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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]

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]

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]