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<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><span>Land-ocean interactions in the coastal zone (LOICZ) are of particular interest regarding the exchange of water and elements, like nutrients, carbon, sulfur, and metals. </span><span>Processes impacting </span><span>groundwater</span><span> fluxes at these boundaries belong to the still unsolved problems in hydrology (Blöschl et al., 2019). </span><span>Stable isotope signatures (H, C, O, S), major and trace element contents in surface waters of a rewetted coastal peatland were investigated to understand the impact of storm-induced flooding by brackish seawater on hydrology and biogeochemical element cycling.</span></p><p><span>The study area is the Hütelmoor, a wetland located at the coastline of the southern Baltic Sea. The area is characterized by a continuous release of fresh water to the Baltic Sea via submarine groundwater discharge (Jurasinski et al., 2018). Surface water is partly drained to a nearby river, but the introduction of brackish waters into the peatland is typically precluded by a small dune and limited to storm-induced flooding events. In the present study, the spatially distributed composition of surface waters was investigated briefly after a flooding event. The results are compared with previous campaigns without actual salt water impact. </span></p><p><span>Conservative elements and water isotopes demonstrate the importance of seasonal variations due to varying evapotranspiration during pre-flood times and allow for a quantification of mixing processes in the post-flood waters. The impact of soil respired CO</span><sub><span>2</span></sub><span>, and/or the mineralization of organic matter or methane on the surface waters is indicated by a shift of the C isotope composition of DIC towards lighter data. The S and O isotopic composition of dissolved sulfate indicates an impact by solutions modified by net microbial sulfate reduction on pre-flood surface waters and a potential oxidation of reduced sulfur species in post-flooding solutions. </span></p><p><span>Previous flooding events already impacted element cycling in the peatland’s past and are also reflected by a sulfidization of peat layers (Fernández-Fernández et al., 2017) and the observation of local areas with enhanced dissolved concentrations in the central part of the peatland.</span></p><p><span>The study is supported by DFG during GK Baltic TRANSCOAST, DAAD, and Leibniz IOW.</span></p><p align="justify"> </p><p><span>References:</span></p><ul><li><span>Blöschl G. et al. (2019) Twenty-three unsolved problems in hydrology (UPH) – a community perspective. Hydrol. Sci. J. 64, 1141-1158.</span></li> <li><span>Jurasinski G. et al. (2018) Understanding the coastal ecocline: Assessing sea-land-interactions at non-tidal, low-lying coasts through interdisciplinary research. Front. Mar. Sci. 5, 1-22</span><span>.</span></li> <li>Fernández-Fernández L.E. et al. <span>(2017) Sulfur isotope biogeochemistry of soils from an episodically flooded coastal wetland, southern Baltic Sea. Geophys. Res. Abs. 19, EGU2017-14335</span><span>.</span></li> </ul>

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<p align="justify">Land-ocean interactions in the coastal zone are of particular interest regarding the exchange of substances, like nutrients, carbon, sulfur, metals, and water. The rising sea level is and will enhance the pressure of salty solutions on previously fresh water ecosystems. We present here new results on the isotope biogeochemistry of a rewetted <span>peatland</span>, at the southern Baltic Sea, that is <span>impacted</span> by event-type flooding with brackish seawater. Sediment cores on transects through the wetland were investigated for the<span>ir</span> pore water and solid phase (mineral and organic matter) composition. Different fractions of the soils and solutions were analyzed for the elemental composition, mineral micro-textures, and the stable isotope composition (H, C, O, S) to understand the changes in water and biogeochemical carbon-sulfur-metal cycles due to flooding and the consequence for the development of sulfur isotope signatures in authigenic mineral phases and organic matter.</p><p align="justify">Flooding events with brackish water <span>increased </span>the availability of sulfate as an electron acceptor for microbial carbon transformations. This added sulfur impacted the remineralization capacity of organic substrate<span>s</span> and created space for mineral authigenesis, with related iron sulfide textures. It yields isotope signals that are indicative for non-steady state biogeochemistry of coastal ecosystems and allow for a transfer of proxy information to other modern and past coastal organic-rich peatlands.</p><p align="justify">The soil cores <span>from</span> the peatland <span>reflects</span> the intense activity of sulfate-reducing bacteria and the associated formation of iron sulfides (essentially pyrite) <span>and provided the </span>isotop<span>e</span> evidence for site-dependent sulfurization of organic matter. Sedimentary sulfur fractions and their stable isotope signatures are controlled by the availability of dissolved organic matter and/or methane, reactive iron, and in particular dissolved sulfate and, thereby, from the relative position with respect to the coast line, and depend on the surface topography and soil characteristics. Further mechanistic investigations consider the role of DOS upon changing sulfur substrate availability.</p><p> </p><p>Acknowledgement for support by DFG-Baltic TRANSCOAST, ERASMUS, DAAD, Leibniz-IOW</p>