Sahlenburg is a small village in the Cuxhaven district, in Lower Saxony, Germany, bordering the southern North Sea. It is located between the Weser and Elbe estuaries, which exert a strong freshwater influence in the region (Czitrom et al., 1988; Buth et al., 2015). This coastal zone of the northern part of Lower Saxony is particularly vulnerable to climate change, for example sea level rise (Sterr, 2008), and an overall increase of storm surges and numbers of storm tides in the German Bight (Danard et al., 2004). The region has relatively flat topography with some hills in the West and the South (Rahman et al., 2018). The watershed is characterized by a moraine consisting of buried valleys refilled with gravel, sand, silt, and clay that was formed by subglacial melt-water erosion during the quaternary glaciations. This structure is highly permeable and contains a large groundwater reservoir. Considering that the neighboring low-lying coastal regions are vulnerable to salt water intrusion, this watershed is particularly important for future supply of drinking water in the region (Steuer et al., 2009). The hydraulic connections to other groundwater reservoirs and the pathways for contaminants from the surface to deeper reservoirs can differ along their path because the filling of the buried valleys is not uniform. Consequently, saltwater intrusions into some groundwater reservoirs of the valleys may occur and this will be of increasing importance as the seawater level is expected to rise in the coming future, affecting the North Sea region (Piotrowski, 1994; Steuer et al., 2009).

The Sahlenburg tidal flats are part of the Wadden Sea National Park of Lower Saxony, bordering the moraine to the Southeast and the Southern North Sea to the Northwest. The study site is relatively sheltered from waves and has a mean tidal range of 3.50 m (Schmidt et al., 2011). Sandy beaches and saltmarshes are located along the coastline, with the latter acting as accumulation sites for finer sediments. During high water, the salt marsh belt is partly submerged ( Figure 1 ), while during low water, the tidal flat is exposed up to several km offshore due to the gentle slope of the coastline. In the intertidal zone of the study site, the glacial sands are overlain by Wadden Sea sediments which are depositions from the nearby Elbe estuary (Reineck and Siefert, 1980). In between, at 1-2 m depth below the sediment surface, a combined peat-clay layer is located which acts as an aquitard for the groundwater from the moraine watershed. Both peat and clay layer are thickest nearshore and thin out offshore (Rodemann et al., 2005), extending approximately 1 km seaward. Through cracks in the layer, meteoric groundwater discharges into the intertidal zone in the form of focused springs (“sand boils”), approximately 20 - 50 cm in diameter. In addition, larger-scale (up to meters) “pancake” structures were previously observed and attributed to broad upwelling of groundwater (Bartsch, 2009). Discharge from sand-boil type springs has been estimated to be ~ 700 L d^-1, with a seawater amount of <12%, for a single spring location (Schlüter and Maier, 2021). The multitude (at least 100) of springs commonly observed in the region (Bartsch, 2009), as well as potential diffusive fluxes, amount to substantial freshwater inputs into the local intertidal zone.

Field campaigns were carried out in June and August 2018, February 2019, November 2019, and July 2020. Porewater from the intertidal springs was sampled during low tide in three different types of locations in the tidal flat area ( Figure 1 ): nearshore, where the springs were located close to the vegetated shoreline (saltmarsh), offshore, approximately 70 meters from the vegetation, and approximately in the middle between both locations. Spring water samples were taken from the surface (<10 cm), at 50 cm, and at 100 cm depth. For comparison, non-spring porewater from the tidal flat was collected from the same depths as spring water. In addition, porewater samples from a nearby sandy beach, and surface water (<20 cm depth) samples from a nearby lake were obtained. Seawater was sampled during high tide approximately 10 m offshore of the sandy beach in the vicinity of the lake location ( Figure 1 ). Sandy beach porewater samples were collected at 50 cm, 100 cm, and 200 cm depth. Two regional groundwater wells were sampled, Spieka II Neufeld (Spieka well) and Sahlenburg III well 1 (Sahlenburg well) with approximately 8 kilometers and 3 kilometers distance respectively from the study area. Additionally, surface water samples were taken from 5 m depth from two rivers in the vicinity of Sahlenburg during the cruise HE527 of RV Heincke on March 2019. The Elbe and Weser River mouth sampling points were located at approximately 8 kilometers and 25 kilometers distance from the study area, respectively.

During the June and August 2018 campaigns, only spring porewater from the offshore zone was collected, since due to a very dry period, barely any springs were found. Beach porewater, lake water, and seawater were not sampled in June and August 2018. In November 2019 water samples from the two groundwater wells were collected, as well as meteoric water from a channel (ditch) next to an unpaved road adjacent to the sampling site. Tidal porewater was sampled only in February 2019 and July 2020.

At the sampling locations, porewater was drawn using pre-rinsed polyethylene (PE) syringes and stainless-steel push-point samplers. Temperature, salinity, and dissolved oxygen concentrations were measured with a Hach (HQ40D) multiparameter probe for the November 2019 and July 2020 campaigns. During June 2018, August 2018, and February 2019 temperature and salinity were measured using a WTW Multi 3430 probe with a TetraCon 925 Conductivity Sensor, and dissolved oxygen was measured with a hand-held flow-through cell and attached temperature probe (FireStringGO2, Pyro Science). Dissolved oxygen saturations were converted to molar concentrations using salinity and temperature data (Garcia and Gordon, 1992). Fluorescent DOM (FDOM) was measured on site after sample filtration using an Aquafluor Handheld Fluorometer/Turbidimeter 8000-010 (Turner Designs). The instrument is pre-set to an excitation center-wavelength of 375 nm and an emission wavelength range >420 nm, which corresponds to Peak C/M (marine/aquatic humic-like, Stubbins et al., 2014). Concentrations were noted as relative fluorescence units (RFU) based on a quinine sulfate standard.

Sample water filtration was conducted using polypropylene (PP) inline filter holders equipped with a sandwich of 0.8/0.2 µm acid washed SUPOR filter membranes (Pall) which were directly connected to the syringes. For the determination of nutrients in the November 2019 and July 2020 campaigns, samples were filtered into 20 mL PE Zinsser vials and poisoned with mercury chloride (HgCl2). In the February 2019 campaign, samples were stored frozen, and in June and August 2018, no samples for nutrients were collected. For Fe(II) determination, 1 mL of filtered sample was pipetted into 2 mL Eppendorf safe-lock PP tubes pre-filled with 100 mL ferrozine solution. For DOC, TDN (total dissolved nitrogen) and DOM analyses, samples were filtered into acid-washed high-density polyethylene (HDPE) sample bottles. From each station, 60 mL sample volumes were collected. Filtered samples were acidified with suprapur HCl to pH 2. Seawater samples were collected during high tide from the nearshore surf zone using acid-washed polycarbonate (PC) bottles before filtration into 60 mL HDPE bottles. All samples were kept onsite in cooling boxes. They were all filtered in between porewater stations during the day and acidified at the end of each day.

The overall low salinities in springs, beach porewater, and tidal flat porewater (<10) compared to seawater (>20) reflect a strong contribution of fresh groundwater in the whole tidal flat area. Overall, seawater salinities in this region of the German Bight are in the brackish range (~18-24) due to freshwater input from the Elbe River (Brase et al., 2017). Among the three spring location types, the lowest salinities were found offshore, and the highest nearshore ( Supplementary Table 1 ). One explanation for this pattern could be local topographic heterogeneity: There was a systematic difference in spring sizes; nearshore springs were usually smaller (diameter ~2-4 cm) and comprised of groups (3-7 springs in clusters), compared to offshore springs (diameter ~5-30 cm, single springs). Furthermore, nearshore springs were located near the high water line and in depressions in the tidal flat relief, which may have favored seawater retention in pools during low tide, as well as addition of recirculating seawater. Another explanation could be the local hydrology: We did not find any springs much further offshore than our “offshore” locations (i.e., ~ 70 m from the salt marsh), in line with an earlier report on most springs being typically located between 25 m and 90 m into the tidal flat (Bartsch, 2009). Considering that the offshore springs were downslope of the nearshore springs, and towards a thinning peat-clay layer, this could indicate that the former is subject to stronger hydraulic gradients and possibly under a weaker confining layer to break through than the latter.

Regardless of spring location, spring salinities in our study were comparatively low and in the low range of freshwater springs from another North German intertidal sand flat in Sylt, Germany (0-16; Zipperle and Reise, 2005). Generally, intertidal spring systems, even those occurring in unconsolidated sediments, appear to be characterized by lower salinities compared to diffuse groundwater discharge systems, which range from brackish to even hypersaline (10-37; Hays and Ullman, 2007; Waska and Kim, 2010; Ahrens et al., 2020). The high hydraulic pressure of these – at least partially – confined aquifer systems seems to effectively prevent substantial seawater intrusion.

The seasonal patterns in spring salinity ( Supplementary Table 1 ) did not co-vary consistently with precipitation rates during the campaigns, which were highest in November 2019 (76-100 mm) and July 2020 (101-125 mm) and lowest in June 2018 and February 2019 (31-40 mm, Deutscher Wetterdienst). Thus, despite the overall low salinities in the tidal flat, which were clearly groundwater-driven, it appears that precipitation had no direct influence on the salinity of the springs. Central Europe including our study site experienced a severe drought period from spring to fall 2018 with overall low rainfalls and high temperatures compared to previous decades (Ionita and Nagavciuc, 2020). Subsequently, we observed lower occurrences of intertidal springs in June and August 2018. Especially, small-scale nearshore springs could not be found, and mostly offshore springs were sampled in these two first campaigns. Despite these extraordinary conditions, the spring biogeochemistry was similar to the other sampled seasons, and even the average temperature was not significantly higher than for example in June 2020. Overall, the weak seasonal trends in spring physico-chemistry were surprising considering the strong temperature oscillations (~8-20°C) which we expected to impact microbial activities. Spring temperatures were weakly negatively correlated with oxygen and TDN (i.e., NO_x) concentrations ( Figure 6 ) and the only two spring samples containing measurable Fe(II) concentrations were collected in the summer campaigns ( Supplementary Table 1 ), indicating enhanced respiration rates. However, DOC concentrations in the groundwater springs were rather low and may not provide enough substrate for the local microbial communities.

FDOM was used as an SGD tracer in coastal environments in previous studies (Kim et al., 2012; Nelson et al., 2015; Kim and Kim, 2017) and in addition applied to differentiate for example terrestrial, anthropogenic or microbial sources of DOM. The handheld device used in this study has emission and excitation spectra targeting mainly terrestrial, “humic-like” FDOM fractions (Coble, 1996). Therefore, our findings suggest that FDOM in the tidal flat could be from terrestrial sources, albeit not necessarily from the meteoric groundwater. The elevated values in nearshore springs might be from saltmarsh plants, but it is also feasible that finer sediments with particulate organic carbon from the nearby rivers preferentially settle in these marshes. In addition, the sediments of the region are characterized by buried Pleistocene marsh peats (Streif, 2002) and their degradation can lead to the discharge of terrestrial FDOM through the springs and porewater in the tidal flats. Indeed, the peat-clay layer was reported as being most extensive nearshore (Bartsch, 2009). Similarly, transport of peat-derived DOM through SGD was also suggested for a barrier island near our study region (Waska et al., 2021).

Overall, our findings suggest that the intertidal groundwater springs in Sahlenburg are not a major source of DOC and FDOM to the local coastal ocean: DOC concentrations in the springs and porewater overall were much lower than reported from diffuse-type systems (e.g. an intertidal creek bank and a sheltered sandy beach, 100 – 2700 µM, Seidel et al., 2014; Seidel et al., 2015) and large tidal flats (60 – 1700 µM, Kim et al., 2012) and in a similar range in a karstic system (40 – 85 μM, Tamborski et al., 2020). Even the nearby groundwater wells in Sahlenburg and Spieka had up to fourfold higher DOC concentrations than e.g. the offshore springs ( Supplementary Table 1 ). Slightly higher average values of DOC and FDOM in the nearshore springs, together with higher salinities compared to offshore samples, could be rather explained by local effects, such as the input of marine DOM from seawater, either surficial or after recirculation through the saltmarsh, or possibly peat leachates. Unpublished tritium-helium dating results (Schlüter pers. comm.) have indicated that this groundwater is decades old, which could mean that DOM has already undergone substantial microbial degradation. Another explanation for the low DOC concentrations in the springs compared to wells or nearby surface waters could be removal via adsorption to minerals in the aquifer (Shen et al., 2015). In contrast, the fine-grained intertidal sediments did not appear to contribute much to DOC production or removal in the springs during their final flow path before discharge, although some mid-salinity concentrations appeared to be higher than expected from the groundwater-seawater mixing curve ( Supplementary Figure 3A ). Perhaps high groundwater velocities resulted in shorter contact times with the sediment matrix, allowing for less time for exchange processes resulting in low concentrations in the springs (Tiemeyer et al., 2017). In support of this argument, we observed strong bubbling (“sand boils”) during the sampling campaign, indicating high fluxes from the springs. Previously, discharge rates of more than 700 mL min^-1 were observed in springs from our study region by Schlüter and Maier (2021) with a released groundwater volume of about 400 L in 7h. In support of this suggestion, some tidal flat and especially the beach porewater samples, presumably subject to longer water residence times, displayed much higher DOC concentrations than expected based on the local groundwater sources ( Supplementary Figure 3A ). Initially, a detailed description of the beach subterranean estuary was not our major research objective, but its extraordinary biogeochemistry clearly warrants a more comprehensive investigation in future studies.

TDN concentrations in the springs in Sahlenburg were at the high end of reported sites under anthropogenic influence. For example, TDN concentrations ranged from 67 – 158 µM in a diffuse estuarine intertidal flat system (Santos et al., 2014) and 51.9 – 148 µM in volcanic submarine springs (Swarzenski et al., 2017). The much higher concentrations of TDN (here essentially comprised of nitrate) in the intertidal springs compared to those found in the seawater and nearby rivers suggest that they are an important source of nitrogen to the surrounding tidal flat area. To illustrate, Schlüter and Maier (2021) estimated a maximum groundwater discharge volume of 1371 L d^-1 for one “sand boil” in Sahlenburg. Using this volumetric flux, the average TDN and Si concentrations of all sampled springs and seasons in our study would yield inputs of 447 mmol TDN d^-1 and 279 mmol Si d^-1 for only a single groundwater point-discharge site. Note that although we did not quantify spring abundances in this study, our observations as well as previous reports indicate the occurrence of at least one point-source discharge per m shoreline (Bartsch, 2009). In comparison, Ahrens et al. (2020) estimated a maximum DIN flux of 102 mmol d m shoreline^-1 for the subterranean estuary of a barrier island high-energy beach, approximately 40 km west of our study site.

High values of nitrogen and especially nitrate are typically observed in watersheds close to agricultural, artificially fertilized land and shallow groundwater below sewage plumes (Jordan et al., 1997, Slomp and Van Cappellen, 2004). The high values found for nitrite and nitrate in the intertidal springs were similar to those from an SGD site in the vicinity of a watershed impacted by agriculture that ranged from 0.02 µM to 460 µM for NO_x (nitrite and nitrate) (Bishop et al., 2017). Furthermore, the high concentrations of nitrate in the intertidal springs suggest a shallow groundwater source since nitrate sources occur on or near the soil surface. Like most parts of Northern Germany, the Sahlenburg area is extensively used by agriculture with chronically high nitrate concentrations particularly in the shallow groundwater as documented by local authorities (Klages et al., 2022) and evidenced by our data from the Sahlenburg monitoring well ( Supplementary Table 1 ). In the Sahlenburg springs, the high average concentrations of nitrate, and lower concentrations of nitrite and ammonium are indicative of an oxygenated system where there is little denitrification (Tamborski et al., 2020), in line with elevated dissolved oxygen concentrations. Additional upstream aspects could impact the nitrogen concentrations in the intertidal springs, such as the time from infiltration and exfiltration of nitrogen loading to the land surface, the proximity of nitrogen sources to the springs, and local climatic variations (Toth and Katz, 2006). For example, at the dune base of a high energy sandy beach, enrichment of groundwater with nitrate was linked to nitrification of remineralized ammonium (Ahrens et al., 2020).

In addition to the high NO 3 − concentrations, our observed high concentrations of dissolved oxygen, which were close to saturation, suggest that the spring groundwater does not originate from a deep aquifer, or at least is in contact with the vadose zone before it reaches the tidal flat. It is also noteworthy in this context that while spring geochemistry did not exhibit distinct seasonal trends, the temperature of the spring water fluctuated highly depending on ambient air conditions. The high oxygen concentrations and fast discharge rates led to low iron concentrations in springs and tidal flat porewater compared to beach porewater, although the tidal flat is comprised of fine-grained sediments and would normally be dominated by sulfate reduction (Seidel et al., 2014), while beach STEs are generally more oxic- to suboxic (e.g. Ahrens et al., 2021). Therefore, we propose that the spring groundwater is exposed to the atmosphere before it enters the tidal flat and does not spend enough time in the tidal flat sediments to become anoxic again.

The observed enrichment of silicate in springs and beach porewater compared to seawater was expected because typically, groundwater is enriched in dissolved silicate compared to surface waters. Thus, dissolved silicate is used to trace groundwater transport processes in the sediment and the coastal aquifer and can complement other conventionally applied tracers, such as radon or radium (Oehler et al., 2019b). Additionally, the higher concentrations of silicate in the intertidal springs and beach porewater compared to the other sampled areas can be linked to the dissolution of biogenic silicate derived from sedimented diatoms, which can be a significant organic matter source for the in situ active microbes (Seidel et al., 2014). Our results were in the high end of ranges when compared with other studies: for example karstic coastal springs in the Mediterranean ranged from 36.2 to 90.3 µM (Garcia-Solsona et al., 2010). In a high-energy beach and a back barrier tidal flat concentrations up to 320 µM were linked to inflow of fresh/brackish groundwater from below, and in porewaters of the tidal flat higher concentrations of silicate were associated with high deposition rates and advective porewater circulation, contributing to the incorporation and subsequent dissolution of biogenic opal (Reckhardt et al., 2015).

Lower concentrations of dissolved phosphate were likely an additional result of the oxic conditions, as phosphate is removed through sorption onto minerals such as oxides of Fe, Al, and Ca, and co-precipitation (Welskel and Howes, 1992; Slomp and Van Cappellen, 2004). Our intertidal springs had higher average concentrations ​​than those in submarine karstic springs at northwest Mediterranean Sea, which ranged 0.15 – 0.46 µM (Tamborski et al., 2020), and higher than those in a diffuse seepage nearshore system with average values of 2.3 µM, and ranging from nearly 0 – 12 µM (Santos et al., 2008). Nevertheless, groundwater springs were depleted compared to surface seawater, and despite nearby agriculture, groundwater phosphate was not a major source due to the prevailing oxic conditions. On the other hand, sub- to anoxic beach porewater may contribute excess phosphate to the coastal ocean ( Table 1 ).

We did not find a distinct seasonal trend in DOM geochemistry of the intertidal springs from Sahlenburg through our data displayed in the PCoAs ( Figures 4 , 5 ). Likewise, spatial patterns of local DOM sources were very heterogeneous. Nevertheless, molecular characteristics of DOM in intertidal springs were distinct from those in seawater and other endmembers. These observations indicate that spatial trends are far more relevant than seasonality for DOM molecular composition in the Sahlenburg groundwater springs. Overall, the land-ocean gradient in Sahlenburg is spatially complex; for example, the nearshore springs were more similar to seawater in their DOM composition than the offshore springs. Furthermore, the tidal flat porewater had similar salinity and molecular composition to the springs, implying that, rather than springs providing terrestrial geochemical “islands” in a marine environment, the whole tidal flat appears to be strongly saturated with meteoric groundwater. Interestingly, the water channel (meteoric water from a ditch) had a DOM composition similar to nearshore spring samples ( Figure 4 ). The sampling location for the water channel was in a vegetation zone near the saltmarsh around ~300 m from the sampling zone. Either this sample was impacted by the same vascular plant-derived inputs, or it was in contact with the groundwater that later discharged into the nearshore springs. Furthermore, the DOM molecular similarities, as well as the slightly elevated salinities, imply that the nearshore springs had not only an imprint from the local vegetation but also from the seawater. Likewise, DOM in the middle springs and tidal flat porewater was likely a mixture of DOM derived from groundwater springs and seawater.

Aromatic DOM compound abundances and AI_mod were elevated in nearshore springs, seawater, and terrestrial endmembers (wells, channel, and lake), compared to offshore groundwater springs, indicating relatively higher amounts of terrestrial DOM in the former compared to the latter. Indeed, AI_mod generally is related to terrestrial DOM derived from degradation of polyphenolic land-derived compounds such as lignin and tannin (Seidel et al., 2017). This could also explain the higher aromaticity in nearshore springs, and the decrease towards offshore spring locations. Surprisingly, seawater had elevated levels in aromaticity and FDOM and thus molecular similarities with the terrestrial endmembers in our study, such as lake water and inland wells. In surface estuaries, generally negative correlations are observed between aromaticity and salinity (Osterholz et al., 2016). But in our study, we observed positive correlations between AI_mod, FDOM, and salinity. This, as well as its overall low average salinity (~14 – 26), suggests that our seawater endmember was influenced by terrestrial DOM from the two adjacent large Weser and Elbe rivers. Concurrently, a relatively high I_Terr index, indicative of a terrigenous signature (Medeiros et al., 2016), was observed in the seawater compared to the springs ( Supplementary Table 2 ). Finally, for the February and March 2019 campaigns, DOM of the two estuaries had a similar molecular composition as seawater ( Figure 5 ). These results further support our observation that seawater is strongly influenced by these estuaries in the area. Nevertheless, some seawater samples formed an additional cluster separated from the estuaries, in the vicinity of beach porewater and lake samples. As the seawater samples were always collected at high tide, directly offshore from the beach porewater and lake locations, we suggest that local heterogeneities in wind- and tidally driven currents may be responsible for these distinct patterns. Furthermore, higher molecular S/C ratios of DOM in seawater and estuaries compared to springs and porewater from Sahlenburg could be related to anthropogenic sources in the estuaries, as well as resuspension of anoxic (sulfidic) estuarine sediments and release of porewater with sulfur-enriched DOM into the water column from surrounding tidal flats along the German coast (Seidel et al., 2014).

The lower molecular mass of DOM in seawater compared to springs could result from microbial degradation, as previously reported from FT-ICR-MS studies of terrestrial and marine DOM exposed to microbial activity (Burdige and Gardner, 1998; Kim et al., 2006). These trends in DOM molecular mass also apply to porewater and seawater from a different intertidal flat area near our study site (Seidel et al., 2014). DOM in beach porewater, seawater and lake water had lower molecular masses and was more oxidized as indicated by higher O/C ratios compared to intertidal springs. DOM in the offshore springs had also higher H/C ratios and as such more aliphatic character than DOM at all other sites. In previous studies high H/C ratios in submarine springs were associated with labile DOM produced autochthonously in the aquifer, or through microbial transformation of vascular-plant-derived DOM during infiltration from the soil, increasing the abundance of aliphatic DOM (Adyasari et al., 2021). Furthermore, selective adsorption of aromatic, oxygen-rich DOM to aquifer minerals may result in higher H/C ratios, and lower O/C ratios in DOM in groundwater (Shen et al., 2015). Contrastingly, the lower H/C ratios in DOM of our beach porewater compared to the seawater were in line with patterns found in the STE of a high energy beach, and possibly due to iron oxidation-reduction cycles which cause enrichment with humic-like FDOM and aromatic compounds (Waska et al., 2021).

Despite the slightly higher H/C ratios, which are generally associated with higher lability (D’Andrilli et al., 2015), offshore groundwater also had the highest molecular degradation index I_DEG, as well as highest abundances of highly unsaturated compounds amongst all sample types ( Figure 3C ; Supplementary Table 2 ; Supplementary Figure 3B ; Flerus et al., 2012), pointing to an extensive DOM turnover in the coastal aquifer. Our results indicate that the use of H/C ratios as an indicator for “freshness” (i.e., recent primary production) or lability (i.e., suitability as a substrate for microbial metabolism) must be evaluated in the environmental context, and ideally together with several other molecular markers. Indeed, the concurrent increase of I_DEG and highly unsaturated compounds suggest increasing amounts of reworked DOM (Adyasari et al., 2021). An overall increase of the relative contribution of highly unsaturated compounds from nearshore to offshore springs, together with an increase of I_DEG from lower to higher degradation state, implies that DOM was actively utilized during transport in the STE (Adyasari et al., 2021). Still, DOC concentrations in spring samples were overall very low. An advanced DOM degradation state, together with overall low DOC concentrations, could explain low aquifer reactivities, preventing the complete exhaustion of oxygen and nitrate along the groundwater flow paths. Previously, Montiel et al. (2019) proposed that organic(peat-)rich subterranean estuaries enhanced denitrification of groundwater from anthropogenically impacted watersheds, while increasing ammonium and dissolved organic nitrogen loads to the coastal water column. Although peat layers are common in North Germany and partially comprise the confined layer in our study site, they appeared to have only a slight impact on DOC concentrations and DOM composition. Instead, the low-DOC, high-I_DEG spring DOM characteristics resembled those of deep-sea DOM (Flerus et al., 2012), which is generally considered as having low reactivity. Furthermore, the groundwater spring samples had a relatively high number of assigned DOM molecular formulae considering their very low DOC concentrations. For comparison, spring samples had approximately 30% less molecular formulae than seawater at > threefold lower DOC concentrations, indicating that the concentrations of single organic compounds in the springs may have been much more diluted than in any other investigated sample type. High DOM molecular complexity in low overall DOC quantities can limit microbial substrate utilization (Dittmar et al., 2021). Under these circumstances, the capacity of the coastal aquifer to mitigate nitrogen pollution via denitrification could be hampered, resulting in the high nitrate loads of discharging groundwater.

We observed different patterns in regard to the distributions of organic and inorganic nitrogen species: The relative abundances of unsaturated DOM compounds with N were highest in offshore springs and showed similar trends as TDN concentrations. This suggests similar sources, since, for example, elevated nitrate concentrations but also unsaturated DOM compounds with N have been related to anthropogenic land use, agriculture and urbanization (Slomp and Van Cappellen, 2004; Roebuck et al, 2019). However, the intensity-weighed molecular N/C ratios indicated that seawater had overall more N-containing formulae while aliphatic DOM compounds with N were more prominent in groundwater. This is probably because seawater contains more phytoplankton-derived DOM that is more enriched in N-containing aliphatic compounds compared to terrigenous DOM, which in turn has overall lower N/C ratios (Seidel et al., 2017). Nonetheless, SGD has been identified as important pathways for the transport of anthropogenically sourced nutrients to coastal oceans (Santos et al., 2021). Our study shows that point-source groundwater discharge systems have the potential of being a “fast-track” transport system for anthropogenic nutrients and DOM impacting the local biogeochemistry of tidal systems. For future work, linking N-containing DOM compounds with other pollution tracers such as stable nitrogen isotopes could be a worthwhile future exploration of the organic nitrogen pool in Sahlenburg groundwater.