The Mekong River is the 10^th 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. 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.

Human activity has disturbed the functioning of river ecosystems all around the globe. The current global climatic fluctuations and local anthropogenic impact lead to rearrangement in the structure and functioning of aquatic communities. One of the most important components of aquatic ecosystems is phytoplankton as the main primary producer of the organic matter, the basis for trophic relations and indicator of changes in the environment. This article presents the first results of a study concerning the peculiarities of quantitative distribution of biomass and species composition of phytoplankton in the delta of the Mekong River at the beginning of the dry season (December of 2018). Diatoms dominated according to biomass practically in all the stations of selection of samples. The total biomass of phytoplankton on average accounted for 0.049 ± 0.007 mg/L at the abundance of 40 ± 7 103 ind./L. In practically all the studied plots, according to biomass, the dominating diatoms were first of all Aulacoseira granulata, A. islandica, Cyclotella meneghiniana, Cyclotella spp., and Oxyneis binalis. Among Chlorophyta, most often we found Chlorella sp. and Scenedesmus quadricauda, but their biomass was insignificant. We determined statistically significant correlation relationships between biomass of phytoplankton and hydrological parameters. Based on the Spearman’s rank correlation coefficient, we determined negative relations between the total biomass of phytoplankton with salinity and pH. Positive correlation was seen between the biomass of diatoms and turbidity, and also between the temperature and the biomass of chlorophytes and Dinophyta. The biomass of golden algae (Chrysophyceae) and Dinophyta positively correlated with the mineralization. Quantitative regression analysis confirmed the close relationship between the total biomass of phytoplankton, hydrophysical and hydrochemical parameters. Besides the importance of scientific data on biological diversity and ecology of plankton algae, the results we obtained are necessary for organizing biological monitoring in the delta of the Mekong River in the future.

This work continues a series of publications on the diversity of the main groups of invertebrates found through the investigation of meiobenthic communities in various inland waterbodies in central and southern Vietnam. Here, a checklist of oligochaetes and aeolosomatids (the annelid worms) discovered for the period 2010–2021 is presented, as well as data on the structure and abundance of their populations found in the country, ecology, local and global distribution, and, where necessary, taxonomic, morphological and other comments. In total, 71 representatives of the species and supra–species ranks from 3 families and 17 genera are listed. In total, 39 representatives were identified to the species level. During the present research, 2 species new to science were discovered and described in previously published works. This paper reports 9 more forms that need further study and may belong to still undescribed species. In addition to the new and still-unidentified species, the checklist reports about 30 more members of species and supra–species ranks that were recorded in Vietnam for the first time. Based on new and previously published data, the currently known diversity of aquatic oligochaetes and aeolosomatids in Vietnam is estimated at approximately 100–113 and 8–9 representatives, respectively. It is shown that members of the family Enchytraeidae remain practically unexplored in the waterbodies of the region. The subfamily Tubificinae and the family Aeolosomatidae also continue to be relatively poorly studied here at the species level.

During recent years, many taxa of water fleas (Crustacea: Cladocera) with wide distribution ranges were revised and finally accepted as groups of cryptic species. Here we provide a redescription of Ilyocryptus raridentatus Smirnov, 1989 (Anomopoda: Ilyocryptidae). Our study clearly confirms that I. raridentatus belongs to the sarsi-group based on incomplete moulting, absence of dorsal keel, structure of antennule, thoracic limb I and postabdomen. The species is widely distributed in Australia and East Asia (Malaysia, Thailand, Vietnam, Central China, Japan, South Korea) and penetrates north to the Far East of Russia. A morphological variability among different populations of I. raridentatus does not exceed an intrapopulational level. I. raridentatus seems to be a relatively rare species with a benthic lifestyle similar to its congeners.

In previous papers, it was shown that the Far East is a territory where moving south, Boreal fauna is fluently changed to Tropical fauna. Kotov (2016) proposed to place all taxa from the northern portion of Far East in several faunistic complexes according to the area of their differentiation in the past, i.e. in Pleistocene refugia. But many daphniids and chydorids were placed in an artificial group of non-revised taxa rather than a certain faunistic complex. Ilyocryptids are less numerous and better studied, and they could be used as a model group to test such an approach. Totally, 10 species were found in the Far East: from the very common I. spinifer group, the relatively common I. acutifrons, I agilis, I. yooni, I. raridentatus and I. cuneatus to the very rare I. cf. bhardwaji, I. isanensis, I. thailandensis and I. uenoi. Note that four species are found in Vietnam for the first time, namely, I. isanensis, I. cf. raridentatus, I. thailandensis and I. yooni. In contrast to the aforementioned daphniids and chydorids, ten Far Eastern ilyocryptid taxa accurately fit three faunistic complexes: WE—widely distributed in North Eurasia; ST—southern tropical; EN—Far Eastern endemic. Ilyocryptid distribution fits well with the “wide transitional zone concept” between “Palaearctic” (in reality, we have found that its separation within the Holarctic does not work well for the cladocerans) and Oriental zones, and such our conclusion is made based on the analysis of the adequately known group.

This work is the third installment in a series of publications devoted to the diversity, distribution and ecology of major groups of hydrobionts discovered during the study of meiobenthic communities in inland waters of central and southern Vietnam. Here, a checklist of cladoceran crustaceans (Cladocera) found for the period 2010–2022 is presented, as well as generalized and updated data on their taxonomy, ecology and global/local distribution, based on the latest studies. A total of 55 representatives from four families (Sididae, Ilyocryptidae, Macrothricidae and Chydoridae) and 29 genera are recorded. Of these, 13 taxa were detected in Vietnamese water bodies for the first time, including the chydorid Paralona pigra, which was the first registered in the Oriental region as a whole. Another four discovered forms from the genera Ilyocryptus, Macrothrix, Prendalona and Ephemeroporus require additional research. They may belong to not-yet-described species because they exhibit noticeable morphological differences from other known relatives. Based on newly obtained and previously published data, the diversity of Cladocera from the bottom and near-bottom ecological groups in Vietnam today can be estimated at around 95 representatives (four Sididae, seven Ilyocryptidae, one Ophryoxidae, nine Macrothricidae and 74 Chydoridae). A review of the available literature, together with the significant proportion of new and still poorly studied taxa found in our material, indicate the need for further comprehensive studies on the cladocerans in the region.