Field observations on different rivers and three cruises on both sides of Kamchatka Peninsula were made from 2022 to 2025, together with satellite data on chlorophyll a concentration, are analyzed to show the hydro-ecological features of different catchments and receiving basins. Unlike other volcanic regions of the world, the catchments of Eastern Kamchatka are characterized by a long spring-summer flood as a result of snowmelt, therefore, the seasonal effect of dilution of Pacific coastal waters is noticeable at depths up to 500 m and at distances of more than 150 km from the coastline. Together with the increasing runoff of the volcanic rivers of Eastern Kamchatka, concentrations of nutrient are systematically increasing due to accumulation of nutrients in the snow and groundwater discharge. As a result, phytoplankton blooms from May to July with the spring-summer stratification of the Pacific coastal waters occurs. The second stage of blooming of phytoplankton, during which the development of harmful dinoflagellates is often, and sometimes large-scale “red tides” occur in September-October. This occurs because snowmelt in the highlands of the catchment areas can continue to the September supplying additional nutrients to the coastal Pacific Ocean. Thus, the most intense phytoplankton blooms should be expected in the adjacent part of the Pacific Ocean in years with both a large amount of snow in the previous winter and sufficiently high air temperatures in summer for intensive snowmelt in the highlands and the establishment of stratification of Coastal Pacific waters.

Volcanic Catchments, Stable Isotopes, Nutrient Fluxes, Phytoplankton Bloom, Coastal Ecosystem