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ORIGINAL RESEARCH article

Front. Earth Sci.

Sec. Interdisciplinary Climate Studies

Volume 13 - 2025 | doi: 10.3389/feart.2025.1638978

This article is part of the Research TopicNew Challenges for Baltic Sea Earth System ResearchView all 11 articles

Recent stagnation period and unprecedented deoxygenation in the Baltic Sea: causes and consequences

Provisionally accepted
Taavi  LiblikTaavi Liblik1*Enriko  SihtEnriko Siht1Fred  BuschmannFred Buschmann1Marlene  KaljumäeMarlene Kaljumäe1Villu  KikasVillu Kikas1Urmas  LipsUrmas Lips1Stella-Theresa  LuikStella-Theresa Luik1Diana  MaslovaDiana Maslova1Kristian  PärtKristian Pärt1Kai  SalmKai Salm1Oliver  SamlasOliver Samlas1Simo-Matti  SiiriäSimo-Matti Siiriä2Sirje  SildeverSirje Sildever1Māris  SkudraMāris Skudra3Kimmo  Ari TikkaKimmo Ari Tikka2Laura  TuomiLaura Tuomi2
  • 1Tallinn University of Technology, Tallinn, Estonia
  • 2Ilmatieteen Laitos, Helsinki, Finland
  • 3Latvian Institute of Aquatic Ecology, Riga, Latvia

The final, formatted version of the article will be published soon.

The properties of the Baltic Sea deep waters are strongly influenced by the alternation between Major Baltic Inflows (MBIs) and the stagnation periods separating them. In the present observation-based study, we report developments in the Central Baltic Sea during the last stagnation period from 2016 to 2024 in the context of changes since the 1960s. There has been a trend towards enhanced oxygen deficiency at the end of consecutive stagnation periods since the late 1960s, with a decline rate of 1.5 µmol l-1 y-1 in the volume-averaged oxygen concentration below 100 m depth in the Eastern Gotland Basin (EGB). The magnitude of deoxygenation in the latest stagnation period remarkably deviates from this trend toward more severe deficiency. The oxygen deficit below 100 m in the EGB was 2.5 x 10^6 t in 2024, which is approximately double compared to the end of the last stagnation in 2013. The current high oxygen deficit is accompanied by high ammonium and phosphate concentrations and unusually warm water in the deep layer. We suggest that the reasons behind the current extensive oxygen deficiency are, besides long-term eutrophication, a dense bottom layer, strong and extended stratification hindering vertical oxygen import, weak horizontal oxygen transport, and higher water temperature affecting oxygen solubility and organic matter mineralization rates. The mentioned factors are likely connected to the overall warming of the Baltic and North Sea and can be attributed to climate change. Our results suggest that, under the current anthropogenic pressure, existing internal phosphorus load and with warmer water due to climate change, it is highly unlikely that deoxygenation will be alleviated in the Baltic Sea over the coming decade.

Keywords: Baltic Sea, Deoxygenation, major Baltic inflow (MBI), Anoxia, hypoxia, climate warming, Eutrophication, stratification

Received: 31 May 2025; Accepted: 29 Sep 2025.

Copyright: © 2025 Liblik, Siht, Buschmann, Kaljumäe, Kikas, Lips, Luik, Maslova, Pärt, Salm, Samlas, Siiriä, Sildever, Skudra, Tikka and Tuomi. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

* Correspondence: Taavi Liblik, taavi.liblik@taltech.ee

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