Bottom Oxygen Research Articles

Using Timescales of Deficit and Residence to Evaluate Near‐Bottom Dissolved Oxygen Variation in Coastal Seas

AbstractWe identify the local, vertical and lateral processes that cause bottom oxygen variation, and characterize timescales for each process using oxygen budget analysis based on a coupled physical‐biogeochemical model for the coastal seas in east China. Local oxygen deficit often occurs in the summer season due to the faster local consumption than the vertical replenishment in seasonal timescale. Lateral transport of oxygen‐rich ambient water replenishes local deficit of bottom dissolved oxygen. Competition between local deficit and lateral exchange determines seasonal hypoxia formation and sustainment. Short local consumption timescale is favorable for hypoxia formation, and transient hypoxia often forms when the local deficit and lateral exchange processes act on comparable timescales, such as the East China Sea in which the bottom hypoxia usually lasts for days. Extremely long lateral exchange timescale suggests that dissolved oxygen variation is predominantly controlled by local processes, and short local consumption timescale often causes increasingly severe seasonal hypoxia until the onset of the relaxation of local deficit, such as the bottom hypoxia in the Bohai Sea. Using these timescales to evaluate local deficit relative to vertical and lateral residence time and their variability is a convenient and potentially powerful general mechanistic framework to evaluate strategies to mitigate coastal hypoxia worldwide.

Seasonal evolution and controlling factors of bottom oxygen depletion in the Bohai Sea

A coupled physical-biogeochemical model is used to investigate the seasonal evolution and controlling factors of oxygen depletion in the Bohai Sea (BS). Comparisons show that the model reproduces observed spatiotemporal variations of important physical and biogeochemical variables well. Bottom oxygen in the BS shows an annual cycle with significant drawdown in summer and enhanced replenishment in fall. Two oxygen-depleted regions off Qinhuangdao (QHD) and the Yellow River estuary (YRE) develop separately and experience higher oxygen depletion rates and longer durations of low-oxygen conditions. The evolution of oxygen depletion is primarily controlled by stratification and biological oxygen consumption but is also modulated by lateral transport. Strong stratification is established earlier than oxygen depletion and maintains its development. The biological oxygen consumption determines the two oxygen-depleted regions under stratified conditions. Lateral transport influenced by anticyclonic circulations favors an expansion of oxygen depletion off QHD but alleviates oxygen depletion off the YRE.

Air-Sea Gas Fluxes and Remineralization From a Novel Combination of pH and O2 Sensors on a Glider

Accurate, low-power sensors are needed to characterize biogeochemical variability on underwater glider missions. However, the needs for high accuracy and low power consumption can be difficult to achieve together. To overcome this difficulty, we integrated a novel sensor combination into a Seaglider, comprising a spectrophotometric lab-on-a-chip (LoC) pH sensor and a potentiometric pH sensor, in addition to the standard oxygen (O2) optode. The stable, but less frequent (every 10 min) LoC data were used to calibrate the high-resolution (1 s) potentiometric sensor measurements. The glider was deployed for a 10-day pilot mission in August 2019. This represented the first such deployment of either type of pH sensor on a glider. The LoC pH had a mean offset of +0.005±0.008 with respect to pH calculated from total dissolved inorganic carbon content, c(DIC), and total alkalinity, AT, in co-located water samples. The potentiometric sensor required a thermal-lag correction to resolve the pH variations in the steep thermocline between surface and bottom mixed layers, in addition to scale calibration. Using the glider pH data and a regional parameterization of AT as a function of salinity, we derived the dissolved CO2 content and glider c(DIC). Glider surface CO2 and O2 contents were used to derive air-sea fluxes, Φ(CO2) and Φ(O2). Φ(CO2) was mostly directed into the ocean with a median of −0.4 mmol m–2 d–1. In contrast, Φ(O2) was always out of the ocean with a median of +40 mmol m–2 d–1. Bottom water apparent oxygen utilization (AOU) was (35±1) μmol kg–1, whereas apparent carbon production (ACP) was (11±1) μmol kg–1, with mostly insignificant differences along the deployment transect. This deployment shows the potential of using pH sensors on autonomous observing platforms such as Seagliders to quantify the interactions between biogeochemical processes and the marine carbonate system at high spatiotemporal resolution.

Benthic Foraminiferal Response to the Millennial-Scale Variations in Monsoon-Driven Productivity and Deep-Water Oxygenation in the Western Bay of Bengal During the Last 45 ka

In this study, we presented a high-resolution benthic foraminiferal assemblage record from the western Bay of Bengal (BoB) (off Krishna–Godavari Basin) showing millennial-scale variations during the last 45 ka. We studied temporal variations in benthic foraminiferal assemblages (relative abundances of ecologically sensitive groups/species, microhabitat categories, and morphogroups) to infer past changes in sea bottom environment and to understand how monsoon induced primary productivity-driven organic matter export flux and externally sourced deep-water masses impacted the deep-sea environment at the core site. Our records reveal a strong coupling between surface productivity and benthic environment on glacial/interglacial and millennial scale in concert with Northern Hemisphere climate events. Faunal data suggest a relatively oxic environment when the organic matter flux to the sea floor was low due to low primary production during intensified summer monsoon attributing surface water stratification and less nutrient availability in the mixed layer. Furthermore, records of oxygen-sensitive benthic taxa (low-oxygen vs. high-oxygen benthics) indicate that changes in deep-water circulation combined with the primary productivity-driven organic matter flux modulated the sea bottom oxygen condition over the last 45 ka. We suggest that the bottom water at the core site was well-ventilated during the Holocene (except for the period since 3 ka) compared with the late glacial period. At the millennial timescale, our faunal proxy records suggest relatively oxygen-poor condition at the sea floor during the intervals corresponding to the cold stadials and North Atlantic Heinrich events (H1, H2, H3, and H4) compared with the Dansgaard/Oeschger (D-O) warm interstadials. The study further reveals oxygen-poor bottom waters during the last glacial maximum (LGM, 19–22 ka) which is more pronounced during 21–22 ka. A major shift in sea bottom condition from an oxygenated bottom water during the warm Bølling–Allerød (B/A) (between 13 and 15 ka) to the oxygen-depleted condition during the cold Younger Dryas (YD) period (between 10.5 and 13 ka) is noticed. It is likely that the enhanced inflow of North Atlantic Deep Water (NADW) to BoB would have ventilated bottom waters at the core site during the Holocene, B/A event, and probably during the D-O interstadials of marine isotope stage (MIS) 3.

Effects of Nutrient Management Scenarios on Marine Eutrophication Indicators: A Pan-European, Multi-Model Assessment in Support of the Marine Strategy Framework Directive

A novel pan-European marine model ensemble was established, covering nearly all seas under the regulation of the Marine Strategy Framework Directive (MSFD), with the aim of providing a consistent assessment of the potential impacts of riverine nutrient reduction scenarios on marine eutrophication indicators. For each sea region, up to five coupled biogeochemical models from institutes all over Europe were brought together for the first time. All model systems followed a harmonised scenario approach and ran two simulations, which varied only in the riverine nutrient inputs. The load reductions were evaluated with the catchment model GREEN and represented the impacts due to improved management of agriculture and wastewater treatment in all European river systems. The model ensemble, comprising 15 members, was used to assess changes to the core eutrophication indicators as defined within MSFD Descriptor 5. In nearly all marine regions, riverine load reductions led to reduced nutrient concentrations in the marine environment. However, regionally the nutrient input reductions led to an increase in the non-limiting nutrient in the water, especially in the case of phosphate concentrations in the Black Sea. Further core eutrophication indicators, such as chlorophyll-a, bottom oxygen and the Trophic Index TRIX, improved nearly everywhere, but the changes were less pronounced than for the inorganic nutrients. The model ensemble displayed strong consistency and robustness, as most if not all models indicated improvements in the same areas. There were substantial differences between the individual seas in the speed of response to the reduced nutrient loads. In the North Sea ensemble, a stable plateau was reached after only three years, while the simulation period of eight years was too short to obtain steady model results in the Baltic Sea. The ensemble exercise confirmed the importance of improved management of agriculture and wastewater treatments in the river catchments to reduce marine eutrophication. Several shortcomings were identified, the outcome of different approaches to compute the mean change was estimated and potential improvements are discussed to enhance policy support. Applying a model ensemble enabled us to obtain highly robust and consistent model results, substantially decreasing uncertainties in the scenario outcome.

Zoophycos and related trace fossils from the Chefar El Ahmar Formation, Upper Emsian-Frasnian Ia-Ib (Ougarta, SW Algeria)

The Upper Emsian to Frasnian Ia-Ib strata of the Marhouma area (or “km 30” outcrop), exposed in the Ougarta Range (SW Algeria) belong to the Chefar El Ahmar Formation. On the basis of distinct lithological and palaeontological features, this formation is subdivided into three members (Lower Marly Limestones Member, Middle Marly Limestones Member, and Upper Marly Limestones Member). The studied beds show low to moderate diversity of trace fossil assemblage which contains thirteen ichnotaxa: Chondrites intricatus, Chondrites isp., Chondrites cf. targionii, Circulichnis cf. montanus, Cochlichnus isp., Neonereites biserialis, Neonereites multiserialis, Nereites isp., Palaeophycus isp., Planolites isp., Thalassinoides isp., Zoophycos aff. cauda-galli, and Zoophycos isp. A. The two latter ichnotaxa are the most common trace fossils in the assemblage and occur at three different levels showing different bioturbation intensities. The first Zoophycos-bearing level (Zl 1) is characterised by an overall high bioturbation intensity reflecting a very high oxygenation rate and nutrient supply, allowing the development of large and dense Zoophycos specimens. The second Zoophycos-bearing level (Zl 2) has a considerable reduction of bioturbation intensity as compared to the previous level, with an abundance of Chondrites, which is probably due to radical palaeoecological changes that suggests dysoxic and stressful conditions. The third Zoophycos-bearing level (Zl 3) is characterised by an overall moderate bioturbation intensity. The distribution of trace fossils was influenced by lithology, sedimentation rate, energy level (storm events), bottom oxygenation, and nutrient supply. The lithofacies and trace fossils of the Chefar El Ahmar Formation both indicate a depositional environment fluctuating from the lower shoreface to lower offshore zone.

Small‐Scale Variability of Bottom Oxygen in the Northern Gulf of Mexico

AbstractWhile the occurrence of seasonal bottom hypoxia in the Northern Gulf of Mexico is an extensively studied subject, most research effort has been put on understanding and quantifying hypoxic extent, and little is known about internal variability and short‐term shifts. We use a realistic hydrodynamic model with a simple oxygen parametrization to demonstrate that hypoxia development in the far‐field of the Mississippi plume is subject to physical processes with spatial scales ranging from (10) to (100 km) and temporal scales from the near‐inertial period to seasonality. We use a budget decomposition of the oxygen equation to explore patterns. The mean spatial structure of the local rate of change reveals features shaped as oxygen loss rings enclosing oxygenation cores. This structure is mainly a balance between net advection (horizontal and vertical) and sediment oxygen demand. In terms of temporal variability, we found a large near‐inertial signal in advection, following a convergence‐divergence pattern, at all spatial scales, and a strong subinertial signal at smaller scales only. Variability in advection increases with decreasing temporal and spatial scales, consistently with a field rich in instabilities introducing small‐scale, strong anomalies. Through a Reynolds decomposition of the budget, we separate anomalies from the main flow and identify the total vertical perturbation flux as the primary counterbalance to sediment oxygen demand during periods when the hypoxic extent is maintained or destroyed. Vertical flux anomalies manifest as bottom water intrusions into the mid water column, which have been captured by high‐resolution observations.

Paleoceanographic evolution of the Gulf of Tehuantepec (Mexican Pacific) during the last ~6 millennia

Oceanographic dynamics in the Gulf of Tehuantepec (GT) are the direct consequence of climate variability, mainly influenced by the strong wind regime locally called “Tehuanos” and the interactions between the Intertropical Convergence Zone (ITCZ) and El Niño Southern Oscillation (ENSO). The area is characterized by intense upwelling driven by the Tehuanos within one of the largest Oxygen Minimum Zones (OMZ) in the world. Upwelling carries nutrient-rich subsurface waters to the surface and provides marine resources to the coasts conforming one of the main economic sectors in the region. In this study, sediment core MD02-2521 is used to perform the first high-resolution paleoceanographic reconstruction of the last 6 millennia in the GT. The main focus is put on the analysis of the benthic foraminifera (BF) assemblages inhabiting within the OMZ, which appear to respond to bottom oxygenation and climate variations of the last ~6000 years. The microfossil assemblages throughout the sediment core revealed, first, intervals where the lack of foraminifera provide evidence of episodes of strong deoxygenation that triggered the dissolution of calcareous foraminiferal tests, second, a long-term decline of bottom-water oxygenation in the last ~2500 years likely responding to the southward migration of the ITCZ. Last, variations in response to the transitions between cold and warm periods occurred during the last 2–2.5 millennia and cyclicities of 1470 years resembling Bond Cycles suggest a climatic connection between the Pacific and Atlantic Oceans during the late-Holocene.

A numerical model study of the main factors contributing to hypoxia and its interannual and short-term variability in the East China Sea

Abstract. A three-dimensional physical-biological model of the marginal seas of China was used to analyze interannual and intra-seasonal variations in hypoxic conditions and identify the main processes controlling their generation off the Changjiang (or Yangtze River) estuary. The model was compared against available observations and reproduces the observed temporal and spatial variability of physical and biological properties including bottom oxygen. Interannual variations of hypoxic extent in the simulation are partly explained by variations in river discharge but not nutrient load. As riverine inputs of freshwater and nutrients are consistently high, promoting large productivity and subsequent oxygen consumption in the region affected by the river plume, wind forcing is important in modulating interannual and short-term variability. Wind direction is relevant because it determines the spatial extent and distribution of the freshwater plume, which is strongly affected by either upwelling or downwelling conditions. High-wind events can lead to partial reoxygenation of bottom waters and, when occurring in succession throughout the hypoxic season, can effectively suppress the development of hypoxic conditions, thus influencing interannual variability. A model-derived oxygen budget is presented and suggests that sediment oxygen consumption is the dominant oxygen sink below the pycnocline and that advection of oxygen in the bottom waters acts as an oxygen sink in spring but becomes a source during hypoxic conditions in summer, especially in the southern part of the hypoxic region, which is influenced by open-ocean intrusions.

Long-term variability of near-bottom oxygen, temperature, and salinity in the Southern Baltic

Abstract Bottom conditions in deep sea basins are strongly connected to the health of marine ecosystems. Due to its enclosed location and shallow bathymetry, the Baltic Sea is very sensitive to eutrophication and climate change. The primary objective of this study is to describe long-term variability of near-bottom oxygen concentration, salinity, and temperature in three basins of the Southern Baltic: the Bornholm Basin (BB), Slupsk Furrow (SF), and Gdansk Deep (GD) based on the analysis of historical hydrographic data, and to demonstrate the impact of inflow events on deep ventilation in those basins. Mean bottom oxygen concentration in the period 1946–2016 was very similar in the deep basins (1.2–1.4 ml l−1), and much higher in the shallower SF (3.2 ml l−1) due to the water mass modification occurring between BB and SF. SF was found to contribute to the ventilation of GD with a 1–3-month lag. The results indicate that vertical mixing in SF and the eastward advection of ventilated waters towards GD directly influence bottom water properties in GD. Therefore, next to Major Baltic Inflows (MBIs), also weaker barotropic and baroclinic inflows are important for the bottom ventilation in this area. The long-term trends are very similar in all of the basins. Near-bottom oxygen gradually decreased from 1946, reaching a minimum in 2000. This coincided with salinity and temperature minimums and was linked to reduced frequency and volume of barotropic inflows due to their decadal variability. Oxygen concentrations have been slowly recovering in all three basins since 2000. At the end of the record, their 5-year mean values were approximate to the levels from the 1970's. In the period 1946–2016, hypoxia (oxygen concentration below 2 ml l−1) occurred more frequently in BB (78%) than in GD (73%). Although the results show slow recovery of dissolved oxygen concentrations based on 5-year long mean values, the occurrence of hypoxia increased over the last two decades, reaching 85% in BB and GD and 24% in SF. Mean and decadal variability of the seasonal cycle of near-bottom water properties were analysed. Mean annual cycles of bottom oxygen in three basins are approximately sinusoidal, with the highest values occurring in March, and lowest in September. The range of the mean oxygen seasonal cycle decreases eastward from 1.6 ml l−1 in BB, through 1.3 ml l−1 in SF, to 1.2 ml l−1 in GD. Furthermore, between 1946–1980 and 1995–2016, the range of the mean DO seasonal cycle decreased by 0.2 ml l−1 in BB, 0.6 ml l−1 in SF, and 0.4 ml l−1 in GD. In BB and SF, minimum bottom temperatures occur in summer, and maximum in winter. The strongest annual temperature variation (3 °C) occurs in SF, and is smaller in BB (1.5 °C) and GD (1 °C). The amplitude of the seasonal salinity variation is approximately 0.5 psu in all basins. In BB and GD, a drop in salinity is observed in autumn.

Contribution of the offshore detached Changjiang (Yangtze River) Diluted Water to the formation of hypoxia in summer

The Changjiang (Yangtze River) Diluted Water (CDW) plume substantially impacts the biogeochemical processes off the estuary and its adjacent area, resulting in considerable environmental and ecological effects. Based on survey data in the northeastern area off the Changjiang Estuary (CE) obtained in the summers of 2008 and 2013, the hypoxia induced by the offshore detached CDW plume and the associated controlling mechanisms were investigated. The results show that the offshore transport of the CDW plume caused a dispersed low-salinity area in the northeastern area off the CE during summer, in sharp contrast with the surrounding high-salinity and high-density waters. There was a hypoxic area with low-pH (i.e., acidification) near the 40-m isobath in bottom waters in the northeastern area off the CE, and its position generally corresponded to the surface offshore CDW plume. In the area affected by the offshore low-salinity water, the surface patch-like phytoplankton bloom and the organic debris produced in situ were the material drivers of the bottom oxygen consumption and led to the corresponding relationship between the bottom hypoxic zone and the high chlorophyll-a (Chl-a) area at the surface. We consider that the local stratification caused by the offshore low-salinity water and the stable environment within the detached CDW plume constituted the external dynamic conditions for maintaining the bottom hypoxia. Our results demonstrate that the offshore detached CDW plume in the northeastern area off the CE may contribute to the formation of a local hypoxic center with low pH. This study would provide basis for understanding of the physical-biogeochemical processes and environmental responses in the offshore areas of the CDW plume.

Estuarine Forecasts at Daily Weather to Subseasonal Time Scales

AbstractMost present forecast systems for estuaries predict conditions for only a few days into the future. However, there are many reasons to expect that skillful estuarine forecasts are possible for longer time periods, including increasingly skillful extended atmospheric forecasts, the potential for lasting impacts of atmospheric forcing on estuarine conditions, and the predictability of tidal cycles. In this study, we test whether skillful estuarine forecasts are possible for up to 35 days into the future by combining an estuarine model of Chesapeake Bay with 35‐day atmospheric forecasts from an operational weather model. When compared with both a hindcast simulation from the same estuarine model and with observations, the estuarine forecasts for surface water temperature are skillful up to about 2 weeks into the future, and the forecasts for bottom temperature, surface and bottom salinity, and density stratification are skillful for all or the majority of the forecast period. Bottom oxygen forecasts are skillful when compared to the model hindcast, but not when compared with observations. We also find that skill for all variables in the estuary can be improved by taking the mean of multiple estuarine forecasts driven by an ensemble of atmospheric forecasts. Finally, we examine the forecasts in detail using two case studies of extreme events, and we discuss opportunities for improving the forecast skill.

Epibenthic megafauna communities in Northeast Greenland vary across coastal, continental shelf and slope habitats

The marine area of Northeast Greenland belongs to the largest national park in the world. Biodiversity assessments and tailored conservation measures often target specific physiographic or oceanographic features of an area for which detailed knowledge on their biological communities is incomplete. This study, therefore, characterizes epibenthic megafauna communities in a priori defined seabed habitats (fjord, shelf, shelf break and slope) and their relationship to environmental conditions in Northeast Greenland waters as a basis for conservation and management planning. Megabenthos was sampled from the Bessel Fjord across the shelf to the upper continental slope between latitudes 74.55°N–79.27°N and longitudes 5.22°W–21.72°W by Campelen and Agassiz trawls at 18 locations (total of 33 samples) at depths between 65 and 1011 m in August 2015 and September 2017. A total of 276 taxa were identified. Gross estimates of abundance ranged from 4 to 854 individuals 1000 m−2 and biomass ranged from 65 to 528 g wet weight 1000 m−2 (2017 only). The phyla Arthropoda and Porifera contributed the most to taxon richness, while Mollusca and Echinodermata were the most abundant, and Echinodermata had the highest biomass of all phyla. Fjord, shelf, shelf break and slope seabed habitats revealed different megafaunal communities that were partly explained by gradients in depth, bottom oxygen concentration, temperature, salinity, and turbidity. The present study provides a current baseline of megabenthos across seabed habitats in Northeast Greenlandic waters and reveals putative connections between Arctic and Atlantic biota.

The response of sediment phosphorus retention and release to reservoir operations: Numerical simulation and surrogate model development

Dam impoundment causes significant phosphorus (P) retention in reservoir sediment. This makes sediment P release a serious water quality threat in areas where external P loads are being controlled. Reservoir operation strategies affect sediment P retention and release through complex influence on hydrodynamic, temperature, algal and sediment conditions. However, the spatiotemporal response of P retention and release to reservoir operations remains unclear. This study uses a high-fidelity hydrodynamic-eutrophication-sediment model to investigate spatiotemporal variation in sediment P retention and release in a large reservoir in China (i.e., the Danjiangkou Reservoir) as well as associated impacts caused by reservoir operations. Modeling results reveal significant sediment P release in the reservoir (5213 tons annually), from June to October in particular. The intermittent increase in submerged soil area surrounding the reservoir is the dominant factor for P release during the initial stage of this period, while an acute decrease in bottom oxygen conditions caused by thermal stratification in the deep-water zone is the dominant factor during the middle and latter stages. Moreover, the inherent complexity of reservoir P cycling along with the large number of state variables limit the usage of high-fidelity P models in optimizing reservoir operations. To resolve this problem, we apply a dynamic artificial neural network (i.e., nonlinear autoregressive network with exogenous inputs) to develop a surrogate model to predict the response of sediment P release to reservoir operations. The surrogate model is successful in predicting the annual time series of P release with a dramatic reduction in computational burden. It can easily be coupled with a reservoir operation optimization model, thereby enabling operators to identify optimal operation rules to support reservoir socioeconomic functions while mitigating the threats from sediment P legacy.

The step-by-step restriction of the Mediterranean (start, amplification, and consolidation phases) preceding the Messinian Salinity Crisis (climax phase) in the Bajo Segura basin

This research explores the Messinian record in a western Mediterranean basin at the eastern end of the Betic Cordillera (the Garruchal section, southern sector of the Bajo Segura basin). Sedimentary facies development and foraminiferal assemblages were used as palaeoenvironmental proxies to characterize the palaeoceanographic evolution of the basin’s progressive restriction. Therefore, a succession of palaeoenvironmental phases can be established as follows. The “start phase” (until ~ 6.72 Ma) records the first ecological evidence of the restriction, evidenced by the sudden appearance of common benthic stress markers. The “amplification phase” of the restriction (up to ~ 6.35 Ma) marks a progressive increase in low bottom oxygen levels related to water-mass stratification in the evolution from upper epibathyal to outer shelf environments. The “consolidation phase” (until the intra-Messinian discontinuity ~ shortly after 6.03 Ma) records basin shallowing (from outer to inner shelf environments) in a context of a restricted basin. And lastly, the “climax phase” (until the end-Messinian discontinuity) records coastal sedimentation (marine coastal lagoon and sandy beaches). The findings of this research reveal that the Garruchal section is a good illustration of the sedimentary and palaeoenvironmental response of the peripheral basins to the progressive restriction of the Mediterranean before the Messinian Salinity Crisis. Finally, since it is widely accepted that the Mediterranean marine restriction has been developed in a stepwise mode, we here propose to establish the following terms for implementation at Mediterranean regional level: “start,” “amplification,” and “consolidation” phases, which finally led to the “climax” phase, that is, the Messinian Salinity Crisis stricto sensu.

Photic-zone euxinia and anoxic events in a Middle-Late Devonian shelfal sea of Panthalassan continental margin, NW Canada: Changing paradigm of Devonian ocean and sea level fluctuations

Abstract The latest Eifelian – Frasnian strata of the Mackenzie Valley, NW Canada, provide an excellent archive of paleoceanographic signals imprinted in oxic and anoxic facies deposited in close proximity. Fondoformic black-shale strata preserve fingerprints of four global anoxic events (Kacak, Frasnes, Middlesex, and Rhinestreet), which receives confirmation with δ13Corg data. The discovery of 2,3,6- and 3,4,5-trimethyl aryl isoprenoids (biomarkers of green sulfur bacteria) at and between the levels of anoxic events contributes to the growing evidence of photic-zone euxinia as a common state of oceanographically open and semi-restricted shelfal basins of the Middle Devonian – Early Mississippian; a condition impossible under present-day vigorous thermohaline circulation, but consistent with models of greenhouse ocean depicting drastic slowdown in watermass turnover, reversals of deep ocean circulation, greatly expanded oxygen minimum zones, and profoundly changed nutrient flows. The rocks under study were deposited in an oceanographically open basin with fluctuating chemocline as attested by unstable presence of gammacerane in GCMS spectra and co-occurrence of signatures of water-column euxinia and pyritized hyalosponge spicules indicating episodes of weak bottom oxygenation. Shallow-water carbonate banks in the same basin show signatures of reduced hydrodynamic activity and do not record sea level changes in excess of several meters, which lines up with the shortage of evidence for high-amplitude base-level fluctuations in coeval strata worldwide. This supports discarding changes in sea level as the principal control over Devonian anoxic events and instead suggests pulsatory expansions of thick, semi-continuous oxygen minimum zones of the greenhouse ocean. Sea level fluctuations could still be involved as non-glacial (thermal and aquifer?) eustatic transgressions of a very modest amplitude.

Water column changes under ice during different winters in a mid-latitude Mediterranean high mountain lake

Winter limnology has been developed mainly in northern-temperate or subarctic lakes, while mid-latitude Mediterranean high mountain lakes have been less studied during the ice-covered season. We present a five-year study in one of these lower latitude high mountain lakes, conducted with multiprobes located at two different depths recording high-frequency data. We have studied highly contrasted winters regarding temperature and snowfall. The results of a principal component analysis and a subsequent hierarchical clustering show that bottom oxygen depletion under ice is strongly influenced by winter cover thickness and transparency. A thick and opaque winter cover (dark mode) leads to an intense oxygen depletion until anoxia, while a thin and clear winter cover (clear mode) allows higher dissolved oxygen saturation due to oxygen production by photosynthesis. Under unusual conditions of cold and dry climate, only a weak decrease in bottom dissolved oxygen was observed. The extent of dissolved oxygen depletion and its relationship with meteorological parameters and climate patterns are further investigated with generalized additive models. Winter cover thickness and phenology are driven by the joint effect of the North Atlantic Oscillation and East Atlantic climatic patterns. Ice phenology could also influence the following ice-free period given the correlation of lake heat content with the ice-off date and the ice cover length. We propose some features jointly occurring in Lake Cimera as a model for mid-latitude Mediterranean high mountain lakes, including (yet not exclusive) the occurrence of very thick winter covers, the prevalence of dark mode, the relatively high and similar through the years oxygen depletion rates, and high water temperatures during the ice-free periods. It is unclear how the ongoing climate change will affect the prevalence of dark over clear modes due to the expected antagonistic effects of warmer temperatures and lower snowfalls.

Linkages Between Phytoplankton and Bottom Oxygen in the Chesapeake Bay

AbstractThe primary cause of coastal hypoxia is generally attributed to phytoplankton blooms and their subsequent benthic decay. However, direct linkages between phytoplankton and hypoxia have been rarely reported. Here using satellite and field data, we show that such linkages exist in the Chesapeake Bay. For deeper ( 10 m) but nonhypoxic stations, bottom dissolved oxygen is significantly correlated with surface algal biomass during the preceding weeks. Optimal correlation exhibits a bimodal seasonal variation with two peaks in April and August, respectively. Significant particulate dislocation effect is also observed: In April, May, August, and September when water column is stratified, the most significant influencer of bottom oxygen is attributable to phytoplankton blooms located down‐estuary; during destratified months of October and November, however, the dislocation effect switches to the opposite direction, that is, phytoplankton from up‐estuary have the biggest impact. We also notice enhanced phytoplankton growth posterior to lower bottom oxygen, and the strongest correlation is during summer when release of recycled ammonium and hypoxia‐induced phosphate is the most intense. These results demonstrate that the decay of phytoplankton exerts a significant impact on bottom oxygen and releases ammonium which sustains further bloom, whereas hypoxia promotes phytoplankton growth by enhancing the efficiency of phosphate recycling. Importantly, the capability of using satellite data to capture these linkages has significant ramifications for improving hypoxia forecasts in coastal systems. It essentially provides a way to characterize and constrain changes in bottom oxygen demand.

Genetic structure ofSufflogobius bibarbatusin the Benguela upwelling ecosystem using microsatellite markers

The bearded goby Sufflogobius bibarbatus is an abundant endemic small fish species on the continental shelf of the northern Benguela. The goby habitat is characterised by generally low bottom oxygen concentrations that vary spatially and seasonally. In the present study of population structure, 13 samples of S. bibarbatus from inner and outer shelf areas between 19°S and 32°S were screened using ten microsatellite loci. The genetic data were analysed in relation to isolation by distance and depth. Furthermore, for the first time, this study examined genetic data in relation to bottom oxygen concentration at the sampling locations. The data show low but significant genetic heterogeneity (G-test; FST = 0.007, p < .05). There was weak but significant genetic differentiation along a latitudinal gradient across all sampling sites from 19.50°S to 32.37°S (Mantel test; r = .464, p = .001), but this disappeared when the southernmost sample was removed. On the other hand, a positive correlation of bottom oxygen concentration with pairwise FST (r = .336; p = .017) was observed among the sampling sites from the Northern Benguela shelf area. Overall, the data are complex but suggest that isolation by distance and bottom oxygen concentration may play a role in the genetic structuring of S. bibarbatus. The findings are discussed in relation to the species’ life history features and oceanographic characteristics of the Benguela upwelling ecosystem.

Scenarios of Deoxygenation of the Eastern Tropical North Pacific During the Past Millennium as a Window Into the Future of Oxygen Minimum Zones

Diverse studies predict global expansion of Oxygen Minimum Zones (OMZs) as a consequence of anthropogenic global warming. While the observed dissolved oxygen concentrations in many coastal regions are slowly decreasing, sediment core paleorecords often show contradictory trends. This is the case for numerous high-resolution reconstructions of oxygenation in the Eastern Tropical North Pacific (ETNP). While major shifts in redox conditions of the ETNP are dominated by glacial-interglacial cycling, important fluctuations also occur in response to minor climatic and oceanographic perturbations. It is important to understand these scenarios of past redox variation, as they are the closest analogues for near future climate and oceanographic change. We present recently collected sediment core proxy records from the Gulf of California in which we reproduce the variability of productivity and oxygenation of the ETNP OMZ during the past millennium. We emphasize paleoproductivity (Corg, Ni, Baexcess) and paleoredox indicators (Mo, V, Uauth) in sediment cores collected in Alfonso and La Paz basins and compare these OMZ records with other archives of the Eastern Pacific. Our findings indicate that the OMZ expanded in response to increased upwelling and productivity during cold intervals around 1830, 1680, and 1500 years AD (evidenced by higher P, Ni, Mo, V, and Uauth). The most hypoxic times corresponded to the beginning of the Little Ice Age (expressed in elevated Mo). Significant OMZ contractions occurred around 1300, 1650, and 1900s AD after reoxygenation events that were instigated by low productivity (lower Ni, Mo, V, and Uauth). The mechanisms that control decadal-to-centennial oxygen variability in the ETNP remain unidentified but are likely influenced by solar forcing not only driving migrations of the Intertropical Convergence Zone (ITCZ) but more importantly changes in the intensity of the Pacific Walker Circulation (PWC). During the Little Ice Age solar irradiance was at its lowest for the past millennium, which strengthened the PWC. This pattern contributed to more frequent La Niña-like conditions, which enhanced upwelling of nutrient-rich waters in the west coast of North America, driving productivity and reducing bottom oxygen levels, as seen in our ETNP records.