31 Mar 2023
 | 31 Mar 2023
Status: this preprint is open for discussion.

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

Karin E. Limburg, Yvette Heimbrand, and Karol Kuliński

Abstract. Ocean acidification is spreading globally as a result of anthropogenic CO2 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 Sea cod (EBC) between the late 1990s and 2021. B : Ca is positively proportional to pH in carbonates, as B in the form of borate is taken up in the CaCO3 matrix. Examining a time series of EBC otoliths, we found varying levels of B : Ca since the 1980s, with the most recent years at an all-time low during 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 healthy 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 anti-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 suggest a general increase in both hypoxia and acidification within the Baltic intermediate and deep waters in the last decade reflected in cod otoliths.

Karin E. Limburg et al.

Status: open (until 07 Jul 2023)

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Karin E. Limburg et al.

Karin E. Limburg et al.


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Short summary
Otoliths (ear-stones) have provided chemical markers for lifetime patterns of such phenomena as migration (via changes in strontium) and hypoxia (changes in manganese). In this study, we found a 3-to-5 fold decline in otolith boron : calcium in eastern Baltic Sea cod and explore its relation to declines in pH. The relationships are complicated with physiology and the complexity of Baltic Sea biogeochemistry, but analyses suggest intensifying hypoxia and acidification may affect otolith chemistry.