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Widespread hypoxia occurs seasonally across the Oregon continental shelf, and the duration, intensity, and frequency of hypoxic events have increased in recent years. In hypoxic regions, iron reduction can liberate dissolved Fe(II) from continental shelf sediments. Fe(II) was measured in the water column across the continental shelf and slope on the Oregon coast during summer 2022 using both a trace metal clean rosette and a high‐resolution benthic gradient sampler. In the summer, Fe(II) concentrations were exceptionally high (40–60 nM) within bottom waters and ubiquitous across the Oregon shelf, reflecting the low oxygen condition (40–70 μM) at that time. The observed inverse correlation between Fe(II) and bottom water oxygen concentrations is in agreement with expectations based on previous work that demonstrates oxygen is a major determinant of benthic Fe fluxes. Rapid attenuation of Fe(II) from the benthic boundary layer (within 1 m of the seafloor) probably reflects efficient cross‐shelf advection. One region, centered around Heceta Bank (~ 44°N) acts a hotspot for Fe release on the Oregon continental shelf, likely due to its semi‐retentive nature and high percent mud content in sediment. The results suggest that hypoxia is an important determinant of the inventory of iron is Oregon shelf waters and thus ultimately controls the importance of continental margin‐derived iron to the interior of the North Pacific Basin. [ABSTRACT FROM AUTHOR]

Oxygen is declining globally in subsurface seawater. Fjords are particularly susceptible to hypoxia, because entrance sills restrict water circulation. We studied the oxygen dynamics of the Kitimat Fjord System on the west coast of Canada, using 3 yr of data from moored sensors and water column profiles. Subsurface oxygen within the fjord system is principally controlled by the connection to Hecate Strait on the outer shelf. Variations in oxygen concentration in Hecate Strait are reflected in the water that is drawn into the fjord system year‐round at mid‐depth (50–150 m). Deep water is renewed annually, beginning in May/June. Remineralization results in a seasonal decline in oxygen at depth. In the absence of deep‐water renewal, the bottom water in the system could become hypoxic in 1–2 yr. Mixing over a shallow inner sill (100 m) replenishes the oxygen in the deep water of Gardner Canal year‐round. The decadal trends in oxygen concentration show an increase from the 1950s to a maximum in the late 1970s, followed by a decline through 2016. The decline over the period 1977–2016 is 0.018 mL L−1 yr−1 (~ 0.83 μmol L−1 yr−1), which is consistent with the decline observed over the upper continental shelf over the same period. [ABSTRACT FROM AUTHOR]

Sponges are animals that feed by filtering water through their perforated body. We examined the in situ diel dynamics of sponge metabolism by continuously measuring the oxygen concentrations in the water inhaled and exhaled by undisturbed sponges. A clear daily pattern of oxygen removal was evident for six of the seven species we studied with their nocturnal oxygen removal being almost double the diurnal values (+ 86 ± 57%). Oxygenic photosynthesis by the sponge's symbiotic or endolithic phototrophic microbes may explain some of the diel difference, but significant day–night differences were also observed in three sponge species for which no evidence of photosynthetic activity (tested with imaging pulse‐amplitude‐modulation Fluorometry) was found. Mean oxygen removal (± 95% confidence interval for the mean) per species ranged from 1.7 ± 1 μmol O2 per liter (hereafter: μmol O2 L−1) for the low microbial abundance (LMA) sponge Callyspongia siphonella to 30.5 ± 10.5 μmol O2 L−1 for the high microbial abundance HMA) sponge Theonella swinhoei with considerable variation in oxygen removal across all scales (minutes to hours, within and among specimens). Events of high oxygen removal (> 50 μmol L−1) were regularly observed for five of the seven species and were predominantly nocturnal, occasionally lasting several hours. The high variability in oxygen removal stresses the need for long‐term in‐situ measurements of benthic suspension feeders metabolism. [ABSTRACT FROM AUTHOR]

Marine cable bacteria (Candidatus Electrothrix) and large colorless sulfur‐oxidizing bacteria (e.g., Beggiatoaceae) are widespread thiotrophs in coastal environments but may exert different influences on biogeochemical cycling. Yet, the factors governing their niche partitioning remain poorly understood. To map their distribution and evaluate their growth constraints in a natural setting, we examined surface sediments across seasons at two sites with contrasting levels of seasonal oxygen depletion in Chesapeake Bay using microscopy coupled with 16S rRNA gene amplicon sequencing and biogeochemical characterization. We found that cable bacteria, dominated by a single phylotype closely affiliated to Candidatus Electrothrix communis, flourished during winter and spring at a central channel site which experiences summer anoxia. Here, cable bacteria density was positively correlated with surface sediment chlorophyll, a proxy of phytodetritus sedimentation. Cable bacteria were also present with a lower areal density at an adjacent shoal site which supports bioturbating macrofauna. Beggiatoaceae were more abundant at this site, where their biomass was positively correlated with sediment respiration, but additionally potentially inhibited by sulfide accumulation which was evident during one summer. A springtime phytodetritus sedimentation event was associated with a proliferation of Beggiatoaceae and multiple Candidatus Electrothrix phylotypes, with cable bacteria reaching 1000 m length cm−2. These observations indicate the potential impact of a spring bloom in driving a hot moment of cryptic sulfur cycling. Our results suggest complex interactions between benthic thiotroph populations, with bioturbation and seasonal oscillations in bottom water dissolved oxygen, sediment sulfide, and organic matter influx as important drivers of their distribution. [ABSTRACT FROM AUTHOR]

Continental shelves are a major source of iron (Fe) and manganese (Mn) to marine waters. Here, we investigate controls on benthic release of Fe and Mn and the impact on the water column in the Baltic Sea. We find high in situ benthic release rates of dissolved Fe and Mn at seasonally hypoxic sites (bottom water oxygen between 0-63 μmol L-1) receiving high inputs of organic matter. We find that benthic Fe and Mn release is sensitive to bottom water oxygen concentrations. Benthic Fe release is likely additionally controlled by Fe-sulfur redox chemistry in the surface sediment. For Mn, benthic release correlates positively with Mn oxide availability in the surface sediment. Benthic release contributes to high dissolved Fe and Mn concentrations in the water column and is amplified by repeated cycling of Fe and Mn between the sediment and overlying water through benthic release, oxidation in the water column, deposition as metal oxides, followed by reductive dissolution. Most water column Fe (~ 80%) is present in particulate form near the seafloor. In contrast to Fe, a large percentage of the Mn remains dissolved (~ 50%). We show that easily reducible Fe and Mn oxides are key forms of particulate Fe and Mn in suspended matter. The Baltic Sea represents a highly eutrophic, low oxygen end-member when compared to other modern coastal systems. Our results imply that, upon continued eutrophication and deoxygenation of the coastal ocean, benthic release of dissolved Fe and Mn from continental shelves could become greater than previously thought. [ABSTRACT FROM AUTHOR]

The effects of bottom water oxygen concentration on sediment oxygen uptake, oxygen penetration depth, nitrate and ammonium fluxes, anammox, denitrification, dissimilatory nitrate reduction to ammonium, nitrifi- cation, and mineralization were investigated off the Changjiang estuary and its adjacent East China Sea, by combining a seasonal comparison with three artificially induced bottom water oxygen conditions (oxic, ambient, and severe hypoxia). A 50% decrease in in-situ bottom water oxygen concentrations between May and August, led to decreases in the average sediment oxygen uptake and oxygen penetration depth by 23% and 29%, respectively. Anammox rates decreased by a factor of 2.5, and the relative contribution of anammox to the total benthic N-loss decreased from 20% to 7.4%. However, denitrification rates increased, leading to an overall benthic N-loss rate of 0.85 mmol N m-2 d-1. At the same time, an increasing contribution of dissimilatory nitrate reduction to ammonium to total nitrate reduction led to higher recycling of inorganic nitrogen during hypoxia in August. Under artificially induced conditions of severe hypoxia, there was a sharp decrease in both sediment oxygen uptake and benthic N-loss rates by 88% and 38%, respectively. Nitrate and ammonium fluxes showed complex behavior at different sites which could be related to the repression of sedimentary nitrifi- cation below a bottom water oxygen threshold of 9.7 μM and increasing dissimilatory nitrate reduction to ammonium. Taken together, our results indicate that changes in benthic nutrient cycling under seasonal hypoxia enhance the retention of both organic and inorganic nitrogen, thereby exacerbating oxygen deficiency. [ABSTRACT FROM AUTHOR]

Abstract: Lake water hydrogen (δD) and oxygen (δ18O) isotopic ratios have been widely used to study the hydrologic cycle. In a given region, δD and δ18O values from multiple lakes are often correlated and define a local evaporation line (LEL). The slope of and extension of each lake along this line contain information of many variables with entangled effects of important lake fluxes (e.g., evaporation). We present a new method to extract regional and local (a particular lake) environmental information based on principal component analyses (PCAs) of the δD‐δ18O relationships. Water of up to 30 lakes in West Greenland was sampled every summer for 5 yr, and measured for δD and δ18O values. For a given year, PCA yields values of the first (Enrichment) and second (Deviance) principal components for each lake. In addition, we obtain the slope of the line along the first eigenvector, which is similar, but not identical, to the LEL. We refer to this line as the regional isotopic enrichment line (RIEL), and demonstrate that the RIEL slope changes interannually with the dominant atmospheric circulation patterns affecting regional humidity, specifically the North Atlantic Oscillation and frequency of sea breezes. The Deviance of a lake correlates strongly with its longitude and surface area, both of which influence local humidity and isotopic ratios of vapor. Conversely, Enrichment carries little extractable environmental information because it is controlled by many variables of competing effects. This method can be widely applied to both modern hydrological studies and paleoclimate reconstructions using lake sediments. [ABSTRACT FROM AUTHOR]

We quantified oxygen flux in a coastal stream in Virginia using a novel combination of the conventional open water technique and the aquatic eddy covariance technique. The latter has a smaller footprint (sediment surface area that contributes to the flux; ∼ 10 m2), allowing measurements to be made at multiple sites within the footprint of the open water technique (∼ 1000 m2). Sites included an unvegetated stream pool with cohesive sediment, a macrophyte bed with sandy sediment, and an unvegetated sand bed with rippled bedforms. Nighttime eddy covariance oxygen uptake was always smaller than uptake produced by the open water technique. At the pool and unvegetated sand bed sites, nighttime eddy covariance uptake was 20-fold smaller than open water uptake. At the macrophyte bed site, gross primary production quantified with the two techniques was similar but eddy covariance uptake was 2.4-fold smaller. The difference in oxygen uptake between eddy covariance and open water techniques could not be accounted for by uncertainties in the gas transfer velocity but could be accounted for by anoxic groundwater inflow through stream banks outside of the eddy covariance footprint. Nighttime oxygen uptake was also measured with eddy covariance in a tidal freshwater part of the stream, where pore space in the sandy sediment near the sediment-water interface was flushed with stream water at peak water velocities. As a result of this advective hyporheic exchange, nighttime oxygen flux increased fourfold with a doubling of water velocity. [ABSTRACT FROM AUTHOR]

Many lakes have positive, heterograde vertical oxygen (O2) profiles with a metalimnetic maximum usually assumed to be the result of biological O2 production. However, supersaturated metalimnetic O2 maxima are formed by biological processes (net photosynthetic production of O2) and physical processes (warming of gasses trapped below the thermocline). Although the mechanisms contributing to supersaturated metalimnetic O2 peaks are understood, the contribution of biological vs. physical processes is not well known in lakes. To examine these contributions, we measured O2 and argon (Ar) saturation anomalies in the metalimnia of 17 lakes. Unlike O2, Ar is biologically inert and, therefore, can be used to quantify physical processes. There was a positive Ar anomaly at the depth of the O2 maximum in every lake. On average, only 14% of the O2 maximum could be attributed solely to biological production of O2, but this percentage varied widely among lakes. Additionally, in a subset of lakes, the relative saturation of O2 at the metalimnetic maximum was lower than Ar due to net biological consumption, creating a weaker O2 maximum than would be expected based on the physical processes alone. Some lakes were sampled multiple times during summer and net ecosystem production (NEP) was also calculated. There were many instances of positive NEP in the metalimnion; however, net autotrophy was usually transient instead of persistent. Overall, biological production of O2 alone is not responsible for metalimnetic O2 maxima as both physical and biological processes contribute substantially to the formation and persistence of O2 maxima in lakes. [ABSTRACT FROM AUTHOR]

We used δ15N-nitrate () and δ18O-to unravel the provenance and fate of in a groundwater-fed estuary. A total of 13 monthly and two time series surveys were undertaken in the Werribee River estuary near Melbourne, Australia. The different survey timescales provided a comprehensive evaluation of the hydrological effects on the biogeochemistry of, which accounted for tidal variability and episodic runoff events. The distribution of δ15N and δ18O of the estuarine along a mixing line between shallow and deep groundwater provided strong evidence for the predominance of groundwater-derived in the estuary. During dry periods when water residence time in the estuary was extended, shallow groundwater contributed 60% to 76% of the (calculated from δ15N- and δ18O-) to the estuary, and assimilation removed ~70% of this groundwater-derived . During wet periods, deep groundwater provided more (62%) than shallow groundwater, and there was no indication of consumption. Occasional sources of, which were also reflected by their δ15N- and δ18O- values, included from nitrification and sewage-derived particularly at the bay entrance to the estuary. Greater emphasis should be placed upon the role of groundwater as a substantial end member when assessing biogeochemistry using methods relying on the dual isotopic composition of. [ABSTRACT FROM AUTHOR]

The widely reported paradox of methane oversaturation in oxygenated water challenges the prevailing paradigm that microbial methanogenesis only occurs under anoxic conditions. Using a combination of field sampling, incubation experiments, and modeling, we show that the recurring mid-water methane peak in Lake Stechlin, northeast Germany, was not dependent on methane input from the littoral zone or bottom sediment or on the presence of known micro-anoxic zones. The methane peak repeatedly overlapped with oxygen oversaturation in the seasonal thermocline. Incubation experiments and isotope analysis indicated active methane production, which was likely linked to photosynthesis and/or nitrogen fixation within the oxygenated water, whereas lessening of methane oxidation by light allowed accumulation of methane in the oxygen-rich upper layer. Estimated methane efflux from the surface water was up to 5 mmol m-2d-1 . Mid-water methane oversaturation was also observed in nine other lakes that collectively showed a strongly negative gradient of methane concentration within 0-20% dissolved oxygen (DO) in the bottom water, and a positive gradient within ≥ 20% DO in the upper water column. Further investigation into the responsible organisms and biochemical pathways will help improve our understanding of the global methane cycle. [ABSTRACT FROM AUTHOR]

The concentration of oxygen in the deep Strait of Georgia has declined at a rate of 0.45-1.3 µmol L-1yr-1 since 1971 and now seasonally approaches thresholds of biological tolerance. The decline has resulted principally from the increasing hypoxia of upwelled Pacific Ocean water. Local anthropogenic loadings have little potential to reduce bottom water oxygen concentrations. Hypoxic, upwelled water mixes vigorously with surface water in the tidal passages of Haro Strait before entering the deep Strait of Georgia during a series of deep-water renewal events in late spring and late summer. Remineralization of at least 22 µmol L-1yr-1, together with diffusive mixing, reduces the bottom water oxygen concentration to a winter minimum of 90-110 µmol L-1. Linear extrapolation of the long-term trend indicates that parts of the Strait could become episodically hypoxic (<62.5 µmol L-1 ; < 1.4 mL L-1) as early as 2042. However, water mass modeling shows that the mixing with surface water in Haro Strait limits the potential of the shelf water to reduce the oxygen concentration in the deep Strait; even should the shelf water become completely anoxic, the concentration of oxygen in bottom waters would level off just above 90 µmol L-1 after 3 yr. Increasing surface water temperature will reduce the solubility of oxygen, but this effect is projected to cause a further decline of only 2.2 µmol L-1 over the next 25 yr. Despite its restricted circulation, the deep Strait of Georgia is less threatened by hypoxia than are many other coastal areas, because of the combination of light-limited primary production and intense tidal mixing in the approaches, which replenishes the deep-water oxygen annually. [ABSTRACT FROM AUTHOR]