Marked recent declines in boron in Baltic Sea cod otoliths – a bellwether of incipient acidification in a vast hypoxic system?

• Published in: Biogeosciences
• Main Authors: Limburg, Karin E., Heimbrand, Yvette, Kuliński, Karol
• Format: Text
• Language: English
• Published: 2023
• Subjects: Ocean acidification
• Online Access: https://doi.org/10.5194/bg-20-4751-2023; https://bg.copernicus.org/articles/20/4751/2023/

Ocean acidification is spreading globally as a result of anthropogenic CO 2 emissions, but the Baltic Sea has until recently been thought to be relatively well-buffered by terrigenous inputs of alkalinity from its watershed. We discovered a 3- to 5-fold decline in boron (as B : Ca) in otoliths of eastern Baltic cod (EBC) between the late 1990s and 2021. Examining a time series of EBC otoliths, we found varying levels of B : Ca starting in the 1980s, with the most recent years showing an all-time low for this period. This trend correlates with declines in pH and dissolved oxygen but not with changes in salinity. We examined possible physiological influences on B : Ca by including a collection of Icelandic cod as an “out-group”. Icelandic cod otoliths showed strongly positive correlations of B : Ca with physiologically regulated P : Ca; this was not the case for EBC. Finally, B : Ca in EBC otoliths is negatively correlated, to some extent, with Mn : Mg, a proposed proxy for hypoxia exposure. This negative relationship is hypothesized to reflect the dual phenomena of hypoxia and acidification as a result of decomposition of large algal blooms. Taken together, the otolith biomarkers Mn : Mg and B : Ca in cod suggest a general increase in both hypoxia and acidification within the Baltic intermediate and deep waters in the last decade.