The paper aims to assess the organic pollution level of the coastal waters of the southern part of the Kamchatka Peninsula based on microbiological and physical and chemical parameters of bottom sediments. During cruise 25/2 aboard PV Professor Multanovsky (July–August 2025), samples of surface-layer bottom sediments were collected at 16 stations in the north-western Pacific Ocean and the Sea of Okhotsk. The grain size distribution, organic matter content (determined by the loss-on-ignition method), physical and chemical parameters of the bottom water (temperature, salinity, pH, redox potential), as well as the abundance of heterotrophic and hydrocarbon-oxidising bacteria (determined by the limiting dilution method) were determined. The study revealed that the organic matter content in silty soils (5.0–14.7%) was significantly higher than in sandy-gravelly soils (less than 2.3%); an inverse relationship was identified between this parameter and the redox potential (ρ = −0.72; p < 0.01). The abundance of heterotrophic bacteria ranged from 10² to 10⁷ cells·g⁻¹, whilst that of hydrocarbon-oxidising bacteria ranged from 10 to 10⁴ cells·g⁻¹. The highest abundance values for both groups were recorded in Avacha Bay (10⁷ and 10³ cl·g⁻¹, respectively) and in the recreational bays (Vilyuchenskaya, Listvennichnaya), indicating a high anthropogenic load. The minimum values (10² and 10 cells·g⁻¹) were recorded in the background area at the mouth of the Nalycheva River. A positive correlation was found between the abundance of hydrocarbon-oxidising bacteria and organic matter content (ρ = 0.6, p < 0.05), as well as a negative relationship was found between the abundance of both groups and the redox potential (for heterotrophic bacteria ρ = −0.7, p < 0.01; for hydrocarbon-oxidising bacteria ρ = −0.8, p < 0.001). The soil type has a statistically significant effect on the abundance of hydrocarbon-oxidising bacteria (p < 0.05). The results confirm the effectiveness of using heterotrophic bacteria and hydrocarbon-oxidizing bacteria abundance indicators as bioindicators of organic pollution and show the self-purification potential of the studied waters.
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