Dredging and dumping impact coastal fluxes of sediment and organic carbon

The disturbance of coastal sediments by human activities constitutes a potentially large disruption to natural sediment and carbon fluxes, but large-scale estimates of these impacts are lacking thus far. Using data analysis and process-based numerical modelling, we estimate the amounts of sediment and organic carbon disturbed, as well as potential CO2 released from North Sea sediments resulting from (1) dredging in the form of mineral aggregate extraction and (2) dumping of material dredged during waterway maintenance. Despite disturbing less sediment than aggregate extraction, dumping has a greater potential for carbon remineralization due to a higher organic carbon content in estuarine muds compared to subtidal sands and gravels. Overall, we estimate carbon disturbance by both activities to be 1-2 magnitudes higher than by marine construction, but 1-2 magnitudes lower than by bottom-contacting fisheries. Simulations show that most material dumped within the German Bight and Wadden Sea deposits within the coastal basins and intertidal areas after one year, the amount being similar in magnitude to natural sedimentation in that area. These results highlight the potential of human activities to shape long-term coastal morphology, and we argue that they should be considered in regional to global sediment and carbon budgets.