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 peatlands represent an interface between marine and terrestrial ecosystems; their hydrology is affected by salt and fresh water inflow alike. Previous studies on bog peat have shown that pore water salinity can have an impact on the saturated hydraulic conductivity (Ks) of peat because of chemical pore dilation effects. In this study, we aimed at quantifying the impact of higher salinities (up to 3.5% NaCl) on Ks of fen peat. Two experiments employing a constant‐head upward‐flow permeameter and differing in measurement and salinity change duration were conducted. Additionally, a third experiment to determine the impact of water salinity on the release of dissolved organic carbon (DOC) of the studied peat type was carried out. The results show a decrease of Ks with time, which does not depend on the water salinity but is differently shaped for different peat types. We assume pore clogging due to a conglomerate of physical, chemical, and biological processes, which rather depend on water movement rate and time than on water salinity. However, an increased water salinity did increase the DOC release. We conclude that salinity‐dependent behaviour of Ks is a function of peat chemistry and that for some peat types, salinity may only affect the DOC release without having a pronounced impact on water flow.

Reactive barriers, such as denitrifying bioreactors, have been identified as a clean-upoption for nutrient-laden agriculture runoff. Here we tested a 20 m long, 3.75 m wide and 2.2 mdeep woodchip bioreactor receiving tile drainage water from a 5.2 ha field site, aiming at testing thehydraulic functioning of a dual-inlet system and quantifying its impact on nutrient loads (nitrogen,reactive phosphorus, organic carbon) in a region with a drainage season taking place in thehydrological winter (November to April). The hydraulic conditions in the dual-inlet bioreactorsystem developed differently than expected; asymmetric flow rates led to long average hydraulicretention times and a highly dispersed residence time distribution, which was revealed by abromide tracer test. With a nitrate load reduction of 51 to 90% over three drainage seasons, thewoodchip bioreactor proved at the same time to be very effective under the winter conditions ofnortheastern Germany. The bioreactor turned from an orthophosphate source in the first year ofoperation into an orthophosphate sink in the second and third year, which was not expected becauseof anoxic conditions (favorable for denitrification) prevailing within the woodchips. Besides anefficient nutrient retention, the woodchip bioreactor contributed to the total organic carbon load ofreceiving waters, which impairs the overall positive role of bioreactors within intensivelyagriculturally used landscapes. We consider this promising low-maintenance biotechnologyparticularly suitable for single drainage pipes with high discharge and high nitrate concentrations.

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.