<p>Here we report on the porewater dissolved organic matter dynamics and underlying benthic biogeochemical processes in a historically brackish, diked, peatland located along the Baltic Sea in northeastern Germany.  The regeneration process of the “Heiligensee and Hütelmoor” includes a return of freshwater inputs as well as increased connection to the sea.  For porewater observations, two stationary multiport (about 0.5 m intervals) lances are located in the coastal sediments coastward of the sand-dune dyke, reaching down to ~5 meters through permeable sediments and peat layers.  Frequent sampling of these porewater lances indicates substantial influences by fresh submarine ground water discharge in the middle depths.  Therefore, we studied the impact by mixing of these groundwater (as, for example, a source of Fe, DOM, DIC, P, Ca) with saltwater (a source of SO<sub>4 </sub>to fuel sulfate reduction) and the role of organic matter in the drowned peat layers.  We were particularly interested in the sulfurization of DOM, as biogenic sulfide can react both with Fe and DOM/POM.  Samples for a suite of analyses were taken in November 2020.  Characterizations included dissolved organic matter (21T FT-ICR-MS, National High Magnetic Field Laboratory), major and trace elements (ICP-OES), nutrient and sulfide concentration, as well as stable isotopes of sulfate, DIC, and water. Results are compared to nearby groundwater wells (a coastal sandy aquifer, a coastal peat layer, and an inland well), the brackish Baltic Sea and  the Hütelmoor surface waters, as well as the river Warnow.  Thus, we characterize the endmembers as well as the mixing zones in order to understand their influence on the chemical alterations of dissolved organic matter in this dynamic region.</p>

<p>The role that of fresh surface and ground water sources play on the coastal water balance, element balances, and the associated biogeochemical processes is currently a matter of intense debate and investigation. The measures of fresh and saline water mixing in coastal areas have been found to be challenging, however stable water isotopes (O-16, O-17, O-18), in combination with further hydrochemical tracers, provide a valuable tool to identify different sources, that are furthermore linked to different biogeochemical processes, e.g. impacting the benthic and pelagic carbon cycle.</p><p>In the present communication, we report on combined investigations in pore and surface waters of Königshafen Bay (North Frisian island Sylt, Germany), a tidal area in the eastern North Sea. In addition, tidal cycles at the outlet of the bay were sampled. Results are compared to potential surface and subterrestrial fresh water endmembers, open North Sea, submarine groundwater discharge in the backbarrier tidal area of Spiekeroog, as well as the Elbe river estuary. Besides dissolved major and minor elements, the stable water isotope composition is used to characterize the temporal and spatial distribution of different water sources to the bay and the seasonal dynamics in the water column. Porewater gradients indicate different degrees of freshening, locally already in the top 50 cm below the seafloor with spatial heterogeneity. Different fresh water endmembers are indicated both by the water isotope and hydrochemical signatures. It turns that at least two fresh water sources can be identified for sediments under SGD impact, that differ in composition from surface water sources draining into the southern North Sea. Further work is on the way to investigate the dynamics in the (sub)surface fresh water sources for the tidal basin and the link to other geochemical tracers, as well as the coupling to the dissolved carbon system on different temporal and spatial scales.</p><p>  </p><p>The investigations are supported by the DFG-project KiSNet, the BMBF project COOLSTYLE (CARBOSTORE), the DAAD, the DFG project Baltic Transcoast, and Leibniz IOW.</p>

<p>Rewetted peatlands can be a significant source of methane, but in coastal systems, input of sulfate-rich seawater could potentially reduce these emissions. The presence of sulfate is known to suppress methanogenesis, by encouraging the growth of sulfate-reducers, which outcompete methanogens for substrate. After a drought in 2018 and a storm surge in the following winter, we investigated the effects of the drought and the brackish water inflow on the microbial communities relative to methane exchange in a rewetted fen at the southern Baltic Sea coast.</p><p>We took peat cores at four previously sampled locations along a salinity gradient to compare the soil and pore water geochemistry as well as the microbial methane and sulfate cycling communities to the common freshwater rewetting state and the drought 2018. We used high-throughput sequencing and quantitative polymerase chain reaction (qPCR) to characterize pools of DNA and cDNA targeting total and putatively active bacteria and archaea. While sequencing was done for the 16S rRNA gene, qPCR was performed on key functional genes of methane production (<em>mcrA</em>), methane oxidation (<em>pmoA</em>) and sulfate reduction (<em>dsrB</em>) in addition to 16S rRNA. Furthermore, we measured local methane (CH<sub>4</sub>) fluxes with closed chambers and retrieved soil plugs to determine the concentrations and isotopic signatures of dissolved gases in the pore water.</p><p>The sequence of the drought and the inflow of brackish water increased the absolute abundance of sulfate reducing bacteria (SRB) by two orders of magnitude. We did not observe a decrease of absolute methanogenic archaea abundance after the inflow as we expected parallel to the increase of SBRs, but saw that changes in the methanogenic communities’ compositions already took place in the drought year 2018. After the inflow, absolute abundance of aerobic methanotrophic bacteria decreased back to their pre-drought level, following an increase during 2018 drought conditions. The expected establishment of methanotrophic archaea (ANME), which are capable of sulfate-mediated anaerobic methane oxidation, was not recorded though. While CH<sub>4</sub> fluxes showed a strong decline of almost 90 % to a new minimum since rewetting in 2009, dissolved CH<sub>4</sub> pore water concentrations and a strong depletion of <sup>13</sup>C-values of CH<sub>4 </sub>and CO<sub>2</sub> (DIC) indicated ongoing methanogenesis and lack of methane oxidation after the brackish water inflow. The observed reduction of CH<sub>4</sub> emissions might be a result of methane oxidation and sulfate reduction in the brackish water column above the peat soil. The legacy effect of the preceding drought likely influenced the microbial communities and pore water geochemistry simultaneously suggesting a mixed effect of drought and inflow. Overall, our study revealed that the sequence of drought conditions followed by the inflow of brackish water enlarged the sulfate reducing microbial communities and substantially reduced the CH<sub>4</sub> emissions in a rewetted fen. However, unlike drought, which is associated with a rapid and irreversible peat degradation through aerobic decomposition processes, brackish water inflow encourages peat preservation by maintaining anaerobic conditions. Still, further research is needed to directly study the complex effects of brackish water rewetting on peatlands.</p>

<p>The impact of submarine groundwater discharge (SGD) on coastal biogeochemistry is currently under intense investigation. SGD can impact diagenesis and in general act as a potential source of elements, especially dissolved carbon, to coastal surface waters. However, qualitative and quantitative assessments of SGD are challenging since it requires the identification of suitable geochemical tracers for the complex hydrological and biogeochemical processes in the subterranean estuary. In this communication, we report on combined investigations carried out in Königshafen Bay (North Frisian island Sylt, Germany), a tidal area in the eastern North Sea. Sampling encompassed vertical porewater gradients, and surface waters collected through transects in the bay, and in tidal cycles at the outlet of the bay. Potential surface and subterrestrial freshwater endmembers are used to assess the results. Besides major and minor elements, this study focuses on the stable carbon isotope composition of dissolved inorganic carbon (DIC) and the activity of radium (Ra) isotopes. Our main aim is to characterize the interaction between diagenesis and the composition of SGD, as well as the resulting impact on the carbon system of the water column, and, via tidal exchange extended to the coastal North Sea. Porewaters showed usually an increase of isotopically light DIC with depth and a freshening already in the top 50 cmbsf at some sites. This indicates that both, carbon diagenesis and mixing of seawater with fresh groundwaters at depth impact the distribution of DIC. The activities of the short-living Ra isotope (<sup>224</sup>Ra<sub>ex</sub>) were higher in the bay compared to the open North Sea. Porewater activities were up to 30 times higher than in the bay’s surface waters with a maximum development at intermediate salinities. In the water column at the outlet of the bay, <sup>224</sup>Ra<sub>ex</sub> and <sup>223</sup>Ra showed maximum activities during low tide as a consequence of the highest contribution of waters in contact with the sediments of the bay. Moreover, due to the high hydraulic gradient developed during low tide more contribution from potential endmembers enriched in Ra can be expected. Further work is on the way to quantify the impact of SGD on the tidal basin and the indirect role for the North Sea carbon system on different temporal and spatial scales.</p><p> The investigations are supported by the DFG-project KiSNet, the BMBF project COOLSTYLE (CARBOSTORE), the DAAD, the DFG RTG Baltic TRANSCOAST, and the Leibniz IOW.</p>

Abstract. Rewetted peatlands can be a significant source of methane (CH4), but in coastal ecosystems, input of sulfate-rich seawater could potentially mitigate these emissions. The presence of sulfate as electron acceptor during organic matter decomposition is known to suppress methanogenesis, by favoring the growth of sulfate-reducers, which outcompete methanogens for substrate. We investigated the effects of a brackish water inflow on the microbial communities relative to CH4 production-consumption dynamics in a freshwater rewetted fen at the southern Baltic Sea coast after a storm surge in January 2019 and analyzed our data in context with the previous freshwater rewetted state (2014 serves as our baseline) and the conditions after a severe drought in 2018. We took peat cores at four previously sampled locations along a brackishness gradient to compare soil and pore water geochemistry as well as the microbial methane and sulfate cycling communities with the previous conditions. We used high-throughput sequencing and quantitative polymerase chain reaction (qPCR) to characterize pools of DNA and cDNA targeting total and putatively active bacteria and archaea. Furthermore, we measured CH4 fluxes along the gradient and determined the concentrations and isotopic signatures of trace gases in the peat. We found that both, the inflow effect of brackish water and in parts also the preceding drought increased the sulfate availability in the surface and pore water. Still, peat soil CH4 concentrations and the 13C compositions of CH4 and total dissolved inorganic carbon (DIC) indicated ongoing methanogenesis and little methane oxidation. Accordingly, we did not observe a decrease of absolute methanogenic archaea abundance or a substantial change in methanogenic community composition following the inflow, but found that the methanogenic community had mainly changed during the precedent drought. In contrast, absolute abundances of aerobic methanotrophic bacteria decreased back to their pre-drought level after the inflow while they had increased during the drought year. In line with the higher sulfate concentrations, the absolute abundances of sulfate reducing bacteria (SRB) increased – as expected – by almost three orders of magnitude compared to the freshwater state and also exceeded abundances recorded during the drought by over two orders of magnitude. Against our expectations, methanotrophic archaea (ANME), capable of sulfate-mediated anaerobic methane oxidation, did not increase in abundance after the brackish water inflow. Altogether, we could find no microbial evidence for hampered methane production or increased methane consumption in the peat soil after the brackish water inflow. Because Koebsch et al. (2020) reported a new minimum in CH4 fluxes at this site since rewetting of the site in 2009, methane oxidation may, however, take place in the water column above the peat soil or in the lose organic litter on the ground. This highlights the importance to consider all compartments across the peat-water-atmosphere continuum to develop an in-depth understanding of inflow events in rewetted peatlands. We propose that the changes in microbial communities and GHG fluxes relative to the previous freshwater rewetting state cannot be explained with the brackish water inflow alone, but was potentially reinforced by a biogeochemical legacy effect of the precedent drought.