Irrespective of the general situation, extreme events may cause higher gradients than expected. Input of seawater to the peatland can be traced by the salinity of the groundwater. Recent measurements in groundwater wells across the peatland yielded salinities between 3 and 5 PSU both in the peat and the underlying sand aquifer Figure 7A. During a previous sampling campaign, which focused on the central areas of the study site, salinities of up to 9 PSU were locally measured in the upper peat layers Figure 7B ; Koebsch et al. Less pronounced seasonal oscillations are presumably due to evaporation and dilution processes.
Observations at groundwater wells behind the dune dyke show that storm events with high seawater levels cause subsurface saltwater intrusions into the sands behind the dune dyke, detectable as sudden increases in salinity with subsequent gradual decrease. However, these small seawater intrusions were only observed where the dune is most narrow and could not be detected at 50 m inland of areas with wider dunes.
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During the decades of drainage, the hydraulic gradients between peatland and Baltic Sea were reversed Figure 6 , suggesting a bigger salt wedge and regular intrusions of saline waters into the peatland by surface flooding via the drainage ditches which are connected to the Baltic Sea via the Warnow estuary at that time supported by Bohne and Bohne, , and an unpublished Master thesis from It is assumed that the high salinities found further inland in the peatland today are still a relic of former floodings.
Peat pore water depth profiles along a transect from the coastline to the forest show that at all four coring locations, except the one closest to the forest, electrical conductivity was increasing with depth, and Selle et al. Only near to the southeastern boundary of the peatland close to the forest, groundwater originated from the year of rewetting These observations suggest a strong legacy effect of flooding with seawater in the peatland, supporting the hypothesis that the impact of sea water on the terrestrial side is expected to have long lasting consequences for biogeochemical cycling on the land side.
A Geological profile through the peatland with salinity distribution PSU ; B Depth distribution of salinity derived from pore water sampling along a transect from the forest Core 1 to close behind the dune dyke Core 4 see Figure 3.
All cores but the one near the forest edge show increasing salinities with depth within the peat. Combined concentration and stable isotope analysis C, S indicates an intense exchange of water and or substances between the soil pore water and the drainage ditches, which can also be seen in the field by enhanced concentrations of metabolites and redox-sensitive metals in surface waters.
DIC was enriched in the light carbon isotope compared to Baltic seawater and dissolved sulfate that was already isotopically modified by net microbial sulfate reduction — processes that only take place within the anoxic parts of the soils. Thus, when assessing the intensity of exchange between land and sea at shallow coasts, the influence of man-made structures like ditches has to be considered since the signal on the land side will likely be the result of both exchange processes across the shoreline, and through canals and ditches, which are widespread structures on shallow coasts.
Fen peatlands and wetlands in general are transitional habitats with associated unique processes occurring between land-based mineral soils and water bodies such as rivers and oceans. However, their buffer function for water and compound fluxes depends on forcing gradients and material properties.
With regards to the latter, the saturated hydraulic conductivity Ks is of mayor importance, especially in rewetted systems where high water tables prevail. It has been suggested that the geochemistry of pore water may influence the hydraulic properties of non-rigid organic soils. We tested the sensitivity of Ks upon shifts in salinity on peat samples in different stages of degradation Gosch et al.
The hydraulic conductivity of the undisturbed peat samples was measured with the constant head method, using water with different electrical conductivities ranging from 1 to 55 mS cm For details, see Gosch et al. Our results suggest that salinisation has only minor and non-directional impact on Ks, no matter how strongly we increased the salinity of the water. Our results showed a decrease of Ks with time, which did not depend on the water salinity but was differently shaped for different peat types ibid.
Interestingly, these findings do not confirm earlier studies in which Ks was found to increase with increasing salinity Ours et al. These studies were, however, conducted on bog peat samples whereas we have analyzed fen peat.
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Although systematic comparisons are missing, it is likely that Ks differs systematically between bog and fen peat since the pore structure of the peats differ strongly. The hydraulic conductivity, as measured in percolation experiments, is to a certain extent an integrative signal of the soil pore structure Rezanezhad et al. It can, thus, be concluded from the given results that varying salinity conditions do not necessarily modify the pore structure of peat soils as had been hypothesized. This research needs to be continued explicitly addressing pore structure employing imaging methods.
A comparative computer tomography analysis on samples from the main peat body and from peat layers outcropping in the sea continuous seawater impact may be a promising way to study the effect of sea water on peat structure. In this sandy environment, hydrodynamic forces mostly related to wave action induce pore water flows when interacting with ripples and similar sedimentary structures.
Overlying water is advected into and pore water driven out of the sediment in distinct areas on spatial scales of cm to dm based on ripple dimensions. Ripples change position in response to hydrodynamic forcing. At the coast off Rostock, these changes occur on time scales ranging from minutes to days.
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From in situ experiments using stirred benthic chambers Huettel and Gust, we found fluxes of nutrients between pore water and overlying water to increase two- to fourfold when the sediment interface was exposed to pressure gradients mimicking typical hydrodynamic conditions in the overlying water. Adding the temporal dynamics of the interface topography this suggests a highly dynamic environment with respect to salinity, oxygen, sulfide, and pore water nutrient concentrations. The groundwater was marked with fluorescent dye and infused through a permeable sea bed made of porous foam over a surface of 0.
The flow measurement was performed by a laser optical PIV-LIF particle image velocimetry — laser induced fluorescence setup. The field is illuminated by a YAG laser at a 7. The LIF camera captures the fluorescent light, which is then quantitatively evaluated indicating the concentration in the groundwater. The PIV camera captures the refracted light from the tracer particles in the flow in double frames. From the time resolved flow velocity and concentration fields the concentration boundary layer and turbulent Reynolds stresses and fluxes were calculated. Concerning the boundary conditions, three scenarios were investigated to cover different levels of agitation in the coastal zone.
The transport coefficients derived from the fields of concentration and velocity for these scenarios can then be used in an improved modeling of the flow and transport in the benthic boundary layer under unsteady conditions. This boundary was observed during a time period of 15 min. Three scenarios with increasing wave amplitude and orbital velocity were investigated. Our results from these measurements show that for a planar topography the vertical transport of the discharged groundwater is dominated by the diffusion velocity, whereas the horizontal transport is governed by the wave motion.
Thus, with increasing wave motion the initially pure vertical transport is substituted by a horizontal efflux of the discharged groundwater. Further analysis will allow derivation of model equations of the distribution of discharged groundwater in the benthic boundary layer with respect to the wave motion and the surface topography. These experimentally validated model equations for the turbulent transport can then be used as improved boundary conditions for the numerical simulation of the distribution process on the entire coast bringing us closer to a better understanding of exchange processes on shallow coasts.
Thickness of the groundwater boundary layer in lab experiment derived from the concentration profile of the laser induced fluorescence LIF tracer in the discharged groundwater. The profile was derived from a horizontal averaging of the LIF-tracer concentration maps example map displayed above. To investigate the seasonal and event-driven effects on the transport and biogeochemical transformation processes, vertical pore water profiles were retrieved regularly from water lances.
Dissolved sulfide was measured spectrophotometrically Specord 40 spectrophotometer according to Cline Temperature and salinity PSU distribution in front of the experimental coastal peatland summer A seasonal study of the Ra isotope distribution in surface waters showed high dynamics, and based on the detection of short-living Ra isotopes, a clear indication for fast and intense benthic-pelagic mixing of Ra-enriched pore waters into the water column.
Along the shore line hydrochemical and isotopic composition of pore waters down to 5 m below sea floor were investigated regularly Figure The found freshening with depth indicates increasing contributions by less saline water that is also isotopically lighter than the Baltic Sea surface water.
In parallel, the pore waters display the typical biogeochemical zonation with an increase in the concentrations of DIC, PO 4 , NH 4 , and H 2 S with depth data not shown. The carbon isotope signature of DIC indicates the addition of biogenic CO 2 derived from the mineralization of reduced carbon, probably superimposed by carbonate dissolution. Sulfide re-oxidation is likely limited to processes in the top part of the sediments and limited at depth due to a lack of reactive iron. The investigations of the long pore water profiles indicate dynamics in the pore water system which support the hypothesis that the near shore exchange through the sediment is a constant source of nutrients and complex organic molecules to the coastal sea.
Examples for vertical profiles of the pore water composition at a submarine groundwater discharge SGD impacted site taken with a mobile pore water lance in spring Whereas the water isotopes and salinity PSU indicate the contribution of fresh and isotopically lighter water at depth, the dissolved carbonate system shows a clear contribution of biogenic CO 2 derived from the anaerobic mineralization of essentially marine organic matter. Dissolved sulfide, the product of microbial sulfate reduction, accumulated about 40 cm below sea floor and also phosphate and silica were substantially enriched in the pore waters when compared to surface water.
The release of DOM from peat layers from both the terrestrial and the marine side varied in leaching experiments. DOM was released in higher concentrations from the terrestrial peat as compared to the outcropping peat layers in the sea. The composition of a typical pore water DOM from peat exposed to saltwater as revealed by pyrolysis-field ionization mass spectrometry Py-FIMS — a non-targeted soft ionization MS method Leinweber et al.
This composition differs from the mean composition of other fen peat DOM samples Leinweber et al. These differences may reflect an influence of salinity on the DOM sampled because saline waters extracted more phenols and lignin monomers and alkylaromatics than freshwater. This finding has important consequences for a future with a predicted increase in flooding events—i. Due to its evolution and land management decisions in the past first drainage, then rewetting , the studied coastal fens biogeochemistry is impacted by both fresh and saline waters.
In the past, the fen must have received substantial sulfate loads from episodic flooding with brackish water and the legacies of those floodings can still be found in deeper layers of the peat. Despite the high sulfate loads, which are known to inhibit methanogenesis Bartlett et al. Our analysis of repeated pore water sampling shows that high sulfate concentrations and high methane concentrations from surface to pore waters are spatially separated Figure 11 suggesting sulfate reduction and methanogenesis to happen in different depth zones and, thus, supporting the hypothesis that episodic flooding with seawater generates strong legacy effects in the biogeochemical cycling on land.
Pore water profiles of DOC black, circle , CH 4 blue, diamond , and SO 4 red, square derived from repeated sampling in the central part of the peatland. Symbols denote median concentrations over monthly samples taken from May to October , shaded polygons stretch from the 1st to the 3rd quartile of the distribution at the specific depths. Lines as well as shading contours are linearly interpolated between depths.
We found a strong increase in salinity with depth Figure 7B at all sampling locations likely caused by residual brackish solutions. This zone of intense sulfur cycling is positioned below a freshened soil zone were methanogenesis takes place, which is dominated by acetotrophic Methanosaeta. Anaerobic oxidation of methane is indicated for the transitional zone. A comparison of pore water and solid phase compositional profiles suggests temporal changes of the reaction zones in the past.
We believe that the development of a shallow methanogenesis zone above a zone of intense sulfur cycling may be a specific feature of episodically flooded coastal wetlands, especially when hydrological changes cause freshening of surface waters. Interestingly this layering of a methanogenesis zone above a sulfate reduction zone is reversal of the anaerobic methane oxidation zone, observed in typical marine sediments e.
Submarine Groundwater Discharge helps making nearshore waters heterotrophic
This highlights the need for a more differentiated perspective on coastal ecosystems as commonly neglected methane sources. The recorded distributions of temperature and salinity in coastal waters reveal a clear impact of distinctly different waters close to shore showing cross-slope gradients with slightly reduced salinities and elevated temperatures after a short period of stagnation Figure 9. To analyze the trace gas distribution at the sediment surface, water was filled bubble-free into crimp vials that were immediately sealed. Trace gas concentrations were analyzed on a gas chromatograph Shimadzu GC with an FID flame ionization detector for methane and ECD electron capture detector for nitrous oxide determination.
Interestingly, the temperature and salinity anomalies are found near the coastal area with emerging peat deposits Kreuzburg et al. Since peat may release DOC with advection of low saline pore water Tiemeyer et al. These findings match new data from the southern North Sea, where increasing concentrations and air-sea fluxes of methane were reported toward the shore in the Belgian coastal zone, in particular in areas of free gas occurrence Borges et al. The free gas occurrence in the Belgian coastal zone has been suggested to be generated by the decomposition of a thin Pleistocene peat layer zone Missiaen et al.
Thus, our results support the hypothesis that SGD is increasing the inflow of DOC and nutrients into the coastal zone and thereby instigating changes in biogeochemical cycling in the coastal sediments. Dissolved methane distribution in the shallow Baltic Sea in front of the study site, composite of several measurements in June Coastal-near dynamics apparently foster the transfer of methane from the sediment toward the water column, which is usually oxidized by effective anaerobic and aerobic oxidation Knittel and Boetius, However, our data suggest that elevated methane concentrations near the shore are hardly oxidized in the water likely due to the shallow water depth suggesting possible methane emissions from the coastal waters to the atmosphere.
In the future we plan to assess whether or not this is contributing substantially to atmospheric methane fluxes. Salinity plays a major role for macrophytobenthos in the eutrophic southern Baltic Sea Volkmann et al.