The hydrological and hydrochemical data of surface and bottom waters of Academy Bay were obtained on two POI FEB RAS cruises carried out from July 11 to 14, 2016 and from September 15 to 19, 2020. Observations in 2016 were carried out during the summer flood and after thawing sea ice in the bay which were confirmed by the hydrological characteristics of the waters. Supersaturation of bottom and surface waters with atmospheric oxygen and undersaturation with atmospheric carbon dioxide indicate that production processes dominate in the bay. However, low contents of nutrients and chlorophyll a in the bay and in the estuaries of the Syran and Ulban rivers characterize the bay as rather an oligotrophic basin. This is supported by the common procedure of eutrophication assessment. In September 2020, the majority of bottom waters were undersaturated with oxygen and supersaturated with carbon dioxide, meaning organic matter degradation processes dominated. In the fall 2020, the apex of Ulban Bay was characterized by very high concentrations of dissolved inorganic phosphorus, reaching 10–14 μmol/L, and by chlorophyl a concentrations of surface waters higher than 10 mg/m 3 . Values of obtained parameters suggest that waters of Academy Bay have high eutrophic status. The difference between observations implemented in 2016 and 2020 cannot be explained by the interannual variability of the hydrochemical parameters of the waters of Academy Bay, rather it can be explained by an additional episodic source of nutrients that occurs in the late summer–autumn season. Salmon that died after spawning can serve as such an source. These dead salmon provide eutrophication of Academy Bay waters. High eutrophic waters probably provide the feeding base for polar whales.

The activity of dissolved radium isotopes 223Ra, 224Ra, 228Ra, the concentrations of nutrients: ammonium, nitrates, nitrites, phosphates, silicates, total nitrogen, and total phosphorus are studied in the Razdolnaya River estuary at suspension concentration in river water >1000 mg/L. During flood, classical two-layer estuarine circulation was observed over the nearshore; this circulation was formed by a discharge current and compensation flow from the depth of 20 m to the depth of 8 m. The main source of 228Ra inflow into the estuary is desorption from river suspension in the beginning of the mixing zone near the mouth bar, where its activity reached 163 ± 0.03 dpm 100 L–1, which is 139 times that in the river water. The zone near the river mouth bar shows higher activity of 224Ra and 223Ra (4 and 17 times greater than that in river water) and an increase in the concentrations of total phosphorus, $${\text{NH}}_{{\text{4}}}^{ + }$$ , DSi, $${\text{NO}}_{{\text{3}}}^{ - }$$ , and total nitrogen. An extremum in the activity of 224Ra (65.41 ± 0.68 dpm 100 L–1) and an increase in 223Ra (1.97 ± 0.11 dpm 100 L–1) were recorded in the bottom water of the nearshore; therefore, the main source of these isotopes is bottom sediments. Direct correlation was found to exist in the pairs $${\text{PO}}_{4}^{{3 - }}$$ – 224Ra and $${\text{NH}}_{{\text{4}}}^{ + }$$ – 224Ra, and simultaneous extremum was recorded in 224Ra, $${\text{PO}}_{4}^{{3 - }}$$ , and $${\text{NH}}_{{\text{4}}}^{ + }$$ in bottom nearshore waters; this extremum is attained in the area with highest density of colonies of polychaetes polychaetes―active bioirrigtors of pore waters.

In February 2020, geochemical tracers along with hydrochemical and hydrological characteristics were used to reveal a discharge site of salt groundwaters in the head of the Razdol’naya R. estuary at a distance of 22 km from its mouth bar. The elevated activity of 224Ra isotopes from 11.1 ± 0.1 to 2.2 ± 0.05 Bq/m3 in the bottom water layer over a distance of 15 km indicates to the propagation of slightly saline waters from a pool 8 m in depth toward the receiving basin. The distribution of stable water isotopes (δ18O and δD) and ions of basic salt composition showed that water of the top aquifer at the site of discharge area can suffer the effect of seawater. The major cause of the observed phenomenon is supposed to be the penetration of seawater into the top aquifer within the estuary during winter low-water season and the seepage of this water in the region with largest depth. The contrast of the temperature of ground and surface water was accompanied by a positive temperature anomaly in the zone of their interaction and a decrease in ice thickness along the mixing zone.