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ABSTRACT ObjectiveThe purpose of this study was to understand the possible provenance of a unique population of Atlantic Cod Gadus morhua that is found in the low-salinity Åland Sea region of the northern Baltic Sea. This population consists of large, healthy individuals, in contrast to the Atlantic Cod in the rest of the Baltic Sea. MethodsWe used laser ablation inductively coupled plasma mass spectrometry to measure levels of boron (as B:Ca) in the otoliths of Atlantic Cod in regions throughout the Baltic Sea. We examined both lifetime chronologies and concentrations in the core region that corresponds to birth and early life. ResultsWe found that B:Ca concentrations were 31 to 348 times higher in the otoliths of cod that occupy the Åland Sea, including in the core region. These concentrations were much higher than expected given that boron is linearly, positively proportional to salinity, which is higher in the southern Baltic Sea, and other populations displayed very low concentrations by comparison. ConclusionsBased on the otolith B:Ca as a unique marker, we suggest that the cod that are sampled in the Åland Sea may be a separate population from those that inhabit the rest of the Baltic Sea. This would not prevent it from mixing with other populations but could point to a separate spawning area. The source of the elevated boron is currently unknown, but the widespread occurrence in cod otoliths from the Åland Sea indicates an extensive nonpoint source. 

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Typical analyses of otolith microchemistry use calcium, a major constituent, as an internal standard, setting its value as a constant and ignoring any potential variations. In fact, patterns do occur in otolith Ca deposition, as can be observed either by repeating the analysis, by creating two-dimensional maps of Ca, or both. Here we present evidence of Ca variations in fish otoliths from analyses using synchrotron-based scanning X-ray fluorescence microscopy, electron microprobe analysis, and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). 2-D maps of otoliths created with LA-ICP-MS indicate that Ca is elevated where especially Zn and P are low, and vice versa, suggesting that spatial variations in protein deposition may affect concentrations of Ca. We encourage others to examine Ca concentrations in their biomineralized samples to check for variations, using LA-ICP-MS and other methods. 

Abstract Anthropogenic deoxygenation of the Baltic Sea caused major declines in demersal and benthic habitat quality with consequent impacts on biodiversity and ecosystem services. Using Baltic cod otolith chemical proxies of hypoxia, salinity, and fish metabolic status and growth, we tracked changes from baseline conditions in the late Neolithic (4500 BP) and early twentieth century to the present, in order to understand how recent, accelerating climate change has affected this key species. Otolith hypoxia proxies (Mn:Mg) increased with expanding anoxic water volumes, but decreased with increasing salinity indexed by otolith Sr:Ca. Metabolic status proxied by otolith Mg:Ca and reconstructed growth were positively related to dissolved oxygen percent saturation, with particularly severe declines since 2010. This long-term record of otolith indicators provides further evidence of a profound state change in oxygen for the worse, in one of the world’s largest inland seas. Spreading hypoxia due to climate warming will likely impair fish populations globally and evidence can be tracked with otolith chemical biomarkers. 

ABSTRACT Fjords provide valuable research opportunities for marine scientists. They are excellent natural infrastructure for climate impact studies associated with hypoxic episodes and consequences for mesopelagic and deep‐sea ecosystems involving oceanographic circulation processes and basin water renewals. Repeated sampling from the same populations is possible, making fjords excellent systems for developing time series of data for climate impact studies. We provide an overview of the 14 years of data from Norwegian West Coast fjords, focusing on Masfjorden, and report major findings from Oslofjorden in Eastern Norway, exhibiting recurrent hypoxia in the basin waters. We document that the oxygen levels in Masfjorden decreased rapidly by over 60% at 450 m depth in < 8 years, which is much faster than the average rate of deoxygenation in the global ocean. We also discuss the increase in the deep‐sea and low‐light‐adapted coronate jellyfishPeriphylla periphyllain view of altered optical conditions of the basin water potentially related to deoxygenation. We argue that fjords like Masfjorden and Oslofjorden are not only macrocosms for ecological processes but also are likely an accelerated version of deep oceans with respect to climate impacts. 

Abstract. Anthropogenic warming and nutrient over-enrichment of our oceans have resulted in significant, and often catastrophic, reductions in dissolved oxygen (deoxygenation). Stress on water-breathing animals from this deoxygenation has been shown to occur at all levels of biological organization: cellular, organ, individual, species, population, community, and ecosystem. Most climate forecasts predict increases in ocean deoxygenation; thus, it is essential to develop reliable biological indicators of low-oxygen stress that can be used by regional and global oxygen monitoring efforts to detect and assess the impacts of deoxygenation on ocean life. This review focuses on responses to low-oxygen stress that are manifest at different levels of biological organization and at a variety of spatial and temporal scales. We compare particular attributes of these biological indicators to the dissolved oxygen threshold of response, timescales of response, sensitive life stages and taxa, and the ability to scale the response to oxygen stress across levels of organization. Where there is available evidence, we discuss the interactions of other biological and abiotic stressors on the biological indicators of low-oxygen stress. We address the utility, confounding effects, and implementation of the biological indicators of oxygen stress for research and societal applications. Our hope is that further refinement and dissemination of these oxygen stress indicators will provide more direct support for environmental managers, fisheries and mariculture scientists, conservation professionals, and policymakers to confront the challenges of ocean deoxygenation. An improved understanding of the sensitivity of different ocean species, communities, and ecosystems to low-oxygen stress will empower efforts to design monitoring programs, assess ecosystem health, develop management guidelines, track conditions, and detect low-oxygen events. 

Abstract: The intensified expansion of the Baltic Sea’s hypoxic zone has been proposed as one reason for the current poor status of cod ( Gadus morhua) in the Baltic Sea, with repercussions throughout the food web and on ecosystem services. We examined the links between increased hypoxic areas and the decline in maximum length of Baltic cod, a demographic proxy for services generation. We analysed the effect of different predictors on maximum length of Baltic cod during 1978–2014 using a generalized additive model. The extent of minimally suitable areas for cod (oxygen concentration ≥ 1 ml l−1) is the most important predictor of decreased cod maximum length. We also show, with simulations, the potential for Baltic cod to increase its maximum length if hypoxic areal extent is reduced to levels comparable to the beginning of the 1990s. We discuss our findings in relation to ecosystem services affected by the decrease of cod maximum length.