ASSESSMENT OF THE PHYSICOCHEMICAL PROPERTIES OF THE MAIN RIVERS OF THE MEKONG DELTA AND THEIR ECOLOGICAL ZONING

The Mekong Delta is one of Asia's largest megadeltas. It is a region with unique biodiversity and the main food source for a large part of the population of Vietnam and Cambodia. The decrease in the biodiversity and resource capacity of the delta, associated with the growth of anthropogenic impact and other negative environmental factors, requires efforts to maintain its ecosystem in a state favorable for the life of aquatic organisms, for which the first step is to organize effective monitoring of aquatic communities. Its organization must consider the complex structure of the delta and related features of the physicochemical characteristics of the habitat. The mean and interval values of the parameters were as follows: transparency was 47.7±16.4 (10-100) cm; in the surface water layer, the temperature was 29.7±1.37 (26.0-33.3) °C, electrical conductivity was 5490.4±8392.5 (133-33623) µS/cm, total dissolved solids were 3556.2±5445.8 (86.5-21853), pH was 7.6±0.3 (6.9-8.4), and dissolved oxygen was 4.8±1.05 (3.0-8.9) mg/L; in the bottom water layer, the temperature was 29.6±1.43 (26.0-34.1) °C, electrical conductivity was 7671.4±11193.2 (125-37870) µS/cm, total dissolved solids were 4905.3±7199.8 (78.0-24615.5), pH was 7.6±0.3 (6.9-8.2), and dissolved oxygen was 4.6±1.00 (2.6-7.7) mg/L. The zoning of the delta, based on the knowledge of the environment, makes it possible to organize the monitoring process in the most rational way. Following the results of the analysis of physicochemical parameters, the boundaries of zones with different environmental conditions that affect the formation of fish communities have been determined. The main contributing factors are salinity (3 zones), oxygen regime (3 zones), and runoff distribution (4 zones). The interrelation between the delta zoning according to the hydrochemical regime and the structural features of the fish communities at the family level has been found, which further supports the idea that such zoning must be taken into consideration when organizing monitoring surveys.