Volcanic catchments and coastal areas near active volcanoes have specific ecosystems in many regions of the world primarily because of availability of nutrients (N, P, Si) and trace elements supplied from volcanism. Isotopic (δD–H₂O, δ¹⁸O–H₂O, δ¹⁵N–NO₃^-, δ¹⁸O–NO₃^-) and hydrochemical data from eight different rivers and results of three cruises on both sides of the Kamchatka Peninsula held from 2022 to 2026 together with satellite data of chlorophyll “a” concentration are discussed in this work. Unlike other volcanic regions of the world, the Eastern Kamchatka catchments have a lengthy spring-summer flooding as a result of snow melting. Seasonal effect of dilution in coastal Pacific waters is identified at the depths of up to 500 m and at the distances of more than 150 km from the shore line. In the period of increasing discharge of volcanic rivers of Eastern Kamchatka, concentrations of DIN and DIP nutrients systematically increase because accumulation in the snow is observed during volcanism. As a result, with spring-summer stratification of coastal Pacific waters, phytoplankton blooms occur from May to July and this is having good correlation with the period of high-water discharge of the rivers. Next bloom stage, which often involves the harmful dinoflagellates and sometimes large-scale “red tides”, occurs in September/October. Our hypothesis for this bloom stage is significant increase of DIN/DIP ratio in years with hot summer. This occurs due to increased concentrations of DIN in the snow near the top of volcanos, which accumulates during the previous winter due to volcanic exhalations. It is explained that snow melting process at the highlands supply additional DIN to the coastal part of the Pacific Ocean. Thus, the most intense phytoplankton bloom in the adjacent part of the Pacific Ocean should be expected in the years with both a large amount of snow in the previous winter and sufficiently high air temperatures in summer for intense snow melting in the highlands and the formation of stratification of the coastal Pacific waters.

This work was supported by Interdepartmental Integrated Program for Scientific Research of the Kamchatka Peninsula and Adjacent waters in 2024-2026 №124072200009-5