to November, the amount of macroplastic at the bottom decreased by an average of 2.5 times. In this study a significant relationship between the number of animals and the mass of macroplastics was statistically proven for most species of fish and decapods. The substrate, containing a significant amount of plastic fragments, attracted aquatic organisms. All in all, we present results for poorly understood processes of transport, deposition and influence of plastic debris in large rivers in regions of monsoon subequatorial climate and show that more efforts should be dedicated to further unravel potentially complex pathways of the plastic exposure to water ecosystems.