A challenging current task is the monitoring of cadmium (Cd) concentrations in natural environments, assessment of its effects on the biotas, and selection of new bioindicators for environmental monitoring. The aim of this work was to estimate the content of Cd in water, bottom sediments, and aquatic animals (mollusks, polychaetes, and crustaceans) in the eastern part of the Gulf of Finland, Baltic Sea, and to experimentally study the effect of different Cd concentrations in water (from 0.1 to 5 mg/L) on the physiological state of the Baltic mollusks Limecola balthica and the level of accumulation of this metal in the digestive gland of the mollusk. Data acquired in 2019−2020 at 31 stations in the Gulf of Finland showed that the Cd content in the water varied from 0.003 to 0.058 μg/L, and in bottom sediments contained from 0.1 to 3.4 mg/kg Cd. The content of Cd in the tissues of aquatic animals in the deep-water part of the Gulf of Finland was below the detection threshold, but this content was as high as 1 to 50 μg/kg of the wet weight of animals in the coastal areas. Experimental evidence suggests that Cd affects the behavior and rate of aerobic metabolism in mollusks. The rate of oxygen consumption by mollusks after ten days of exposure to 0.1 mg/L Cd and 48 h of exposure to 0.5 mg/L Cd significantly decreased compared to the control. At 5 mg/L Cd, the following toxic effects were observed: pathological changes in the state of mollusks, such as decreased respiration intensity, abnormal behavior, and a high mortality rate (>50%). In the course of the experiments, the accumulation of Cd in the digestive gland of the mollusks increased from 12 to 99 μg/kg with increasing exposure time and Cd concentration of in the water. At most stations, Cd concentrations in bottom sediments did not exceed the threshold value of a good ecological status. Thus, at present, the problem of Cd pollution is not critical to the eastern part of the Gulf of Finland; however, the data obtained indicate that the content of Cd > 0.1 mg/L in water can lead to the suppression of the populations of aquatic animals.
