Knowledge of trace elements content and their behavior in aquatic ecosystems is important for their sustainable use. There is a lack of such data for saline and, especially, hypersaline lakes and lagoons. Concentrations of more than 20 elements were evaluated in bottom sediments of 15 saline/hypersaline lakes and Lagoon Sivash in Crimea. An average salinity varied from 4 to 335 g/L in studied water bodies. The concentration of the trace elements varied from lake to lake. The highest variability was recorded for Cd, from 4.13 mg/kg to below the detectable level (CV = 1.463), and for Se, from 5.52 to 0.05 mg/kg (CV = 1.053). The lowest variability demonstrated by Cr, from 368 to 17 mg/kg (CV = 0.463), and by V, from 67.8 to 1.7 mg/kg (CV = 0.481). According to the found content of studied elements, all lakes were separated into three groups, and Lagoon Sivash was not included in these clusters. Salinity affected the concentration of some elements in bottom sediments, and this effect was not linear or unidirectional. In some cases, the action of other factors, often unknown, masked the effect of salinity. The geochemical background affects the structure and functioning of aquatic ecosystems, but the state of these ecosystems can significantly modify this background. An understanding of the differences in the elemental composition of bottom sediments in different lakes is possible only based on an integrated consideration of the interaction of all landscape, intra-ecosystem, and anthropogenic processes and factors that can influence this.

Chemoecological studies were carried out in the water areas of the Kagau and Longtau rivers in the Can Gio Biosphere Reserve (Vietnam). Mass-spectrometry analysis was used to determine the concentrations of 15 microelements (heavy metals and metalloids) in water and bottom sediments. The overall pollution level in the Can Gio rivers was found to be moderate. However, the concentrations of some heavy metals in several cases exceeded the maximum permissible concentration at short-time exposure; also, the limits of safe concentrations at chronic exposure were exceeded for some elements. The concentrations of some heavy elements amount to 30 to 80% of the MPC for water bodies used for fishery. Primarily, these are copper, zinc, arsenic, vanadium, iron, and molybdenum. It was found that, according to the Regulations of Vietnam, the waters of the examined water areas showed concentrations of suspended matter in excess of the standards for category A1 sources, and the concentration of dissolved oxygen in water was below the standard. The obtained results suggest the need to monitor the chemical–ecological conditions of rivers in Can Gio as protected natural objects, which are also used to some extent for social and economic purposes.