Bottom Water Oxygen Concentrations Research Articles

Cable Bacteria Activity Modulates Arsenic Release From Sediments in a Seasonally Hypoxic Marine Basin.

Eutrophication and global change are increasing the occurrence of seasonal hypoxia (bottom-water oxygen concentration <63 μM) in coastal systems worldwide. In extreme cases, the bottom water can become completely anoxic, allowing sulfide to escape from the sediments and leading to the development of bottom-water euxinia. In seasonally hypoxic coastal basins, electrogenic sulfur oxidation by long, filamentous cable bacteria has been shown to stimulate the formation of an iron oxide layer near the sediment-water interface, while the bottom waters are oxygenated. Upon the development of bottom-water anoxia, this iron oxide “firewall” prevents the sedimentary release of sulfide. Iron oxides also act as an adsorption trap for elements such as arsenic. Arsenic is a toxic trace metal, and its release from sediments can have a negative impact on marine ecosystems. Yet, it is currently unknown how electrogenic sulfur oxidation impacts arsenic cycling in seasonally hypoxic basins. In this study, we presented results from a seasonal field study of an uncontaminated marine lake, complemented with a long-term sediment core incubation experiment, which reveals that cable bacteria have a strong impact on the arsenic cycle in a seasonally hypoxic system. Electrogenic sulfur oxidation significantly modulates the arsenic fluxes over a seasonal time scale by enriching arsenic in the iron oxide layer near the sediment-water interface in the oxic period and pulse-releasing arsenic during the anoxic period. Fluxes as large as 20 μmol m−2 day−1 were measured, which are comparable to As fluxes reported from highly contaminated sediments. Since cable bacteria are recognized as active components of the microbial community in seasonally hypoxic systems worldwide, this seasonal amplification of arsenic fluxes is likely a widespread phenomenon.

Characterization of dissolved organic matter processing between surface sediment porewater and overlying bottom water in the Yangtze River Estuary

Dissolved organic matter (DOM) exchange in the sediment-water interface of estuaries is essential for the global elemental cycle. To clarify the interface DOM processing, this study applies optical techniques and ultrahigh-resolution mass spectrometry to assess DOM composition of surface sediment porewater and bottom (overlying) water across the Yangtze River Estuary (YRE). Results suggested that DOM exchange in the sediment-water interface mainly followed from sediment porewater to bottom water driven by a significant dissolved organic carbon concentration gradient and hydrodynamic force. We also characterized two porewater DOM sources, including microbial production and byproducts of processed sediments. High microbial activities resulted in the enrichment of protein-like fluorescent components and N-bearing compounds in porewater, potentially decreasing the oxygen concentration of bottom water due to the high lability. And the deamination of N-bearing compounds in the sediment-water interface could likely serve as a N-bearing nutrient source to bottom water. Moreover, due to sediment-specific features in different areas driven by hydrologic sorting and local phytoplankton supply, porewater DOM of muddy areas accumulated more aromatic substances from the degradation of terrestrial organic matter. The release and oxic transformation of oxygen-deficient aromatic compounds could contribute to the refractory carbon pool of estuarine water (carboxyl-rich alicyclic molecules, CRAM), modulating the quality of organic carbon mobilized from the land to the coastal ocean. Considering strong hydrodynamic force in numerous estuaries worldwide, DOM exchange and processing at the sediment-water interface has a meaningful influence on the biogeochemistry of estuarine water columns, which warrants further studies.

Bathymetric and regional benthic foraminiferal distribution on the Espírito Santo Basin slope, Brazil (SW Atlantic)

To understand the main environmental factors controlling the benthic foraminiferal distribution along continental slopes, and corroborate paleoenvironmental interpretations, we investigated benthic foraminiferal assemblages (total fauna >63 μm) and sedimentological data along six transects on the continental slope of the Espírito Santo Basin (ESB) between 18°20′ and 21°20′ S. Surface sediment samples (0–2 cm) were collected perpendicularly to the coast, from the upper (400 m) to the lower slope (3000 m). The density, taxonomic diversity and assemblage composition change with depth. The genus Globocassidulina dominates the upper and lower slope, whereas Bolivina is the most abundant taxon in the deepest lower slope. The differences in ecological preferences among the most abundant taxa allowed us to recognize five benthic foraminiferal groups, distributed in three main bathymetric sectors: upper, middle – lower, and lower slope. Group I (upper slope, 400 m) is characterized by the highest mean relative abundances of Globocassidulina rossensis, Trifarina spp. and Trifarina angulosa. Group II (middle – lower slope, 1000–1300 m) shows the highest mean relative abundances of Globocassidulina subglobosa, Bolivina albatrossi, Bulimina aculeata and Uvigerina peregrina. Group III (middle – lower slope, 1000–1300 m) is characterized by the highest mean relative abundances of Globocassidulina crassa, Gavelinopsis versiformis, Epistominella exigua and the unilocular group. Group IV (lower slope, 1900 m) shows the highest mean relative abundances of Alabaminella weddellensis, Bolivina inflata and E. exigua. Group V (lower slope, 2500–3000 m) is dominated by the genus Bolivina (B. lowmani, B. pseudoplicata and Bolivina spp). The most abundant taxa along the ESB slope are ecologically associated to the organic matter flux, bottom water oxygen concentration, and hydrodynamics conditions. The quantity and quality of food supply is the primary factor controlling the distribution of benthic foraminiferal assemblages along depth. Secondary factors include the properties of water masses, and intermediate and deep boundary currents. The increased fragmentation of benthic and planktic foraminifera tests (>125 μm) on the upper and middle – lower slope is related to abrasion during transport from shallower regions to the slope.

Global Change Modulated Asian Inland Climate Since 7.3 Ma: Carbonate Manganese Records in the Western Qaidam Basin

Late Cenozoic drying of the Asian inland has not only exerted a profound impact on the regional environment but also affected global climate as an important source of global atmospheric dust. Continuous and accurately dated sediment records from the Asian interior are pivotal to a better understanding of the evolutionary history of Asian inland drying and the associated driving mechanisms. In this study, we present a continuous record of climate change in the Asian interior spanning the past 7.3 Myr, reconstructed by the redox evolution of a paleolake in the western Qaidam Basin, NE Tibetan Plateau. The paleolake redox conditions are linked to the oxygen concentration of lake bottom water and lake level, and were revealed by the manganese (Mn) concentration in the carbonate fraction (leached by the diluted acetic acid) of the carbonate-rich lacustrine sediments retrieved from two drill-cores (SG-1 and SG-1b). The reconstructed regional climate in the western Qaidam Basin shows long-term fluctuations, consistent with the secular evolution of the coeval global climate, especially the sea surface temperature variation in the high latitude North Atlantic. Three transitions of the paleolake hydrochemical system occurred at 6.2, 5.3, and 2.6 Ma, with a short drying stage at 6.2–5.3 Ma and prolonged Quaternary drying since 2.6 Ma. We argue that drying of the Asia interior has been dominantly forced by global cooling, in particular, the high-latitude cooling of the Northern Hemisphere.

Sedimentary Organic Matter Shapes Macrofaunal Communities but Not Benthic Nutrient Fluxes in Contrasting Habitats Along the Northwest Atlantic Continental Margin

The heterogeneous topography of continental margins can influence patterns of resource availability and biodiversity in deep-sea sediments, potentially altering ecosystem functioning (e.g., organic matter remineralization). Noting a lack of studies that address the latter, we contrasted spatial patterns and drivers of benthic nutrient fluxes and multiple characteristics of macrofaunal communities in shelf, slope, canyon and inter-canyon sedimentary habitats along the Northwest Atlantic continental margin. Replicate sediment push cores were collected from 10 stations (229–996 m depth), incubated for ∼48 h to estimate fluxes of nitrate, nitrite, ammonium, phosphate, and silicate (as a measure of organic matter remineralization) and subsequently analyzed to characterize macrofaunal communities. We also considered various environmental factors, including sedimentary organic matter quantity and quality, and assessed their influence on fluxes and macrofauna. Comparatively high macrofaunal density and distinct community composition and trait expression characterized Georges Canyon, where elevated sedimentary organic matter suggested important lateral transport mechanisms along this canyon axis, with deposition of organic matter strongly affecting biological communities but not benthic nutrient fluxes. Lower penetration of macrofauna into the sediments, distinct community composition, biological traits, and higher nutrient flux rates characterized inter-canyon habitats compared to slope habitats at similar depths. Within inter-canyons, intermediate to low organic matter suggested hydrodynamic forces inhibiting organic matter deposition, affecting biological and functional processes. The input of fresh phytodetritus to the seafloor was the best predictor of macrofaunal density and diversity and contributed to variation in macrofaunal community composition and biological trait expression, together with latitude, depth, and other measures of organic matter quantity and quality. Benthic nutrient fluxes revealed complex variation, with disproportionate effects of few key macrofaunal taxa, together with bottom water oxygen concentration, and sediment granulometry. Our results suggest a relationship between resource availability and macrofaunal density, diversity, and taxonomic and trait composition, whereas organic matter remineralization exhibited a more complex response, which we suggest reflected variation in hydrodynamics and/or physical disturbance in heterogeneous continental margin habitats.

Linkages Between East China Sea Deep‐Sea Oxygenation and Variability in the East Asian Summer Monsoon and Kuroshio Current Over the Last 400,000 years

AbstractThe East China Sea (ECS) seasonally receives a high organic input due to the terrestrial organic matter influx, which is controlled by the East Asian Summer Monsoon (EASM), and the increased productivity driven by upwelling of the subsurface Kuroshio Current (KC). Changes in benthic foraminiferal assemblage composition in combination with paleoceanographic proxy data (CaCO3 (%), TOC (%), δ13Cpf, and δ18Obf) are used to reconstruct bottom water oxygenation and organic export flux variability over the last 400 kyr in the ECS. Multivariate analyses of benthic foraminiferal census data identified six biofacies characteristic of varying environmental conditions. These results suggest that enhanced EASM precipitation and KC upwelling directly influenced organic export flux and bottom water oxygen content in the ECS. The ECS bottom water was suboxic during Marine Isotope Stage (MIS) 11 to 8; suboxic to dysoxic between MIS 7 and 6, strongly dysoxic between mid‐MIS 5 and 4, and exhibited high variability between MIS 3 and 1. Spectral analysis of relative abundances of representative genera Quinqueloculina (oxic), Bulimina (suboxic), and Globobulimina (dysoxic) reveals a robust 23 kyr signal, which we attribute to precessionally‐paced changes in surface productivity and bottom water oxygenation related to KC variability over the past 400 kyr.

Abyssal Benthic Rover, an autonomous vehicle for long-term monitoring of deep-ocean processes.

The deep-ocean carbon cycle is poorly quantified. An abyssal benthic rover was developed to make long time-series measurements of seafloor processes related to organic carbon remineralization and sequestration. Benthic Rover II (BR-II) is an autonomous dual-tracked vehicle that measures bottom water temperature and oxygen concentration, current velocity, and sediment community oxygen consumption (SCOC; respiration). BR-II is programmed to transit with low surface-contact pressure across the seafloor, photograph bottom conditions, and stop regularly to occupy respirometer incubation sites, with deployment periods up to 1 year. Now, continuously operational at a 4000-m station in the northeast Pacific over 5 years, substantial weekly, seasonal, annual, and episodic events have been recorded, which are critical to assessing the deep-ocean carbon cycle. There was a significant increase in phytodetritus cover (P < 0.01) arriving on the seafloor from the overlying water column between 2015 and 2020 that was negatively correlated with bottom water dissolved oxygen concentration (P < 0.01). Over the continuous 5-year monitoring period from November 2015 to November 2020, SCOC was positively correlated with phytodetritus cover (P < 0.01) and increased significantly from 2015 to 2020 (P < 0.01). These results show important influences of biological processes on the carbon cycle. The demonstrated success of BR-II now creates opportunities to expand the long-term monitoring of the deep sea to resolve the coupling of water column and benthic processes key to understanding the oceanic carbon cycle on a planet engulfed in a changing climate.

Geothermal heating and episodic cold-seawater intrusions into an isolated ridge-flank basin near the Mid-Atlantic Ridge

Six-year records of ocean bottom water temperatures at two locations in an isolated, sedimented deep-water (∼4500 m) basin on the western flank of the mid-Atlantic Ridge reveal long periods (months to >1 year) of slow temperature rises punctuated by more rapid (∼1 month) cooling events. The temperature rises are consistent with a combination of gradual heating by the geothermal flux through the basin and by diapycnal mixing, while the sharper cooling events indicate displacement of heated bottom waters by incursions of cold, dense bottom water over the deepest part of the sill bounding the basin. Profiles of bottom water temperature, salinity, and oxygen content collected just before and after a cooling event show a distinct change in the water mass suggestive of an incursion of diluted Antarctic Bottom Water from the west. Our results reveal details of a mechanism for the transfer of geothermal heat and bottom water renewal that may be common on mid-ocean ridge flanks.

Trace Metal Evidence for Deglacial Ventilation of the Abyssal Pacific and Southern Oceans

AbstractThe deep ocean has long been recognized as the reservoir that stores the carbon dioxide (CO2) removed from the atmosphere during Pleistocene glacial periods. The removal of glacial atmospheric CO2 into the ocean is likely modulated by an increase in the degree of utilization of macronutrients at the sea surface and enhanced storage of respired CO2 in the deep ocean, known as enhanced efficiency of the biological pump. Enhanced biological pump efficiency during glacial periods is most easily documented in the deep ocean using proxies for oxygen concentrations, which are directly linked to respiratory CO2 levels. We document the enhanced storage of respired CO2 during the Last Glacial Maximum (LGM) in the Pacific Southern Ocean and deepest Equatorial Pacific using records of deglacial authigenic manganese, which form as relict peaks during increases in bottom water oxygen (BWO) concentration. These peaks are found at depths and regions where other oxygenation histories have been ambiguous, due to diagenetic alteration of authigenic uranium, another proxy for BWO. Our results require that the entirety of the abyssal Pacific below approximately 1,000 m was enriched in respired CO2 and depleted in oxygen during the LGM. The presence of authigenic Mn enrichment in the deep Equatorial Pacific for each of the last five deglaciations suggests that the storage of respired CO2 in the deep ocean is a ubiquitous feature of late‐Pleistocene ice ages.

Proxies for paleo-oxygenation: A downcore comparison between benthic foraminiferal surface porosity and I/Ca

Benthic foraminiferal surface porosity (the mean percentage of surface area covered by pores; higher porosity: lower oxygenation) and iodine to calcium ratio (I/Ca, higher I/Ca: higher oxygenation) are both promising paleoceanographic proxies that will advance through testing in down-core studies. Here we report the first down-core comparison (~45 kyr) of these proxies for a core from the southern Brazilian margin (26°40.22′ S, 46°26.46′ W, 475 m water depth). Both proxies are most sensitive to low-O2 conditions (<50 μmol/kg), and not well-constrained at higher O2 concentrations. Porosity values are generally low (<15%) and I/Ca ranges between ~4 and ~ 6 μmol/mol throughout the core. The two proxies are overall consistent, suggesting that bottom-water oxygen concentrations at the site remained above 50 μmol/kg during the last 45 kyr. Several non-O2 factors (e.g., iodate reduction rates, water mass mixing, temperature, foraminiferal shell robustness) could influence the proxies and require further investigation.

Multi-trophic level responses to environmental stressors over the past ~150 years: Insights from a lake-rich region of the world

Multiple stressors affect water quality and biodiversity in lakes worldwide. However, our understanding of which combinations of stressors are of greatest impact and how lakes have shifted from their pre-industrial baselines is fragmented. Questions remain regarding how multiple trophic groups are affected by global change stressors and whether region-specific differences need to be considered. Here, we apply multiple factor analyses together with latent variable modeling to quantify potential interactions within and among taxa that span multiple trophic levels (i.e. diatoms, cladocerans and chironomids) to improve our understanding of paleo-environmental dynamics from a suite of Canadian lakes spanning four ecozones: the Boreal Shield, the Mixedwood Plains, the Atlantic Highlands, and the Atlantic Maritimes. Across all ecozones, multi-trophic assemblages were distinguished in a multiple factor analysis along a land-use gradient, with mesotrophic/eutrophic diatoms and profundal chironomids tolerant of low bottom-water oxygen concentrations recorded in more disturbed sites. Functional units across three indicator groups formed distinct networks of co-responses to key environmental and land-use gradients, although cladocerans seemed to be driven by additional (i.e., residual) gradients. Applying a temporal beta-diversity approach between modern and pre-industrial assemblages, we detected significantly greater turnover of diatom functional groups in high versus low human impact sites (p = 0.03, n = 57), with similar but non-significant trends apparent with the chironomid and cladoceran groups. Ecozonal differences in temporal turnover were also evident in diatom (p = 0.003, n = 57) and chironomid functional groups (p = 0.04, n = 41). The ecozonal differences may be partially due to differential sensitivities of the pre-industrial assemblages and may also be driven by historical disturbances. Our work highlights how the magnitude and direction of ecological change vary across ecozones and can modify responses to human impacts, with a general trend of higher species turnover in sites with greater human disturbance.

Polycyclic Aromatic Hydrocarbons Have Adverse Effects on Benthic Communities in the Baltic Sea: Implications for Environmental Status Assessment

Changes in benthic macrofaunal communities are indicative of environmental stressors, including eutrophication and hypoxia. However, some species are sensitive not only to hypoxia but also to various environmental contaminants. We tested which of the environmental predictors (sediment organic carbon, sediment concentrations of metals and polyaromatic hydrocarbons [PAHs], bottom water oxygen, salinity, temperature, and surface chlorophyll-a concentration) that best explained the following response variables: (1) macrofauna community composition, (2) abundance of a benthic sentinel species, the amphipod Monoporeia affinis; and (3) the Benthic Quality Index (BQI). All data originated from 29 reference monitoring stations in the Baltic Sea and the statistical tests included both uni- and multivariate analyses. The community composition and BQI were best explained by the same combination of salinity, depth, temperature and PAH concentrations. The abundance of M. affinis, which is sensitive to hypoxia and chemical exposure, was best explained by PAHs as a single predictor. Our findings suggest that benthic communities in the Baltic Sea are influenced by anthropogenic contaminants, which should be taken into account when benthos is used for eutrophication status assessment.

Phosphorus cycling from a coastal upwelling zone in the Southeastern Arabian Sea.

The present study examined the seasonal cycling of phosphorous (P) along the southwest coast of India (SWCI) based on two cruises during the southwest monsoon (SWM) and northeast monsoon (NEM) of 2018. During SWM, the entire SWCI experienced intense upwelling manifested by the incursion of cold, nutrient-rich, and hypoxic waters. During NEM, the region was transformed into a warm, well-oxygenated and nutrient-deplete environment. Dissolved inorganic phosphorus (DIP) was significantly high in the subsurface during SWM due to its release from sediments. The sediment P was high and showed an increasing trend towards the south, principally dependent on the sediment texture, organic carbon, and Fe concentrations. Bioavailable P, the sum of exchangeable (PEx) and reducible (PFe) fractions, was almost consistent (5-20%) over seasons, though PFe showed a marked reduction during SWM. Authigenic fraction (PAut) was the most dominant (46%), followed by detrital (PDet 41%) and residual (PRes 8%) fractions. Principal component analysis (PCA) of geochemical parameters for SWM was indicative of the high dissolution of Fe (oxy)hydroxides under hypoxia releasing P and its complexation with organic matter and Fe. PCA results for the NEM were different, as it indicated increased preservation of P-associated organic matter and Fe, alternately favouring the formation of PAut in sediments. The study's significance is the observation that the bottom water oxygen concentration can significantly influence sedimentary P cycling in tropical coastal upwelling zones.

Appraisal of sedimentary alkenones for the quantitative reconstruction of phytoplankton biomass

Marine primary productivity (PP) is the driving factor in the global marine carbon cycle. Its reconstruction in past climates relies on biogeochemical proxies that are not considered to provide an unequivocal signal. These are often based on the water column flux of biogenic components to sediments (organic carbon, biogenic opal, biomarkers), although other factors than productivity are posited to control the sedimentary contents of the components, and their flux is related to the fraction of export production buried in sediments. Moreover, most flux proxies have not been globally appraised. Here, we assess a proxy to quantify past phytoplankton biomass by correlating the concentration of C37 alkenones in a global suite of core-top sediments with sea surface chlorophyll-a (SSchla) estimates over the last 20 y. SSchla is the central metric to calculate phytoplankton biomass and is directly related to PP. We show that the global spatial distribution of sedimentary alkenones is primarily correlated to SSchla rather than diagenetic factors such as the oxygen concentration in bottom waters, which challenges previous assumptions on the role of preservation on driving concentrations of sedimentary organic compounds. Moreover, our results suggest that the rate of global carbon export to sediments is not regionally constrained, and that alkenones producers play a dominant role in the global export of carbon buried in the seafloor. This study shows the potential of using sedimentary alkenones to estimate past phytoplankton biomass, which in turn can be used to infer past PP in the global ocean.

Persistent oxygen depletion of bottom waters caused by methane seepage: Evidence from the South China Sea

Seepage of methane and oil is widespread at active and passive continental margins worldwide. Large-scale methane release may have caused bottom water oxygen depletion and acidification in the geological past, but direct evidence of low-oxygen conditions caused by methane leakage in natural systems is scarce. Here we report on the geochemical composition of sediments of two deepwater seep sites (>1700 m) from the South China Sea. The δ34S values of chromium reducible sulfur (δ34SCRS), δ13C values of total inorganic carbon (δ13CTIC), total sulfur (TS) and total organic carbon (TOC) contents, as well as redox-sensitive elements (Fe, U, and Mn) are used to constrain the intensity of fluid seepage and its impact on bottom water redox conditions. Sediment horizons affected by methane seepage are characterized by relatively high TS/TOC ratios (>1.4), low δ13CTIC values (<−27‰), and positive δ34S values (>22‰). The identified horizons show elevated authigenic U contents, high U/Th, as well as low Mn/Al ratios, suggesting oxygen depletion during their deposition. It is shown that local oxygen concentration of bottom waters was related to methane flux, with periods of larger methane release initiating during the Last Glacial Maximum. Much oxygen was apparently consumed during times of intense methane seepage, resulting in low oxygen conditions of bottom waters – conditions that persisted for ten thousand years and more. Our observations suggest that the interplay of flow intensity and microbial processes at seeps can locally affect the oxygen concentration in the deep ocean. The impact of microbial methane consumption on the oxygen concentration in the deep ocean should consequently be considered when looking into potential effects of future warming of the ocean, as more gas hydrate is susceptible to destabilization during ocean warming.

Seasonal dynamics of the activities of dissolved 137Cs and the 137Cs of fish in a shallow, hypereutrophic lake: Links to bottom-water oxygen concentrations

Remobilization of radiocesium from anoxic sediments can be an important mechanism responsible for long-term contaminations of lakes. However, it is unclear whether such remobilization occurs in shallow lakes, where concentrations of dissolved oxygen in the hypolimnion (bottom DO) change temporally in response to meteorological conditions, and whether remobilized radiocesium influences the activity in fish. We examined the seasonal dynamics of the activities of dissolved 137Cs and 137Cs in fish (pond smelt and crucian carp) from Lake Kasumigaura, a shallow, hypereutrophic lake, five years after the Fukushima Daiichi Nuclear Power Plant accident. The activities of both dissolved 137Cs and 137Cs in fish declined during that time, but the declines showed a clear seasonal pattern that included a summer peak of 137Cs activity. The activity of dissolved 137Cs increased when the bottom DO concentration decreased, and a nonlinear causality test revealed significant causal forcing of dissolved 137Cs activity by bottom DO. The fact that NH4-N concentrations in bottom waters were higher in the summer suggested that remobilization of 137Cs from sediments could result from highly selective ion-exchange with NH4-N. Despite the shallow depth of Lake Kasumigaura, winds had little influence bottom DO concentrations or dissolved 137Cs activities. The fact that seasonal means of 137Cs activities in pond smelt and crucian carp were positively correlated with the seasonal means of dissolved 137Cs activities suggested that remobilized 137Cs may have influenced the seasonal dynamics of radiocesium in fish through food-chain transfer, but higher feeding rates in warm water could may have also contributed to the seasonal dynamics of 137Cs activity in fish. Our findings suggest that in shallow lakes, intermittent but repeated hypoxic events may enhance remobilization of radiocesium from sediments, and remobilized radiocesium may contributed to long-term retention of radiocesium in aquatic organisms.

Coastal hypoxia and eutrophication as key controls on benthic release and water column dynamics of iron and manganese

AbstractContinental 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.

Continental margin sediments underlying the NE Pacific oxygen minimum zone are a source of nitrous oxide to the water column

AbstractContinental margin sediments are important sites of marine nitrogen cycling and potential contributors to atmospheric N2O emissions. We employed trace‐level N2O microsensors to measure vertical N2O profiles at submillimeter resolutions in intact cores from outer continental margin sediments underlying the NE Pacific oxygen minimum zone. We used mathematical modeling to estimate depth‐dependent rates of N2O production and fluxes to the overlying water along a transect of diminishing bottom water oxygen concentrations. Net sediment efflux was observed at all sites on the outer continental margin, with a mean value of 524 nmol m−2 d−1. N2O efflux increased with decreased oxygen penetration depth in sediments. Enhanced N2O production and efflux were obtained when outer continental shelf sediments were experimentally exposed to lower bottom‐water O2 concentrations, to simulate upwelling conditions. Our results underline the need for further investigation of the drivers of N2O production in continental margin sediments, and the relative importance of these environments to the global N2O budget.

Response of benthic nitrogen cycling to estuarine hypoxia

AbstractThe effects of bottom water oxygen concentration on sediment oxygen uptake, oxygen penetration depth, nitrate and ammonium fluxes, anammox, denitrification, dissimilatory nitrate reduction to ammonium, nitrification, 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 nitrification 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.

Effects of hypolimnetic oxygenation on fish tissue mercury in reservoirs near the new Almaden Mining District, California, USA.

Almaden, Calero, and Guadalupe reservoirs (San Jose, CA, USA) are small (<13 million m3) surface water reservoirs polluted by the former New Almaden Mining District, North America's most productive historical mercury (Hg) mine. Stevens Creek Reservoir (Cupertino, CA, USA) also has elevated fish Hg concentrations, but no historical mining source. We report a 15-year dataset to evaluate the effectiveness of line diffuser hypolimnetic oxygenation systems (HOSs) in reducing methylmercury (MeHg) concentrations in reservoir water and fish after four consecutive years of operation. HOSs were installed in each reservoir to increase dissolved oxygen concentrations in bottom water, thereby suppressing the activity of anaerobic bacteria (e.g., sulfate-reducing bacteria) known to produce MeHg. Before HOS operation, MeHg concentrations increased in bottom waters of all four reservoirs during periods of thermal stratification and profundal hypoxia. MeHg concentrations decreased significantly in bottom waters during HOS operation, with mean reductions of 63%-85% below pre-oxygenation concentrations. However, MeHg concentrations were unchanged or increased in surface waters. This could be the result of enhanced mixing between surface and bottom waters as a result of line diffuser oxygenation, or continued Hg methylation occurring in the oxic water column and littoral sediments. Despite little change in whole water column MeHg concentrations, we observed modest but significant declining trends in fish tissue Hg in Guadalupe and Stevens Creek reservoirs. Results suggest that oxygenation, rather than directly lowering MeHg in water, may have mixed nutrients into surface waters, thereby enhancing primary productivity and indirectly affecting Hg bioaccumulation by diluting concentrations in phytoplankton.