Dredging and dumping impact coastal fluxes of sediment and organic carbon

Domain: earth_science
Published: 2026-01-07
Editorial outcome: major_revision

Abstract
The disturbance and relocation of coastal sediments by human activities constitutes a potentially large disruption to natural sediment and organic carbon fluxes, but large-scale estimates of these impacts are lacking. Here, we estimate the amounts of sediment and organic carbon disturbed and relocated in the North Sea resulting from (1) dredging in the form of mineral aggregate extraction and (2) material dumping during waterway maintenance. We show that despite disturbing less sediment than aggregate extraction, dumping causes greater carbon disturbance. We estimate carbon disturbance by both activities to be higher than by marine construction, but lower than by bottom-contacting fisheries. Simulations indicate that most dumped material re-deposits near the coast. Globally, dumping of organic carbon is estimated to 0.09–0.46 Gt C yr−1, and disturbance by material extraction to 0.04−0.08 Gt C yr−1. Comparison to natural processes suggests that these activities should be considered in regional to global sediment and carbon budgets.

Introduction
As the interface between terrestrial and marine environments, coasts and marginal seas play an important role in regional to global sediment and carbon cycles. Therein, the seafloor acts both as a zone of transformation and long-term sequestration1. Recently, various budget estimates have aimed to quantify coastal particle and solute fluxes on regional or global scales by accounting for natural processes such as river loads, coastal erosion, redistribution by ocean currents, and sedimentation2–4. Studies of anthropogenic impacts on these fluxes have largely been limited to effects related to watershed modifications such as river damming, channel deepening, and erosion due to changes in climate and land-use5–10, as well as associated impacts on ecosystem production and elemental cycles11,12. Several studies have also focused on the role of bottom-contacting fisheries in sediment and carbon disturbance13–16. In contrast, other activities that cause direct mechanical disturbance and/or relocation of sediment have received little attention.

When sediment is disrupted mechanically, organic matter within the suspension plumes can be transported, exposed to oxygenated waters and remineralised more readily than when trapped in the sediment, effectively increasing nutrient turnover rates and thereby reducing the efficacy of marine ecosystems as natural nutrient and carbon pumps (“Blue Carbon”)17. In addition, seafloor disturbances can alter benthic habitats, with adverse impacts on ecosystem functions through changes to both macrobenthic and microbial communities18–20.

In this study, we focus on two activities for which large-scale quantifications are lacking thus far: (1) aggregate extraction in the form of sand and gravel mining and (2) dumping of material originating from the dredging of waterways and harbours. We estimate the amounts of bulk sediment and organic carbon (OC) disturbed or relocated by dredging and dumping in the North Sea, a marginal sea under intense human pressures (Fig. 1). Based on publicly available data (see “Methods”) and numerical simulations, we estimate amounts of disturbed bulk sediment and OC and potential OC remineralisation. We analyse the results with an emphasis on the Wadden Sea, Earth’s largest tidal flat system that stretches along the Dutch, German, and Danish coasts. Dredging and dumping have been identified as main risks for maintaining the Outstanding Universal Value of the Wadden Sea World Heritage Site21, with impacts including smothering of benthos, increased contaminant concentrations, and locally decreasing oxygen concentrations. By comparison to estimates of natural fluxes, we show that fluxes induced by humans can play a major role in shaping the overall sediment and carbon budgets of coastal oceans.

Results
Aggregate extraction
Our analysis (see “Methods”) shows that in total, an order of 100,000 kt of marine aggregates in the form of sand and gravel are extracted each year in the North Sea for use in construction, land reclamation, and beach nourishment, with a slight increasing trend during the past three decades, albeit with considerable inter-annual variability (Fig. 2). In 2009 and 2010, the amount more than doubled compared to the multi-year average. The sudden increase in extraction starting in 2009 coincides with large coastal engineering efforts with high raw material requirements, such as the construction of the deep sea harbour Jade-Weser Port in Germany in 2009 and the Dutch sand engine, a large beach nourishment effort completed in 201122. This indicates that individual projects can be responsible for the bulk of the disturbance through aggregate extraction.

The average concentration of sediment OC at the extraction sites is calculated to be 0.10% based on a published sediment carbon map23. We can therefore estimate the recent amount of annually disturbed OC from aggregate extraction in the North Sea to the order of 100 kt C. According to the TOCMAIM model24, 87% of this disturbed OC is of low reactivity. The fractions of medium and high reactivity, which are more readily remineralised upon disturbance, make up 10.0% and 3.0% of OC, respectively. In our simulations, aggregate extraction results in an excess of about 1 kt C being remineralized after one year compared to the undisturbed scenario (see Supplementary Fig. 1), with minimal difference between a scenario with continuous (1.12 kt C) and one with weekly (0.97 kt C) extraction, which correspond to about 3.67 kt CO2 released to the seawater (aqueous emissions) if all lost OC is remineralised to CO2. The average relative reduction of sediment OC within extraction areas amounts to 5.7% within the upper 30 cm, being partially remineralised and partially resuspended and redistributed around the extraction sites (Supplementary Fig. 1).

It is important to note that the separation of OC into three reactivity classes is a simplification. It has been shown that OC of low reactivity may be made more bioavailable when exposed to oxygen, especially when mixed with more labile compounds (“priming”)25. This priming effect is not well understood and was not included in our model, but has been estimated to result in a 23.4%–91.6% increase in bulk mineralisation rates25. Therefore, the longer-term potential of aggregate extraction to cause excess OC remineralisation may indeed be somewhat higher, but should not exceed aqueous emissions on the order of 10 kt CO2 yr−1. For comparison, the recent fuel emissions of the European dredging fleet was estimated at 600–800 kt CO2 yr−1 according to an industry report26.

Material dumping
Recent dumping of dredged material in the North Sea is 50,000–100,000 kt yr−1, according to our analysis. Despite interannual variance, the data show no clear trend during the past decades, with the apparent onset of increase after 2008 attributable to the inclusion of data from the UK in that year. Our estimate for the amount of OC contained in dumped material is in the range of 500–5000 kt C yr−1, reflecting the uncertainty in the OC content of dumped material (Fig. 3b).

In our dumping simulations, a large portion of the dumped fine-grained (and therefore OC-bearing) material remains close to the coast in both simulations with slowly sinking (0.05 mm s−1) and more rapidly sinking (1 mm s−1) particle tracers. Overall, 92.3−99.6% of dumped OC is deposited on the seabed after one year, while the remainder of is in suspension in the water column according to the simulation. Fig. 4 shows the resulting dumped OC concentration on the seabed after one year for assumed OC contents in dumped sediment of 5% in the fine fraction and 1% in the coarse fraction (see Supplementary Fig. 2 for full ranges). The slowly sinking tracers (Fig. 4a, b) distribute more toward the northeast along the direction of the residual circulation, while the rapidly sinking tracers (Fig. 4c) stay closer to the dumping locations. Simulated redistribution is also sensitive to whether dumped material is initially placed at the bottom or at the surface of the water column; when placed at the surface (Fig. 4a), transport farther toward the offshore occurs, while the offshore distribution in the case of bottom-injected material (Fig. 4b) appears more gradual, with a smoother gradient and less patchiness.

The simulations reveal two distinct responses between the energetic Southern Bight and the more sheltered Wadden Sea; the Southern Bight is nearly completely winnowed by strong currents and associated high bottom-shear stresses in the shallow, coarse-grained connection to the Strait of Dover (see Supplementary Fig. 3), with some local deposition occurring near the muddy Belgian and Dutch coasts. The absence of stratification during much of the year, combined with intense tidal currents in the area, lead to recurrent deposition-resuspension cycles and a dispersion of the dumped material to considerable distances (>100 km) from the dumping sites. Meanwhile, dumped material does not spread as far from dumping sites in the Wadden Sea. A majority (59.4−69.2%) of the fine-grained sediment dumped within the German Bight and the Wadden Sea area deposits within the Wadden Sea World Heritage area, and 25.3−38.8% deposits within the 24 German Wadden Sea basins27 (Table 1). Notably, 22.0−34.0% deposits at depths of <1.5 m, which corresponds to the mean low water level in the Wadden Sea and is roughly equivalent to its intertidal zone, meaning that dumping may contribute to the observed growth of intertidal flats27 of the Wadden Sea.

As opposed to the Southern Bight, which is mainly non-depositional, the Wadden Sea is a net sink of sediment in the Southern North Sea, allowing a quantitative comparison between net import and redistribution by dumping. The amount of bulk sediment deposited in the Wadden Sea in the model after one year is on the order of 8000 kt, which is nearly the same amount as the total estimated mud deposition in the Wadden Sea (7950 kt yr−1) from riverine and marine sources4. This suggests that dumping can be as important for the internal distribution of mud in the Wadden Sea as natural sedimentation processes.

Estimating the potential remineralisation and corresponding climate impact of dumped material is complicated by technical and conceptual challenges. Firstly, a large proportion of the dumping sites are located close to or within the estuaries (Fig. 3a), where it is quickly transported back toward the harbours by tidal pumping as part of the residual estuarine circulation, and after which it may be dredged and dumped again. Though this phenomenon, which has been termed “circular dredging”28, is a known issue in harbour management, it has not been well-documented in the scientific literature to date. Our simulations confirm this process, suggesting that a large part (roughly 25–40%) of dumped material is transported back to the nearshore and deposited there. This implies that the same material can be dredged and redeposited several times throughout the year and would thus be tallied repeatedly in the total quantities shown in Fig. 3b, even if the sediment budget of the overall system remained unchanged. This issue may also carry over to the dumping simulations, in which removal by dredging is not considered. As a result, the simulations may somewhat overestimate nearshore accumulation locally, but the dispersal and accumulation patterns outside of those dredged areas will not be affected. Therefore, the simulations give an indication of the overall transport patterns of dumped OC.

Secondly, the reactivity, i.e., the mineralisation rate of the OC in the dumped material, is largely unknown. Although it has been shown that large estuaries and harbour basins act as efficient bioreactors and thus OC exported from the estuary is largely inert and slowly microbially remineralised29, dredged material may still be at the beginning of the mineralisation process and can therefore contain more reactive OC30. Nevertheless, our results allow an estimate of the lower and upper limits of remineralisation of 15 to 500 kt C, assuming that between 3% and 10% of the dumped OC is remineralised following dredging and dumping, which is the range of oxic remineralisation found in a 21-day incubation experiments using mud from the port of Hamburg30. If remineralised to CO2, this would equate to emission from the seabed sediment to the water column of 55 to 1835 kt CO2 yr−1.

Comparison to other impacts
Table 2 lists the estimated impacts on sediment and OC in comparison to other sediment-disturbing activities, as well as natural sediment fluxes for which estimates have been compiled. Since the sandy seabed of the North Sea is mostly relict, and coastal erosion is the only considerable modern source of coarse-grained sediment to the North Sea, we compare mainly the fine-grained fluxes.

Both material extraction and dumping are at least equal in magnitude to the natural sediment fluxes. While large-scale OC fluxes in the North Sea are dominated by exchange with the North Atlantic, dumping potentially contributes as much OC as riverine export, and maybe in the same magnitude as natural sedimentation. The largest total net impact on OC remineralisation is from bottom-contacting fisheries15,24, estimated to aqueous emissions on the order of 1000 kt CO2 yr−1. However, this impact has decreased during the last decade with an overall decrease of fishing effort in the area due to declining fish stocks and fishing quotas. The impact of aggregate extraction on its net OC remineralisation is found to be comparatively low.

Most prominent among on-going marine infrastructure construction is the deployment of vast offshore windfarms in coastal waters globally. The initial disruption during the placement of pile foundations and cables, lasting days to weeks, is followed by an equilibration phase during which the ecosystem adjusts to the newly formed artificial reef over the next years to decades, and by a decommissioning phase, during which additional sediment disturbance may occur for days to weeks. For a typical twenty-year life cycle of commissioned and planned offshore wind farms in the southern North Sea, OC disturbance and remineralisation were estimated to 20 kt C yr−1 and 20 kt CO2 yr−1, respectively31, somewhat higher than our estimated impact for aggregate extraction and substantially less than for bottom-contacting fisheries and for disturbance by dumping. The same magnitude of OC disturbance as for wind farms has been estimated for the installation of subsea communication cables32.

A comparison to human impacts on riverine fluxes is not straightforward, since contrasting effects on riverine export to the coast have been determined, with increased solute fluxes due to land use changes and decreased particulate fluxes due to retention by damming. In addition, riverine nutrient loads may play a more important role for the coastal carbon cycle than direct carbon exports by enhancing primary production and thereby increasing biological carbon fixation12. Considering the total OC export from rivers in the entire North Sea33 of around 1100 kt C yr−1 along with global estimates for the effect of damming (13% decrease34) and of European climate and land use change (20% increase7) brings the human impact on direct riverine export to the order of 100−200 kt C yr−1, which is smaller than disturbances by both bottom-contacting fisheries and dumping, and similar to the disturbance by aggregate extraction.

Methods
Aggregate extraction data
The amount of material extracted from the European seabed has increased drastically, by two orders of magnitude since the 1970’s52. Aggregate extraction is typically conducted using suction dredgers that pump a sediment-water mixture from the seafloor into hoppers onboard the vessels. The suction heads themselves create resuspension plumes at the seafloor. On the vessels, the excess, sediment-laden water spills back into the ocean as overflow discharge along with any unwanted, fine particulates that are filtered out, creating highly concentrated surface sediment plumes that can be detected up to several kilometres from the extraction sites53. These plumes of fine-grained sediment contain most of the organic matter present in the dredged material54. Extraction sites are concentrated in the southern North Sea and at the Danish coast (Fig. 1).

Data on 217 existing extraction areas55 and extraction mass and/or volume52,55,56 from 1992–2023 were analysed. Only extraction areas with a status classified as "Active" were considered, and a large shell extraction area spanning a large stretch of the Dutch coast was excluded from analysis.

In cases where extraction amounts were given in volume, they were converted to mass using the averaged porosity of all extraction sites of 0.36 and a grain density of 2650 kg m−³ . Porosity was derived by interpolation of more than 2000 sediment samples collected in the region23. The amount of OC impacted was calculated for three OC classes of different reactivities based on results of a numerical model (see "Numerical models"). As an exact colocation of extracted amounts to extraction sites was not possible for all data points, we assumed that the extracted amount was evenly distributed across all extraction sites. For the same reason, it was not always possible to distinguish between extracted amounts inside and outside of the North Sea for the UK, Denmark, Germany, France, and Norway. Therefore, the total amounts may be overestimated since they also contain extraction from the English Channel and the Baltic, Irish and Celtic Seas. However, since the largest extraction sites are located within the North Sea, and the largest amounts by far are extracted from the Dutch coast, we do not expect this to affect the magnitude of our estimate.

Dumping data
The majority of dumped material in the North Sea originates from maintenance dredging in navigation channels and harbour basins, with only a small amount of dredged material being disposed of on land4. Most dumping is concentrated at the southern North Sea coast near large ports such as those of Antwerp, Rotterdam, and Hamburg (Fig. 1). Dumping data57 compiled by the OSPAR Commission were analysed for information on dumping locations and quantities, containing data for 1995–2021, except for the years 2016 and 2020, as well as information on material composition (such as grain size) starting 2013. Beach nourishment has been included in monitoring efforts starting from 2013. No data for the UK is contained before 2008, and activities in Denmark and Norway appear from 2013.

Deposited amounts were located using deposit site codes for the years 2014, 2015, 2017–2019, and 2021 and filtered for the North Sea area. For the latter three years, no deposit site codes are included for activities in Denmark, and for 2018, no deposit site codes are included for some activities in Belgium, preventing the location of those activities. Though grain size was not specified for those activities, nearly all of them were classified as land reclamation, beach nourishment or construction, indicating that this dumped material comprised coarse-grained sediment. In total, 11,200, 3200 and 1600 kt of dumping are omitted in the years 2018, 2019 and 2021, respectively, amounting to a few percent of the multi-year average.

Because of the heterogenous nature of dumping data documentation, dumped material was categorised into three types based on sediment descriptions in the data: "coarse" for sand and coarser material (22.0% of activities), "fine" for silt and finer material (12.8% of activities), and "mixed" for mixtures of coarse and fine material (13.4% of activities). Material with no sediment type information (51.7% of all activities) was categorised as "mixed" (see Supplementary Table 1 and Supplementary Fig. 4). The "mixed" material was assumed to comprise coarse and fine sediment in equal parts.

Data on OC contents of dredged and dumped material is lacking. Therefore, a range of OC contents of 2–8% and 0–2% was assumed in the fine and coarse fraction, respectively, based on the composition of harbour muds sampled at the port of Hamburg58. Similar ranges for OC contents have been found in sediments of other dredged areas in the region, such as in the harbours of Dunkirk59 and in the Scheldt Estuary60.

Numerical models
We use a suite of process-based numerical models to simulate the impacts of aggregate extraction and the distribution of dumped material (Table 3 and Fig. 6).

The first model is the Total Organic Carbon Macrobenthos Interaction Model (TOCMAIM)61, which accounts for OC reactivity through three sediment classes with different remineralisation rate constants, as well as lithogenic sediment dynamics. This model applies the phytodetritus fields of the NPZD-type ecosystem model ECOSMO62 to assign fresh input of OC at the seafloor, where it is consumed and mixed vertically by bioturbating macrobenthos and remineralised through microbial respiration. The model is run for 63 years, at which point a quasi-equilibrium between the OC reactivity classes has established. There is a strong seasonal signal of fresh (highly reactive) OC at the seafloor surface following the phytoplankton blooms, most of which is remineralised by the end of the year. Therefore, we use the end-of year fields of the year 2013 and average the OC reactivity contents over the upper 30 cm, corresponding to the typical depth of furrows observed following trailer head suction dredging63,64. The outputs of this model are used to initialise the sediment and macrobenthos of a 3D coupled hydrodynamics and sediment transport model based on the SCHISM modelling system65. The validated setup covers the entire Northwest European shelf using an unstructured grid with a horizontal resolution of a few km in the North Sea66.

For the simulation of material extraction, total extracted amounts were spread evenly across all extraction areas and implemented as a removal of the sand component along with corresponding additional resuspension of the remaining grain size classes (clay, silt and the three OC classes; see Supplementary Table 2 for details). Thereby, the simulated impact represents both the enhanced oxygen exposure due to resuspension and the erosion of the upper bed layer, exposing deeper layers to higher remineralisation rates. Using the hydrographic conditions of the year 2000, one full year of extraction is simulated for two scenarios: one with continual extraction throughout the entire year and another with extraction once per week for a full day (see Supplementary Methods 1 for details). The results are compared to a simulation of the same period without extraction.

For the dumping simulations, the spatial resolution of the SCHISM model grid was increased in the Wadden Sea and the German Bight in order to adequately represent the flow dynamics there67. The resolution in the German Bight ranges from 0.7 km to 5 km and is around 1 km in the Wadden Sea area with local refinements at complex features such as flow-confining channels. For a representative picture of recent activities, dumping was assumed to be continuous, and the dumping volumes of 2014–2021 were averaged over time. Based on the estimated dumped OC amounts, dumping was implemented as an additional constant source of water and fine-grained sediment tracer at each model grid cell nearest to each dumping location (see Supplementary Methods 2 for details). It is assumed that OC and fine-grained sediment (mud) behave similarly in the water column, since OC is typically attached to fine mineral grains in natural suspensions, and therefore the results represent both OC and mud dispersal.

The mineralisation rates of OC in harbour muds vary widely and cannot be feasibly estimated from the available data. As we are mainly interested in the general distribution of dumped OC and mud, we do not assign a degradation rate to the dumped material in the model, and instead treat it as an inert, mass-conservative tracer. As opposed to material extraction, the dumping of OC does not directly affect OC in the subsurface layers of the existing sediment bed. Therefore, no other grain size classes are set for this experiment.

Some dumping points are located close to the shore or within estuaries that are outside of the model domain. Where the distance from a dumping site to the nearest grid cell was more than 0.2°, the dumping site was not considered in the model. This cutoff is chosen such that upstream and inshore dumping activities are discarded, while including nearshore or estuarine dumping activities that lay just outside the model domain. The disregarded dumping activities amount to 3.6% of the total dumped amount and are mostly located within UK estuaries, which are not well resolved in the model. The amount of excluded dumping in the Wadden Sea is only 0.3% of the total dumped amount.

The dynamic behaviour of dumped material is not well known. In the dumping model experiment, OC is treated as a sinking tracer with a constant sinking speed of 0.05 mm s−1 (see Supplementary Table 3 for details). However, in natural suspensions, particles with high organic matter content form larger aggregates at high concentrations with significantly higher sinking velocities68. Therefore, a sensitivity experiment was carried out in which the dumped material was injected at the bottom model layer, directly above the seabed, instead of at the surface layer. This is motivated by observations and high-resolution simulations showing that dumped material initially collapses quickly at the seafloor69. As an alternative approach to account for the possible influence of aggregation, an additional simulation was carried out with a sinking rate increased by a factor of twenty. This high sinking speed of 1 mm s-1 corresponds approximately to the maximum sinking speeds measured in the coastal southern North Sea70. We simulate one full year of dumping using the hydrographic conditions of 2012.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "I have read the new version of the manuscript, and I am happy with how the authors have addressed all comments. I have a few small suggestions for improving clarity, but I think this manuscript is a great addition to the scientific literature and will attract a lot of attention from the scientific and policy-making communities.\nKind regards\nSebastiaan van de Velde\nL78: define ‘aqueous CO2 emissions’ for non-specialists\nL84: 21-65% relates to the total mineralisation rate or the mineralisation of the unreactive fraction alone? If it is the unreactive fraction, you would have to first get a rate for the unreactive fraction, and then calculate the increase – not just multiply the whole rate by 1.65.\nIn any case, it is a bit unclear if phrased this way.\nL224: I know I mentioned 0.6 in relation to the buffering capacity of seawater – but that was a mistake on my part – 0.6-0.7 related to the CO2 release for calcium carbonate precipitation. In perfect equilibrium with the atmosphere, and assuming the pCO2 in the atmosphere does not change, a DIC added to the water-column will always degass, since the alkalinity and pCO2 set the DIC concentration. The 0.6-0.7 relates to the fact that not all water will re-equilibrate with the atmosphere due to circulation, and is thus a global number – which is not the same as the buffering capacity.\nL238: I am not sure what riparian means, and I suspect I am not alone – maybe replace with a more common word?", "reviewer_3": "The revised article has improved in quality, but I argue that with this advanced model and the extensive and detailed databases for dredging and dumping, the manuscript needs to expand the analysis of simulation results and balance it between both the Wadden Sea and the Southern Bight as two distinct regions. Model runs with two different sinking speeds of particles are not enough for me to believe that this process is as important for carbon disturbance as rivers, even on the scale of the Wadden Sea. I understand the challenges coming from quantification and parameterization of processes as dredging and dumping, but with a model like that their impacts should be analyzed deeper for the purpose of transferability of model results:\nMajor questions:\n1. You model the entire North Sea, but the results disproportionately focus on the Wadden Sea, while the Southern Bight, which is equally busy in terms of dumping/dredging, is almost ignored or counted as a part of the greater North Sea (e.g. “Wadden Sea” or “German Bight” are mentioned 24 times in the manuscript, “Southern Bight” and “Southern North Sea” (which I believe also includes the Wadden) - less than 10). What are the differences between the Wadden Sea and SBNS for the fate of dumped carbon? How hydrodynamics, composition of the sediment bed and local biogeochemistry shape the fate of the dumped matter? Can Wadden sea depocenter absorb some scattered deposited material?\n2. The flocculation process, that is disproportionately important for dumping, is implemented in the model with a sinking speed x20 than a real one. What is the source of this value? Regardless, I would either create another class of flocculated mud and define flocculation intensity as a function of mud content in the water, or, alternatively, directly parameterize the sinking speed as this function. It will allow a more realistic spread.\n3. In Supplement: “For the dumping experiments, the seafloor is left bare, i.e., no sediment is initialized in the model”. Does your model account for different properties of sand and mud in terms of deposited carbon retention (or it’s only the shear stress)? Does your model (in this dumping experiment) simulate the natural carbon cycle (it was not clear for me)? Bioturbation/bioirrigation brings carbon into deeper layers, but if naturally deposited carbon is absent, irrigation will exclusively act on the dumped material, overestimating its degradation and burial.\n4. It would be really interesting to see a relative impact (%) of dumping compared to the natural deposition as it is in the North Sea, with a significance horizon of 5%. It will show how localized the impact of dumping is, regardless of its absolute contribution to the carbon budgets on the regional scale. Same stands for dredging.\n5. I thought about sensitivity tests for different proportions of sediments and carbon in the dumped material, but you have created a long realistic database. In this case, how are the sites where OC-rich mud is dumped, different from OC-poor sandy sediment sites? I believe that if we go into details of dumping materials, an analysis or elaboration should be done about impacts of those different materials.\n6. Methodology: it needs a comprehensive sketch of the processes, represented in the model, with some additional info about sediment classes, organic carbon content range etc. As a reader, I had a hard time switching constantly between explanation in methodology chapter, supplement and numerous references to the previous setups to understand the modelling logic of the setup.\n7. Figure 1 (minor): Zoom-in is still needed for the relevant regions: the SBNS and the Wadden Sea to demonstrate the sites. I didn’t understand your comment about the Wadden sea and vulnerability map.\nThe current analysis for the North Sea is quite region-specific thus limiting manuscript’s relevance to mostly the North Sea. Expanding the analysis will help a reader from any part of the world to project the findings on their region of interest and get first-hand estimates without a need to run a coupled model. Therefore I believe, addressing this criticism will align the manuscript with the standards of Nature.\nRecommended: major revision.", "reviewer_4": "The authors provided detailed and substantive updates to their manuscript in response to the 5 reviews. These changes have resulted in a clearer and more robust product. All of my concerns and questions have been addressed."}, "author_response": "We thank the reviewer for their insightful comments that have helped us greatly in improving the quality of our manuscript. We have addressed all comments in our revised manuscript and provide point-to-point responses below. Responses indicating changes to the manuscript are indicated in italics.\nL78: define ‘aqueous CO2 emissions’ for non-specialists\nDefined: “… , or about 3.67 ktCO2 released to the seawater (aqueous emissions) if all OC is remineralised to CO2 ,…“\nL84: 21-65% relates to the total mineralisation rate or the mineralisation of the unreactive fraction alone? If it is the unreactive fraction, you would have to first get a rate for the unreactive fraction, and then calculate the increase – not just multiply the whole rate by 1.65. In any case, it is a bit unclear if phrased this way.\nSince the priming effect is usually operationally defined as an increase in remineralisation rate of the bulk material (i.e., the mixture), the relative contributions of different reactivity fractions have not been separated in the cited studies. According to Sanches et al. (2021), this increase is 23.4%–91.6% (21-65 was the range of the log-response ratio). Our upper limit of 10 ktCO2/yr for mining still holds under this assumption.\nWe have corrected and re-worded this to clarify that it is the bulk sediment OC that is affected.\nL224: I know I mentioned 0.6 in relation to the buffering capacity of seawater – but that was a mistake on my part – 0.6-0.7 related to the CO2 release for calcium carbonate precipitation. In perfect equilibrium with the atmosphere, and assuming the pCO2 in the atmosphere does not change, a DIC added to the water-column will always degass, since the alkalinity and pCO2 set the DIC concentration. The 0.6-0.7 relates to the fact that not all water will re-equilibrate with the atmosphere due to circulation, and is thus a global number – which is not the same as the buffering capacity.\nThank you for the clarification.\nRephrased: “It is also important to note that the numbers for excess carbon remineralisation presented here should not be equated to atmospheric CO2 emissions, as only a portion of remineralised OC will directly outgas due to the buffering capacity of the seawater carbonate system. Nevertheless, a corresponding effect on air-sea CO2 exchange is likely, as eventual equilibration with the atmosphere is expected in the shallow, mixed coastal zone.”\nL238: I am not sure what riparian means, and I suspect I am not alone – maybe replace with a more common word?\nChanged to “…other nations bordering the North Sea.“"}, {"round": 2, "reviewer_comments": {"reviewer_3": "MINOR NOTES\nI think the manuscript is well-written and all my notes were taking into account. I just remind to be aware, that the SBNS is not as non-depositional as it might appear in some setups. Of course it’s not the Trench, but De Borger’s campaign have shown OC content in the sediment bed. Some of the PP is deposited during the leap tide and is not resuspended back.\nI also highlight very good schematics and images. I think the authors have done a great work on understanding the processes of dredging and dumping and I recommend the manuscript for the publication.\nPlease consider the notes down below as suggestions to improve the manuscript.\nMAIN TEXT\nAdd explicit statement that dumping material is inert, no OC remineralisation included.\nAdd justification why dumping uses only one tracer class while extraction uses six.\nInsert mechanistic hydrodynamic contrast between Wadden Sea and Southern Bight.\nAdd fine = OC-bearing explanation in dumping introduction.\nAdd global assumptions link (OC content) to Methods.\nClarify circular dredging does not bias spatial patterns.\nAdd note on log-scale inflation in Fig. 4 caption.\nAdd tidal basin definition reference near Table 1.\nAdd hydrographic year mismatch explanation (2000 vs 2012).\nAdd 0.2° exclusion rule explanation to main Methods.\nSUPPLEMENT\nClarify fine/coarse OC roles in Table 1.\nAdd “bare seafloor” explanation.\nJustify 0.2° cutoff.\nClarify C = 296 kg/m³ derivation.\nAdd rationale for 0.05 and 1 mm/s.\nAdd log-scale note for Supplement Fig. 2.\nAdd explanation for Dec 30, 2014 shear stress date.\nLink Supplement Fig. 4 to OC-bearing logic.\nFIGURES\nAdd missing Wadden Sea blank-vulnerability note to Fig. 1 caption.\nAdd midpoint OC-range information to Fig. 3a/b captions."}, "author_response": "We thank the reviewer for their insightful comments that have helped us greatly in improving the quality of our manuscript. We have addressed all remaining comments in our revised manuscript and provide point-to-point responses below. Responses indicating changes to the manuscript are indicated in italics."}]

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