TO THE QUESTION OF ACCUMULATION OF HEAVY METALS (Pb, Hg, As) IN THE COMPONENTS OF AQUATIC ECOSYSTEMS OF ESTUARINE AREA OF THE RED RIVER (VIETNAM)
The paper focuses on the peculiarities of accumulating heavy metals (Pb, Hg, As) in the system "water - bottom sediments - mollusks" in the estuary of the Red river (Vietnam). The objectives established in the work were to determine concentration and accumulation of heavy metals in different components of the estuarine area of the Red river (water, suspended solids, sediments, soft tissues and intestines of shellfish) and to analyze the dynamics of heavy metals. The studies were conducted during the main hydrological seasons in 2014-2016 (high water and low water periods). The suspended Pb, Hg, As concentration in the waters of the mouth area of the Red river is maximum in the mouths of the arms and gradually decreases towards the sea. The distribution of Pb, Hg, As in suspended solids is characterized by a considerable unevenness, the maximum concentration of these elements was observed in the surface water layer on the river-sea frontier. According to the average values of the enrichment coefficient K<sub>SS/BS, </sub>the studied heavy metals arrange in the following order: Pb>Hg>As, i.e. the maximum contents of Hg and As in the mouth area of the Red river were observed in sediments, maximum content of Pb - in suspension. There has been found a mechanism of distributing heavy metals in different forms in sediments. Accumulating the studied heavy metals in mollusks Meretrix lyrata is uneaven, which is both stipulated by physical abilities of different body organs of the mollusks and movability of the metals. According to the content of heavy metals in the soft tissues and intestines of mollusks, heavy metals are distributed in the following order: As>Pb>Hg. Accumulation of Pb in the intestine is 3 times greater than in soft tissues, but accumulation of As and Hg in the intestine and soft tissues are almost identical. Mollusks take most heavy metals from the suspended matter and organic fraction of bottom sediments.