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Anthropogenic nutrient input has caused a rapid expansion of bottom water hypoxia in the Baltic Sea over the past century. Two earlier intervals of widespread hypoxia, coinciding with the Holocene Thermal Maximum (HTM HI ; 8–4 ka before present; BP) and the Medieval Climate Anomaly (MCA HI ; ~1200–750 years BP), have been identified from Baltic Sea sediments. Here we present sediment records from two sites in the Baltic Sea, and compare the trace metal (As, Ba, Cd, Cu, Mo, Ni, Pb, Re, Sb, Tl, U, V, Zn) enrichments during all three hypoxic intervals. Distinct differences are observed between the intervals and the various elements, highlighting the much stronger perturbation of trace metal cycles during the modern hypoxic interval. Both Mo and U show a strong correlation with C org and very high absolute concentrations, indicative of frequently euxinic bottom waters during hypoxic intervals. During the modern hypoxic interval (Modern HI ) comparatively less Mo is sequestered relative to C org than in earlier intervals. This suggests partial drawdown of the water column Mo inventory in the modern water column due to persistent euxinia and only partial replenishment of Mo through North Sea inflows. Molybdenum contents in modern sediments are likely also affected by the recent slowdown in input of Mo in association with deposition of Fe and Mn oxides. Strong enrichments of U in recent sediments confirm that the Modern HI is more intense than past intervals. These results suggest that U is a more reliable indicator for the intensity of bottom water deoxygenation in the Baltic Sea than Mo. Sedimentary Re enrichment commences under mildly reducing conditions, but this element is not further enriched under more reducing conditions. Enrichments of V are relatively minor for the MCA HI and Modern HI, possibly due to strong reservoir effects on V in the water column, indicating that V is unreliable as an indicator for the intensity of bottom water hypoxia in this setting. Furthermore, Ba profiles are strongly influenced by post-depositional remobilization throughout the Holocene. The strong relationship between C org and Ni, Tl and particularly Cu suggests that these trace metals can be used to reconstruct the C org flux into the sediments. Profiles of As, Sb and Cd and especially Pb and Zn are strongly influenced by anthropogenic pollution. [ABSTRACT FROM AUTHOR]

Hydrogen sulfide (H 2 S) is linked to water quality deterioration in the Baltic Sea, with widespread seafloor hypoxia. We examined the vertical and temporal variability of in situ [H 2 S], oxygen concentration ([O 2 ]), temperature (T) and pH at weekly, hourly and minute intervals at 13 locations in the western Gulf of Finland in 2013–2014. The main target was the 60–100 m water depth range, containing 3.2–290 μM O 2 and 6.3–22.6 μM H 2 S. Where gas was detected by acoustic surveys, the structure of the H 2 S layer was more complex compared to stations devoid of gas. Local minima and maxima in pH frequently occurred near the H 2 S upper boundary (redox transition zone). Except for the homogeneous, tranquil zone above the seafloor at some stations, substantial rapid changes in hydrographic conditions were common. Typically, a layer of marked temporal T variability was present atop or within the topmost H 2 S layers. The largest temporal changes over a weekly period were − 0.44 °C/− 10.8 μM H 2 S/− 0.12 pH units (at seafloor level), + 0.18 °C/+7.9 μM H 2 S between casts (1 h) and + 0.03 °C/− 2.5 μM H 2 S per minute (high resolution logging). Abrupt [H 2 S] changes were recorded at two stations with sediments containing free gas. The T and [H 2 S] changes were synchronous at several layers, reflecting water movement. We conclude that rapid changes occur in hydrographic conditions in the near-bottom H 2 S layer in the northern Baltic Sea, especially at locations where free gas is present in the underlying sediments. [ABSTRACT FROM AUTHOR]

Internal phosphorus (P) loading from sediments, controlled by hypoxia, is often assumed to hamper the recovery of lakes and coastal areas from eutrophic conditions. We use a box-model to calculate seasonal and annual inputs, export, retention and internal cycling of P in the inner archipelago of Stockholm, Sweden (Baltic Sea) in 1968-2015. The area receives freshwater from Lake Mälaren and treated sewage from the greater Stockholm area. The sewage treatment plants (STPs) have improved their nutrient removal in steps, starting with P in 1972 and nitrogen in 1996. In the first 10-20 years after the main P load reduction in 1972-76, the model shows, in comparison to the load, a small negative annual P balance, probably due to release from legacy sediment P stores. The now stabilized, near neutral P balance indicates no continued internal loading from legacy P, but P retention is low, despite improved oxygen conditions. Seasonally, sediments are a P sink in spring and a P source in summer and autumn. Most of the deep-water P release from sediments in summer-autumn appears to be derived from the settled spring bloom and is exported during winter. Oxygen consumption and P release in the deep water are generally tightly coupled, indicating limited control by P binding to iron-oxyhydroxides under oxic conditions. However, in years of deep-water hypoxia enhanced P release suggest contribution from redox-sensitive P stores. The oxygen conditions in the area have generally improved, probably due both to lower sedimentation of organic matter from the 1970s and lower STP ammonium loads from the late 1990s. Increased oxygen inputs to the intermediate and deep waters due to weakened stratification and enhanced vertical mixing have probably also contributed, while increased respiration rates due to elevated bottom water temperatures probably explain worsened oxygen conditions during the 1990s. Since the P turnover time is short and legacy P minute, measures to bind P in Stockholm inner archipelago sediments would primarily accumulate P imported from the Baltic Sea and from Lake Mälaren inflow, and management here should focus on reducing external nutrient inputs. [ABSTRACT FROM AUTHOR]

The recent proliferation of harmful cyanobacterial blooms (cyanoHABs) in the Baltic and other marginal seas poses a severe threat for the health of infested ecosystems as e.g. the massive export and decay of cyanobacterial biomass facilitates the spread of bottom water hypoxia. There is evidence that cyanoHABs occurred repeatedly in the Baltic Sea but knowledge of their spatiotemporal distribution and the cyanobacteria that contributed to them is limited. In this study, we examined representatives of the major bloom-forming heterocystous cyanobacteria (i.e. Aphanizomenon, Dolichospermum (formerly Anabaena) and Nodularia) to establish lipid fingerprints that allow tracking these environmentally important diazotrophs in the modern and past Baltic Sea. The distribution of normal and mid-chain branched alkanes, fatty acid methyl esters, bacteriohopanepolyols and heterocyst glycolipids permitted a clear chemotaxonomic separation of the different heterocystous cyanobacteria but also indicated a close phylogenetic relationship between representatives of the genera Aphanizomenon and Dolichospermum. Compared to the discontinuous nature of phytoplankton surveys studies, the distinct lipid profiles reported here will allow obtaining detailed spatiotemporal information on the frequency and intensity of Baltic Sea cyanoHABs as well as their community composition using the time-integrated biomarker signatures recorded in surface and subsurface sediments. As heterocystous cyanobacteria of the genera Aphanizomenon, Dolichospermum and Nodularia are generally known to form massive blooms in many brackish as well as lacustrine systems worldwide, the chemotaxonomic markers introduced in this study may allow investigating cyanoHABs in a great variety of contemporary environments from polar to tropical latitudes. [ABSTRACT FROM AUTHOR]

Chemical Munitions Search & Assessment (CHEMSEA) project has performed studies on chemical weapon (CW) detection, sediment pollution and spreading as well as biological effects of chemical warfare agents (CWAs) dumped in the Baltic Sea. Results suggest that munitions containing CWAs are more scattered on the seafloor than suspected, and previously undocumented dumpsite was discovered in Gdansk Deep. Pollution of sediments with CWA degradation products was local and close to the detected objects; however the pollution range was larger than predicted with theoretical models. Bottom currents observed in the dumpsites were strong enough for sediment re-suspension, and contributed to the transport of polluted sediments. Diversity and density of the faunal communities were poor at the dumping sites in comparison to the reference area, although the direct effects of CWA on benthos organisms were difficult to determine due to hypoxic or even anoxic conditions near the bottom. Equally, the low oxygen might have affected the biological effects assessed in cod and caged blue mussels. Nonetheless, both species showed significantly elevated molecular and cellular level responses at contaminated sites compared to reference sites. [ABSTRACT FROM AUTHOR]

Although bottom water hypoxia (O < 2 mg L) is presently widespread in the Baltic Sea coastal zone, there is a lack of insight into past changes in bottom water oxygen in these areas on timescales of millennia, and the possible driving factors. Here, we present a sediment-based environmental reconstruction of surface water productivity, salinity and bottom water oxygen for the past 5400 years at Gåsfjärden, a coastal site in SE Sweden. As proxies, we use dinoflagellate cysts, benthic foraminifera, organic carbon (C), biogenic silica (BSi), C/N, C/P, Ti/Al, K/Al and grain size distribution. The chronology of the sediment sequence is well constrained, based on Pb, Cs and AMS C dates. Between 3400 and 2100 BCE, isostatic conditions favored enhanced deep water exchange between Gåsfjärden and the open Baltic Sea. At that time, Gåsfjärden was characterized by relatively high productivity and salinity, as well as frequently occurring hypoxic-anoxic bottom water, despite the relatively large connection with the Baltic Sea. The most severe interval of oxygen depletion is recorded between 2400 and 2100 BCE, and appears to coincide with a similar hypoxic event in the Gotland Basin in the open Baltic Sea. As regional climate became wetter and colder between 2100 BCE and 700 BCE, salinity declined and bottom water oxygen conditions improved. Throughout the record, grain size, Ti/Al and K/Al data indicate an evolution towards a more enclosed coastal system, as suggested by reconstructions of the post-glacial shoreline regression. Gåsfjärden shifted to close to modern conditions after 700 BCE, and was characterized by less hypoxia and lower salinity compared with 3400-700 BCE. The timing of the shift corresponds with the Sub-Boreal/Sub-Atlantic transition in Europe. Human-induced erosion in the catchment is observed as early as 600 CE, and is particularly prominent since regional copper mining activity increased around 1700 CE. A sharp increase in sediment C concentration is recorded since the 1950s, indicating significant anthropogenic impact on biogeochemical cycles in the coastal zone, as observed elsewhere in the Baltic Sea. [ABSTRACT FROM AUTHOR]

Expanding hypoxia in the Baltic Sea over the past century has led to the development of anoxic and sulfidic (euxinic) deep basins that are only periodically ventilated by inflows of oxygenated waters from the North Sea. In this study, we investigate the potential consequences of the expanding hypoxia for manganese (Mn) burial in the Baltic Sea using a combination of pore water and sediment analyses of dated sediment cores from eight locations. Diffusive fluxes of dissolved Mn from sediments to overlying waters at oxic, hypoxic and euxinic sites are consistent with an active release of Mn from these areas. Although the present-day fluxes are significant (ranging up to ca. 240 µmol m-2 d-1), comparison to published water column data suggests that the current benthic release of Mn is small when compared to the large pool of Mn already present in the hypoxic and anoxic water column. Our results highlight two modes of Mn carbonate formation in sediments of the deep basins. In the Gotland Deep area, Mn carbonates likely form from Mn oxides that are precipitated from the water column directly following North Sea inflows. In the Landsort Deep, in contrast, Mn carbonate andMn sulfide layers appear to form independently of inflow events, and are possibly related to the much larger and continuous input of Mn oxides linked to sediment focusing. Whereas Mn-enriched sediments continue to accumulate in the Landsort Deep, this does not hold for the Gotland Deep area. Here, a recent increase in euxinia, as evident from measured bottom water sulfide concentrations and elevated sediment molybdenum (Mo), coincides with a decline in sediment Mn content. Sediment analyses also reveal that recent inflows of oxygenated water (since ca. 1995) are no longer consistently recorded as Mn carbonate layers. Our data suggest that eutrophication has not only led to a recent rise in sulfate reduction rates but also to a decline in reactive Fe input to these basins. We hypothesize that these factors have jointly led to higher sulfide availability near the sediment--water interface after inflow events. As a consequence, the Mn oxides may be reductively dissolved more rapidly than in the past and Mn carbonates may no longer form. Using a simple diagenetic model for Mn dynamics in the surface sediment, we demonstrate that an enhancement of the rate of reduction of Mn oxides is consistent with such a scenario. Our results have important implications for the use of Mn carbonate enrichments as a redox proxy in marine systems. [ABSTRACT FROM AUTHOR]

Baltic Sea species live in a complex, variable environment characterized by highly fluctuating hydrology, including large seasonal and diel pH variations. For decades, oxygen deficiency caused by anthropogenic eutrophication has affected the Baltic Sea, and large areas of the seafloor are permanently hypoxic resulting in severely degraded benthic communities. Species living in this system are thus potentially tolerant and adapted to this fluctuating environment, but also vulnerable as illustrated by high mortality of benthic species as a result of hypoxia. In the future, the frequency and extent of regularly co-occurring low oxygen and low pH levels will likely increase with on-going climate change. A key species in the Baltic Sea soft-bottom communities, the bivalve Macoma balthica (L.), experiences such conditions throughout its life-cycle, and therefore serves as a good model organism for studying the combined effects of oxygen and pH conditions. To study the response of M. balthica to multiple changes occurring in the benthic environment, we conducted an experiment to investigate the survival and shell growth of newly settled juveniles simultaneously exposed to two pH levels (7.85 and 7.35) and two oxygen levels (8.5 and 3.0 mg/l) for 29 days in a fully factorial design. Survival was high in all treatments (> 60%), but significantly higher in the two low oxygen treatments (> 70%). Although positive growth was observed in all treatments, pH and oxygen as well as their interaction significantly affected relative growth. The highest growth was observed in the “low O 2 /high pH” treatment, which was 2.4 times higher than in both treatments with high oxygen. Although the mechanism for these differences remains unknown, hypoxia-induced metabolic depression likely plays a role. Our results highlight the need to know more about the occurrence and performance of benthic species regularly exposed to changing conditions, and of the range and conditions encountered in situ. With the predicted future negative changes in oxygen availability as well as pH, the adaptive responses of benthic species to multiple stressors will be critical in understanding ecosystem dynamics in the face of change. [ABSTRACT FROM AUTHOR]

Recent reports have demonstrated that hypoxia is widespread in the coastal zone of the Baltic Sea. Here we evaluate the long-term trends of dissolved oxygen in bottom waters and of the drivers of coastal hypoxia. Eleven of the 33 sites evaluated had increasing trends of bottom water dissolved oxygen, but only the Stockholm Archipelago presents a consistent positive increasing trend in time. The vast majority of sites continue to worsen, especially along the Danish and Finnish coasts, in spite of remediation efforts to reduce nutrients. Surface temperatures were relatively comparable across the entire coastal Baltic Sea, whereas bottom water temperatures varied more strongly among sites, most likely due to differences in mixing (or stratification) and water exchange with the open Baltic Sea. Nutrient concentrations varied by factors 2–3 with highest levels at sites with restricted water exchange and higher land based nutrient loading. None of the sites were permanently stratified during the summer seasonal window although most of the sites were stratified more than half of the time. The frequency of hypoxia was also quite variable with sites in Gulf of Bothnia almost never experiencing hypoxia to enclosed sites with more than 50% chance of hypoxia. There are many factors governing hypoxia and the complexity of interacting processes in the coastal zone makes it difficult to identify specific causes. Our results demonstrate that managing nutrients can create positive feedbacks for oxygen recovery to occur. In the absence of nutrient reductions, the recovery from hypoxia in coastal marine ecosystems is unlikely. [ABSTRACT FROM AUTHOR]

Decreased phosphorus (P) retention in aquatic sediments during hypoxic periods results in increased P recycling to the water column. To revert to less productive conditions in the enclosed bays of the Baltic Sea archipelago, increased sediment P burial capacity is needed. Aluminum (Al) addition is considered to be a cost-effective lake restoration method, as it improves sediment P burial capacity. However, little is known about its ability to permanently bind P in brackish systems. In summer 2000, Al sulfate granules were added to a hypoxic bottom area in the Östhammar bay, Sweden. Sediment core samples from the area were collected 10 years later. A peak in Al and P was detected at 20 cm sediment depth, reflecting the added Al and P trapped to it. Only part of the added Al was recovered, but the recovered Al (8 g Al/m 2 ) trapped P at a ratio of 5:1 (molar). Chemical fractionation showed that P extracted as “Al–P” constituted 55% of the trapped P, indicating that Al added also trapped P extracted as other P forms. [ABSTRACT FROM AUTHOR]

Deoxygenation is a global problem in coastal and open regions of the ocean, and has led to expanding areas of oxygen minimum zones and coastal hypoxia. The recent expansion of hypoxia in coastal ecosystems has been primarily attributed to global warming and enhanced nutrient input from land and atmosphere. The largest anthropogenically induced hypoxic area in the world is the Baltic Sea, where the relative importance of physical forcing versus eutrophication is still debated. We have analyzed water column oxygen and salinity profiles to reconstruct oxygen and stratification conditions over the last 115 y and compare the influence of both climate and anthropogenic forcing on hypoxia. We report a 10-fold increase of hypoxia in the Baltic Sea and show that this is primarily linked to increased inputs of nutrients from land, although increased respiration from higher temperatures during the last two decades has contributed to worsening oxygen conditions. Although shifts in climate and physical circulation are important factors modulating the extent of hypoxia, further nutrient reductions in the Baltic Sea will be necessary to reduce the ecosystems impacts of deoxygenation. [ABSTRACT FROM AUTHOR]

Nutrient over-enrichment is one of the classic triggering mechanisms for the occurrence of cyanobacteria blooms in aquatic ecosystems. In the Baltic Sea, cyanobacteria regularly occur in the late summer months and form nuisance accumulations in surface waters and their abundance has intensified significantly in the past 50 years attributed to human-induced eutrophication. However, the natural occurrence of cyanobacteria during the Holocene is debated. In this study, we present records of cyanobacteria pigments, water column redox proxies, and nitrogen isotopic signatures for the past ca. 8000 years from Baltic Sea sediment cores. Our results demonstrate that cyanobacteria abundance and nitrogen fixation are correlated with hypoxia occurring during three main intervals: (1) ca. 7000-4000 B.P. during the Littorina transgression, (2) ca. 1400-700 B.P. during the Medieval Climate Anomaly, and (3) from ca. 1950 A.D. to the present. Issues of preservation were investigated, and we show that organic matter and pigment profiles are not simply an artifact of preservation. These results suggest that cyanobacteria abundance is sustained during periods of hypoxia, most likely because of enhanced recycling of phosphorus in low oxygen conditions. [ABSTRACT FROM AUTHOR]

Hypoxia (oxygen concentrations of <2 ml/L) and so-called "dead zones" are a growing concern in coastal marine environments. The Baltic Sea is a shelf sea that is highly sensitive to hypoxia, and may serve as a laboratory for studying the interplay between natural and anthropogenic forcing of redox conditions in the global coastal zone. Past occurrences of hypoxia in the Baltic Sea have been shown by previous studies, but high-resolution, quantitative reconstructions of past hypoxia intensity are lacking. Here we present bulk sediment geochemical records from the deep basins of the Baltic Sea that show multicentennial oscillations during intervals of past hypoxia, suggesting rapid alternations between hypoxic and relatively oxic conditions. While the onset of past hypoxic events was likely forced by climatic variability, these events intensified and terminated rapidly due to feedbacks in the phosphorus (P) cycle. The modern intensity of hypoxia is similar to several past events, suggesting that hypoxia in the Baltic Sea has a maximum potential intensity. However, using ultrahigh-resolution laser ablation--inductively coupled plasma--mass spectrometry scanning of sediment blocks, we show that modern hypoxia intensified more rapidly than any past event. This confirms the role of anthropogenic nutrient loading in driving this system into its current hypoxic state. [ABSTRACT FROM AUTHOR]

Vertical diffusivity and oxygen consumption in the basin water, the water below the sill level at about 59 m depth, have been estimated by applying budget methods to monitoring data from hydrographical stations BY4 and BY5 for periods without water renewal. From the vertical diffusivity, the mean rate of work against the buoyancy forces below 65 m depth is estimated to about 0.10 mW m. This is slightly higher than published values for East Gotland Sea. The horizontally averaged vertical diffusivity κ can be approximated by the expression κ = a N where N is the buoyancy frequency and a ≈ 1.25 × 10 m s, which is similar to values for a used for depths below the halocline in Baltic proper circulation models for long-term simulations. The contemporary mean rate of oxygen consumption in the basin water is about 75 g O m year, which corresponds to an oxidation of 28 g C m year. The oxygen consumption in the Bornholm Basin doubled from the 1970s to the 2000s, which qualitatively explains the observed increasing frequency and vertical extent of anoxia and hypoxia in the basin water in records from the end of the 1950s to present time. A horizontally averaged vertical advection-diffusion model of the basin water is used to calculate the effects on stratification and oxygen concentration by a forced pump-driven vertical convection. It is shown that the residence time of the basin water may be reduced by pumping down and mixing the so-called winter water into the deepwater. With the present rate of oxygen consumption, a pumped flux of about 25 km year would be sufficient to keep the oxygen concentration in the deepwater above 2 mL O L. [ABSTRACT FROM AUTHOR]

Abstract: Hypoxic conditions (O2 <2mgL−1) are frequently observed in the relatively shallow and stratified North Sea–Baltic Sea transition zone. Inter-annual variability with more extensive hypoxia has been observed in years with calm weather conditions during late summer. A future warmer climate may increase hypoxia in the area due to combined effects from decreased oxygen solubility and increased respiration rates. Feedbacks from climate change can, therefore, amplify negative effects from eutrophication, such as hypoxia. Here we apply a high resolution three-dimensional ocean circulation model with a simple pelagic and benthic oxygen consumption model (OXYCON), based on the seasonal organic carbon budget in the area, and demonstrate that the model is able to simulate the temporal and spatial variability of the observed oxygen concentration in the bottom waters during a three-year period. The potential impact from a warmer climate was analysed in a sensitivity study with a 3°C warmer climate, and showed a significant increase of hypoxic bottom areas compared to present day conditions. The relative role of increased respiration and decreased oxygen solubility in the inflowing bottom water and at the surface was analysed and it was found that decreased solubility accounted for about 25% of the simulated decrease in bottom water oxygen concentration in the centre of the area in the early fall. A sensitivity study showed that the simulated effect from a 3°C temperature increase on the bottom water oxygen concentration could be compensated by a 30% reduction in the export production. The model simulations of the North Sea–Baltic Sea transition zone indicate a significant expansion of the hypoxic areas and a lengthening of the hypoxic period under a warmer climate. [Copyright &y& Elsevier]

In the sedimental organic matter of eutrophic continental seas, such as the largest dead zone in the world, the Baltic Sea, bacteria may directly participate in nutrient release by mineralizing organic matter or indirectly by altering the sediment’s ability to retain nutrients. Here, we present a case study of a hypoxic sea, which receives riverine nutrient loading and in which microbe-mediated vicious cycles of nutrients prevail. We showed that bacterial communities changed along the horizontal loading and vertical mineralization gradients in the Gulf of Finland of the Baltic Sea, using multivariate statistics of terminal restriction fragments and sediment chemical, spatial and other properties of the sampling sites. The change was mainly explained by concentrations of organic carbon, nitrogen and phosphorus, which showed strong positive correlation with Flavobacteria, Sphingobacteria, Alphaproteobacteria and Gammaproteobacteria. These bacteria predominated in the most organic-rich coastal surface sediments overlain by oxic bottom water, whereas sulphate-reducing bacteria, particularly the genus Desulfobacula, prevailed in the reduced organic-rich surface sediments in the open sea. They correlated positively with organic nitrogen and phosphorus, as well as manganese oxides. These relationships suggest that the bacterial groups participated in the aerobic and anaerobic degradation of organic matter and contributed to nutrient cycling. The high abundance of sulphate reducers in the surficial sediment layers reflects the persistence of eutrophication-induced hypoxia causing ecosystem-level changes in the Baltic Sea. The sulphate reducers began to decrease below depths of 20 cm, where members of the family Anaerolineaceae (phylum Chloroflexi) increased, possibly taking part in terminal mineralization processes. Our study provides valuable information on how organic loading affects sediment bacterial community compositions, which consequently may maintain active nutrient recycling. This information is needed to improve our understanding on nutrient cycling in shallow seas where the dead zones are continuously spreading worldwide. [ABSTRACT FROM AUTHOR]

The reversal of estuarine circulation caused by southwesterly wind forcing may lead to vanishing of stratification and oxygenation of deep layers during the cold season in the Gulf of Finland. Six CTD+oxygen transects (130 km long, 10 stations) were conducted along the thalweg from the western border to the central gulf (21 December 2011-8 May 2012). Two bottom-mounted ADCP, near the western border and in the central gulf were installed. A CTD with dissolved oxygen sensor was deployed close to the western ADCP. Periods with typical estuarine circulation were characterized by strong stratification; high salinity, hypoxic conditions and inflow to the gulf in the near bottom layer. Two circulation reversals were observed, in December-January and February-March. The first well-developed reversal event caused the vanishing of stratification and oxygen concentrations that were almost over 270 µmol L-1 (6mLL-1) in the entire water column along the transect; and lasted for about 1.5 months. Shifts from estuarine circulation to reversed circulation and vice versa were both associated with strong currents (up to 40 cms-1) in the deep layer. In the western area of study, near-bottom oxygen conditions strongly depended on salt wedge intrusions (hypoxic water) from the NE Baltic Proper, while in the eastern part good oxygen conditions caused by reversals remained for a few months. Change from oxygenated to hypoxic conditions in the entrance area to the Gulf might occur very rapidly, within less than a day. [ABSTRACT FROM AUTHOR]

This study presents benthic data from 12 samplings from February to December 2010 in a 28m deep channel in the southwest Baltic Sea. In winter, the distribution of solutes in the porewater was strongly modulated by bioirrigation which efficiently flushed the upper 10 cm of sediment, leading to concentrations which varied little from bottom water values. Solute pumping by bioirrigation fell sharply in the summer as the bottom waters became severely hypoxic (<2 µMO2). At this point the giant sulfide-oxidizing bacteria Beggiatoa was visible on surface sediments. Despite an increase in O2 following mixing of the water column in November, macrofauna remained absent until the end of the sampling. Contrary to expectations, metabolites such as dissolved inorganic carbon, ammonium and hydrogen sulfide did not accumulate in the upper 10 cm during the hypoxic period when bioirrigation was absent, but instead tended toward bottom water values. This was taken as evidence for episodic bubbling of methane gas out of the sediment acting as an abiogenic irrigation process. Porewater--seawater mixing by escaping bubbles provides a pathway for enhanced nutrient release to the bottom water and may exacerbate the feed- back with hypoxia. Subsurface dissolved phosphate (TPO4) peaks in excess of 400 µM developed in autumn, resulting in a very large diffusive TPO4 flux to the water column of 0.7±0.2 mmol m-2 d-1. The model was not able to simulate this TPO4 source as release of iron-bound P (Fe--P) or organic P. As an alternative hypothesis, the TPO4 peak was reproduced using new kinetic expressions that allow Beggiatoa to take up porewater TPO4 and accumulate an intracellular P pool during periods with oxic bottom waters. TPO4 is then released during hypoxia, as previous published results with sulfide-oxidizing bacteria indicate. The TPO4 added to the porewater over the year by organic P and Fe--P is recycled through Beggiatoa, meaning that no additional source of TPO4 is needed to explain the TPO4 peak. Further experimental studies are needed to strengthen this conclusion and rule out Fe--P and organic P as candidate sources of ephemeral TPO4 release. A measured C/P ratio of <20 for the diffusive flux to the water column during hypoxia directly demonstrates preferential release of P relative to C under oxygen-deficient bottom waters. This coincides with a strong decrease in dissolved inorganic N/P ratios in the water column to ~1. Our results suggest that sulfide oxidizing bacteria could act as phosphorus capacitors in systems with oscillating redox conditions, releasing massive amounts of TPO4 in a short space of time and dramatically increasing the internal loading of TPO4 to the overlying water. [ABSTRACT FROM AUTHOR]

Abstract: Nutrient loading of aquatic ecosystems results in more food for benthic macrofaunal communities but also increases the risk of hypoxia, resulting in a reduction or complete loss of benthic biomass. This study investigates the interaction between eutrophication, hypoxia and benthic biomass with emphasis on the balance between gains and loss of benthic biomass due to changes in nutrient loadings. A physiological fauna model with 5 functional groups was linked to a 3D coupled hydrodynamic–ecological Baltic Sea model. Model results revealed that benthic biomass increased between 0 and 700% after re-oxygenating bottom waters. Nutrient reduction scenarios indicated improved oxygen concentrations in bottom waters and decreased sedimentation of organic matter up to 40% after a nutrient load reduction following the Baltic Sea Action Plan. The lower food supply to benthos reduced the macrofaunal biomass up to 35% especially in areas not currently affected by hypoxia, whereas benthic biomass increased up to 200% in areas affected by eutrophication-induced hypoxia. The expected changes in benthic biomass resulting from nutrient load reductions and subsequent reduced hypoxia may not only increase the food supply for benthivorous fish, but also significantly affect the biogeochemical functioning of the ecosystem. [Copyright &y& Elsevier]

Primary production in the eutrophic Baltic Sea is limited by nitrogen availability; hence denitrification (natural transformation of nitrate to gaseous N) in the sediments is crucial in mitigating the effects of eutrophication. This study shows that dissimilatory nitrate reduction to ammonium (DNRA) process, where nitrogen is not removed but instead recycled in the system, dominates nitrate reduction in low oxygen conditions (O <110 μM), which have been persistent in the central Gulf of Finland during the past decade. The nitrogen removal rates measured in this study show that nitrogen removal has decreased in the Gulf of Finland compared to rates measured in mid-1990s and the decrease is most likely caused by the increased bottom water hypoxia. [ABSTRACT FROM AUTHOR]

Background: Protist communities inhabiting oxygen depleted waters have so far been characterized through both microscopical observations and sequence based techniques. However, the lack of cultures for abundant taxa severely hampers our knowledge on the morphology, ecology and energy metabolism of hypoxic protists. Cultivation of such protists has been unsuccessful in most cases, and has never yet succeeded for choanoflagellates, even though these small bacterivorous flagellates are known to be ecologically relevant components of aquatic protist communities. Results: Quantitative data for choanoflagellates and the vertical distribution of Codosiga spp. at Gotland and Landsort Deep (Baltic Sea) indicate its preference for oxygen-depleted zones. Strains isolated and cultivated from these habitats revealed ultrastructural peculiarities such as mitochondria showing tubular cristae never seen before for choanoflagellates, and the first observation of intracellular prokaryotes in choanoflagellates. Analysis of their partial 28S rRNA gene sequence complements the description of two new species, Codosiga minima n. sp. and C. balthica n. sp. These are closely related with but well separated from C. gracilis (C. balthica and C. minima p-distance to C. gracilis 4.8% and 11.6%, respectively). In phylogenetic analyses the 18S rRNA gene sequences branch off together with environmental sequences from hypoxic habitats resulting in a wide cluster of hypoxic Codosiga relatives so far only known from environmental sequencing approaches. Conclusions: Here, we establish the morphological and ultrastructural identity of an environmental choanoflagellate lineage. Data from microscopical observations, supplemented by findings from previous culture-independent methods, indicate that C. balthica is likely an ecologically relevant player of Baltic Sea hypoxic waters. The possession of derived mitochondria could be an adaptation to life in hypoxic environments periodically influenced by small-scale mixing events and changing oxygen content allowing the reduction of oxygen consuming components. In view of the intricacy of isolating and cultivating choanoflagellates, the two new cultured species represent an important advance to the understanding of the ecology of this group, and mechanisms of adaptations to hypoxia in protists in general. [ABSTRACT FROM AUTHOR]

Abstract: This study investigates the biogeochemical processes that control the benthic fluxes of dissolved nitrogen (N) species in Boknis Eck – a 28 m deep site in the Eckernförde Bay (southwestern Baltic Sea). Bottom water oxygen concentrations (O2−BW) fluctuate greatly over the year at Boknis Eck, being well-oxygenated in winter and experiencing severe bottom water hypoxia and even anoxia in late summer. The present communication addresses the winter situation (February 2010). Fluxes of ammonium (NH4 +), nitrate (NO3 −) and nitrite (NO2 −) were simulated using a benthic model that accounted for transport and biogeochemical reactions and constrained with ex situ flux measurements and sediment geochemical analysis. The sediments were a net sink for NO3 − (−0.35 mmol m−2 d−1 of NO3 −), of which 75% was ascribed to dissimilatory reduction of nitrate to ammonium (DNRA) by sulfide oxidizing bacteria, and 25% to NO3 − reduction to NO2 − by denitrifying microorganisms. NH4 + fluxes were high (1.74 mmol m−2 d−1 of NH4 +), mainly due to the degradation of organic nitrogen, and directed out of the sediment. NO2 − fluxes were negligible. The sediments in Boknis Eck are, therefore, a net source of dissolved inorganic nitrogen (DIN = NO3 − + NO2 − + NH4 +) during winter. This is in large part due to bioirrigation, which accounts for 76% of the benthic efflux of NH4 +, thus reducing the capacity for nitrification of NH4 +. The combined rate of fixed N loss by denitrification and anammox was estimated at 0.08 mmol m−2 d−1 of N2, which is at the lower end of previously reported values. A systematic sensitivity analysis revealed that denitrification and anammox respond strongly and positively to the concentration of NO3 − in the bottom water (NO3 − BW). Higher O2−BW decreases DNRA and denitrification but stimulates both anammox and the contribution of anammox to total N2 production (%Ramx). A complete mechanistic explanation of these findings is provided. Our analysis indicates that nitrification is the geochemical driving force behind the observed correlation between %Ramx and water depth in the seminal study of . Despite remaining uncertainties, the results provide a general mechanistic framework for interpreting the existing knowledge of N-turnover processes and fluxes in continental margin sediments, as well as predicting the types of environment where these reactions are expected to occur prominently. [Copyright &y& Elsevier]

Hypoxia is a well-described phenomenon in the offshore waters of the Baltic Sea with both the spatial extent and intensity of hypoxia known to have increased due to anthropogenic eutrophication, however, an unknown amount of hypoxia is present in the coastal zone. Here we report on the widespread unprecedented occurrence of hypoxia across the coastal zone of the Baltic Sea. We have identified 115 sites that have experienced hypoxia during the period 1955-2009 increasing the global total to ca. 500 sites, with the Baltic Sea coastal zone containing over 20% of all known sites worldwide. Most sites experienced episodic hypoxia, which is a precursor to development of seasonal hypoxia. The Baltic Sea coastal zone displays an alarming trend with hypoxia steadily increasing with time since the 1950s effecting nutrient biogeochemical processes, ecosystem services, and coastal habitat. [ABSTRACT FROM AUTHOR]

The bottom water of the North Sea-Baltic Sea transition zone suffers from seasonal hypoxia, usually during late summer and autumn. These hypoxic events are critical for the benthic ecosystems and the concentration of dissolved oxygen is an important measure of the water quality. However, to model the subsurface dissolved oxygen is a major challenge, especially in estuaries and coastal regions. In this study a simple oxygen consumption model is coupled to a 3-D hydrodynamical model in order to analyse oxygen variations in the transition zone. The benthic and pelagic consumption of oxygen is modelled as a function of water temperature and oxygen concentration. A quantitative assessment of the model demonstrates that the model is able to resolve both seasonal and interannual variations in dissolved oxygen. Results from several experimental simulations highlight the importance of physical processes in the regulation of dissolved oxygen. Advective oxygen transport and wind induced mixing are two key processes that control the extent of hypoxia in the transition zone. [ABSTRACT FROM AUTHOR]

The Baltic Sea is characterised by a heterogeneous oceanographic environment. The deep water layers forming the habitat of Baltic cod ( Gadus morhua callarias L.) are subjected to frequently occurring pronounced anoxic conditions. Adverse oxygen conditions result in physiological stress for organisms living under these conditions. For cod e.g. a direct relationship between oxygen availability and food intake with a decreasing ingestion rate at hypoxia could be revealed. In the present study, the effects of oxygen deficiency on consumption rates were investigated and how these translate to stock size estimates in multi-species models. Based on results from laboratory experiments, a model was fitted to evacuation rates at different oxygen levels and integrated into the existing consumption model for Baltic cod. Individual mean oxygen corrected consumption rates were 0.1-10.9% lower than the uncorrected ones. At the currently low predator stock size, however, the effect of oxygen-reduced consumption on the total amount of eaten prey biomass and thus predation mortalities was only marginal. But should successful management lead to higher cod stock sizes in the future, then total predation mortalities will greatly increase and thus improved precision of these estimates would be valuable for the assessment of prey stocks. [ABSTRACT FROM AUTHOR]

Abstract: The Neva Estuary situated in the eastern Gulf of Finland is one of the largest estuaries of the Baltic Sea with a large conurbation, St. Petersburg, situated on its coast. Eutrophication, alien species and large-scale digging and dumping of bottom sediment are the most prominent anthropogenic impacts on its ecosystem. However, many ecosystem responses, which are traditionally attribute to these impacts, are related to natural dynamics of the ecosystem. Fluctuations in discharge of the Neva River, intrusions of bottom hypoxic waters from the western part of the Gulf of Finland, higher summer temperatures and a shorter period of ice cover are climatic mediated factors inducing adverse changes in its ecosystem from the 1980s onwards. The main ecosystem responses to these factors are 2–3-fold increase of trophic status, deterioration of native zoobenthic communities and establishment of alien species, as well as the many fold decrease of fish catch and the population of ringed seal in the region. [Copyright &y& Elsevier]

Abstract: The biomass dynamics of the green alga Cladophora glomerata was studied in the shallow-water littoral zone of the Neva estuary during May–October of 2003–2005. Additionally, the average production rate of C. glomerata, the biomass of drifting algae and oxygen depletion were examined during period of algae decomposition. Two peaks in C. glomerata biomass, in July and in September, were observed during all years studied, reaching a maximum 300±100gDWm−2. The primary production of C. glomerata varied from 3.6 to 7.9gCm−2 contributing to around 90% of the total primary production in this habitat. From the middle of July to late August the biomass of drifting C. glomerata exceeded the biomass of the attached algae, accounting for 62% of the total algal biomass in shallow areas (0–1m). The hypoxia (a minimum oxygen level of 5% or 0.62mgdm−3) in the water was recorded near shore (to 20-m distance from the shore), negatively influencing the quality of shallow-water habitats. [Copyright &y& Elsevier]

During the last century (1900s) industrialized forms of agriculture and human activities have caused extensive eutrophication of Baltic Sea waters. As a consequence, the Baltic Sea developed a hypoxic zone that has caused serve ecosystem distur- bance. Climate forcing has also been proposed to be responsible for the reported trends in hypoxia (<2 mg/l O2) both during the last c. 100 years and during the Medieval Period. By contrast, investigations on the degree of anthropogenic forcing on the ecosystem on long time-scales (millennial) have not been thoroughly addressed. This paper critically examines evidence for anthropogenic disturbance of the marine environment beyond the last century through the analysis of the population growth, technological development and land-use changes in the drainage area. Natural environmental changes, i.e. changes in the morphology and depths of the Baltic basin and the sills, were probably the main driver for large-scale hypoxia during the early Holocene (8000-4000 cal. yr BP). We show that hypoxia during the last two millennia has followed the general expansion and contraction trends in Europe and that human perturbations have been an important driver for hypoxia during that time. Hypoxia occurring during the Medieval Period coincides with a doubling of the population (from c. 4.6 to 9.5 million), a massive reclamation of land in both established and marginal cultivated areas and significant increases in soil nutrient release. The role of climate forcing on hypoxia in the Baltic Sea has yet to be convincingly demonstrated, although it could have contributed to sustain hypoxia through enhanced salt water inflows or through changes in hydrological inputs. In addition, cyanobacteria blooms are not natural features of the Baltic Sea as previously hypothesized, but are a consequence of enhanced phosphorus release that occurs together with hypoxia. [ABSTRACT FROM AUTHOR]

Abstract: In this study, redox-dependent phosphorus (P) recycling and burial at 6 sites in the Baltic Sea is investigated using a combination of porewater and sediment analyses and sediment age dating (210Pb and 137Cs). We focus on sites in the Kattegat, Danish Straits and Baltic Proper where present-day bottom water redox conditions range from fully oxygenated and seasonally hypoxic to almost permanently anoxic and sulfidic. Strong surface enrichments of Fe-oxide bound P are observed at oxic and seasonally hypoxic sites but not in the anoxic basins. Reductive dissolution of Fe-oxides and release of the associated P supports higher sediment–water exchange of PO4 at hypoxic sites (up to ∼800μmolPm−2 d−1) than in the anoxic basins. This confirms that Fe-bound P in surface sediments in the Baltic acts as a major internal source of P during seasonal hypoxia, as suggested previously from water column studies. Most burial of P takes place as organic P. We find no evidence for significant authigenic Ca–P formation or biogenic Ca–P burial. The lack of major inorganic P burial sinks makes the Baltic Sea very sensitive to the feedback loop between increased hypoxia, enhanced regeneration of P and increased primary productivity. Historical records of bottom water oxygen at two sites (Bornholm, Northern Gotland) show a decline over the past century and are accompanied by a rise in values for typical sediment proxies for anoxia (total sulfur, molybdenum and organic C/P ratios). While sediment reactive P concentrations in anoxic basins are equal to or higher than at oxic sites, burial rates of P at hypoxic and anoxic sites are up to 20 times lower because of lower sedimentation rates. Nevertheless, burial of reactive P in both hypoxic and anoxic areas is significant because of their large surface area and should be accounted for in budgets and models for the Baltic Sea. [Copyright &y& Elsevier]

Hypoxia, a growing worldwide problem, has been intermittently present in the modern Baltic Sea since its formation ca. 8000 cal. yr BP. However, both the spatial extent and intensity of hypoxia have increased with anthropogenic eutrophication due to nutrient inputs. Physical processes, which control stratification and the renewal of oxygen in bottom waters, are important constraints on the formation and maintenance of hypoxia. Climate controlled inflows of saline water from the North Sea through the Danish Straits is a critical controlling factor governing the spatial extent and duration of hypoxia. Hypoxia regulates the biogeochemical cycles of both phosphorus (P) and nitrogen (N) in the water column and sediments. Significant amounts of P are currently released from sediments, an order of magnitude larger than anthropogenic inputs. The Baltic Sea is unique for coastal marine ecosystems experiencing N losses in hypoxic waters below the halocline. Although benthic communities in the Baltic Sea are naturally constrained by salinity gradients, hypoxia has resulted in habitat loss over vast areas and the elimination of benthic fauna, and has severely disrupted benthic food webs. Nutrient load reductions are needed to reduce the extent, severity, and effects of hypoxia. [ABSTRACT FROM AUTHOR]

In this article the authors discuss hypoxia in the Baltic Sea and whether engineering is a preventative solution. The authors provide an overview of hypoxia in the Baltic Sea, which is the lack of dissolved oxygen (O2) in bottom water, and the efforts taken to prevent it. They cite the frustration regarding the lack of progress in improving water quality and discuss large-scale engineered remediation projects to increase O2 in bottom waters.

Abstract: The hypoxic zone in the Baltic Sea has increased in area about four times since 1960 and widespread oxygen deficiency has severely reduced macro benthic communities below the halocline in the Baltic Proper and the Gulf of Finland, which in turn has affected food chain dynamics, fish habitats and fisheries in the entire Baltic Sea. The cause of increased hypoxia is believed to be enhanced eutrophication through increased anthropogenic input of nutrients, such as nitrogen and phosphorus. However, the spatial variability of hypoxia on long time-scales is poorly known: and so are the driving mechanisms. We review the occurrence of hypoxia in modern time (last c. 50 years), modern historical time (AD 1950–1800) and during the more distant past (the last c. 10000 years) and explore the role of climate variability, environmental change and human impact. We present a compilation of proxy records of hypoxia (laminated sediments) based on long sediment cores from the Baltic Sea. The cumulated results show that the deeper depressions of the Baltic Sea have experienced intermittent hypoxia during most of the Holocene and that regular laminations started to form c. 8500–7800 cal. yr BP ago, in association with the formation of a permanent halocline at the transition between the Early Littorina Sea and the Littorina Sea s. str. Laminated sediments were deposited during three main periods (i.e. between c. 8000–4000, 2000–800 cal. yr BP and subsequent to AD 1800) which overlap the Holocene Thermal Maximum (c. 9000–5000 cal. yr BP), the Medieval Warm Period (c. AD 750–1200) and the modern historical period (AD 1800 to present) and coincide with intervals of high surface salinity (at least during the Littorina s. str.) and high total organic carbon content. This study implies that there may be a correlation between climate variability in the past and the state of the marine environment, where milder and dryer periods with less freshwater run-off correspond to increased salinities and higher accumulation of organic carbon resulting in amplified hypoxia and enlarged distribution of laminated sediments. We suggest that hydrology changes in the drainage area on long time-scales have, as well as the inflow of saltier North Sea waters, controlled the deep oxic conditions in the Baltic Sea and that such changes have followed the general Holocene climate development in Northwest Europe. Increased hypoxia during the Medieval Warm Period also correlates with large-scale changes in land use that occurred in much of the Baltic Sea watershed during the early-medieval expansion. We suggest that hypoxia during this period in the Baltic Sea was not only caused by climate, but increased human impact was most likely an additional trigger. Large areas of the Baltic Sea have experienced intermittent hypoxic from at least AD 1900 with laminated sediments present in the Gotland Basin in the Baltic Proper since then and up to present time. This period coincides with the industrial revolution in Northwestern Europe which started around AD 1850, when population grew, cutting of drainage ditches intensified, and agricultural and forest industry expanded extensively. [Copyright &y& Elsevier]

Eutrophication of the Baltic Sea has potentially increased the frequency and magnitude of cyanobacteria blooms. Eutrophication leads to increased sedimentation of organic material, increasing the extent of anoxic bottoms and subsequently increasing the internal phosphorus loading. In addition, the hypoxic water volume displays a negative relationship with the total dissolved inorganic nitrogen pool, suggesting greater overall nitrogen removal with increased hypoxia. Enhanced internal loading of phosphorus and the removal of dissolved inorganic nitrogen leads to lower nitrogen to phosphorus ratios, which are one of the main factors promoting nitrogen-fixing cyanobacteria blooms. Because cyanobacteria blooms in the open waters of the Baltic Sea seem to be strongly regulated by internal processes, the effects of external nutrient reductions are scale-dependent. During longer time scales, reductions in external phosphorus load may reduce cyanobacteria blooms; however, on shorter time scales the internal phosphorus loading can counteract external phosphorus reductions. The coupled processes inducing internal loading, nitrogen removal, and the prevalence of nitrogen-fixing cyanobacteria can qualitatively be described as a potentially self-sustaining "vicious circle." To effectively reduce cyanobacteria blooms and overall signs of eutrophication, reductions in both nitrogen and phosphorus external loads appear essential. [ABSTRACT FROM AUTHOR]

The relationships between the concentration of water nutrients and the biomass of benthic invertebrate feeding guilds were examined at 46 sites in the northern Baltic Sea during 1993–2003. We analysed whether and how degree of exposure, presence of fronts, salinity, hypoxia, nutrient concentrations, depth, sediment type and structure of invertebrate communities contributed to these relationships. In general macrozoobenthos did not respond to the changing nutrient concentrations in the areas that were regularly impacted by fronts (river estuaries, bank slopes, straits). Macrobenthic species diversity, depth, 11-year average of nutrient concentration and sediment type explained best how strong the nutrient-invertebrate relationships were. The deposit feeders, that inhabited more diverse communities, were less sensitive to the increased concentration of nutrients than those in less diverse communities. On the other hand, the sensitivity of suspension feeders to rising nutrient load increased with benthic diversity. The response of macrozoobenthos to nitrogen level decreased with increasing depth. Our data did not support the hypothesis that there was a significant difference in the occurrence of nutrient-invertebrate relationships between hypoxic and normoxic conditions. The probability of finding negative nutrient-invertebrate relationships increased with depth. The results pointed to nitrogen limitation in the coarse and fine sediments and phosphorus limitation in the mixed sediments. Increased nitrogen values strengthened the response of suspension feeders to the concentration of phosphorus. Increasing phosphorus level dampened the relationships between benthic functions and concentration of phosphorus. This study confirmed that depth and sediment type were the best regularly monitored abiotic variables that could be used to determine the type areas within the northern Baltic Sea in sensu the European Community Water Framework Directive. As the nutrient-invertebrate relationships were significantly modified by macrobenthic diversity, the environmental classification should incorporate specific biological measures such as benthic diversity in order to better describe the quality status of the water body. [ABSTRACT FROM AUTHOR]

Analyzes the hypoxia in the Baltic Sea and basin-scale changes in phosphorus biogeochemistry. Oxygen in bottom waters; Phosphorus biogeochemistry; Importance of phosphorus in eutrophication.

Abstract: A significant relation was established between the maximum extent of sea ice covering the Baltic Sea and the hypoxic area in the deeper parts of the Baltic Proper, with a lag of 2 years: for the period 1970–2000, less ice was correlated with a smaller anoxic area. At the same time, maximum ice extent is subject to a long-term climate-related decline, due to higher air temperatures and an increased frequency of westerly winds. Together, this suggests that the hypoxic area will decrease in the coming decades. Internal sedimentary phosphorus-loading is closely related to the hypoxic area and hence would decrease as well. Wind strength variability did not have a significant additive effect to the variance in hypoxic area already explained by sea ice extent. The observed lag is in agreement with the order of magnitude of estimated residence times in the shallower sub-basins feeding the Baltic Proper, where ice extent varies most. [Copyright &y& Elsevier]

The article reports on a study which determines the association between hypoxia in the Baltic Sea and chemical composition within the ear-stones, or otoliths, of Baltic Sea cod Gadus morhua. Researchers are exploiting trace elements and isotopes incorporated within the otoliths to understand the environmental conditions in which a fish lived. The study found the association of otolith chemistry to hypoxia.