Bottom Water Oxygen Concentrations Research Articles

Parameterisation of oxygen dynamics in the bottom water of the Baltic Sea-North Sea transition zone

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 481:25-39 (2013) - DOI: https://doi.org/10.3354/meps10220 Parameterisation of oxygen dynamics in the bottom water of the Baltic Sea-North Sea transition zone Jørgen L. S. Hansen1,*, Jørgen Bendtsen2 1Department of Bioscience, University of Aarhus, Frederiksborgvej 399, 4000 Roskilde, Denmark 2ClimateLab, Symbion Science Park, Fruebjergvej 3, boks 98, 2100 Copenhagen Ø, Denmark *Email: joh@dmu.dk ABSTRACT: An organic carbon budget for the Kattegat and Belt Seas in the North Sea-Baltic Sea transition zone was used to parameterise the pelagic and benthic respiration in a new oxygen model, OXYCON, which describes the influence of temperature-dependent pelagic and benthic respiration on bottom water oxygen conditions. The significance of respiration versus physical mixing and advection processes for the bottom water oxygen concentration was analysed through a sensitivity study where the OXYCON model was implemented in 2 transport models: a Lagrangian model of bottom water transport, based on an age-tracer of the bottom water transit-time through the area, and a 3-dimensional circulation model of the transition zone. The solutions of both models were in accordance with the observed spatial and temporal distribution of oxygen in the area during the period 2001 to 2003. In particular, the temporal and spatial dynamics of a severe hypoxic event in 2002 were well described. The inter-annual variability of hypoxia during this period could therefore be explained by changes in physical mixing and ventilation of the bottom layer with oxygen-rich surface water and by the bottom water temperature. Variability in the sources of organic material available for remineralisation in the bottom water seems to have less influence on the inter-annual variation in hypoxia but instead determines the background conditions and the long-term trend in oxygen dynamics. KEY WORDS: Hypoxia · Carbon mineralisation · Respiration · Baltic Sea · Kattegat · Modelling Full text in pdf format PreviousNextCite this article as: Hansen JLS, Bendtsen J (2013) Parameterisation of oxygen dynamics in the bottom water of the Baltic Sea-North Sea transition zone. Mar Ecol Prog Ser 481:25-39. https://doi.org/10.3354/meps10220 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 481. Online publication date: May 07, 2013 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2013 Inter-Research.

Impact of global change on coastal oxygen dynamics and risk of hypoxia

Abstract. Climate change and changing nutrient loadings are the two main aspects of global change that are linked to the increase in the prevalence of coastal hypoxia – the depletion of oxygen in the bottom waters of coastal areas. However, it remains uncertain how strongly these two drivers will each increase the risk of hypoxia over the next decades. Through model simulations we have investigated the relative influence of climate change and nutrient run-off on the bottom water oxygen dynamics in the Oyster Grounds, an area in the central North Sea experiencing summer stratification. Simulations were performed with a one-dimensional ecosystem model that couples hydrodynamics, pelagic biogeochemistry and sediment diagenesis. Climatological conditions for the North Sea over the next 100 yr were derived from a global-scale climate model. Our results indicate that changing climatological conditions will increase the risk of hypoxia. The bottom water oxygen concentration in late summer is predicted to decrease by 24 μM or 11.5% in the year 2100. More intense stratification is the dominant factor responsible for this decrease (58%), followed by the reduced solubility of oxygen at higher water temperature (27%), while the remaining part could be attributed to enhanced metabolic rates in warmer bottom waters (15%). Relative to these climate change effects, changes in nutrient runoff are also important and may even have a stronger impact on the bottom water oxygenation. Decreased nutrient loadings strongly decrease the probability of hypoxic events. This stresses the importance of continued eutrophication management in coastal areas, which could function as a mitigation tool to counteract the effects of rising temperatures.

Oxygen consumption in the water column and sediments of the northern Gulf of Mexico hypoxic zone

Hypoxia is a global problem resulting from excessive nutrient inputs to coastal regions, but the biogeochemical mechanisms of hypoxia development are not well understood. The primary location of oxygen consumption (i.e., sediments versus water column) is still debated and may depend on the analytical approach used. In this study, oxygen respiration was measured using incubations combined with membrane inlet mass spectrometry in sediments, water overlying sediments, and the water column in the Gulf of Mexico hypoxic zone. Water column respiration ranged from 0.09 to 4.42 μmol O2 l−1 h−1 (mean = 0.77 ± 0.07 (standard error)) and was significantly higher shortly after two hurricanes. Overlying water respiration ranged from 0.31 to 2.46 μmol O2 l−1 h−1 (mean = 0.70 ± 0.09) and accounted for 3.7 ± 0.8% of total below-pycnocline respiration. Sediment oxygen consumption, measured using a continuous-flow incubation technique, was lowest after the two hurricanes and ranged from 408 to 1800 μmol O2 m−2 h−1 (mean = 834 ± 83.8 μmol O2 m−2 h−1). Sediments accounted for 25 ± 5.3% of total below-pycnocline respiration, and sediment oxygen consumption was related negatively to ambient bottom-water oxygen concentration. This negative relationship contradicts previous literature and suggests that high sediment oxygen consumption is driven by abundant, fresh organic material and regulates bottom-water oxygen concentration, rather than the common assumption that bottom-water oxygen concentration determines sediment oxygen consumption. The results from this study suggest that storms and mixing events may lead to conditions suitable for hypoxia redevelopment in as little as two days after disturbances, with the water column playing a critical role in system hypoxia development and maintenance.

Occurrence of benthic microbial nitrogen fixation coupled to sulfate reduction in the seasonally hypoxic Eckernförde Bay, Baltic Sea

Abstract. Despite the worldwide occurrence of marine hypoxic regions, benthic nitrogen (N) cycling within these areas is poorly understood and it is generally assumed that these areas represent zones of intense fixed N loss from the marine system. Sulfate reduction can be an important process for organic matter degradation in sediments beneath hypoxic waters and many sulfate-reducing bacteria (SRB) have the genetic potential to fix molecular N (N2). Therefore, SRB may supply fixed N to these systems, countering some of the N lost via microbial processes, such as denitrification and anaerobic ammonium oxidation. The objective of this study was to evaluate if N2 fixation, possibly by SRB, plays a role in N cycling within the seasonally hypoxic sediments from the Eckernförde Bay, Baltic Sea. Monthly samplings were performed over the course of one year to measure nitrogenase activity (NA) and sulfate reduction rates, to determine the seasonal variations in bioturbation (bioirrigation) activity and important benthic geochemical profiles, such as sulfur and N compounds, and to monitor changes in water column temperature and oxygen concentrations. Additionally, at several time points, the active N-fixing community was examined via molecular tools. Integrated rates of N2 fixation (approximated from NA) and sulfate reduction showed a similar seasonality pattern, with highest rates occurring in August (approx. 22 and 880 nmol cm−3 d−1 of N and SO42−, respectively) and October (approx. 22 and 1300 nmol cm−3 d−1 of N and SO42− respectively), and lowest rates occurring in February (approx. 8 and 32 nmol cm−3 d−1 of N and SO42−, respectively). These rate changes were positively correlated with bottom water temperatures and previous reported plankton bloom activities, and negatively correlated with bottom water oxygen concentrations. Other variables that also appeared to play a role in rate determination were bioturbation, bubble irrigation and winter storm events. Molecular analysis demonstrated the presence of nifH sequences related to two known N2 fixing SRB, namely Desulfovibrio vulgaris and Desulfonema limicola, supporting the hypothesis that some of the nitrogenase activity detected may be attributed to SRB. Overall, our data show that Eckernförde Bay represents a complex ecosystem where numerous environmental variables combine to influence benthic microbial activities involving N and sulfur cycling.

Effects of transient bottom water currents and oxygen concentrations on benthic exchange rates as assessed by eddy correlation measurements

AbstractEddy correlation (EC) measurements in the benthic boundary layer (BBL) allow estimating benthic O2 uptake from a point distant to the sediment surface. This noninvasive approach has clear advantages as it does not disturb natural hydrodynamic conditions, integrates the flux over a large foot‐print area and allows many repetitive flux measurements. A drawback is, however, that the measured flux in the bottom water is not necessarily equal to the flux across the sediment‐water interface. A fundamental assumption of the EC technique is that mean current velocities and mean O2 concentrations in the bottom water are in steady state, which is seldom the case in highly dynamic environments like coastal waters. Therefore, it is of great importance to estimate the error introduced by nonsteady state conditions. We investigated two cases of transient conditions. First, the case of transient O2 concentrations was examined using the theory of shear flow dispersion. A theoretical relationship between the change of O2 concentrations and the induced vertical O2 flux is introduced and applied to field measurements showing that changes of 5–10 μM O2 h−1 result in transient EC‐fluxes of 6–12 mmol O2 m−2 d−1, which is comparable to the O2 uptake of shelf sediments. Second, the case of transient velocities was examined with a 2D k‐ε turbulence model demonstrating that the vertical flux can be biased by 30–100% for several hours during changing current velocities from 2 to 10 cm s−1. Results are compared to field measurements and possible ways to analyze and correct EC‐flux estimates are discussed.

Sensitivity of hypoxia predictions for the northern Gulf of Mexico to sediment oxygen consumption and model nesting

Every summer, a large area (15,000 km2 on average) over the Texas–Louisiana shelf in the northern Gulf of Mexico turns hypoxic due to decay of organic matter that is primarily derived from nutrient inputs from the Mississippi/Atchafalaya River System. Interannual variability in the size of the hypoxic zone is large. The 2008 Action Plan put forth by the Mississippi River/Gulf of Mexico Watershed Nutrient Task Force, an alliance of multiple state and federal agencies and tribes, calls for a reduction of the size of the hypoxic zone through nutrient management in the watershed. Comprehensive models help build mechanistic understanding of the processes underlying hypoxia formation and variability and are thus indispensable tools for devising efficient nutrient reduction strategies and for building reasonable expectations as to what responses can be expected for a given nutrient reduction. Here we present such a model, evaluate its hypoxia simulations against monitoring observations, and assess the sensitivity of the hypoxia simulations to model resolution, variations in sediment oxygen consumption, and choice of physical horizontal boundary conditions. We find that hypoxia simulations on the shelf are very sensitive to the parameterization of sediment oxygen consumption, a result of the fact that hypoxic conditions are restricted to a relatively thin layer above the bottom over most of the shelf. We show that the strength of vertical stratification is an important predictor of dissolved oxygen concentration in bottom waters and that modification of physical horizontal boundary conditions can have a large effect on hypoxia simulations because it can affect stratification strength.

Sediment quality assessment using Gmelinoides fasciatus and Monoporeia affinis (Amphipoda, Gammaridea) in the northeastern Baltic Sea

Crustaceans in the order Amphipoda are sensitive organisms for the assessment of sediment quality. In this work we performed 10-day toxicity tests on muddy sediments collected from a total of 29 sites in the Gulf of Finland, the Gulf of Riga and the Gulf of Bothnia (northeastern Baltic Sea) using Baltic Sea species such as the native amphipod Monoporeia affinis (Bousfield, 1989) and the invasive amphipod Gmelinoides fasciatus (Stebbing, 1899), and also compared these results with those of bioassays carried out using the standard test species, laboratory-cultivated amphipod Hyalella azteca (Saussure, 1858). The sediment samples (three cm of the upper layer) were collected by a GEMAX Dual Corer during the R/V “Aranda” cruises in August and September of 2009 and 2010. Toxicity of sediments in bioassays with M. affinis and G. fasciatus gave varied results depending on the amphipod species used. The lowest quality of sediments determined using M. affinis was recorded at sites located in the offshore and deepwater areas (60-100 m depths) of the Gulf of Finland characterized by hypoxic/anoxic conditions. Toxicity testing applying G. fasciatus showed that sediments at >50% of the study sites in the Gulf of Finland and in the Gulf of Riga can be assessed as highly contaminated. Males of G. fasciatus were significantly more sensitive to potential contamination in sediments than females. The lower survival of males under contaminant stress may result in a skewed sex ratio in natural populations and in a decline of reproduction success. The survival rate of G. fasciatus in the toxicity tests correlated positively with the Shannon diversity index (calculated for macrozoobenthos at the study sites), weight losses on ignition (%) in sediments, and it also showed a negative relation with the bottom water oxygen content (mg/l). The results suggest that G. fasciatus is the more sensitive species of the three amphipods tested and can be used as a indicator of sediment quality in the Baltic Sea and other water bodies.

Relationship between pore density in benthic foraminifera and bottom-water oxygen content

Reliable estimates of bottom-water oxygen contents are crucial to understanding the formation of past oxygen-depleted environments. Here, we investigate the relationship between pore density in calcareous benthic foraminiferal tests and environmental factors like bottom-water oxygen and nitrate concentration, water depth, and temperature in living (Rose Bengal stained) specimens of the shallow-infaunal species Bolivina pacifica, and the two deep-infaunal species Fursenkoina mexicana, and Chilostomella oolina. Used samples span an oxygen-gradient (0.10 to 4.62mlL−1) across oxygen minimum zones (OMZ) off Namibia and Pakistan.Bolivina pacifica and F. mexicana display an inverse correlation between pore density and in-situ bottom-water oxygen content (BW-O2), indicating a morphological response of the foraminifers to decreasing oxygenation. Supporting previous results, we suggest that both species may increase their pore numbers to improve the ability of oxygen uptake in low-oxygen environments. Comparison of the calculated pore densities for B. pacifica and F. mexicana with bottom-water nitrate concentration (BW-NO3−) and temperatures, however, illustrates that these factors might also influence the pore density. Our results for the deep-infaunal species C. oolina show no significant relationship between pore density and BW-O2. This suggests that C. oolina, rather than increasing its pore density, has another life-strategy to survive sustained low-oxic conditions, possibly nitrate respiration. The non-correlation between pore densities and BW-NO3−, however, suggests that pores are not involved in the denitrification process.According to our data we suggest that the pore density of some benthic foraminiferal species is controlled by BW-O2. This relation is, however, species-specific. Overall, our data suggest that this morphological response could provide the basis for an independent proxy for BW-O2 in future studies.

Atlas of modern dinoflagellate cyst distribution based on 2405 data points

Dinoflagellate cysts are useful for reconstructing upper water conditions. For adequate reconstructions detailed information is required about the relationship between modern day environmental conditions and the geographic distribution of cysts in sediments. This Atlas summarises the modern global distribution of 71 organic-walled dinoflagellate cyst species. The synthesis is based on the integration of literature sources together with data of 2405 globally distributed surface sediment samples that have been prepared with a comparable methodology and taxonomy. The distribution patterns of individual cyst species are being compared with environmental factors that are known to influence dinoflagellate growth, gamete production, encystment, excystment and preservation of their organic-walled cysts: surface water temperature, salinity, nitrate, phosphate, chlorophyll-a concentrations and bottom water oxygen concentrations. Graphs are provided for every species depicting the relationship between seasonal and annual variations of these parameters and the relative abundance of the species. Results have been compared with previously published records; an overview of the ecological significance as well as information about the seasonal production of each individual species is presented.The relationship between the cyst distribution and variation in the aforementioned environmental parameters was analysed by performing a canonical correspondence analysis. All tested variables showed a positive relationship on the 99% confidence level. Sea-surface temperature represents the parameter corresponding to the largest amount of variance within the dataset (40%) followed by nitrate, salinity, phosphate and bottom-water oxygen concentration, which correspond to 34%, 33%, 25% and 24% of the variance, respectively. Characterisations of selected environments as well as a discussion about how these factors could have influenced the final cyst yield in sediments are included.

New insights into the formation and burial of Fe/Mn accumulations in Lake Baikal sediments

Lake Baikal is the deepest and oldest lake on Earth. Extraordinary features of the lake are manganese and iron‐enriched layers and crusts occurring at different depths within the sediment. They can be broadly subdivided into an upper accumulation at the O2/Mn(II) redox boundary and one or more layers buried within the reducing part of the sediment. The processes leading to their formation and peculiar distribution within the sediment have remained subject of debate, in particular whether the burial of vast amounts of Mn and Fe-oxides results from a steady-state process or if it is the consequence of singular events, such as changes in sedimentation rate, bottom water oxygen concentrations, or the mass accumulation rate (MAR) of organic carbon (Corg), Mn or Fe. We retrieved short cores from the South basin, the North Basin, and Academician Ridge, determined sedimentation rates, contents of Corg, Mn and Fe, and estimated pore‐water fluxes from concentration profiles of O2, NO3−, Mn(II), Fe(II), SO42− and CH4. A consistent picture emerged from the data showing that the upper Fe/Mn layer formed at the lower end of the oxygen penetration depth as a dynamic pattern, moving upward with the growing sediment. Thereby, reductive dissolution of Mn(IV) occurred at the lower margin. Upward diffusing Mn(II) was oxidized with O2 forming the upper boundary of the Fe/Mn accumulation. The buried Fe/Mn layers were immobilized within the sediment and underwent slow reductive dissolution mainly driven by the anaerobic oxidation of CH4. The process leading to the detachment of the ‘active’ Fe/Mn layer from the top redox interface is not unambiguously clear. However, we suggest a cyclic pattern where the burial of a Fe/Mn layer is accompanied by the generation of a new enrichment at the O2/Mn(II) redox boundary, which is subsequently nourished by the slowly dissolving old layer.

Authigenic uranium in foraminiferal coatings: A proxy for ocean redox chemistry

The rate of uranium accumulation in oceanic sediments from seawater is controlled by bottom water oxygen concentrations and organic carbon fluxes—two parameters that are linked to deep ocean storage of CO2. To investigate glacial‐interglacial changes in what is known as authigenic U, we have developed a rapid method for its determination as a simple addition to a procedure for foraminiferal trace element analysis. Foraminiferal calcite acts as a low U substrate (U/Ca < 15 nmol/mol) upon which authigenic U accumulates in reducing sediments. We measured a downcore record of foraminiferal U/Ca from ODP Site 1090 in the South Atlantic and found that U/Ca ratios increase by 70–320 nmol/mol during glacial intervals. There is a significant correlation between U/Ca records of benthic and planktonic foraminiferal species and between U/Ca and bulk sediment authigenic U. These results indicate that elevated U/Ca ratios are attributable to the accumulation of authigenic U coatings in sediments. Foraminiferal Mn/Ca ratios were lower during the glacial intervals, suggesting that the observed U accumulation on the shells is not directly linked to U incorporation into secondary manganese phases. Thus, foraminiferal U/Ca ratios may provide useful information on past changes in sediment redox conditions.

Phosphorus diagenesis in deep-sea sediments: Sensitivity to water column conditions and global scale implications

The geological record bears evidence of various periods of wide-scale oceanic anoxia that are associated with perturbations of the marine carbon and phosphorus (P) cycles. In this study, we examine the impact of changes in bottom water oxygen and organic matter (OM) input on burial of P in deep-sea sediments using a reactive-transport model. Results show that the burial of key reactive P phases, namely authigenic calcium associated P minerals (Ca–P), organic P (org P) and iron-bound P (Fe–P), responds non-linearly to both water column forcings, namely water column oxygenation and OM loading. High organic matter (OM) flux with either very low or high oxygen (~180μM) favor the formation of authigenic Ca–P, while low oxygen and intermediate to high OM fluxes promote org P burial. Iron-bound P is only preserved in the sediment when oxygen levels are high and OM fluxes low. The conditions for maximum P recycling (or minimum P burial) are low bottom water oxygen concentrations and low OM fluxes to the sediment–water interface (hypoxic, oligotrophic deep-sea settings). The bivariate dependence of P burial on oxygen and OM flux was implemented in an existing box model of the global marine P, oxygen and organic carbon cycles, replacing simple empirical redox functions for P burial. The response of the original and new box model to decreased ocean mixing was then assessed, in the context of a long-term response. In the new model, org P instead of authigenic Ca–P is the dominant burial phase of P in deep-sea environments during periods of oceanic anoxia. Nevertheless, reduced ocean mixing leads to a similar response in total P burial and, as a consequence, to a similar increase in deep water anoxia and decrease in open ocean productivity whether an empirical or mechanistic description of phosphorus diagenesis is considered.

Role of sediment denitrification in water column oxygen dynamics: comparison of the North American East and West Coasts

Abstract. Low oxygen concentrations, either natural or anthropogenically driven, can severely affect coastal marine ecosystems. A deeper understanding of oxygen dynamics is required in order to improve numerical models, eventually to predict the timing and severity of hypoxia. In this study we investigate the effect of sediment denitrification on oxygen concentrations in bottom waters over the continental shelf. We used two coupled physical-biological models based on the Regional Ocean Modelling System (ROMS) to compare summer simulations with and without denitrification within the sediments for two North American shelves: the Middle Atlantic Bight (MAB) and the Vancouver Island Shelf (VIS). These regions belong to western and eastern boundary current systems, respectively, and are characterized by different physical and biological dynamics. Both models assume coupled nitrification-denitrification within the sediments. Denitrification represents a loss of bioavailable nitrogen through the production of dinitrogen gas, with the potential to affect biogeochemical cycles. In our MAB model, this loss of regenerated nutrients through denitrification within the sediments significantly affects primary production, since recycled nitrogen supports most of the primary production in that region. The diminished primary production and consequent decrease of organic matter flux to the seafloor leads to less sediment oxygen consumption and higher oxygen concentrations in bottom waters. However, changes in regenerated nitrogen on the VIS barely affect primary production due to the efficient supply of new nutrients through wind-driven upwelling during summer and the nutrient-rich coastal current. We recommend that modelling experiments focusing on oxygen dynamics (as well as oxygen budget calculations) should include sediment denitrification in coastal regions where regenerated primary production dominates productivity.

Sedimentary phosphorus and iron cycling in and below the oxygen minimum zone of the northern Arabian Sea

Abstract. In this study, we investigate phosphorus (P) and iron (Fe) cycling in sediments along a depth transect from within to well below the oxygen minimum zone (OMZ) in the northern Arabian Sea (Murray Ridge). Pore-water and solid-phase analyses show that authigenic formation of calcium phosphate minerals (Ca-P) is largely restricted to where the OMZ intersects the seafloor topography, likely due to higher depositional fluxes of reactive P. Nonetheless, increased ratios of organic carbon to organic P (Corg/Porg) and to total reactive P (Corg/Preactive) in surface sediments indicate that the overall burial efficiency of P relative to Corg decreases under the low bottom water oxygen concentrations (BWO) in the OMZ. The relatively constant Fe/Al ratio in surface sediments along the depth transect suggest that corresponding changes in Fe burial are limited. Sedimentary pyrite contents are low throughout the ~25 cm sediment cores at most stations, as commonly observed in the Arabian Sea OMZ. However, pyrite is an important sink for reactive Fe at one station in the OMZ. A reactive transport model (RTM) was applied to quantitatively investigate P and Fe diagenesis at an intermediate station at the lower boundary of the OMZ (bottom water O2: ~14 μmol L−1). The RTM results contrast with earlier findings in showing that Fe redox cycling can control authigenic apatite formation and P burial in Arabian Sea sediment. In addition, results suggest that a large fraction of the sedimentary Ca-P is not authigenic, but is instead deposited from the water column and buried. Dust is likely a major source of this Ca-P. Inclusion of the unreactive Ca-P pool in the Corg/P ratio leads to an overestimation of the burial efficiency of reactive P relative to Corg along the depth transect. Moreover, the unreactive Ca-P accounts for ~85% of total Ca-P burial. In general, our results reveal large differences in P and Fe chemistry between stations in the OMZ, indicating dynamic sedimentary conditions under these oxygen-depleted waters.

Effects of global warming on hypoxia in the Baltic Sea–North Sea transition zone

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

Modelling macrofaunal biomass in relation to hypoxia and nutrient loading

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

The ecology and biogeography of Discospirina tenuissima (Foraminifera) in the Atlantic and Indian Oceans

The large (≥1cm diameter) miliolid foraminifera Discospirina tenuissima (Carpenter and Jeffreys, 1870) is common at four sites (NW, NE, SW, and SE), located on either side of the Mid-Atlantic Ridge to the north (54°N) and south (49°N) of the Charlie–Gibbs Fracture Zone. The white discoidal tests of this epifaunal species were visible in video surveys of flat and gently (10°) sloping, sediment-covered areas of seafloor (replicate 500-m-long transects, 1000m2 surface area) obtained using the Remote Operated Vehicle Isis. Average densities varied from 0.07 (SE site) to 1.12(NW)indm−2 for sloped transects and 0.02 (NW) to 1.75(SW)indm−2 for flat transects. Considerable variation was also evident between individual transects (0–2.25indm−2). The tests displayed no consistent dispersion pattern; both significantly random and clumped patterns were observed, in some cases within a single site. Isis was also used to make detailed in situ observations of D. tenuissima and to collect individual specimens. The delicate test margin sometimes exhibited angular notches and other signs of damage, presumably a result of megafaunal activity; in some cases the damage had been repaired. Specimens perforated by a large central hole occurred at the SE site. Smaller sessile organisms, including agglutinated foraminifera and occasional brachiopods, use D. tenuissima tests as a substratum for attachment. In all areas, some tests were surrounded by a ring of sediment, presumably surface material collected by pseudopodia. We interpret these features as being comparable to the feeding cysts created by other foraminiferal species. They were particularly common at the SE site, where one or two abandoned rings indicated that some tests had moved distances of several centimetres across the seafloor. Most previous records of D. tenuissima are from well-oxygenated sites in the NE Atlantic. We provide the first records from the Indian Ocean. Here, this species occurs at bathyal depths in the northwest (1980m) and northeast (938m) Arabian Sea, where bottom-water oxygen concentrations are depressed.

The nitrogen isotope effect of benthic remineralization-nitrification-denitrification coupling in an estuarine environment

Abstract. The nitrogen (N) stable isotopic composition of pore water nitrate and total dissolved N (TDN) was measured in sediments of the St. Lawrence Estuary and the Gulf of St. Lawrence. The study area is characterized by gradients in organic matter reactivity, bottom water oxygen concentrations, as well as benthic respiration rates. N isotope effects on the water column associated with the benthic exchange of nitrate (εapp) and TDN (εsed) during benthic nitrification-denitrification coupling were investigated. The sediments were a major sink for nitrate and a source of reduced dissolved N (RDN = DON + NH4+). We observed that both the pore water nitrate and RDN pools were enriched in 15N relative to the water column, with increasing δ15N downcore in the sediments. As in other marine environments, the biological nitrate isotope fractionation of net fixed N loss was barely expressed at the scale of sediment-water exchange, with &amp;amp;varepsilon;app values &lt;3‰. The strongest under-expression (i.e. lowest εapp) of the biological N isotope fractionation was observed at the most oxygenated sites with the least reactive organic matter, indicating that, through their control on the depth of the denitrification zone, bottom water oxygen concentrations and the organic matter reactivity can modulate εapp. For the first time, actual measurements of δ15N of pore water RDN were included in the calculations of εsed. We argue that large fractions of the sea-floor-derived DON are reactive and, hence, involved in the development of the δ15N of dissolved inorganic N (DIN) in the water column. In the St. Lawrence sediments, the combined benthic N transformations yield a flux of 15N-enriched RDN that can significantly elevate εsed above εapp. Calculated εsed values were within the range of 4.6 ± 2‰ and were related to organic matter reactivity and oxygen penetration depth in the sediments. &amp;amp;varepsilon;sed reflects the δ15N of the N2 lost from marine sediments and thus best describes the isotopic impact of fixed N loss from sediments on the oceanic fixed N pool. Our mean value for εsed is larger than assumed by earlier work, questioning current ideas with regards to the state of balance of the modern N budget.

Dissolved oxygen and suspended particles regulate the benthic flux of iron from continental margins

We present ex situ sediment incubation results from the California and Oregon shelves and compare the calculated benthic flux of dissolved Fe with those from in situ incubations and pore water concentration profiles. We also examine the influence of oxygen depletion and sediment re-suspension on benthic Fe exchange. Ex situ incubation of the California and Oregon shelf sites yielded average benthic Fe fluxes of 3.5 and 8.3μmolm−2day−1, respectively, compared to 17 and 55μmolm−2day−1 from the in situ Lander determinations, and 73 and 103μmolm−2day−1 from modeling of pore water concentration profiles. Differences between benthic Fe flux estimates are primarily accounted for by [1] differences in Fe (II) oxidation kinetics, which result from distinct oxygen consumption rates between incubation methods, and the absence of kinetic considerations in the overlying bottom water in pore water flux calculations, and [2] the effects of biological sediment irrigation that are best represented by in situ incubations due to their sampling area and preservation of bottom water conditions. Bottom water oxygen concentrations were higher at the California shelf site than that at the Oregon shelf site, and probably accounted for the greater discrepancy between methods used to determine benthic Fe flux. The comparison of techniques used to determine benthic Fe flux indicates that the concentration of bottom water oxygen exerts a principle control over the fate of dissolved Fe entering the overlying bottom water — supporting the view that the expansion of coastal hypoxia has the potential to enhance the benthic supply of Fe (II) to shelf waters.An episode of surface sediment re-suspension during ex situ incubation led to a rapid removal of 76–89% of dissolved Fe from seawater, followed by a steady return towards initial seawater concentrations during particle settling, indicating that diffusive inputs of dissolved Fe from sediment pore water are rapidly adsorbed and desorbed by particles during periods of benthic re-suspension. The findings suggest that dissolved Fe concentrations in bottom waters may reflect an equilibrium concentration of non-stabilized aqueous Fe and particle-adsorbed Fe phases — where the addition of suspended particles to bottom waters leads to scavenging of dissolved Fe into labile particulate Fe phases. Thus we suggest that suspended particles are a significant buffer of dissolved Fe released from shelf sediments, an important transport mechanism for benthic Fe inputs, and a regulator of dissolved Fe concentrations in seawater.

Impact of resuspension of cohesive sediments at the Oyster Grounds (North Sea) on nutrient exchange across the sediment–water interface

Benthic-pelagic exchange processes are recognised as important nutrient sources in coastal areas, however, the relative impact of diffusion, resuspension and other processes such as bioturbation and bioirrigation are still relatively poorly understood. Experimental ship-based data are presented showing the effects of diffusion and resuspension on cohesive sediments at a temperate shelf location in the North Sea. Measurements of diffusive fluxes in both spring (1.76, 0.51, −0.91, 17.6 μmol/m2/h) and late summer (8.53, −0.03, −1.12, 35.0 μmol/m2/h) for nitrate, nitrite, phosphate and dissolved silicon respectively, provided comparisons for measured resuspension fluxes. Increases in diffusive fluxes of nitrate and dissolved silicon to the water column in late summer coincided with decreases in bottom water oxygen concentrations and increases in temperature. Resuspension experiments using a ship board annular flume and intact box core allowed simultaneous measurement of suspended particulate matter, water velocity and sampling of nutrients in the water column during a step wise increase in bed shear velocity. The resuspension of benthic fluff led to small but significant releases of phosphate and nitrate to the water column with chamber concentration increasing from 0.70–0.76 and 1.84–2.22 μmol/L respectively. Resuspension of the sediment bed increased water column concentrations of dissolved silicon by as much as 125% (7.10–15.9 μmol/L) and nitrate and phosphate concentrations by up to 67% (1.84–3.08 μmol/L) and 66% (0.70–1.15 μmol/L) respectively. Mass balance calculations indicate that processes such as microbial activity or adsorption/desorption other than simple release of pore water nutrients must occur during resuspension to account for the increase. This study shows that resuspension is potentially an important pathway for resupplying the water column with nutrients before and during phytoplankton blooms and should therefore be considered along with diffusive fluxes in future ecosystem models.