<p>The flow path of a river draining a lowland in the southern Baltic Sea, the Warnow River, was investigated to evaluate its freshwater composition as a source of dissolved substances to regional coastal waters. A spatial study was carried out once to follow the variations from the source to the estuary. A temporal study in the composition as a function of the season, during 6 years (2017-2022), was carried out at a site just before the river reaches the estuary. Surface water was sampled to analyze major and tracer elements, stable (H, C, O, S), and unstable (Ra) isotopes. The results show that the composition of the Warnow River along the flow path is controlled by a complex interplay between in-situ processes, exchange with the atmosphere, diffuse groundwater, and surface water inlets. On a temporal scale, pH, nutrient, and redox sensitive trace element concentrations are strongly impacted by pelagic primary production in spring. During summer and autumn, influences occurred by benthic microbial activity, associated diffusive release from soils/sediments, and surface water inlets. Throughout the investigation period, the Warnow River was a source of isotopically light CO2 to the atmosphere and DIC to the estuarine waters. The delivered DIC concentrations seem to vary with the season due to changes in biological pelagic and benthic activity. DOC was derived from a mixture of C3 organic sources and fertilizers. From concentration-discharge relationships, examples of dilution, mobilization, and chemostasis trends were found. Discharge-controlled seasonal trends are superimposed by system-internal processes and the hydrological consequences of river and drainage management. Our analysis thus provides new insights into the controls on the variations of water and solutes in a managed river at the land-sea interface as part of the regional hydrological cycle of a lowland catchment coastal water system.</p> <p> </p> <p>The study was supported by the DFG research training group BALTIC TRANSCOAST, DAAD ,  and the BMBF project CARBOSTORE/COOLSTYLE</p>

<p>Biogeochemical processes and microbial community structure were investigated in sediment cores from three pockmarks in Hanko, Finland, in the northern Baltic Sea, and compared to groundwater and seawater measurements. Three studied pockmarks varied with the rate of submarine groundwater discharge (SGD). Based on e.g., chloride and DIC concentrations from sediment porewaters, pockmark D had the strongest groundwater influence, while in pockmark E SGD had ceased and therefore this pockmark resembled typical Baltic Sea water and sediment. The pockmark B was the intermediate representative of SGD. The inactive pockmark E had orders of magnitude higher methane concentrations compared to the active pockmarks, but interestingly, this did not reflect on the copy numbers of methanogenesis marker gene (<em>mcr</em>A) results, as pockmark B had equal methanogenesis gene pool as the pockmark E. Sulfate reducer numbers measured with <em>dsr</em>B marker gene was highest in pockmark E sample but also many orders of magnitude higher in other pockmark sediments compared to seawater and groundwater, where the sulfate reducer numbers were only negligible. Reactive transport modeling (RTM) established that the porewater systems in pockmarks D and B were dominated by groundwater advection pushing reactants for biogeochemical reaction into a narrow zone at sediment surface. The advection reduced the organic matter accumulation which results in absence of sulfate-methane transition zone in these pockmarks and concentrates the microbial activity to these habitats. Microbial community structure revealed with phylogenetic marker gene amplicon sequencing reflects the groundwater in active pockmarks, as notable populations of ammonia-oxidizing archaea and nitrifying bacteria in pockmarks are mainly originating from groundwater. RTM also estimated low rates of sulfate consumption and low rates of methane, ammonium and DIC in the active pockmarks.</p>