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Location, specific topography, and hydrographic setting together with climate change and strong anthropogenic pressure are the main factors shaping the biogeochemical functioning and thus also the ecological status of the Baltic Sea. The recent decades have brought significant changes in the Baltic Sea. First, the rising nutrient loads from land in the second half of the 20th century led to eutrophication and spreading of hypoxic and anoxic areas, for which permanent stratification of the water column and limited ventilation of deep-water layers made favourable conditions. Since the 1980s the nutrient loads to the Baltic Sea have been continuously decreasing. This, however, has so far not resulted in significant improvements in oxygen availability in the deep regions, which has revealed a slow response time of the system to the reduction of the land-derived nutrient loads. Responsible for that is the low burial efficiency of phosphorus at anoxic conditions and its remobilization from sediments when conditions change from oxic to anoxic. This results in a stoichiometric excess of phosphorus available for organic-matter production, which promotes the growth of N2 -fixing cyanobacteria and in turn supports eutrophication. This assessment reviews the available and published knowledge on the biogeochemical functioning of the Baltic Sea. In its content, the paper covers the aspects related to changes in carbon, nitrogen, and phosphorus (C, N, and P) external loads, their transformations in the coastal zone, changes in organic-matter production (eutrophication) and remineralization (oxygen availability), and the role of sediments in burial and turnover of C, N, and P. In addition to that, this paper focuses also on changes in the marine CO2 system, the structure and functioning of the microbial community, and the role of contaminants for biogeochemical processes. This comprehensive assessment allowed also for identifying knowledge gaps and future research needs in the field of marine biogeochemistry in the Baltic Sea. [ABSTRACT FROM AUTHOR]

Location, specific topography and hydrographic setting together with climate change and strong anthropogenic pressure are the main factors shaping the biogeochemical functioning and thus also the ecological status of the Baltic Sea. The recent decades have brought significant changes in the Baltic Sea. First, the rising nutrient loads from land in the second half of the 20th century led to eutrophication and spreading of hypoxic and anoxic areas, for which permanent stratification of the water column and limited ventilation of deep water layers made favourable conditions. Since the 1980s the nutrient loads to the Baltic Sea have been continuously decreasing. This, however, has so far not resulted in significant improvements in oxygen availability in the deep regions, which has revealed a slow response time of the system to the reduction of the land-derived nutrient loads. Responsible for that is the low burial efficiency of phosphorus at anoxic conditions and its remobilization from sediments when conditions change from oxic to anoxic. This results in a stoichiometric excess of phosphorus available for organic matter production, which promotes the growth of N2-fixing cyanobacteria and in turn supports eutrophication. This assessment reviews the available and published knowledge on the biogeochemical functioning of the Baltic Sea. In its content, the paper covers the aspects related to changes in carbon, nitrogen and phosphorus (C, N and P) external loads, their transformations in the coastal zone, changes in organic matter production (eutrophication) and remineralization (oxygen availability), and the role of sediments in burial and turnover of C, N and P. In addition to that, this paper focuses also on changes in the marine CO2 system, structure and functioning of the microbial community and the role of contaminants for biogeochemical processes. This comprehensive assessment allowed also for identifying knowledge gaps and future research needs in the field of marine biogeochemistry in the Baltic Sea. [ABSTRACT FROM AUTHOR]

Methane (CH) concentrations and CH stable carbon isotopic composition ( $$ \delta^{13} {\text{C}}_{{{\text{CH}}_{4} }} $$) were investigated in the water column within Jaco Scar. It is one of several scars formed by massive slides resulting from the subduction of seamounts offshore Costa Rica, a process that can open up structural and stratigraphical pathways for migrating CH. The release of large amounts of CH into the adjacent water column was discovered at the outcropping lowermost sedimentary sequence of the hanging wall in the northwest corner of Jaco Scar, where concentrations reached up to 1,500 nmol L. There CH-rich fluids seeping from the sedimentary sequence stimulate both growth and activity of a dense chemosynthetic community. Additional point sources supplying CH at lower concentrations were identified in density layers above and below the main plume from light carbon isotope ratios. The injected CH is most likely a mixture of microbial and thermogenic CH as suggested by $$ \delta^{13} {\text{C}}_{{{\text{CH}}_{4} }} $$ values between −50 and −62 ‰ Vienna Pee Dee Belemnite. This CH spreads along isopycnal surfaces throughout the whole area of the scar, and the concentrations decrease due to mixing with ocean water and microbial oxidation. The supply of CH appears to be persistent as repeatedly high CH concentrations were found within the scar over 6 years. The maximum CH concentration and average excess CH concentration at Jaco Scar indicate that CH seepage from scars might be as significant as seepage from other tectonic structures in the marine realm. Hence, taking into account the global abundance of scars, such structures might constitute a substantial, hitherto unconsidered contribution to natural CH sources at the seafloor. [ABSTRACT FROM AUTHOR]

Within the German integrated project SUGAR, aiming for the development of new technologies for the exploration and exploitation of submarine gas hydrates, the option of gas transport by gas hydrate pellets has been comprehensively re-investigated. A series of pVT dissociation experiments, combined with analytical tools such as x-ray diffraction and cryo-SEM, were used to gather an additional level of understanding on effects controlling ice formation. Based on these new findings and the accessible literature, knowns and unknowns of the self-preservation effect important for the technology are summarized. A conceptual process design for methane hydrate production and pelletisation has been developed. For the major steps identified, comprising (i) hydrate formation; (ii) dewatering; (iii) pelletisation; (iv) pellet cooling; and (v) pressure relief, available technologies have been evaluated, and modifications and amendments included where needed. A hydrate carrier has been designed, featuring amongst other technical solutions a pivoted cargo system with the potential to mitigate sintering, an actively cooled containment and cargo distribution system, and a dual fuel engine allowing the use of the boil-off gas. The design was constrained by the properties of gas hydrate pellets, the expected operation on continental slopes in areas with rough seas, a scenario-defined loading capacity of 20,000 m3 methane hydrate pellets, and safety as well as environmental considerations. A risk analysis for the transport at sea has been carried out in this early stage of development, and the safety level of the new concept was compared to the safety level of other ship types with similar scopes, i.e., LNG carriers and crude oil tankers. Based on the results of the technological part of this study, and with best knowledge available on the alternative technologies, i.e., pipeline, LNG and CNG transportation, an evaluation of the economic competitiveness of the methane hydrate transport technology has been performed. The analysis considers capital investment as well as operational costs and comprises a wide set of scenarios with production rates from 20 to 800 103 Nm3.h-1 and transport distances from 200 to 10,000 km. In contrast to previous studies, the model calculations in this study reveal no economic benefit of methane hydrate transportation versus competing technologies. [ABSTRACT FROM AUTHOR]

Abstract: The release of methane from the seafloor into the water column within the hydrate stability field (HSF) is a natural and widely observed process. The subsequent bubble dissolution rate determines how far upwards gas is transported and the vertical distribution of this methane source to the water column. Understanding this process is essential to describing natural deep sea seeps, to assess the hazard potential from blowouts in offshore drilling activities for gas and oil, and to refine past and future scenarios of global change involving large-scale destabilization of gas hydrates and free methane gas. We report on in situ experiments on single methane and argon bubbles within and above the HSF for depths from 400 to 1500 m. Single bubbles were injected from the ROV Ventana into an attached, back-illuminated, flow-through imaging box. The ascent of individual bubbles within the imaging box was recorded and analyzed for rise velocity, V B , and radius shrinkage rate, dr/dt. For all bubbles V B =~30 cms−1. Methane bubbles released within the HSF had markedly enhanced lifetimes, attributed to hydrate skin formation. dr/dt varied from −7.5 μm s−1 above the HSF to –1 μm s−1 at 1500 m, well within the HSF. Bubble longevity within the HSF increased with distance into the P/T-space from the hydrate phase boundary. There was a delay prior to the slower dissolution, where dr/dt for methane bubbles was comparable to dr/dt above the HSF, which was interpreted as a time delay before the onset of hydrate skin formation. Although variable, the onset time generally decreased with distance from the hydrate phase boundary. No delay was observed for the deepest releases. We relate these findings to formal calculations of methane solubility and density as a function of pressure. Pressure-dependent deviations from ideal gas law and Henry''s law were implemented in a numerical bubble-propagation model which incorporated the effects of decreased solubility and surface mobility after hydrate formation. Inclusion of these effects greatly improved model prediction of observed methane bubble behavior within the HSF. The effect of these individual depth-related effects on bubble dissolution rate and longevity then was assessed quantitatively, and assumptions of different gas solubility prior and after hydrate nucleation or an effect of hydrates at the interface on the hydrodynamics at the surface were tested. Here, our approach implements our current knowledge on physicochemical and hydrodynamic control and does not seek the best fit for the given data sets, thus it also reveals current uncertainties in methane bubble processes in the HSF. [Copyright &y& Elsevier]

In a laboratory-based test series, seven experiments along a simulated Pacific hydrotherm at 152°W, 40°N were carried out to measure the rise velocities of liquefied CO2 droplets under (clathrate) hydrate forming conditions. The impact of a hydrate skin on the rising behavior was investigated by comparing the results with those from outside the field of hydrate stability at matching buoyancy. A thermostatted high-pressure tank was used to establish conditions along the natural oceanic hydrotherm. Under P-/T-conditions allowing hydrate formation, the majority of the droplets quickly developed a skin of CO2 hydrate upon contact with seawater. Rise rates of these droplets support the parametrisation by Chen et al. (Tel/us 2003, 55B, 723-730), which is based on empirical equations developed to match momentum of hydrate covered, deformed droplets. Our data do not support other parametrisation recently suggested in the literature. In the experiments from 5.7 MPa, 4.8 °C to 11.9 MPa, 2.8°C positive and negative deviations from predicted rise rates occurred, which we propose were caused by lacking hydrate formation and reflect intact droplet surface mobility and droplet shape oscillations, respectively. This interpretation is supported by rise rates measured at P-/T- conditions outside the hydrate stability field at the same liquid CO2- seawater density difference (Δp) matching the rise rates of the deviating data within the stability field. The results also show that droplets without a hydrate skin ascend up to 50% faster than equally buoyant droplets with a hydrate skin. This feature has a significant impact on the vertical pattern of dissolution of liquid CO2 released into the ocean. The experiments and data presented considerably reduce the uncertainty of the parametrisation of CO2 droplet rise velocity, which in the past emerged partly from their scarcity and contradictions in constraints of earlier experiments. [ABSTRACT FROM AUTHOR]

To help constrain models involving the chemical stability and lifetime of gas clathrate hydrates exposed at the seafloor, dissolution rates of pure methane and carbon-dioxide hydrates were measured directly on the seafloor within the nominal pressure-temperature (P/T) range of the gas hydrate stability zone. Other natural boundary conditions included variable flow velocity and undersaturation of seawater with respect to the hydrate-forming species. Four cylindrical test specimens of pure, polycrystalline CH4 and CO2 hydrate were grown and fully compacted in the laboratory, then transferred by pressure vessel to the seafloor (1028 m depth), exposed to the deep ocean environment, and monitored for 27 hours using time-lapse and HDTV cameras. Video analysis showed diameter reductions at rates between 0.94 and 1.20 μm/s and between 9.0 and 10.6 · 10−2 μm/s for the CO2 and CH4 hydrates, respectively, corresponding to dissolution rates of 4.15 ± 0.5 mmol CO2/m2s and 0.37 ± 0.03 mmol CH4/m2s. The ratio of the dissolution rates fits a diffusive boundary layer model that incorporates relative gas solubilities appropriate to the field site, which implies that the kinetics of the dissolution of both hydrates is diffusion-controlled. The observed dissolution of several mm (CH4) or tens of mm (CO2) of hydrate from the sample surfaces per day has major implications for estimating the longevity of natural gas hydrate outcrops as well as for the possible roles of CO2 hydrates in marine carbon sequestration strategies. [Copyright &y& Elsevier]

Total alkalinity (TA) has increased in the Baltic Sea, with implications for atmospheric CO2‐induced acidification and CO2 uptake. We compiled extensive data of TA in surface waters of the Baltic Sea, aiming to (i) identify new tendencies in the relationship between TA and salinity (TA–S relationship), (ii) update the TA trend analysis, (iii) investigate spatial–temporal patterns, and (iv) discuss potential drivers and implications. We observed a progressive decrease in the slopes and increase in the intercepts of the TA–S overtime due to the persistent process of TA enhancement. A weak seasonal pattern was identified, with warmer months presenting lower salinity and TA. Lower rates of TA increase were observed in high salinities (Skagerrak–Kattegat; +1.00 to +2.20 μmol kg−1 yr−1), intermediate trends in low salinities (Gulf of Bothnia; +3.28 to +3.57 μmol kg−1 yr−1), and maximal trends in the Central Baltic Sea (+3.70 to +4.57 μmol kg−1 yr−1) and Bornholm Basin (+4.82 to +5.32 μmol kg−1 yr−1). The increase in the intercept of the TA–S in the Gulf of Bothnia suggests a progressive increase in the external supply of TA, although lower than previously thought. The maximum trend in the Bornholm Basin suggests an increase in external supply from the Southern catchment and/or the accumulation of internal production. The positive TA–phosphorus correlations underscore a significant internal source. The TA increase amplifies the CO2 uptake by 1.8–7.8% during spring/summer and reduces the CO2 outgassing by 3.4–7.7% in autumn/winter. The TA enhancement has the potential to buffer CO2‐induced acidification by 39–60% by 2050. [ABSTRACT FROM AUTHOR]

Coastal nutrient loads from point sources such as rivers are mostly well-monitored. This is not the case for diffuse nutrient inputs from coastal catchments unconnected to rivers, despite the potential for high inputs due to intensive land use. The German Baltic Sea coastline consists of numerous peatlands that have been diked and drained. However, some of the dikes have been removed in order to re-establish the hydrological connection to the Baltic Sea, restore local biodiversity, and promote natural CO2 uptake. Since these peatlands were used for agriculture, their rewetting may release accumulated nutrients, leading to nutrient export into the Baltic Sea and intensified coastal eutrophication. Data on these potential nutrient exports are mostly lacking. Therefore, this study investigated nutrient exports from two former agricultural, coastal peatlands: Drammendorfer Wiesen, rewetted in 2019, and Karrendorfer Wiesen, rewetted in 1993. Nutrients (NO3–, NO2–, NH4+, PO43–), nitrous oxide (N2O), particulate organic matter (POM, comprising POC and PON; δ13C-POC), chlorophyll-a, and nitrification rates were analyzed in surface water and porewater sampled weekly to monthly in 2019 and 2020 to compare the effects of different time scales after rewetting on nutrient cycling and potential exports. NH4+, NO2−, and PO43− concentrations were higher in the porewater than in the overlying water at both sites, while nutrient concentrations were generally higher at the recently rewetted Drammendorfer Wiesen than at the Karrendorfer Wiesen. NO3− concentrations in porewater, however, were lower than in the overlying water, indicating NO3− retention within the peat, likely due to denitrification. Nitrification rates and N2O concentrations were generally low, except for a high N2O peak immediately after rewetting. These results suggest that denitrification was the dominant process of N2O production at the study sites. Both peatlands exported nutrients to their adjacent bays of the Baltic Sea; however, N exports were 75% lower in the longer-rewetted peatland. Compared to major Baltic Sea rivers, both sites exported larger area-normalized nutrient loads. Our study highlights the need to monitor the impact of rewetting measures over time to obtain accurate estimates of nutrient exports, better assess negative effects on coastal waters, and to improve peatland management. [ABSTRACT FROM AUTHOR]

Ocean surface pCO 2 estimates are of great interest for the calculation of air–sea CO 2 fluxes, oceanic uptake of anthropogenic CO 2 , and eventually the Global Carbon Budget. They are accessible from direct observations, which are discrete in space and time and thus always sparse, or from biogeochemical models, which only approximate reality. Here, a combined method for the extrapolation of pCO 2 observations is presented that uses (1) model-based patterns of variability from an empirical orthogonal function (EOF) analysis of variability with (2) observational data to constrain EOF pattern amplitudes in (3) an ensemble approach, which locally adjusts the spatial scale of the mapping to the density of the observations. Thus, data-constrained, gap- and discontinuity-free mapped fields including local error estimates are obtained without the need for or dependence on ancillary data (e.g. satellite sea surface temperature maps). This extrapolation approach is generic in that it can be applied to any oceanic or coastal region covered by a suitable model and observations. It is used here to establish a regional pCO 2 climatology of the Baltic Sea (: 10.1594/PANGAEA.961119), largely based on ICOS-DE ship of opportunity (SOOP) Finnmaid surface pCO 2 observations between Lübeck-Travemünde (Germany) and Helsinki (Finland). The climatology can serve as improved input for atmosphere–ocean CO 2 flux estimation in this coastal environment. [ABSTRACT FROM AUTHOR]

Increasing atmospheric CO2 drives ocean acidification globally. In coastal seas, acidification trends can however be either counteracted or enhanced by other processes. Ecosystem effects of acidification are so far small in the Baltic Sea, but changes should be anticipated unless CO2 emissions are curbed. Possible future acidification trends in the Baltic Sea, conditional on CO2 emissions, climate change, and changes in productivity, can be assessed by means of model simulations. There are uncertainties regarding potential consequences for marine organisms, partly because of difficulties to assign critical thresholds, but also because of knowledge gaps regarding species' capacity to adapt. Increased temporal and spatial monitoring of inorganic carbon system parameters would allow a better understanding of current acidification trends and also improve the capacity to predict possible future changes. An additional benefit is that such measurements also provide quantitative estimates of productivity. The technology required for precise measurements of the inorganic carbon system is readily available today. Regularly updated status evaluations of acidification, and the inorganic carbon system in general, would support management when assessing climate change effects, eutrophication or characteristics of the pelagic habitats. This would, however, have to be based on a spatially and temporally sufficient monitoring program. [ABSTRACT FROM AUTHOR]

Ocean surface pCO2 estimates are of great interest for the calculation of air-sea CO2 fluxes, oceanic uptake of anthropogenic CO2, and eventually the Global Carbon Budget. They are accessible from direct observations, which are discrete in space and time and thus always sparse, or from biogeochemical models, which only approximate reality. Here, a combined method for the extrapolation of pCO2 observations is presented that uses (1) model-based patterns of variability from an EOF analysis of variability with (2) observational data to constrain EOF pattern amplitudes in (3) an ensemble approach, 5 which locally adjusts the spatial scale of the mapping to the density of the observations. Thus, data-constrained, gap- and discontinuity-free mapped fields including local error estimates are obtained without the need for or dependence on ancillary data (like, e.g., satellite sea surface temperature maps). This extrapolation approach is generic in that it can be applied to any oceanic or coastal region covered by a suitable model and observations. It is used here to establish a regional pCO2 climatology of the Baltic Sea, largely based on ICOS-DE SOOP Finnmaid surface pCO2 observations between Lübeck-Travemünde 10 (Germany) and Helsinki (Finland). The climatology can serve as improved input for atmosphere-ocean CO2 flux estimation in this coastal environment. [ABSTRACT FROM AUTHOR]

Anthropogenic CO2 emissions currently decrease open ocean pH, but on multi-millennial time scales intensified continental weathering is expected to contribute to increasing oceanic alkalinity ( AT) and thus mitigate the acidification signal. The Baltic Sea is an ideal study site for such AT dynamics, due to its direct link to terrestrial processes, short water residence time and long history of AT measurements dating back to the early 20th century. We compiled an extensive AT data set that revealed the highest data quality and coverage for the past two decades. Within that period, surface water AT levels increased throughout the Baltic Sea. The rates of change were highest in the low-saline, northern areas and decreased gradually toward constant levels in the North Sea. The AT increase observed in the Central Baltic Sea (+3.4 µmol kg−1 yr−1) and the Gulf of Bothnia (+7 µmol kg−1 yr−1) has compensated CO2-induced acidification by almost 50% and 100%, respectively. Further, the AT trends enhanced the CO2 storage capacity and stabilized the CaCO3 saturation state of the Baltic Sea over the past two decades. We discuss the attribution of the AT trends to potential changes in precipitation patterns, continental weathering driven by acidic rain and increasing atmospheric CO2, agricultural liming and internal AT sources. [ABSTRACT FROM AUTHOR]

Marine biogeochemical models based on Redfield stoichiometry suffer from underestimating carbon fixation by primary production. The most pronounced indication of this is the overestimation of the dissolved inorganic carbon (DIC) concentration and, consequently, the partial pressure of carbon dioxide in surface waters. The reduced production of organic carbon will impact most biogeochemical processes. We propose a marine biogeochemical model allowing for a non-Redfieldian carbon fixation. The updated model is able to reproduce observed partial pressure of carbon dioxide and other variables of the ecosystem, like nutrients and oxygen, reasonably well. The additional carbon uptake is realized in the model by an extracellular release (ER) of dissolved organic matter (DOM) from phytoplankton. Dissolved organic matter is subject to flocculation and the sinking particles remove carbon from surface waters. This approach is mechanistically different from existing non-Redfieldian models which allow for flexible elemental ratios for the living cells of the phytoplankton itself. The performance of the model is demonstrated as an example for the Baltic Sea. We have chosen this approach because of a reduced computational effort which is beneficial for large-scale and long-term model simulations. Budget estimates for carbon illustrate that the Baltic Sea acts as a carbon sink. For alkalinity, the Baltic Sea is a source due to internal alkalinity generation by denitrification. Owing to the underestimated model alkalinity, an unknown alkalinity source or underestimated land-based fluxes still exist. [ABSTRACT FROM AUTHOR]

This study examined the CO 2 system in the estuary of the Oder River, one of the largest rivers entering the Baltic Sea. Three measurable parameters describing the CO 2 system, namely total alkalinity (A T), total CO 2 (C T), and the partial pressure of CO 2 (pCO 2), were investigated together with dissolved oxygen, salinity (S), and temperature during two RV Oceania cruises, in May and November of 2016. Large spatial variabilities of A T (1771–2940 μmol kg−1) and C T (1676–2972 μmol kg−1) were determined along the S gradient between the open Baltic Sea and river mouth. In November, the relationships of A T –S and C T -S indicated conservative mixing whereas in May both were strongly affected by biomass production and calcium carbonate formation. The waters of the Oder were oversaturated with CO 2 compared to the atmosphere , irrespective of the season, with pCO 2 values of 1351 ± 42 μatm in May and 1120 ± 32 μatm in November. In the Szczecin Lagoon, however, pCO 2 levels dropped significantly, to 63 μatm, in May, accompanied by an O 2 saturation of up to 134% during the same period. The inverse correlation of pCO 2 and O 2 saturation indicated that the distribution of CO 2 and O 2 in the estuary at the time of sampling was controlled mostly by biological activity. The very large drop in the pCO 2 of the Szczecin Lagoon induced an extreme oversaturation of CaCO 3 that triggered mineral calcite precipitation. The mineral precipitation of carbonates in the lagoon may have accounted for as much as 40% of the C T depletion determined in May, with the remaining 60% attributed to the joint effect of net ecosystem production and CO 2 air/water gas exchange. • The CO 2 system in the Oder River Estuary was investigated for the first time. • OM production and remineralization affect the CO 2 system in the Oder River Estuary. • Extreme primary production may initiate mineral precipitation of calcite in high A T. • Estuarine processes may modify the riverine loads of A T and C T to the Baltic Sea. [ABSTRACT FROM AUTHOR]

Although the majority of coastal sediments consist of sandy material, in some areas marine ingression caused the submergence of terrestrial carbon‐rich peat soils. This affects the coastal carbon balance, as peat represents a potential carbon source. We performed a column experiment to better understand the coupled flow and biogeochemical processes governing carbon transformations in submerged peat under coastal fresh groundwater (GW) discharge and brackish water intrusion. The columns contained naturally layered sediments with and without peat (organic carbon content in peat 39 ± 14 wt%), alternately supplied with oxygen‐rich brackish water from above and oxygen‐poor, low‐saline GW from below. The low‐saline GW discharge through the peat significantly increased the release and ascent of dissolved organic carbon (DOC) from the peat (δ13CDOC − 26.9‰ to − 27.7‰), which was accompanied by the production of dissolved inorganic carbon (DIC) and emission of carbon dioxide (CO2), implying DOC mineralization. Oxygen respiration, sulfate (SO42−) reduction, and methane (CH4) formation were differently pronounced in the sediments and were accompanied with higher microbial abundances in peat compared to sand with SO42−‐reducing bacteria clearly dominating methanogens. With decreasing salinity and SO42− concentrations, CH4 emission rates increased from 16.5 to 77.3 μmol m−2 d−1 during a 14‐day, low‐saline GW discharge phase. In contrast, oxygenated brackish water intrusion resulted in lower DOC and DIC pore water concentrations and significantly lower CH4 and CO2 emissions. Our study illustrates the strong dependence of carbon cycling in shallow coastal areas with submerged peat deposits on the flow and mixing dynamics within the subterranean estuary. [ABSTRACT FROM AUTHOR]

Sediments play an important role in organic matter mineralisation and nutrient recycling, especially in shallow marine systems. Marine ecosystem models, however, often only include a coarse representation of processes beneath the sea floor. While these parameterisations may give a reasonable description of the present ecosystem state, they lack predictive capacity for possible future changes, which can only be obtained from mechanistic modelling. This paper describes an integrated benthic–pelagic ecosystem model developed for the German Exclusive Economic Zone (EEZ) in the western Baltic Sea. The model is a hybrid of two existing models: the pelagic part of the marine ecosystem model ERGOM and an early diagenetic model by Reed et al. (2011). The latter one was extended to include the carbon cycle, a determination of precipitation and dissolution reactions which accounts for salinity differences, an explicit description of the adsorption of clay minerals, and an alternative pyrite formation pathway. We present a one-dimensional application of the model to seven sites with different sediment types. The model was calibrated with observed pore water profiles and validated with results of sediment composition, bioturbation rates and bentho-pelagic fluxes gathered by in situ incubations of sediments (benthic chambers). The model results generally give a reasonable fit to the observations, even if some deviations are observed, e.g. an overestimation of sulfide concentrations in the sandy sediments. We therefore consider it a good first step towards a three-dimensional representation of sedimentary processes in coupled pelagic–benthic ecosystem models of the Baltic Sea. [ABSTRACT FROM AUTHOR]

Abstract: We report on methane enrichments that were observed during summer in the upper water column of the Gotland Basin, central Baltic Sea. In the eastern part of the basin, methane concentrations just below the thermocline varied between 15 nM and 77 nM, in contrast to the western part where no methane enrichments could be detected. Stable carbon isotope ratios of methane (δ13C‐CH4 of −67.6‰) indicated its in situ biogenic origin from CO2 reduction, which was supported by clonal sequences that clustered with Methanomicrobiaceae, a family of methanogenic Archaea. Incubation experiments with a Temora longicornis dominated seston fraction obtained from the relevant depth showed a positive correlation between seston concentration and methane production rates. Our results, in combination with previous literature outcomes, suggest that the methane enrichment in the eastern basin might be sustained by a diet‐consumer relationship between the dinoflagellate Dinophysis norvegica and the copepod T. longicornis. However, our mass balance indicates that a local methane production of 110 pmol L−1 d−1 was needed to maintain the methane enrichment, and that the estimated production rate from our incubation experiments of 0.3 pmol CH4 d−1 per adult T. longicornis (about 1 pmol L−1 d−1) was too low to maintain the methane enrichment by zooplankton associated methane production only. These calculations also showed that methane was consumed below the thermocline and not transported into the upper‐ocean, suggesting that other sources in the mixed layer in the range of 95 pmol L−1 d−1 are needed to maintain the observed methane air–sea flux. [ABSTRACT FROM AUTHOR]

Large quantities of methane are stored in hydrates and permafrost within shallow marine sediments in the Arctic Ocean. These reservoirs are highly sensitive to climate warming, but the fate of methane released from sediments is uncertain. Here, we review the principal physical and biogeochemical processes that regulate methane fluxes across the seabed, the fate of this methane in the water column, and potential for its release to the atmosphere. We find that, at present, fluxes of dissolved methane are significantly moderated by anaerobic and aerobic oxidation of methane. If methane fluxes increase then a greater proportion of methane will be transported by advection or in the gas phase, which reduces the efficiency of the methanotrophic sink. Higher freshwater discharge to Arctic shelf seas may increase stratification and inhibit transfer of methane gas to surface waters, although there is some evidence that increased stratification may lead to warming of sub-pycnocline waters, increasing the potential for hydrate dissociation. Loss of sea-ice is likely to increase wind speeds and sea-air exchange of methane will consequently increase. Studies of the distribution and cycling of methane beneath and within sea ice are limited, but it seems likely that the sea-air methane flux is higher during melting in seasonally ice-covered regions. Our review reveals that increased observations around especially the anaerobic and aerobic oxidation of methane, bubble transport, and the effects of ice cover, are required to fully understand the linkages and feedback pathways between climate warming and release of methane from marine sediments. [ABSTRACT FROM AUTHOR]

Extremely intense bubble plumes, like the North Sea 22/4b blowout megaplume (defined as more than 10 6 L day −1 ), create very strong upwelling flows (>1 m s −1 ), which lead to detrainment of methane-enriched water, but leave direct bubble-mediated transport unaffected. Dissolved CH 4 depth profiles and atmospheric measurements during a fall 2011 survey of the 22/4b site suggest strong constraint of seabed CH 4 below the thermocline. Seabed bubbles were nearly pure CH 4 . The effect of the upwelling flow on the fate of bubble plume CH 4 was investigated with a numerical bubble-propagation model. The model considered different representative bubble plume size distributions, ϕ , and a global (total) megaplume bubble size distribution, Φ, synthesized from video survey data and ϕ from the literature. Simulations showed that none of the literature plumes or variations in the upwelling flow could constrain CH 4 sufficiently below the thermocline. Two new bubble megaplume processes were simulated, vortical bubble trapping (slow rise) and a hypothesized, enhanced bubble gas exchange, k BE , an enhancement factor applied to the normal bubble gas exchange rate, k B . The latter could arise from plume turbulence increasing bubble boundary-layer turbulence and thus its gas exchange. Observations could not be reproduced solely by slow rise, however, simulations with k BE ∼6 reproduced observational constraints, as could weaker k BE in conjunction with slow rise. Field validation of k BE is needed given its implications for the fate of megaplume CH 4 emissions (anthropogenic or natural) for stratified and unstratified conditions. k BE suggests marine CH 4 geologic contributions to the atmosphere from all but shallow waters primarily arises from bubble plumes that are less than megaplume size. [ABSTRACT FROM AUTHOR]

A fully automated measurement system for recording of the surface water CO 2 partial pressure, pCO 2 , was deployed on VOS (voluntary observation ship) “Finnpartner/Finnmaid” in 2003. Since 2009, an amendment of the system also allows for the continuous detection of the surface water partial pressure of methane, pCH 4 . The ship commutes regularly at 2–3 day intervals between the Gulf of Finland (Helsinki) and the Mecklenburg Bight (Lübeck) in the southwest of the Baltic Sea. The pCO 2 data in the central Gotland Sea showed a pronounced seasonality that was mainly controlled by the biological production and decomposition of organic matter in combination with stratification/mixing of the water column. CO 2 consumption in spring/summer caused pCO 2 that were by up to 300 μatm below the atmospheric level. In contrast, the pCO 2 exceeded the atmospheric values during autumn/winter when deep mixing transports CO 2 -enriched water to the surface. To identify the central Baltic Sea as a sink or source for atmospheric CO 2 , an air–sea CO 2 gas exchange balance was established for three selected years (2005, 2008 and 2009). During each year the surface water acted as a net sink for atmospheric CO 2 with uptake rates ranging between 0.60 and 0.89 mol m − 2 yr − 1 . The rates correspond approximately to the enhanced carbon burial in sediments during the last century and suggest a link between eutrophication and CO 2 uptake. The data of the surface methane concentration are used to focus on situations were extraordinarily high methane concentrations were observed. Temporary methane peaks were observed south of the Island of Gotland, which could clearly be attributed to frequent upwelling events. Between spring 2012 and 2013, Finnmaid went at a few occasions to St. Petersburg in the east of the Gulf of Finland. Methane concentration of up to 130 nmol L − 1 where found close to the River Neva mouth but decreased rapidly to the west. The plume of methane-enriched waters was observed farthest to the west during the winter period. This was attributed to air–sea gas exchange that was most effective during summer but inhibited during winter because of the ice coverage. [ABSTRACT FROM AUTHOR]

Cold seep ecosystems can support enormous biomasses of free-living and symbiotic chemoautotrophic organisms that get their energy from the oxidation of methane or sulfide. Most of this biomass derives from animals that are associated with bacterial symbionts, which are able to metabolize the chemical resources provided by the seeping fluids. Often these systems also harbor dense accumulations of non-symbiotic megafauna, which can be relevant in exporting chemosynthetically fixed carbon from seeps to the surrounding deep sea. Here we investigated the carbon sources of lithodid crabs (Paralomis sp.) feeding on thiotrophic bacterial mats at an active mud volcano at the Costa Rica subduction zone. To evaluate the dietary carbon source of the crabs, we compared the microbial community in stomach contents with surface sediments covered by microbial mats. The stomach content analyses revealed a dominance of epsilonproteobacterial 16S rRNA gene sequences related to the free-living and epibiotic sulfur oxidiser Sulfurovum sp. We also found Sulfurovum sp. as well as members of the genera Arcobacter and Sulfurimonas in mat-covered surface sediments where Epsilonproteobacteria were highly abundant constituting 10% of total cells. Furthermore, we detected substantial amounts of bacterial fatty acids such as i-C15∶0 and C17∶1ω6c with stable carbon isotope compositions as low as −53‰ in the stomach and muscle tissue. These results indicate that the white microbial mats at Mound 12 are comprised of Epsilonproteobacteria and that microbial mat-derived carbon provides an important contribution to the crab's nutrition. In addition, our lipid analyses also suggest that the crabs feed on other 13C-depleted organic matter sources, possibly symbiotic megafauna as well as on photosynthetic carbon sources such as sedimentary detritus. [ABSTRACT FROM AUTHOR]

Sediment-hosting hydrothermal systems in the Okinawa Trough maintain a large amount of liquid, supercritical and hydrate phases of CO2 in the seabed. The emission of CO2 may critically impact the geochemical, geophysical and ecological characteristics of the deep-sea sedimentary environment. So far it remains unclear whether microbial communities that have been detected in such high-CO2 and low-pH habitats are metabolically active, and if so, what the biogeochemical and ecological consequences for the environment are. In this study, RNA-based molecular approaches and radioactive tracer-based respiration rate assays were combined to study the density, diversity and metabolic activity of microbial communities in CO2-seep sediment at the Yonaguni Knoll IV hydrothermal field of the southern Okinawa Trough. In general, the number of microbes decreased sharply with increasing sediment depth and CO2 concentration. Phylogenetic analyses of community structure using reverse-transcribed 16S ribosomal RNA showed that the active microbial community became less diverse with increasing sediment depth and CO2 concentration, indicating that microbial activity and community structure are sensitive to CO2 venting. Analyses of RNA-based pyrosequences and catalyzed reporter deposition-fluorescence in situ hybridization data revealed that members of the SEEP-SRB2 group within the Deltaproteobacteria and anaerobic methanotrophic archaea (ANME-2a and -2c) were confined to the top seafloor, and active archaea were not detected in deeper sediments (13-30 cm in depth) characterized by high CO2. Measurement of the potential sulfate reduction rate at pH conditions of 3-9 with and without methane in the headspace indicated that acidophilic sulfate reduction possibly occurs in the presence of methane, even at very low pH of 3. These results suggest that some members of the anaerobic methanotrophs and sulfate reducers can adapt to the CO2-seep sedimentary environment; however, CO2 and pH in the deep-sea sediment were found to severely impact the activity and structure of the microbial community. [ABSTRACT FROM AUTHOR]

Abstract: The surface water CO2 partial pressure, pCO2, was measured during nine cruises in the Gulf of Bothnia at different seasons between 1999 and 2010, with continuous recording during most of the cruises using a bubble-type equilibrator and infrared CO2 detection. The data were used to construct the mean seasonality of the pCO2 in characteristic sub-areas. The low pCO2 measured in spring/summer and the high pCO2 measured in fall/winter indicated that biological production, stratification, mineralization and deep mixing, controlled the seasonal pCO2 cycle in both the Bothnian Bay and the Bothnian Sea. To calculate CO2 exchange with the atmosphere, the gas exchange transfer velocity was computed as a function of wind speed according to . Based on the mean seasonal pCO2 distribution, CO2 fluxes were calculated, taking into account the differing wind conditions and the varying ice coverage. The resulting mean annual flux of +140mmolm−2 yr−1 in the Bothnian Bay indicated that this area is a weak source for atmospheric CO2. In contrast, the Bothnian Sea was found to act as a sink for atmospheric CO2, with a mean flux of −730mmolm−2 yr−1. [Copyright &y& Elsevier]

Air–sea gas fluxes are commonly estimated using wind-based parametrizations of the gas transfer velocity. However, neglecting gas exchange forcing mechanisms – other than wind speed – may lead to large uncertainties in the flux estimates and the carbon budgets, in particular, in heterogeneous environments such as marginal seas and coastal areas. In this study we investigated the impact of including relevant processes to the air–sea CO 2 flux parametrization for the Baltic Sea. We used six parametrizations of the gas transfer velocity to evaluate the effect of precipitation, water-side convection, and surfactants on the net CO 2 flux at regional and sub-regional scale. The differences both in the mean CO 2 fluxes and the integrated net fluxes were small between the different cases. However, the implications on the seasonal variability were shown to be significant. The inter-annual and spatial variability were also found to be associated with the forcing mechanisms evaluated in the study. In addition to wind, water-side convection was the most relevant parameter controlling the air–sea gas exchange at seasonal and inter-annual scales. The effect of precipitation and surfactants seemed negligible in terms of the inter-annual variability. The effect of water-side convection and surfactants resulted in a reduction of the downward fluxes, while precipitation was the only parameter that resulted in an enhancement of the net uptake in the Baltic Sea. • Variability of air–sea CO 2 fluxes is modulated by mechanisms other than wind speed. • Gas transfer velocity parameterizations are not adequate for coastal/marginal seas. • Water-side convection enhances air–sea fluxes during winter in the Baltic Sea. • Surfactants suppress CO 2 downward fluxes during summer. • Precipitation represents a net increase of the carbon uptake. [ABSTRACT FROM AUTHOR]

Abstract: Chemoherm carbonates, as well as numerous other types of methane seep carbonates, were discovered in 2004 along the passive margin of the northern South China Sea. Lithologically, the carbonates are micritic containing peloids, clasts and clam fragments. Some are highly brecciated with aragonite layers of varying thicknesses lining fractures and voids. Dissolution and replacement is common. Mineralogically, the carbonates are dominated by high magnesium calcites (HMC) and aragonite. Some HMCs with MgCO3 contents of between 30–38 mol%–extreme-HMC, occur in association with minor amounts of dolomite. All of the carbonates are strongly depleted in δ 13C, with a range from −35.7 to −57.5‰ PDB and enriched in δ 18O (+4.0 to +5.3‰ PDB). Abundant microbial rods and filaments were recognized within the carbonate matrix as well as aragonite cements, likely fossils of chemosynthetic microbes involved in carbonate formation. The microbial structures are intimately associated with mineral grains. Some carbonate mineral grains resemble microbes. The isotope characteristics, the fabrics, the microbial structure, and the mineralogies are diagnostic of carbonates derived from anaerobic oxidation of methane mediated by microbes. From the succession of HMCs, extreme-HMC, and dolomite in layered tubular carbonates, combined with the presence of microbial structure and diagenetic fabric, we suggest that extreme-HMC may eventually transform into dolomites. Our results add to the worldwide record of seep carbonates and establish for the first time the exact locations and seafloor morphology where such carbonates formed in the South China Sea. Characteristics of the complex fabric demonstrate how seep carbonates may be used as archives recording multiple fluid regimes, dissolution, and early transformation events. [Copyright &y& Elsevier]

Measures the rise and dissolution rates of freely released carbon dioxide droplets in the open ocean to provide fundamental data for carbon sequestration options. Carbon dioxide in the liquid phase; Liquid-gas transition; Presence of a hydrate shell.

Both the frequency and intensity of drought events are expected to increase, with unresolved alterations to peatland methane cycling and the involved microbial communities. While existing studies have assessed drought effects via experimental approaches under controlled conditions, to our knowledge, no studies have examined the in-situ effects of natural drought in restored temperate fens. In this study, we used quantitative polymerase chain reaction (qPCR) and high throughput 16S rRNA gene amplicon sequencing of DNA and complementary DNA (cDNA) to determine the abundances and community structure of total and putatively active microbial communities following the 2018 European summer drought. Together with geochemical and methane flux data, we compared these results to a non-drought reference dataset. During drought, water level and methane flux rates decreased to a new recent minimum in both fens. This corresponded with pronounced shifts in porewater geochemistry. Microbial community composition in the drought year differed markedly, and was characterized by a greater relative and total abundance of aerobic methanotrophs, and, in one of the two sites, by a decrease in total methanogen abundance. In contrast to the non-drought reference years, type I methanotrophs were clearly more dominant than type II methanotrophs in both fens. cDNA sequencing confirmed the activity of type I methanotrophs during drought, with Methylomonaceae having the highest average relative abundance of bacterial cDNA transcripts. We show that changes in microbial community dynamics, porewater geochemistry, and ecosystem methane fluxes can be substantial following natural drought in restored fens, and provide the first in-situ evidence from a natural drought which suggests type I methanotroph populations are more active than type II methanotrophs in response to drought effects. Type I methanotrophs may represent a key microbial control over methane emissions in restored temperate fens subject to natural drought. • Drought decreased water level and ecosystem methane fluxes substantially. • Microbial community dynamics reflected drought conditions at the sites. • Methanogen gene abundances decreased in one of the two fens. • Proportion of type II methanotrophs was low and similar to a non-drought year. • Type I methanotrophs dominated the active bacterial community profile at both sites. [ABSTRACT FROM AUTHOR]

Through frequent inundation, erosion and solution processes, sea level rise, which isconsidered to be one of the main impacts of climate change, alters areal extensions of coastalpeatlands. Coastline retreat shifts adjacent ecosystems and results in the submergence ofterrestrial, organic carbon rich peat sediments in the sea. The coastal peatland Heiligenseeand Hütelmoor near Rostock-Warnemünde is a very unique ecosystem due to its strongconnection with the Baltic Sea. Using sediment cores and shallow geo-acoustic surveys, wefound Holocene peat deposits (Corg of 37-53%, 14C-dated to 6725 ± 87 cal yr BP) tocontinue more than 90 m (areal extend: 0.16-0.2 km2) offshore from the recent peatland(Kreuzburg et al., 2018, doi: 10.3389/feart.2018.00103). The location of outcropping peatdeposits coincides with temperature, salinity and bottom water methane anomalies (CH4= 42.4 ± 12.9 nmol L−1) most likely originating from submarine groundwaterdischarge.Motivated by these findings in the field, we investigated the release of peat-derived dissolvedorganic carbon (DOC) and biogeochemical processes that govern carbon transformations by a50-day column experiment, simulating alternating episodes of brackish water (S ∼16)intrusion and seepage of anoxic, low-saline groundwater. Results imply that discharge oflow-saline groundwater through the peat soils significantly increased the release of DOC,with the source confirmed by 13CDOC signatures of -26.9 to -27.7 ‰Ẇe further observed thatthe concentrations of DOC correlate positively with dissolved inorganic carbon(DIC), suggesting rapid organic matter mineralization. In contrast, oxygenatedbrackish water intrusions show a contrary effect with decreasing DOC and DICconcentrations in the pore water and significantly lower CH4 and CO2 emissions.These, to date unmonitored dynamics, reveal a potential source of CO2 and CH4from coastal sediments with submerged peat, and show that the production andrelease of carbon is significantly dependent on the flow regime and on the sedimentcharacteristics. [ABSTRACT FROM AUTHOR]

A letter to the editor is presented in response to the article "Fate of Rising Carbon Dioxide Droplets in Seawater," by Y. Zhang in the December 2005 issue.