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To date, very little is known about the relative importance of physicochemical factors structuring the benthic diatom community in the flood-pulse tropical ecosystems. This paper aims to investigate the spatial distribution of the benthic diatom community in a poorly studied river catchment of the Tonle Sap Basin in Cambodia. Diatom samples were collected at nineteen (19) monitoring sites from 2018 to 2023 along the Sangker River, and the hierarchical clustering technique was applied to characterize the variation in diatom community structure based on the similarity of diatom genera composition. Seventy-eight (78) diatom genera belonging to 36 families, 18 orders, and 5 classes of diatom flora were recorded. Gomphonema Ehrenberg, Nitzschia Hassall, and Navicula Bory were the most dominant diatom genera in the catchment area. Accordingly, three main diatom assemblages (Ia, Ib, and II) were classified, and 11 indicator genera were identified. Indeed, the identified assemblages reflect the change of diatiom community composition from the upstream assemblage (Ia), middle course (Ib), and floodplain assemblage (II). Dissolved oxygen (DO), pH, conductivity (EC), total phosphorus (TP), and silicon dioxide (SiO2) were the physico-chemical determinants of benthic diatom communities, while physical factors (DO, pH, and nutrients) seemed to be more important than chemical factors. Consequently, any disturbances to the physical condition of the river would drive an abrupt shift in the diatom community’s diversity and structure. These findings pave an essential step towards the establishment of sustainable biomonitoring programs in developing countries such as Cambodia to protect aquatic ecosystems from global change.

Understanding how habitat heterogeneity is linked to biodiversity patterns within flood–pulse catchments is needed for their effective conservation. To study those communities, researchers have begun to explore how local environmental factors influence species composition patterns at relatively fine scales within complex habitat mosaics. However, a robust description of the link between communities' composition and their floodplain habitat characteristics remains poorly articulated.In the Tonle Sap Lake (TSL) of Cambodia, we evaluated whether mesohabitat‐scale (1–5 km) factors throughout the floodplain structured the fish assemblage temporally and spatially. First, we determined whether TSL fish assemblage changed seasonally and, if so, what assemblage components were driving that change. Second, we determined whether environmental factors structured fish composition across floodplain mesohabitat patches during the wet season. Third, we evaluated whether dominant species displayed affinities for specific areas within the floodplain during the wet season, potentially identifying critical mesohabitat patches for the fish assemblage.Fish were collected from nine sites along an elevational gradient traversing the floodplain every 3 months from March 2014 through January 2015. Correspondence analysis and analysis of variance were used to assess seasonal variation in the species composition. Canonical correspondence analysis and a mantel test were then used to identify the significance of environmental factors structuring the fish assemblage.Fish assemblages changed across seasons; in particular, migratory species occurrence decreased during the dry season. During the wet season, the fish assemblage was structured spatially by mesohabitat‐scale factors such as vegetation and water quality. Furthermore, the flooded forest mesohabitat directly adjacent to the permanent lake displayed particularly high species richness and abundance. However, a majority of TSL species appeared to be highly mobile, occurring across multiple floodplain mesohabitats.We concluded that fish assemblage structure and floodplain mesohabitat use within TSL may depend strongly upon the maintenance of the natural flow regime. Since the flow regime will probably be modified by upstream dam development, trade‐offs will emerge between the cycle of fishing on which local residents depend, and the hydropower and agricultural benefits of damming the Mekong River system. [ABSTRACT FROM AUTHOR]

Although the Mekong River is one of the world's 35 biodiversity hot spots, the large-scale patterns of fish diversity and assemblage structure remain poorly addressed. This study aimed to investigate the fish distribution patterns in the Lower Mekong River ( LMR) and to identify their environmental determinants. Daily fish catch data (i.e. from December 2000 to November 2001) at 38 sites distributed along the LMR were related to 15 physicochemical and 19 climatic variables. As a result, four different clusters were defined according to the similarity in assemblage composition and 80 indicator species were identified. While fish species richness was highest in the Mekong delta and lowest in the upper part of the LMR, the diversity index was highest in the middle part of the LMR and lowest in the delta. We found that fish assemblages changed along the environmental gradients and that the main drivers affecting the fish assemblage structure were the seasonal variation of temperature, precipitation, dissolved oxygen, pH and total phosphorus. Specifically, upstream assemblages were characterised by cyprinids and Pangasius catfish, well suited to low temperature, high dissolved oxygen and high pH. Fish assemblages in the delta were dominated by perch-like fish and clupeids, more tolerant to high temperatures, and high levels of nutrients (nitrates and total phosphorus) and salinity. Overall, the patterns were consistent between seasons. Our study contributes to establishing the first holistic fish community study in the LMR. [ABSTRACT FROM AUTHOR]