Water Dynamics and Nutrients Response of Penzhina Bay and Shelikhov Gulf (Sea of Okhotsk) to Strong Tides and River Runoff

Water dynamics and nutrients are widely recognized as the main triggers of phytoplankton blooms. These factors may control the stability of marine ecosystems. Penzhina Bay and the Shelikhov Gulf are famous for their high tidal dynamics in comparison with the basins of the World Ocean and for being the feeding places of Bowhead whales. Here, we study the dynamics and thermohaline structure of water; nutrients; isotopic signatures of δ15N–NO3− and δ18O–NO3−; as well as chlorophyll a in Penzhina Bay, the Shelikhov Gulf, and the Penzhina River to understand the features of an ecosystem with intense tidal dynamics in the subpolar region. This work is based on data obtained in three cruises of the R/V “Akademik Oparin” in the period from 2023 to 2025, with speed boat observations in the Penzhina River from May to October, including the flooding peak in June. The observations covered cases with tides from 7 to 13.4 m in height. The interaction between tides and river runoff was observed to supply dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) from the sea and dissolved silicate (DSi) from the river. The “white nights” in July, combined with the increased supply of nutrients, are good conditions for phytoplankton blooms, and as a result, the concentration of chlorophyll a in the study area was observed to be up to 39 µg/L. High primary production supports the food chain, and this is probably the main reason why Bowhead whales come to feed in the summer. The DIN/DIP ratio indicates DIN as a limiting factor in most of Penzhina Bay and throughout the Shelikhov Gulf. At the same time, the DSi/DIP ratio at a significant distance from the mouth of the Penzhina River is close to 0, indicating unfavorable conditions for diatoms. The DSi limit can cause the blooming of dinoflagellates, which sometimes occurs in this region.