Catchment areas on volcanic territories in different regions are of great interest since they are enriched with nutrients that contribute significantly to coastal ecosystems. The Kamchatka Peninsula is one of the most active volcanic regions of the world；however, to date, the chemistry of its river waters and the state of its coastal ecosystems remain understudied in connection with volcanism. In this study, we report high concentrations of DIP and P org in the Kamchatka River comparable to many rivers in urbanized areas with sewerage and agricultural sources of nutrients. A distinct increase in DIP, P org , and DSi is systematically manifested in all seasons, especially in spring and summer, in the area directly influenced by the Kliuchevskaya group of volcanoes and Shiveluch Volcano. This feature is directly related to snow melting in the river valley and on the slopes of volcanoes that were covered with ash—a source of nutrients. We believe that DIP, P org , DSi, DIN, and N org fluxes in river runoff from volcanic catchment areas in east Kamchatka are a major trigger for spring and summer phytoplankton blooms and subsequent high zooplankton biomass, using Kamchatka Gulf as an example. This study demonstrates the connection between nutrient fluxes from a catchment area and the formation of seasonal phytoplankton blooms and high zooplankton biomass in the coastal area. We also study seasonal, year-to-year, and climatic variability of water discharges and hydrometeorological conditions to understand how nutrient fluxes can change in the foreseeable future and influence coastal ecosystems.