Cod otoliths document accelerating climate impacts in the Baltic Sea.

Area of Science:

• Marine Biology
• Paleoceanography
• Climate Science

Background:

• The Baltic Sea suffers from anthropogenic deoxygenation, impacting marine ecosystems.
• Declining habitat quality affects biodiversity and ecosystem services.
• Climate change exacerbates deoxygenation, posing a threat to marine life.

Purpose of the Study:

• To investigate the long-term effects of climate change and deoxygenation on Baltic cod.
• To reconstruct historical changes in hypoxia, salinity, and fish health using otolith chemistry.
• To understand the impact of these changes on a key species in the Baltic Sea.

Main Methods:

• Utilized chemical proxies (Mn:Mg, Sr:Ca, Mg:Ca) from Baltic cod otoliths.
• Reconstructed historical hypoxia, salinity, metabolic status, and growth from the late Neolithic to the present.
• Compared otolith data with historical environmental conditions.

Main Results:

• Otolith hypoxia proxies (Mn:Mg) increased with expanding anoxic water volumes.
• Otolith salinity proxies (Sr:Ca) showed an inverse relationship with hypoxia.
• Fish metabolic status (Mg:Ca) and growth declined significantly since 2010, correlating with dissolved oxygen levels.

Conclusions:

• Baltic cod otoliths provide a long-term record of environmental change and fish health.
• Accelerating climate change has led to a severe state change in Baltic Sea oxygen levels.
• Otolith chemical biomarkers are effective tools for tracking hypoxia impacts on fish populations globally.