Decline In Oxygen Concentration Research Articles

Continuing long-term expansion of low-oxygen conditions in the Eastern Gulf of Finland

To develop an oxygen indicator for the eastern part of the Gulf of Finland (EGOF), a dataset covering 1900–2021 was compiled. The analysis revealed a long-term declining trend in dissolved oxygen concentrations in the EGOF deep layer of 0.022 mg L–1 a–1 and multi-decadal variations associated with the observed changes in hydrographic conditions. About 27% of the decline in oxygen concentrations for 1900–2021 and 40% for 1990–2021 can be explained by the decrease in solubility due to the temperature increase and changes in hydrographic conditions. The water volume and bottom area under low oxygen conditions in 2016–2021, characterized by dissolved oxygen concentrations <= 6 mg L–1, have increased, compared to the selected reference period with almost no human impact in the 1920s–1950s, from 9.8 km3 to 78.0 km3 (from 2.6% to 20.9% of the EGOF total volume) and from 1190 km2 to 4950 km2 (from 13.4% to 56.0% of the EGOF total area), respectively. The environmental status of the EGOF was assessed as not good based on the introduced oxygen indicator. We conclude that, in the long-term, low oxygen conditions have expanded mostly due to the excess load and accumulation of nutrients in the system and temperature-related changes in biogeochemical processes and fluxes. However, on a decadal scale, changes in hydrographic conditions, i.e. stratification and mixing, can significantly impact the sub-surface oxygen conditions in the EGOF and similar estuarine basins.

Heterogeneity Governs 3D‐Cultures of Clinically Relevant Microbial Communities

AbstractThe intrinsic heterogeneity of bacterial niches should be retained in in vitro cultures to represent the complex microbial ecology. As a case study, mucin‐containing hydrogels ‐CF‐Mu3Gel ‐ are generated by diffusion‐induced gelation, bioinspired on cystic fibrosis (CF) mucus, and a microbial niche challenging current therapeutic strategies. At breathing frequency, CF‐Mu3Gel exhibits a G′ and G″ equal to 24 and 3.2 Pa, respectively. Notably, CF‐Mu3Gel exhibits structural gradients with a gradual reduction of oxygen tension across its thickness (280–194 µmol L−1). Over the culture period, a steep decline in oxygen concentration occurs just a few millimeters below the air–mucus interface in CF‐Mu3Gel, similar to those of CF airway mucus. Importantly, the distinctive features of CF‐Mu3Gel significantly influence bacterial organization and antimicrobial tolerance in mono‐ and co‐cultures of Staphylococcus aureus and Pseudomonas aeruginosa that standard cultures are unable to emulate. The antimicrobial susceptibility determined in CF‐Mu3Gel corroborates the mismatch on the efficacy of antimicrobial treatment between planktonically cultured bacteria and those in patients. With this example‐based research, new light is shed on the understanding of how the substrate influences microbial behavior, paving the way for improved fundamental microbiology studies and more effective drug testing and development.

Southern Hemisphere Westerly Winds have modulated the formation of laminations in sediments in Lago Fagnano (Tierra del Fuego, Argentina) over the past 6.3 ka

Tierra del Fuego in Argentina is a unique location to examine past Holocene wind variability since it intersects the core of the Southern Hemisphere Westerly Winds (SHWW). The SHWW are the most powerful prevailing winds on Earth. Their variation plays a role in regulating atmospheric CO2 levels and rainfall amounts and distribution, both today and in the past. We obtained a piston core (LF06‐PC8) from Bahía Grande, a protected sub‐basin at the southern margin of Lago Fagnano, the largest lake in Tierra del Fuego. This article focuses on the uppermost 185 cm of this core, corresponding to laminated sediment from the last ~6.3 ka. Laminations consist of millimetre‐scale paired dark and light layers. Previous studies and new geochemical analysis show that the dark and light layers are characterized by differing concentrations of Mn and Fe. We attribute the distribution of Mn and Fe to episodic hypolimnic oxic–anoxic variations. The age model suggests an approximately bidecadal timescale for the formation of each layer pair. We propose a new model of these redox changes with the SHWW variations. The most likely phenomenon to produce complete water‐column mixing is thermobaric instability, which occurs in colder winters with low‐intensity SHWW (El Niño‐like conditions). In contrast, windier winters are characterized by higher temperatures and reduced mixing in the water column, facilitating a decline in oxygen concentration. Laminations, and the inferred presence of periodic hypolimnion redox changes, are common features of the past ~6.3 ka. Geochemical proxy variability is compatible with an intensification of El Niño/Southern Oscillation activity during the past ~2 ka.

Recent expansion and intensification of hypoxia in the Arabian Gulf and its drivers

The Arabian Gulf (also known as Persian Gulf, hereafter Gulf) is a shallow semi-enclosed subtropical sea known for its extreme physical environment. Recent observations suggest a decline in oxygen concentrations in the Gulf over the past few decades accompanied by an expansion of seasonal near-bottom hypoxia. Here, we reconstruct the evolution of dissolved oxygen in the Gulf from 1982 through 2010 and explore its controlling factors. To this end, we use an eddy-resolving hindcast simulation forced with winds and heat and freshwater fluxes from an atmospheric reanalysis. We show that seasonal near-bottom hypoxia (O2&amp;lt; 60 mmol m-3) emerges in the deeper part of the Gulf over summer and peaks in autumn in response to enhanced vertical stratification inhibiting mixing and O2 replenishment at depth. We also find a significant deoxygenation in the Gulf over the study period, with the Gulf O2 content dropping by nearly 1% per decade and near-bottom O2 decreasing by between 10 and 30 mmol m-3 in the deeper part of the Gulf between the early 1980s and the late 2000s. These changes result in the horizontal expansion of seasonal bottom hypoxia with the hypoxia-prone seafloor area increasing from less than 20,000 km2 in the 1980s to around 30,000 km2 in the 2000s. The expansion of hypoxia is also accompanied by a lengthening of the hypoxic season with hypoxia emerging locally 1 to 2 months earlier in the late 2000s relative to the early 1980s. Furthermore, declining near-bottom O2 levels result in the expansion of suboxic conditions (O2&amp;lt; 4 mmol m-3) and the emergence and amplification of denitrification there. An analysis of the Gulf oxygen budget demonstrates that deoxygenation is essentially caused by reduced oxygen solubility near the surface and enhanced respiration near the bottom. While reduced solubility results from the warming of the Gulf waters, enhanced respiration is mostly driven by an increased supply of nutrients imported from the Arabian Sea due to the weakening of winter Shamal winds over the study period. Our findings suggest that recent changes in local climate are not only altering the Gulf physical environment but are also having a strong impact on the Gulf biogeochemistry with profound potential implications for the ecosystems and the fisheries of the region.

Simulation of oxygen transport and estimation of tissue perfusion in extensive microvascular networks: Application to cerebral cortex

Advanced imaging techniques have made available extensive three-dimensional microvascular network structures. Simulation of oxygen transport by such networks requires information on blood flow rates and oxygen levels in vessels crossing boundaries of the imaged region, which is difficult to obtain experimentally. Here, a computational method is presented for estimating blood flow rates, oxygen levels, tissue perfusion and oxygen extraction, based on incomplete boundary conditions. Flow rates in all segments are estimated using a previously published method. Vessels crossing the region boundary are classified as arterioles, capillaries or venules. Oxygen levels in inflowing capillaries are assigned based on values in outflowing capillaries, and similarly for venules. Convective and diffusive oxygen transport is simulated. Contributions of each vessel to perfusion are computed in proportion to the decline in oxygen concentration along that vessel. For a vascular network in the mouse cerebral cortex, predicted tissue oxygen levels show a broad distribution, with 99% of tissue in the range of 20 to 80 mmHg under reference conditions, and steep gradients near arterioles. Perfusion and extraction estimates are consistent with experimental values. A 30% reduction in perfusion or a 30% increase in oxygen demand, relative to reference levels, is predicted to result in tissue hypoxia.

New approaches to the study of cell vital activity cultivated in different growing conditions with analysis of oxygen in the medium

Objectives - to develop new approaches to the study of morphofimctional state of chondroblasts, cultured at 37°C on a 3D carrier in different environments: in a CO2 incubator with 5% of CO2 and in a thermostat in an air-proof tube. Material and methods. The study cell culture - chondroblasts, isolated from the cartilage of the articular surfaces of the extra-fingers' phalanges. 3D carrier for cells - the demineralized lyophilized human spongiosa Lioplast®. The resulting tissue-engineered structures were grown in a complete cell culture medium at 37°С under different conditions: in a closed system in thermostat and in an open system in CO2 incubator (5% CO2). To assess the morphofunctional state of the cells on the surface of the 3D carrier, the picrosirius red staining, a LIVE/ DEAD® fluorescent dye kit, and scanning electron microscopy were used. The oxygen concentration in the culture medium was evaluated by the modified Winkler titration method. Results. The complex of morphological methods revealed the presence of living cells on the surface of human spongiosa within the 7-day period of cultivation. The cells either are fusiform or have a polygonal form and have a capacity to grow in 2 or more layers. The titrimetric analysis has demonstrated a decline in the concentration of dissolved oxygen in the medium with cellular tissue material in 7 days of cultivation. The concentration declined by 72.4% in a thermostat and by 63.5% in a CO2 incubator. In the tests tubes which contained only the medium and no cells, there was a similar decline in oxygen concentration by 47.3% in a thermostat and by 66.1% in a CO2 incubator. Conclusion. 1. A method of measuring the amount of dissolved oxygen in a culture medium, during the adhesive cell cultivation on a 3D carrier, was developed, based on the Winkler titration method. 2. A comparative analysis of the amount of dissolved oxygen in the medium in the process of chondroblast cultivation on a 3D human spongiosa carrier, both in a CO2 incubator and in a closed test tube, revealed an overall tendency to a decrease in the concentration of oxygen within 7 days of cultivation. 3. A decrease in oxygen concentration in the test tubes with human spongiosa samples (without cells), within the 7 days of cultivation, was registered. 4. An efficient and cost-saving method of graft manufacturing for the purposes of chondroplasty is the transfer of juvenile joint cartilage chondroblasts to 3D human spongiosa carriers and their further cultivation in air-proof test tubes copletely filled with medium within a period of 7 days.

Climate variability promotes unprecedented cyanobacterial blooms in a remote, oligotrophic Ontario lake: evidence from paleolimnology

Dickson Lake, located in Algonquin Provincial Park, Ontario, is a remote, oligotrophic lake, where cyanobacterial blooms of the genus Dolichospermum (Ralfs ex Bornet & Flahault) P. Wacklin, L. Hoffmann and J. Komarek were reported for the first time in the fall of 2014, and subsequently in the late spring of 2015. To investigate the potential environmental triggers of these bloom events, we assessed long-term trends in water quality using a multi-proxy paleolimnological approach, examining sedimentary diatoms, chironomids, cladocerans, spectrally inferred chlorophyll a, and cyanobacterial akinetes preserved in a 210Pb-dated sediment core. Assemblage changes were modest in all biological proxies. A subtle increase in the abundance of warm-water chironomid taxa (Topt > 15 °C) commences in the year ~ 2000, with further increases in the most recent years of the sediment record (~ 2013–2015). End-of-summer volume-weighted hypolimnetic oxygen concentrations (CI-VWHO), inferred from chironomid remains, reveal a decline in oxygen concentrations over the last two decades coincident with the highest levels of sedimentary chlorophyll a and cyanobacterial akinetes in the sediment record. These paleolimnological findings corroborate observed reports of the onset of cyanobacterial blooms in Dickson Lake in late 2014 and are consistent with increasingly favourable bloom-forming conditions over the past few decades that are related to warmer air temperatures, sharp declines in wind speed, and a lengthening of the ice-free season by 2 weeks since 1975. It is plausible that late ice-out and a quick onset to stratification in 2014 may have resulted in incomplete spring mixing, early onset of hypolimnetic anoxia, and increased internal nutrient loading, that, occurring during a period when climate conditions were particularly ideal for cyanobacterial proliferation, may have fueled the unprecedented algal blooms in this remote lake. Collectively, the factors causing algal blooms in remote lakes such as Dickson Lake are not yet fully understood, and it is worrisome that with continued warming the triggering conditions may become a more common feature of Algonquin Park and other minimally impacted Boreal Shield lakes in the coming years.

Seasonal and interannual oxygen variability on the Washington and Oregon continental shelves

AbstractThe coastal waters of the northern portion of the California Current System experience a seasonal decline in oxygen concentrations and hypoxia over the summer upwelling season that results in negative impacts on habitat for many organisms. Using a regional model extending from 43°N to 50°N, with an oxygen component developed in this study, drivers of seasonal and regional oxygen variability are identified. The model includes two pools of detritus, which was an essential addition in order to achieve good agreement with the observations. The model was validated using an extensive array of hydrographic and moored observations. The model captures the observed seasonal decline as well as spatial trends in bottom oxygen. Spatially, three regions of high respiration are identified as locations where hypoxia develops each modeled year. Two of the regions are previously identified recirculation regions. The third region is off of the Washington coast. Sediment oxygen demand causes the region on the Washington coast to be susceptible to hypoxia and is correlated to the broad area of shallow shelf (&lt;60 m) in the region. Respiration and circulation‐driven divergence contribute similar (60, 40%, respectively) amounts to the integrated oxygen budget on the Washington coast while respiration dominates the Oregon coast. Divergence, or circulation, contributes to the oxygen dynamics on the shelf in two ways: first, through the generation of retention features, and second, by determining variability.

Predicting the potential long-term influence of climate change on vendace (Coregonus albula) habitat in Bassenthwaite Lake, U.K.

. The long-term suitability of Bassenthwaite Lake as a habitat for vendace (Coregonus albula) was assessed using two models. The first was the phytoplankton model (PROTECH) that provided temperature and phytoplankton biomass outputs that were used to drive a second model of lake oxygen (LOX). 2. Both temperature and oxygen concentrations were used to define the available habitat for the adult vendace, using 18 °C as an upper and 2 mg L−1 as a lower threshold, respectively. The outputs of both models were compared with 4 years of observed data for the purposes of validation and produced good simulations of water temperature, total chlorophyll a and oxygen concentrations in the epilimnion, hypolimnion and at the lake bottom. 3. Using the outputs of a regional climate model (RCM) simulating 20 years of both present and future climate conditions for this part of the United Kingdom, both models were re-run. These data suggest the future climate will cause a mean increase of >2 °C in water temperature, little change in overall phytoplankton biomass and a 10% decline in oxygen concentration. 4. Using the thresholds defined above, the habitat volume will decline greatly under the future climate scenarios, with all of the 20 years simulated having periods of zero habitat volume for >7 consecutive days, primarily caused by high temperature. These results suggest that the long-term viability of the lake as a habitat for this rare fish is extremely low.

Long‐term changes in dissolved oxygen concentrations in the ocean caused by protracted global warming

In the Earth's geological record massive marine ecological change has been attributed to the occurrence of widespread anoxia in the ocean [Jahren, 2002; White, 2002; Wignall and Twitchett, 1996]. Climate change projection till the end of this century predict a 4 to 7% decline in the dissolve oxygen in the ocean [Bopp et al., 2002; Matear et al., 2000; Plattner et al., 2001; Sarmiento et al., 1998] suggesting the potential for global warming to eventually drive the deep ocean anoxic. To examine the multicentury impact of protracted global warming on oceanic concentrations of dissolved oxygen, we use a climate system model and a low‐order oceanic biogeochemical model. The models are integrated for an atmospheric equivalent CO2 concentration, which is specified to triple according to a standard scenario from the late nineteenth to the late twenty‐first century, and then is subsequently held constant at that elevated level for an additional 6 centuries. For the present day, the model successfully reproduced the large‐scale features of the dissolved oxygen field in the ocean. In the global warming simulation, the physical model displays marked changes in high‐latitude oceanic stratification and overturning, including near‐cessation of deep water renewal for depths greater than about 1.5 km during the period of elevated stable CO2 concentration. Our model predicts a decline in oxygen concentration through most of the subsurface ocean. Concentration changes in the thermocline waters result mainly from solubility changes in the upstream source waters, while changes in the deep waters result mainly from lack of ventilation and ongoing consumption of oxygen by remineralization of sinking particulate organic matter. Changes in the upper 2 km of the ocean generally show signs of equilibration by the end of the integration, but at greater depths, there occurs a slow but steady decline through to the end of the integration. By the end of the integration, we simulate a doubling of the volume of hypoxic water (less than 10 μmol/kg) in the thermocline of the eastern equatorial Pacific Ocean. During the integration deep ocean oxygen concentrations generally decline by between 20 and 40%, but, significantly, no extensive deep ocean anoxia develops during the period of integration, nor does it appear that it would likely do so for at least a further 4000 years of integration. Subsurface oxygen decline is moderated by an overall reduction in export production of particulate organic matter, which reduces oxygen consumption in the ocean interior due to the remineralization of this material.

Microbially mediated sulphide production in a thermal, acidic algal mat community in Yellowstone National Park.

Our objective in this study was to characterize prokaryotic sulphide production within the oxygenic, predominantly eukaryotic algal mat in an acidic stream, Nymph Creek, in Yellowstone National Park (YNP). We used microsensors to examine fluctuations in H2S and O2 concentrations over time through the vertical aspect of the approximately 3 mm mat in a 46-48 degrees C region of the creek. We also used analyses of PCR-amplified 16S rRNA gene sequences obtained from denaturing gradient gels, and PCR-amplified sequences of a functional gene associated with microbial sulphate respiration (dsrA) to characterize the bacterial community in the same region of the mat. During midday, photosynthesis rates were high within the first 500 micro m interval of the mat and high oxygen concentrations (600% air saturation) penetrated deeply (>1800 micro m) into the mat. During early evening and night, oxygen concentrations within the first 1100 micro m of the mat decreased over time from 60% air saturation (a.s) to 12% a.s. A precipitous decline in oxygen concentration occurred at a depth of 1100 micro m in all night measurements and anoxic conditions were present below 1200 micro m. Within this anoxic region, sulphide concentrations increased from nearly 0 micro M at 1200 micro m depth to 100 micro M at 2400 micro m depth. Enrichment cultures inoculated with Nymph Creek mat organisms also produced H2S. Sequence analyses of 16S rRNA and dsrA genes indicated the presence of at least five bacterial genera including species involved in dissimilative sulphate or sulphur reduction.

Permian-Triassic boundary interval as a model for forcing marine ecosystem collapse by long-term atmospheric oxygen drop

Ecological traits of reefs across the Permian-Triassic boundary interval coincide with a modeled decline of atmospheric oxygen throughout the Permian Period. Selective extinction and recovery patterns within the reef system are observed both at the end of the middle Permian (end-Guadalupian) and at the Permian-Triassic boundary. The end-Guadalupian event selectively affected corals and broke down the cool-water carbonate factory. Sponges, however, were largely unaffected and bloomed in reefs toward the end of the Permian. The end-Permian total destruction of the metazoan reef system only left behind poorly diverse microbial communities. The temporal reef patterns are thus similar to spatial patterns of modern benthic communities approaching oxygen minimum zones. This observation suggests that a decline in oxygen concentrations was at least partly involved in the destruction of reefs, even where there is no direct evidence of oceanic anoxia.

Hypoxia induces transcription of the plasminogen activator inhibitor-1 gene through genistein-sensitive tyrosine kinase pathways in vascular endothelial cells.

A decline in oxygen concentration perturbs endothelial function, which promotes local thrombosis. In this study, we determined whether hypoxia in the range of that observed in pathophysiological hypoxic states stimulates plasminogen activator inhibitor-1 (PAI-1) production in bovine aortic endothelial cells. PAI-1 production, measured by ELISA, was increased by 4.7-fold (P<0.05 versus normoxic control, n=4) at 12 hours after hypoxic stimulation. Northern blot analysis showed the progressive time-dependent increase in the steady-state level of PAI-1 mRNA expression by hypoxia, which reached a 7.5-fold increase (P<0.05 versus control, n=4) at 12 hours. Deferoxamine, which has been known to bind heme protein and to reproduce the hypoxic response, induced PAI-1 production at both the mRNA and protein levels. The half-life of PAI-1 mRNA, as determined by a standard decay assay, was not affected by hypoxia, suggesting that induction of PAI-1 mRNA was regulated mainly at the transcriptional level. Transient transfection assays of the human PAI-1 promoter-luciferase construct indicates that a hypoxia-responsive region lies between -414 and -107 relative to the transcription start site, where no putative hypoxia response element is found. The hypoxia-mediated increase in PAI-1 mRNA levels was attenuated by the tyrosine kinase inhibitors genistein (50 micromol/L) and herbimycin A (1 micromol/L), whereas PD98059 (50 micromol/L, MEK1 inhibitor), SB203580 (10 micromol/L, p38 mitogen-activated protein kinase inhibitor), and calphostin C (1 micromol/L, protein kinase C inhibitor) had no effect on the induction of PAI-1 expression by hypoxia and deferoxamine. Genistein but not daidzein blocked the production of hypoxia- and deferoxamine-induced PAI-1 protein. Thus, we conclude that hypoxia stimulates PAI-1 gene transcription and protein production through a signaling pathway involving genistein-sensitive tyrosine kinases in vascular endothelial cells.

Oxygen-dependent low-temperature 12 (n6)-desaturase induction and alteration of fatty acid composition in Acanthamoeba castellanii

The influence of dissolved oxygen on the synthesis and activity of Δ12-desaturase in Acanthamoeba castellanii was investigated. A decline in oxygen concentration during batch growth at 30° was correlated with a decline in the degree of cellular fatty acid unsaturation. Chilling of early-stationary-phase cultures to 15° led to increased dissolved oxygen levels (from &lt; 1 μM to 305 μM) and increased fatty acid unsaturation, which has been shown previously [Avery, S. V., Harwood, J. L. &amp; Lloyd, D. (1994) Microbiology 140, 2423–2431] to be due mainly to δ12-desaturase induction. In contrast, chilling of mid-exponential-phase cultures, where the dissolved oxygen concentration prior to chilling was high (&gt; 160 μM), gave no change in cellular fatty acid unsaturation. Measurement of [1-14C]acetate incorporation by oxygen-limited A. castellanii revealed that labelling of the Δ12-desaturase product, linoleate (18:2), increased with oxygen concentration. Microsomal levels of the Δ12-desaturase enzyme were found to increase by up to 10-fold during aeration of A. castellanii cultures; a transient elevation in oxygen was sufficient to induce Δ12-desaturase synthesis that was still fully detectable 1 h later. In addition, the activity of pre-existing Δ12-desaturase, measured in isolated microsomal membranes, increased by up to fivefold with increases in the oxygen concentration of assay mixtures. These results demonstrate for the first time that (i) oxygen availability alone can regulate de novo Δ12-desaturase synthesis in A. castellanii, and that (ii) oxygen can limit the activity of pre-existing Δ12-desaturase. These responses can occur independently of temperature changes.

Effects of Climate Warming on Dissolved Oxygen Concentrations in Lake Erie

Abstract A coupled hydrodynamic and water quality model was used to examine the response of dissolved oxygen concentrations to warming of the central basin of Lake Erie. An area-averaged hydrodynamic model was used to estimate the lake temperatures and thermocline variability as forced by surface heating and winds. Vertical turbulence mixing processes were incorporated by a second-moment, turbulence closure submodel. The water quality model comprised a set of 15 mass balance equations that predicted distributions of phytoplankton biomass, nutrient concentration, and dissolved oxygen. A synthesis of the results from the coupled model forced by climate warming scenarios from three atmospheric general circulation models suggested that there will be a substantial decline in oxygen concentrations in the central basin. Although forecasts of future conditions that are beyond established experiences are uncertain, it appears likely that climate warming will lead to such a decline regardless of details in changes ...

CHEMICAL EXCHANGES BETWEEN SEDIMENTS AND WATER IN THE GREAT LAKES‐SPECULATIONS ON PROBABLE REGULATORY MECHANISMS1

As long as the oxygen concentration at the sediment surface does not fall below 1 or 2 mg/liter, typical deep‐water sediments in the Great Lakes will probably be found to exert a measurable but quantitatively unimportant influence on the chemistry of the overlying waters. This conclusion is supported by the case of Windermere.The sequence of more conspicuous changes that take place when the oxygen does fall below 1 mg/liter at the interface is illustrated by the case of Esthwaite Water, representative of events in lakes where biological production or organic pollution is high, where the subthermocline volume is relatively small, or both. A progressive decline in oxygen concentration from 2 mg/liter to analytical zero at the interface was accompanied by a fall in electrode potential in the upper few millimeters of sediment, correlated with mobilization and transfer into the water first of manganese and later of iron. There is a concurrent transfer into the water of substantial quantities of phosphate, previously held in complex form, which may have important biological consequences. Other changes include liberation into the water of ammonia and silicate. Further reduction of the water‐sediment system permits microbial reduction of sulfate.A third example of sediment‐water exchange occurs during winter stratification under ice cover. In Torneträsk, in the course of 95 days of ice cover, a relatively thin layer of bottom water, initially at 1C, gained heat and bicarbonate from and lost oxygen to the sediments. The density increase, arising from the heat gain, set density currents in motion that drained this contact water into the deepest parts of topographically isolated subbasins.