Geoenvironmental specifics in the microtidal mouths of small rivers with deltas in the Russian Arctic Zone during the summer-fall low water season by the example of the Kad River mouth, the White Sea

The hydrological, hydrochemical and hydrobiological conditions of the Kad River mouth are considered that flows to the Sukhoe More lagoon, the Dvina Bay, the White Sea. It is revealed that suspension sedimentation is most developed in the river delta and on its shallow estuarine seashore. Extensive thickets of halophytes are formed in these areas, and the maximum accumulation of heavy metals is noted in bottom sediments. Manganese, zinc and nickel show the highest concentrations in the bottom sediments within the Kadi River mouth. A similar pattern is observed for the snow cover pollution in the considered area (the southwestern part of the Belomorsko-Kuloiskoe plateau). This indicates a significant contribution of snow to the heavy metal accumulation in the estuary area of the Kadi River and the adjacent section of the Sukhoe More lagoon. Perhaps this situation is typical for the river mouths in the Arctic zone. Under the observed climate warming, the processes noted in the Kadi River mouth suggest that they will lead to waterlogging and a reducing the lagoon area in the micro-tidal estuaries of the rivers of the Arctic seas. They will accumulate suspended material carried out with river runoff during the destruction of permafrost soils. It can contain both biogenic substances that contribute to an increase in the trophic capacity of estuarine ecosystems, and toxic substances that limit their development. At the same time, marginal filters of the estuaries of the rivers in the Arctic islands will increase their spatial extent within the lagoons, primarily due to coagulation-sorption and biological stages.