Reply on RC1

on release and flux dynamics of CO 2 , CH and N 2 in a coastal with

Reply: Thank you for your remark regarding the statistical analysis. We intentionally wanted to focus mostly on the differences between the rewetted peatland and the inner bay to investigate the terrestrial-marine connection and the potential impact of the rewetting onto both sides. However, to determine the impact of rewetting, one would naturally expect a comparison between pre-and post-rewetting conditions, but a comparison between the inner bay and the peatland after rewetting is also indicative of the effects of rewetting. Consequently, when we statistically examined the differences between the inner bay and the rewetted peatland, we found clear significant differences for many variables (i.e. variables influenced by biological activity, in particular in spring and summer), but not for physical water properties (i.e. surface water temperature, salinity, oxygen) and these results are summarized in Table 2. To make this focus clearer and understandable, we intend to rephrase the heading of Table 2 to: "Seasonal comparison of the surface water means (± standard deviation) in the peatland ("peat") as opposed to the inner bay ("bay") for all in situ variables. The number of observations is shown in parentheses, and significant seasonal differences between the inner bay and the peatland are indicated in bold." For us, it seems obvious to compare stations of both the peatland and the inner bay that were sampled before and after rewetting, but we initially considered a statistical analysis of pre-vs. post-rewetting nutrient concentrations to be difficult because of low data availability before rewetting.
However, after consulting a statistician, we now included a statistical pre-vs. postrewetting analysis of nutrient and GHG flux data within the same seasons sampled before and after rewetting despite low data availability for nutrient data (summer and autumn 2019 vs. summer and autumn 2020; see new Table 3 in the supplement). We intend to adapt the material and method section 2.3 "Data processing, statistics, and definition of seasons and means" as follows in line 208: "To describe temporal patterns during the entire sampling period, we defined two pre-and four post-rewetting periods, roughly akin to seasons (Table 1). For a direct comparison between the pre-and post-rewetting periods, we compared nutrient and GHG flux data from summer and autumn 2019 with those from summer and autumn 2020 (Table 3) by using the Mann-Whitney-U test." The sentence "For direct comparisons between [...]" in lines 208 and 209 will be removed.
To strengthen the interpretation of the results with respect to the pre-vs. post-rewetting analysis, as suggested by the reviewer, we propose to make the following changes in the manuscript: To add in results section 3.1 "Surface water properties", line 334: "Additionally, no significant differences between summer and autumn 2019 and summer and autumn 2020 were found in the inner bay. After rewetting, temperature and salinity measurements near the peat surface [...]" To include the following table 3 (please see the supplement) right after Table 2: " Table 3: Statistical comparison of pre-and post-rewetting nutrient concentrations and GHG fluxes. For pre-and post-phases, summer and autumn seasons were used (June to November 2019 and 2020, respectively). Nutrient concentrations are compared for the inner bay and GHG fluxes for the peatland site. *** and "n.s" indicate p < 0.001 and not significant, respectively." In results section 3.2.1 "Pre-and post-rewetting spatio-temporal dynamics and comparison with a nearby monitoring station" we will replace the sentence "However, as there were fewer measurements before rewetting, [...]" in line 358 with: "However, this finding could not be confirmed statistically (Mann-Whitney-U-test, see Table 3)." In results section 3.4.2 "Pre-and post-rewetting GHG fluxes" we will refer to the new Table 3 and add: in line 477: "After rewetting, formerly terrestrial CO 2 fluxes decreased in amplitude (-0.5 to 1.4 g m −2 h −1 ), while the summer and autumn averages were unchanged compared to the pre-rewetting fluxes (Table 3)." in line 484:"In summer and autumn 2020, after rewetting, average CH 4 fluxes on formerly terrestrial land increased slightly but significantly (1.74 ± 7.59 mg m −2 h −1 ), whereas in the ditch they decreased significantly (8.5 ± 26.9 mg m −2 h −1 )." Furthermore, we applied a more robust statistical analysis to investigate the influence of temporal vs. spatial dynamics that justified the usage of means for the peatland and the inner bay, respectively. Thus, we suggest to add in line 213: "The difference between spatial (sampling stations) and temporal (sampling seasons) data variability was tested by using a Two-Way ANOVA and showed a higher temporal variability (p < 0.05)." Finally, we are planning to include the following changes within the discussion: In line 683: "At our study site, [...] terrestrial locations increased significantly by 1 order of magnitude, the overall increase [...]." Finally, we need to correct a small mistake in the standard deviation of the pre CO 2 flux in the lines 28 and 475: [...] 0.29 ± 0.82 g m −2 h −1 . (former value: 0.29 ± 0.74 g m −2 h −1 ).

Calculation of lateral transport rates
Sampling and quantifying lateral fluxes in coastal systems is a difficult task, given the potentially huge temporal (and spatial) hydrological variability. This system is not tidal, but still exhibits considerable temporal variability in water level, possibly wind-driven. The authors use a combination of hydrological and topographical information to estimate discharge in relatively high temporal resolution. It is less clear, though, how the manually sampled water constituent data is integrated with this discharge time series. Given the temporal variation in water level, was this taken into account for the water sampling? Or do the authors calculate seasonal or general concentration means? The export rates are given with uncertainty ranges, but it is not explained how this uncertainty range is generated. The uncertainty range is quite high, typically of equal order of magnitude as the mean export rate. I believe that that is indeed realistic and raises the question of how confident we can be about the quantification of these fluxes. Finally, the sign convention for import and export fluxes in equations 4 and 5 are not well explained. I thought that the Qin is a positive flux and Qout negative (equations 2 and 3). However, in equations 4 and 5, this seems to have been flipped: Qin is explicitly a negative flux, and presumably Qout is positive, although that is not clearly defined. This reverse step seems unnecessary and potentially confusing to me.
Reply: Thank you very much for your valuable feedback on this topic. Concerning the high temporal variations of the water level, water levels ranged from ~ -0.1 to 0.6 m above sea level during our study period ( Figure A1). For the calculation of the export rates, we summarized the nutrient concentrations into seasonal concentration means for all nutrient species (DIN-N, PO 4 -P) each for the peatland, the inner bay, and the reference station (central bay) separately. To make this clearer within the method section 2.4.3 "Nutrient transport calculation", we intend to adjust the sentence starting in line 252: "Seasonal mean values of nutrient concentrations (DIN and PO 4 3-) were calculated and converted from µmol L −1 to kg m −3 by using the molecular masses of the basic elements N and P to derive DIN-N and PO 4 -P." To better explain the calculation of the uncertainty range, we propose to add the following text in line 259: "Uncertainty ranges for the seasonal NNTs (uNNT, as 95 % confidence level) were calculated as standard errors (SE) by using an error propagation according to Eq. (6): (please see the supplement for Eq. 6) where terms with "u" denote the respective SE as 95 % confidence level. To gain the annual SE of the NNT, all seasonal SE were added up. " It is right that the uncertainty range is high and "raises the question of how confident we can be about the quantification of these fluxes". Since another of your comments further below addresses this topic as well, we will give a combined answer to both comments and post it after the second comment in the "minor comments" part (concerning lines 522-533).
Concerning the sign convention of equations 4+5, we double-checked these and removed the minus signs in both equations because these were simply wrong. Thank you very much for pointing that out.

Use of reference data (2016-2020)
The authors state that they use 4 years of data from the monitoring station in the central bay, but it is not clear to me how several years are being used as opposed to 'only' the 2019 and 2020 data used in the results. Given the short sampling period, it may be helpful to see how much inter-annual variability occurs in the water chemistry in the central bay and whether the concentrations can possibly get as high as in the peatland area.
Reply: Thank you for pointing out that this seems confusing and hard to understand. The inter-annual variability you mentioned is exactly the reason why we chose to consider more years for the reference data instead of only the two sampling years we have for the study site. Accordingly, in section 3.2.1, we compared our 2019/2020 nutrient data of the inner bay with 5-year reference data (2016-2020) of the nearby monitoring station ("central bay"). By doing so, it became visible that we could neglect the inter-annual variability and focus on the effect of rewetting only. We will clarify this by adding the following sentences in results section 3.2.1 "Pre-and post-rewetting spatio-temporal dynamics and comparison with a nearby monitoring station" starting in line 368: "Nutrient concentrations of the monitoring station ("central bay") showed a low interannual variability during the years 2016-2020 and often lower concentrations than the inner bay ( Figure 6). A detailed comparison of nutrient data from the monitoring station with those from the inner bay showed that before rewetting, only the NH 4 + concentrations were significantly higher in the inner bay. " The sentence "Compared to the monitoring station, [...] shortly before rewetting ( Figure  6)" will be removed.
Additionally, we also used these 5-year-data in Figure 6, line 380. We stressed out in the figure caption that 5-year-data are shown for the central bay. However, we intend to adapt the figure legend of the central bay to "central bay (2016-2020)" to additionally highlight that there are 5-year-data included for this area.

Air-sea exchange
The authors describe very late in the paper (section 3.4.1), that they compared their methods in determining air-sea exchange, i.e. comparing floating chamber estimates to k based estimates. This should be moved up from the results into the methods section. I may even suggest to put the method comparison in the appendix and only note in the methods that they have done this comparison, with reasonable agreement.
Reply: Thank you very much for this good suggestion. Since we had the same discussion among the authors about the best position in the manuscript prior to submission, we will follow the advice to move section 3.4.1 into the appendix C "Appendix C: Comparability of two independent approaches to atmospheric flux determination". Accordingly, the results section will continue with section 3.4 "Pre-and post-rewetting GHG fluxes (CO 2 , CH 4 , N 2 O)".
Moreover, we intend to include a short notice about this comparison in the methods section and create a new headline within section 2.5.3 (line 329) "Comparability of two independent approaches to atmospheric flux determination" with the following content: "We evaluated the comparability of the two previously described methods by comparing the results of a representative station (BTD7) for each post-rewetting season. The comparison showed no significant differences between the fluxes of CO 2 and CH 4 derived with the different methods and therefore, it seems appropriate to combine the fluxes for each GHG into one pooled post-rewetting data set. The pooled post-rewetting flux values were compared with the pre-rewetting values to investigate the effect of rewetting on CH 4 and CO 2 fluxes (Table 3). For more details concerning the comparability approach, see Appendix C."

Peatland CO 2 fluxes
From the information given, it is not clear to me how much of the vascular vegetation remains after flooding and how their possible disappearance is taken into account: The authors take light, dark and shaded measurements before the flooding, presumably when the vegetation was active. They stop doing that after flooding, presumably because no vegetation has survived the flooding. However, in the analysis of the fluxes, it looks to me, that this impact on CO 2 fluxes (more directly on the vegetation itself) is not analyzed or discussed at all. Are the CO 2 fluxes prior to flooding just taken as an average? And -given the light dependence -would it not make sense that those values are more variable than after flooding?
Reply: Thank you for this comment. Yes, vascular vegetation died completely after the flooding. Therefore, we assumed that photosynthesis by the macroflora would not significantly take place after rewetting. This was also the main reason why we conducted the post-rewetting measurements with opaque floating chambers. We will make that clearer in the method section and change the sentence in line 199 to: "Since the flooding caused most plants to die, and almost all measurement locations were covered by water during the study period, we skipped the NEE measurements with transparent chambers." The sentence "Since transparent chambers were no longer used, PPFD variation was no longer considered" will be removed.
We indeed used averages for the gas fluxes, but we included the ranges too, see lines 474-475. In this way, we can show the variability of the CO 2 fluxes. It is also displayed in Figure 10a. We also mentioned in the text that the amplitude of the CO 2 fluxes decreased after rewetting (line 477). Thus, we indirectly take the vegetational die-back into account. If we would exclude the pre-rewetting transparent measurements and only take opaque chamber data into account, we would have an average of 0.62 ± 0.63 g m -2 h -1 from June-November pre-rewetting instead of 0.29 ± 0.82 g m -2 h -1 as mentioned in line 475. Thus, the pre-rewetting CO 2 emissions would be much higher without the consideration of photosynthesis and hence, the activity and presence of vascular plants. The lack of photosynthesis by the macroflora is also visible in the shrinking variability (at least the negative amplitude, which is the CO 2 uptake), from a range of -0.38 to 3.0 g m -2 h -1 when looking at the opaque chamber fluxes only compared to pre-rewetting fluxes from all chambers, which have a range of: -3.3 to 3.0 g m -2 h -1 . Thus, in general, we overestimate the post-rewetting net fluxes since we do not include, likely minor, uptake by for instance algae floating in the water. Therefore, our estimate on how much the rewetting decreases overall GHG emissions is conservative which is the common approach in the literature on rewetting and its impact on GHG emissions.

Minor comments
Lines 389-341: It is worth mentioning that along the 15km distance between peatland and central bay station, some of the nitrogen will be transformed and lost to the atmosphere, so that this is a maximum estimate.
Reply: It is absolutely true that nitrogen undergoes transformations and might also be lost to the atmosphere along the way. We indirectly pointed this out in the method section 2.4.2, lines 234-236 and in the results section 3.2.2, lines 390-391 by calling it <total possible export>. However, we will add the following sentence in method section 2.4.2, line 236 to make this clearer: "Due to transformations and potential losses along the way to the monitoring station, especially of the nitrogen species, the calculated total possible export is a maximum estimate."

Lines 504-511: This is a repetition of the results.
Reply: Thank you for pointing this out. We are going to remove lines 506-511. The transition will be changed to the following in line 506: Lines 522-533: I like the comparison to the river, but I think it would be important to discuss the different range of uncertainties for the two sources to the coastal ocean. I do not doubt that coastal peatlands are hot spots and relevant despite their small scale, but we still have real difficulties quantifying their lateral exchange.
Reply: It is right that the uncertainty range of our calculated exports is high and that the ranges of our values and the ones from the river we used for comparison are highly different. To address this important issue, we intend to include the following sentences in discussion section 4.1 "Nutrient dynamics and export" at the end of line 533: "However, we also want to shortly address the reasons for the high uncertainty range of our calculated nutrient exports. Firstly, they derive from high fluctuating nutrient concentrations in the surface water within the seasons. This is also visible in the high standard deviations (Table 2). Therefore, the 95 % confidence level of the nutrient exports is high and reflects the natural dynamic. Secondly, we conducted default error propagation during the export calculation which leads to even higher ranges on top of the high natural dynamic.
Compared to the Warnow river, it is noticeable that the range of uncertainties is highly different for the two sources. While our uncertainties are mostly higher and in the same order of magnitude compared to the means, the uncertainties of the river data are one order of magnitude lower. This is likely due to the different time scales of the two data sets. Our export data were generated by taking only the first post-rewetting year into account in which the system was still in a transition state and thus, showed very dynamic nutrient concentrations. The uncertainties of the river exports were generated by using 25 years of data, leading to lower uncertainties than using data from only one year. However, the uncertainty range of the river exports was calculated as standard deviation and not as standard error, as was done for the exports of our study site. Therefore, this has to be considered when their uncertainty ranges are compared directly." Line 540: It is worth pointing out that the 'seafloor' includes the now wetted peatland. Anoxic decomposition processes, such as sulfate reduction will produce alkalinity, if the sulfide is removed from active cycling (e.g. via building ironsulfides). It is also worth separating 'primary production' in the different components of phytoplankton and vascular plants. The proportion and relevance of either contribution should change with the flooding.
Reply: This comment shows your profound understanding of coastal wetland biogeochemistry. And yes, there is many interesting changes going on. Indeed, separating primary production in the components of phytoplankton and vascular plants would be helpful and likely change with the flooding. Unfortunately, our data set does not allow to distinguish between these contributions, as this was not within the scope of the work and therefore, we cannot make any estimates on the individual impacts and changes with the flooding. However, we will include the following change: " [...] or can be introduced by mineralization processes from the seafloor, which includes both, the seafloor in the inner bay and the flooded peatland." Lines 559-562: See above, possible influence of anoxic decomposition in the peat.
Reply: We will add the following sentence in line 562: "C T and A T values during this period [...] the recently inundated peat. Besides, local CaCO 3 weathering as well as local anoxic processes, such as sulfate reduction may have increased the A T in the submerged soil and finally contributed to higher A T values compared to the inner bay." Lines 579-581: To me this is an observation that is worthwhile to put in the site description.
Reply: We think that this is a great idea and therefore intend to make the following changes: Line 579-581 will be removed and changes will begin in line 147: "Therefore, minor changes in the water level lead [...] from 0.08 to 0.7 km 2 (Figure 3, Figure A2). The ditch system was only partly removed and hence, some deeper areas with water depths of up to 4 m remained. It is noteworthy that in the first months after rewetting, former grassland and ditch vegetation (Elymus repens L. (Gould) (Couch grass), Phragmites australis (Cav.) Trin. ex Steud. (Common reed)) died almost completely and the cover of emergent macrophytes was then negligible." Line 775: Which vegetation is supposed to expand under these hydrological conditions? If the authors have information on this, that would be helpful. Presumably the grass will die back but maybe Phragmites can withstand the water level height?
Reply: Yes, the grass died back completely because of the permanent inundation. At first, Phragmites also died back in most spots. But over time, it grew back and expanded around the ditches where it was established already before the rewetting. We intend to include this information in section 2.1 "Study site" after the new sentences we wrote as a reply to the previous comment: " [...] and the cover of emergent macrophytes was negligible. However, Phragmites was able to grow back during the growing season and expanded especially around the ditches." Line 776: That may well have depended on the amount of soil moisture/position of water level in the drained peatland, on which there seems to be no information.
Reply: It is right that the soil moisture and the position of the water level are influencing the amount of N 2 O emissions from soils. In our study site, the water level was permanently below the soil surface prior to rewetting. Therefore it seems very likely that the drained peatland was a source of N 2 O, as was already shown for other similar sites (e.g. Martikainen et al., 1993;Regina et al., 1999;Goldberg et al., 2010).
To make this clearer and give some more information about our study site, we intend to add in line 750: "[...] grasslands, respectively (Augustin et al., 1998). N 2 O fluxes in drained peatlands are due to a low water level which allows the penetration of oxygen into the peat to fuel N 2 O producing processes (Martikainen et al., 1993;Regina et al., 1999). As the water level in our study site was permanently below the soil surface before rewetting, it is likely that it was a source of N 2 O." Line 779-782: These implications for future (or adjacent) land development are interesting. However, in my opinion a lot will depend on whether vascular vegetation is going to be established, otherwise I do not see the potential for increasing carbon storage (high positive CO2 fluxes). If the group intends to continue with these measurements on site, it is worthwhile to say that.
Reply: Of course, the potential for carbon storage depends on the development of vascular vegetation and its burial in anoxic sediments, but also on continued scientific research in the future. Therefore, we intend to do the following changes beginning in line 782: "Nonetheless, because degraded peat is both nutrient [...] OM as was demonstrated by other studies. In addition, whether or not the area will act as a C sink in the future, depends on the success and speed of the establishment of vascular vegetation and its burial in the anoxic parts of the sediment.".
Furthermore, the investigations are continuing and hence, we will add a comment in line 788: "The investigations addressed in this study will continue in the study site during the next years in the framework of the DFG funded graduate school Baltic TRANSCOAST." Please also note the supplement to this comment: https://bg.copernicus.org/preprints/bg-2022-117/bg-2022-117-AC1-supplement.pdf