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.

The article presents the results of generalizing the data of 20 years of research in the river–sea transition zone of the Northern Dvina River and White Sea (marginal filter zone), in which sedimentation and biogeochemical transformation of riverine sedimentary material lead to significant losses on its way to the open area of the sea. For the first time, based on a large amount of material, the average-weighted (in terms of runoff) quantities of the so-called “pure” discharge of suspended particulate matter and dissolved and suspended particulate Fe and Mn were obtained. These are discharges of these substances closest to reality with account for losses in the marginal filter zone depending on the season. It was determined that on average in a year from the total volumes transported by river waters to the boundary with the sea, only about 40% of suspended sediments, 50% of total iron, and almost 60% of total manganese reach its open areas. Precisely these volumes should be taken into account in computing the geochemical balance and in ecological research in the sea.

The results of continuous four-year-long investigations (from May 2015 to April 2019) of the elemental composition of water and suspended matter in the Northern Dvina River are given. Water was sampled every month at two sites in the lower reaches of the river. More than 50 major and trace elements are determined in water filtered through Nuclepore filters and in suspended matter on the filters. The methods of inductively coupled plasma mass spectrometry and atomic absorption spectrophotometry were used. In contrast to most similar publications previously, the mean concentrations of elements weighted to water discharge (dissolved forms) and to suspended matter discharge (suspended forms) are obtained, which enables us to consider them as the closest to reality. Comparison with the mean concentrations in world rivers shows that the Corg and Fe concentrations in the water of the Northern Dvina River exceed them by 3–5 times. This is related to natural factors, primarily to specific features of the catchment basin (widely spread bogs). The presence or absence of correlation between trace elements and organoferric colloids significantly affects their concentration. The content of elements in the suspended matter of the river is similar to the mean global level, except for Corg and P. We calculate the discharges of dissolved and suspended elements for every year of research and the mean for the four-year period.

The results of four years of continuous investigations (from May 2015 through April 2019) of the elemental composition of the water of the Northern Dvina River are presented; the water sampling was performed monthly year around at two sites in the lower reaches. More than 50 major and trace elements were determined in filtered water (0.45-µm membrane filter) and in suspended matter on filters, as well as Corg on GF/F glass fiber filters. The ICP MS and AAS methods were applied. The most important outcome is that the average weighted concentrations of elements in the water discharge (dissolved forms) and in suspended matter discharge (suspended form) were obtained in contrast to most similar previous publications; the average weighted concentrations allowed us to consider these results to be the most close to real. The Corg and Fe concentrations in waters of the Northern Dvina River are three- to fivefold higher compared to the average concentrations of these elements in the rivers of the world. This has a natural explanation, which includes the features of the watershed basin (widespread distribution of wetlands). The presence or absence of a correlation between elements and organoferric colloids significantly determines their concentration. The average content of elements in riverine suspended matter is very similar to the global levels, except for organic carbon and P. The annual element discharge for every year of study and the four-year average for two forms of matter (dissolved and suspended) were calculated.

The paper presents new data on concentrations and fluxes of organic carbon, iron, and manganese in waters of the mouth area of the Northern Dvina River. The work was conducted within the project "Northern Dvina River Marginal Filter Observatory" from May 2015 to May 2019 with monthly collection of water samples. Systematic determinations of the concentrations of the main carrier elements of many trace elements in dissolved and particulate forms were used to establish the average weighted of C-org, Fe, and Mn concentrations. The results of 4 years of systematic investigations established the main regularities of seasonal variations of dissolved and particulate forms of element concentrations and the relationships between. Significant variations were observed not only between concentrations but also between fluxes of elements in both forms. However, more than half the annual fluxes of all elements in all years studied was transported to the sea in the spring flood period. The results also demonstrated significant variations between element concentrations and their fluxes in different years. This is unsurprising, since even during the 4 years of investigations, the difference between water runoff and SPM fluxes in different years was up to, respectively, 1.4 and 2.6 times.

Изучение химического состава нерастворимых частиц в снежном покрове дает возможность оценить выпадения из атмосферы в зимнее время. В работе рассмотрены особенности элементного состава нерастворимых частиц (рассеянного осадочного вещества) в снеге и озерном льду фоновых и импактных районов Архангельской области. Для проб, отобранных вблизи автодороги и в городе, характерны более высокие концентрации нерастворимых частиц. Рассеянное осадочное вещества в снеге и льду обогащено тяжелыми металлами (Ni, Cr, Pb, Cd). Наибольшее обогащение получено для среднего прослоя льда. В работе приведены оценки потоков тяжелых металлов на поверхность. Наибольшие содержания и потоки свинца получены для фоновой станции оз. Пикалево.

The results of the first three years of work in the frames of the project «Observatory –the Marginal Filter of Severnaya Dvina River» are presented. The main aim of this project is the regular investigations of sedimentological, geochemical and biological processes in the Severnaya Dvina River lower flow. The samples of surface water were taken every month during three years in two points in the Arkhangelsk city (Yacht-club) and in the port Economya. On the basis of collected by authors of the materials concentration of a suspended matter (SPM), the particulate organic carbon (POC) and the dissolved organic carbon (DOC) during different seasons of year. It is shown that the maximal concentration of a suspended matter on the period of a spring flood and reach 46 mg/l, and minimum are characteristic of the period of a winter low water – to 1,5 mg/l that corresponds to the maximal and minimum river drain. Average concentration of a suspended matter for the three-year period made 7,8 mg/l. Distribution of concentration of POC expressed in mg/l within a year repeats distribution of a suspended matter, and in three years averages 0,82 mg/l. The maintenance of POC expressed in % in three years averaged 8,5% the Seasonal maximum is noted during the spring and summer period when it reached 24,7%. The maintenance of DOC varies in the range of 5,2 – 29,2 mg/l, averaging 15,3 mg/l in three years that by 3 times exceeds mean global value – 5,75 mg/l.

The paper considers tidal effects on the formation of the hydrological and hydrochemical characteristics of the mouth of the Northern Dvina River in the presence of ice cover were considered. The results of natural surveys at the river delta and near-mouth area in March 2014 were analyzed. No surveys as such were performed in winter formerly. It was found that short-term variations of flow rates during the winter low-water time might be traced at 85–95 km from the seaward border of the delta. The tidal effects upon the short-term variability of hydrochemical parameters appeared at 70 km from the delta seaward border. The tidal variations of all the considered parameters were most intense in the mixing zone of riverine and marine waters at the delta.