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Significant research has been carried out in the last decade to describe the CO2 system dynamics in the Baltic Sea. However, there is a lack of knowledge in this field in the NE Baltic Sea, which is the main focus of the present study. We analysed the physical forcing and hydrographic background in the study year (2018) and tried to elucidate the observed patterns of surface water CO2 partial pressure (pCO2) and methane concentrations (cCH4). Surface water pCO2 and cCH4 were calculated from continuous measurements during six monitoring cruises onboard R/V Salme, covering the Northern Baltic Proper (NBP), the Gulf of Finland (GoF) and the Gulf of Riga (GoR) and all seasons in 2018. The general seasonal pCO2 pattern showed oversaturation in autumn-winter and undersaturation in spring-summer in all three areas, but it locally reached the saturation level during the cruises in April, May and August in the GoR and in August in the GoF. cCH4 was oversaturated during the entire study period, and the seasonal course was not well exposed on the background of high variability. Surface water pCO2 and cCH4 distributions showed larger spatial variability in the GoR and GoF than in the NBP for all six cruises. We linked the observed local maxima to river bulges, coastal upwelling events, fronts, and occasions when vertical mixing reached the seabed in shallow areas. Seasonal averaging over the CO2 flux based on our data suggest a weak sink for atmospheric CO2 for all basins, but high variability and the long periods between cruises (temporal gaps in observation) preclude a clear statement. [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]

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]

The rewetting of drained peatlands supports long-term nutrient removal in addition to reducing emissions of carbon dioxide (CO2) and nitrous oxide (N2O). However, rewetting may lead to short-term nutrient leaching into adjacent water and high methane (CH4) emissions. The consequences of rewetting with brackish water on nutrient and greenhouse gas (GHG) fluxes remain unclear, although beneficial effects such as lower CH4 emissions seem likely. Therefore, we studied the actively induced rewetting of a coastal peatland with brackish water, by comparing pre- and post-rewetting data from the peatland and the adjacent bay. Both the potential transport of nutrients into adjacent coastal water and the shift of GHG fluxes (CO2, CH4, N2O) accompanying the change from drained to inundated conditions were analyzed based on measurements of the surface water concentrations of nutrients (dissolved inorganic nitrogen (DIN), phosphate (PO43-)), oxygen (O2), components of the CO2 system, CH4, and N2O together with manual closed-chamber measurements of GHG fluxes. Our results revealed higher nutrient concentrations in the rewetted peatland than in the adjacent bay, indicating that nutrients leached out of the peat and were exported to the bay. A comparison of DIN concentrations of the bay with those of an unaffected reference station showed a significant increase after rewetting. The total nutrient export out of the peatland ranged between 12.5 and 36.5 t yr−1 for DIN-N and 0.2 ± 0.5 t yr−1 for PO4-P. The peatland was also a source of GHG in the first year after rewetting. However, the spatial and temporal variability decreased and high CH4 emissions, as reported for freshwater rewetting, did not occur. CO2 fluxes decreased slightly from 0.29 ± 0.74 g m−2 h−1 (pre-rewetting) to 0.26 ± 0.29 g m−2 h−1 (post-rewetting). The availability of organic matter (OM) and dissolved nutrients were likely the most important drivers of continued CO2 production. Pre-rewetting CH4 fluxes ranged from 0.13 ± 1.01 mg m−2 h−1 (drained land site) to 11.4 ± 37.5 mg m−2 h−1 (ditch). After rewetting, CH4 fluxes on the formerly dry land increased by 1 order of magnitude (1.74 ± 7.59 mg m−2 h−1 ), whereas fluxes from the former ditch decreased to 8.5 ± 26.9 mg m−2 h−1 . These comparatively low CH4 fluxes can likely be attributed to the suppression of methanogenesis by the available O2 and sulfate, which serve as alternative electron acceptors. The post-rewetting N2O flux was low, with an annual mean of 0.02 ± 0.07 mg m−2 h−1. Our results suggest that rewetted coastal peatlands could account for high, currently unmonitored nutrient inputs into adjacent coastal water, at least on a short time scale such as a few years. However, rewetting with brackish water may decrease GHG emissions and might be favored over freshwater rewetting in order to reduce CH4 emissions. [ABSTRACT FROM AUTHOR]

The direction and magnitude of carbon dioxide fluxes between the atmosphere and the sea are regulated by the gradient in the partial pressure of carbon dioxide (pCO2) across the air–sea interface. Typically, observations of pCO2 at the sea surface are carried out by using research vessels and ships of opportunity, which usually do not resolve the diurnal cycle of pCO2 at a given location. This study evaluates the magnitude and driving processes of the diurnal cycle of pCO2 in a coastal region of the Baltic Sea. We present pCO2 data from July 2018 to June 2019 measured in the vicinity of the island of Utö at the outer edge of the Archipelago Sea, and quantify the relevant physical, biological, and chemical processes controlling pCO2. The highest monthly median of diurnal pCO2 variability (31 µatm) was observed in August and predominantly driven by biological processes. Biological fixation and mineralization of carbon led to sinusoidal diurnal pCO2 variations, with a maximum in the morning and a minimum in the afternoon. Compared with the biological carbon transformations, the impacts of air–sea fluxes and temperature changes on pCO2 were small, with their contributions to the monthly medians of diurnal pCO2 variability being up to 12 and 5 µ atm, respectively. During upwelling events, short-term pCO2 variability (up to 500 µ atm within a day) largely exceeded the usual diurnal cycle. If the net annual air–sea flux of carbon dioxide at our study site and for the sampled period is calculated based on a data subset that consists of only one regular measurement per day, the bias in the net exchange depends on the sampling time and can amount up to ±12 %. This finding highlights the importance of continuous surface pCO2 measurements at fixed locations for the assessment of the short-term variability of the carbonate system and the correct determination of air–sea CO2 fluxes. [ABSTRACT FROM AUTHOR]

Organic matter production by cyanobacteria blooms is a major environmental concern for the Baltic Sea, as it promotes the spread of anoxic zones. Partial pressure of carbon dioxide (p CO2) measurements carried out on Ships of Opportunity (SOOP) since 2003 have proven to be a powerful tool to resolve the carbon dynamics of the blooms in space and time. However, SOOP measurements lack the possibility to directly constrain depth-integrated net community production (NCP) in moles of carbon per surface area due to their restriction to the sea surface. This study tackles the knowledge gap through (1) providing an NCP best guess for an individual cyanobacteria bloom based on repeated profiling measurements of p CO2 and (2) establishing an algorithm to accurately reconstruct depth-integrated NCP from surface p CO2 observations in combination with modelled temperature profiles. Goal (1) was achieved by deploying state-of-the-art sensor technology from a small-scale sailing vessel. The low-cost and flexible platform enabled observations covering an entire bloom event that occurred in July–August 2018 in the Eastern Gotland Sea. For the biogeochemical interpretation, recorded p CO2 profiles were converted to CT* , which is the dissolved inorganic carbon concentration normalised to alkalinity. We found that the investigated bloom event was dominated by Nodularia and had many biogeochemical characteristics in common with blooms in previous years. In particular, it lasted for about 3 weeks, caused a CT* drawdown of 90 µmolkg-1 , and was accompanied by a sea surface temperature increase of 10 ∘C. The novel finding of this study is the vertical extension of the CT* drawdown up to the compensation depth located at around 12 m. Integration of the CT* drawdown across this depth and correction for vertical fluxes leads to an NCP best guess of ∼1.2 molm-2 over the productive period. Addressing goal (2), we combined modelled hydrographical profiles with surface p CO2 observations recorded by SOOP Finnmaid within the study area. Introducing the temperature penetration depth (TPD) as a new parameter to integrate SOOP observations across depth, we achieve an NCP reconstruction that agrees to the best guess within 10 % , which is considerably better than the reconstruction based on a classical mixed-layer depth constraint. Applying the TPD approach to almost 2 decades of surface p CO2 observations available for the Baltic Sea bears the potential to provide new insights into the control and long-term trends of cyanobacteria NCP. This understanding is key for an effective design and monitoring of conservation measures aiming at a Good Environmental Status of the Baltic Sea. [ABSTRACT FROM AUTHOR]

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]

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]