Coastal zones connect terrestrial and marine ecosystems forming a unique environment that is under increasing anthropogenic pressure. Rising sea levels, sinking coasts, and changing precipitation patterns modify hydrodynamic gradients and may enhance sea-land exchange processes in both tidal and non-tidal systems. Furthermore, the removal of flood protection structures as restoration measure contributes locally to the changing coastlines. A detailed understanding of the ecosystem functioning of coastal zones and the interactions between connected terrestrial and marine ecosystems is still lacking. Here, we propose an interdisciplinary approach to the investigation of interactions between land and sea at shallow coasts, and discuss the advantages and the first results provided by this approach as applied by the research training group Baltic TRANSCOAST. A low-lying fen peat site including the offshore shallow sea area on the southern Baltic Sea coast has been chosen as a model system to quantify hydrophysical, biogeochemical, sedimentological, and biological processes across the land-sea interface. Recently introduced rewetting measures might have enhanced submarine groundwater discharge (SGD) as indicated by distinct patterns of salinity gradients in the near shore sediments, making the coastal waters in front of the study site a mixing zone of fresh-and brackish water. High nutrient loadings,

Coastal low-lying areas along the southern Baltic Sea provide good conditions for coastal peatland formation. At the beginning of the Holocene, the Littorina Sea transgression caused coastal flooding, submergence and erosion of ancient coastlines and former terrestrial material. The present Heiligensee and Hütelmoor peat deposits (located near Rostock in Northern Germany) were found to continue more than 90 m in front of the coastline based on on-and offshore sediment cores and geo-acoustic surveys. The seaward areal extent of the coastal peatland is estimated to be around 0.16-0.2 km 2 . The offshore boundary of the former peatland roughly coincides with the offshore limit of a dynamic coast-parallel longshore bar, with peat deposits eroded seawards. While additional organic-rich layers were found further offshore below a small sand ridge system, no connection to the former peatlands can be established based on 14 C age and C/N ratios. The preserved submerged peat deposits with organic carbon contents of 37% in front of the coastal peatland Heiligensee and Hütelmoor was radiocarbondated to 6725 ± 87 and 7024 ± 73 cal yr BP, respectively, indicating an earlier onset of the peatland formation as presently published. The formation time of the peat layers reveals information about the local sea level rise. The local sea level curve derived from our 14 C-dated organic-rich layers is in general agreement to nearby sea level reconstructions (North Rügen and Fischland, Northern Germany), with differences explained by slightly varying local isostatic movements.

Sea-level rise coupled with land subsidence from wetland drainage exposes increasingly large areas of coastal peatlands to seawater intrusion. Seawater contains high concentrations of sulfate (SO 4 2−), which can alter the decomposition of organic matter thereby releasing organic and inorganic solutes from peat. In this study, a flow-through reactor system was used in order to examine the transport of SO 4 2− through peat as well as its effect on solute release. Moderately-decomposed fen peat samples received input solutions with SO 4 2− concentrations of 0, 100, 700, and 2,700 mg L −1 ; sample effluent was analyzed for a variety of geochemical parameters including dissolved organic carbon (DOC), dissolved inorganic carbon (DIC) and total dissolved nitrogen (TDN) as well as the concentrations of major cations and anions. The input solution remained anoxic throughout the experiment; however, no signs of a pronounced SO 4 2− reduction were detected in the effluent. SO 4 2− transport in the fen peat resembled non-reactive bromide (Br −) transport, indicating that in the absence of SO 4 2− reduction the anion may be considered a conservative tracer. However, slightly elevated concentrations of DOC and TDN, associated with raised SO 4 2− levels, suggest the minor desorption of organic acids through anion exchange. An increased solute release due to stimulated decomposition processes, including SO 4 2− reduction, was observed for samples with acetate as an additional marine carbon source included in their input solution. The solute release of peats with different degrees of decomposition differed greatly under SO 4 2−-enriched conditions where strongly-decomposed fen peat samples released the highest concentrations of DOC, DIC and TDN.

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 AE 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 (δ 13 C DOC − 26.9‰ to − 27.7‰), which was accompanied by the production of dissolved inorganic carbon (DIC) and emission of carbon dioxide (CO 2 ), implying DOC mineralization. Oxygen respiration, sulfate (SO 2 − 4 ) reduction, and methane (CH 4 ) formation were differently pronounced in the sediments and were accompanied with higher microbial abundances in peat compared to sand with SO 2 − 4 -reducing bacteria clearly dominating methanogens. With decreasing salinity and SO 2− 4 concentrations, CH 4 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 CH 4 and CO 2 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.Sea-level rise is considered to be one of the main impacts of climate change, with significant implications for mineralization processes within coastal wetlands (Nicholls and Cazenave 2010;Neubauer 2013;Plag and Jules-Plag 2013;Hahn et al. 2015;Wang et al. 2016). In particular, coastline retreat may cause submergence of terrestrial, organic carbonrich peat sediments. The extent of submarine peat and the process-based impacts on carbon transformation processes and exchange of trace gases in shallow coastal areas have been poorly addressed. Sediment column experiments are powerful tools to investigate subprocesses and simulate changes of environmental and hydrological conditions. These changes are accelerated by land subsidence of peatland caused by their large-scale drainage for agricultural use. Land subsidence alters the hydrologic exchange processes across the land-sea interface (Nieuwenhuis and Schokking 1997;Hooijer et al. 2012) including changes in surficial runoff, subsurface mixing, and submarine groundwater discharge (SGD).SGD is comprised of all flow of water from the seabed into the coastal ocean, predominantly recirculated seawater (SW), driven by wave action, density gradients, and sea-level dynamics (Robinson et al.

<p>Enhanced silicate weathering (ESW) in coastal environments is a promising method for ocean alkalinity enhancement. The idea behind ESW is to generate alkalinity by application of silicate minerals in coastal areas, where waves, currents and bioturbation can speed up the weathering rate. Due to its potentially large CO<sub>2</sub> sequestration capacity and relatively high technological readiness, allowing rapid upscaling, coastal ESW currently receives substantial interest from researchers and policymakers. However, the vast majority of studies on ESW have been conducted in idealised laboratory conditions, while research on the method in natural environments is lacking. As a result, the CO<sub>2</sub> sequestration efficiency and environmental risks when applying ESW in the field remain largely unknown.</p> <p>Here we present results from the first and longest-running mesocosm experiment investigating ESW and associated CO<sub>2</sub> uptake in coastal marine sediments. Using tanks containing one square meter of natural seafloor each, we have studied biogeochemical cycling in sediment treated with the fast-weathering silicate mineral olivine. Lugworms (<em>Arenicola marina</em>) were added to some tanks to investigate the effect of bioturbation on the olivine dissolution rate, as well as the impact of olivine addition on biota. In the mesocosms, we quantified the sedimentary release of alkalinity and other weathering end-products (trace metals and dissolved silicate). Five years into the experiment, olivine dissolution is obvious from an elevated sedimentary alkalinity release and decreased average olivine grain size. The elevated alkalinity release has further led to higher CO<sub>2</sub> sequestrations in tanks with olivine. Based on the results from this unique mesocosm setup, we will discuss the large-scale effect of ESW on biogeochemical cycling in coastal ecosystems.</p>