Abstract
The Baltic Sea contains the world’s largest anthropogenically caused deoxygenated zone, with increasing episodes and areal extent of hypoxia/anoxia. The iconic Atlantic cod in the Baltic has suffered a loss in condition which has been attributed mainly to hypoxia, with habitat “squeeze” contributing to density dependent (crowding) effects as well. Otoliths, the aragonitic structures that form part of the hearing/balance system in fishes, accumulate Mn in the presence of hypoxia and other reducing environments. Otoliths grow over the lifetime of fishes, and thus lifetime records of hypoxia exposure exist for each individual fish. However, otolith Mn/Ca ratios are also sensitive to growth effects. We tested a new proxy that at least partly accounts for growth: Mn/Mg, since Mg reflects metabolic activity but not hypoxia. This and other elemental proxies were parsed annually from the otoliths to reconstruct lifetime histories of mean, maximum, and cumulative values of this proxy as well as others (Sr/Ca) that inform us about salinity conditions. We analyzed cod from five different time periods: Neolithic (4500 YBP, a normoxic baseline), 1980s, 1990s, 2000s, and 2010s – under different hypoxia intensities, assessing fish growth and condition in relation to hypoxia experience recorded by otolith proxies. Fish grew more poorly with increasing hypoxia exposure; condition at capture (measured by Fulton’s K index) showed a strongly positive relation to growth indexed by magnesium (Mg/Ca). We conclude that cod otolith chemistry proxies not only inform about the hypoxia, growth, and metabolic status of cod, retrospectively throughout life, but also reflect the worsening situation for cod in the Baltic.
Highlights
Hypoxia, or the occurrence of low oxygen levels in water, is spreading rapidly worldwide (Breitburg et al, 2018)
We develop a heuristic model as a dynamic hypothesis, that manganese uptake is a function of both exogenous and endogenous controls
Assuming that these fish remained in the Baltic and did not migrate out into the North Sea, our results suggest that fish that migrated into deeper, saltier Baltic water had a higher probability of encountering hypoxia, while those that remained in less saline habitats were less exposed
Summary
The occurrence of low oxygen levels in water, is spreading rapidly worldwide (Breitburg et al, 2018). The Baltic Sea ranks among the largest marine areas to have increased in hypoxic intensity and extent, from 5,000 to >60,000 km in its major basins over the past century (Carstensen et al, 2014). Freshwater inflows from the drainage basin contain nutrients that stimulate production (Hong et al, 2017), eventually resulting in microbial respiration that depletes oxygen (Conley et al, 2009). Major inflows of oxygenated seawater through the Danish Straits provide temporary relief, but reoxygenated episodes have in the past two decades been rare or of shorter duration than anticipated, due to complex mixing and stagnation (Conley et al, 2009; Schmale et al, 2016). As warming continues, Baltic hypoxia is likely to continue and intensify, with increased areas of hypoxic, anoxic, and sulfidic waters
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