Macrozoobenthos of the Mekong River Delta under Conditions of Extremely Low Water

In 2019, due to changes in the hydrograph caused by the construction of dams upstream, the Mekong River Basin experienced a decline in delta water level. In this year, macrozoobenthos communities of the Bassac (Hau) and Mekong (Tien) Rivers in Vietnam were studied in April and December. The total species richness, abundance, and biomass of studied macrozoobenthos assemblages did not depend on any environmental factors. The beta-diversity estimation for species abundance shows the existence of three groups of species assemblages, related mostly to salinity. Most assemblages were characterized by low species richness (3.2 on average) and similarity (general mean Bray-Curtis = 8%), a large number of species, which were dominants in certain assemblages (31-42% of the total list), with the absence of a relationship with hydrological zones or seasons. This allowed us to consider these groupings as "pseudocommunities", randomly formed in the extreme conditions of a sharp change in the hydrological regime of the Mekong River Delta in 2019.