Exploring nitrogen dynamics in coastal sediments: A study of permeable sands in the southern North Sea

Abstract

Coastal systems are immensely impacted by mobile ground touching fisheries, global change related effects (e.g. increasing water temperatures, sea level rise and ocean acidification) and other human induced pressures (e.g. pollution and sand extractions). These stressors contribute to a worsening of the environmental status of the southern North Sea. To prevent further degradation, three marine protected areas have been established in the German Exclusive Economic Zone (EEZ). This thesis investigates benthic biogeochemical processes, with a focus on benthic oxygen and nitrogen dynamics in permeable sands of the southern North Sea and explores potential effects of the exclusion of mobile ground touching fisheries. First (chapter 2), various sediment characteristics (e.g. chlorophyll-a, phaeophytin-a, total organic carbon and mud content) and their application as proxies for benthic oxygen consumption and nitrogen turnover are explored. A parameterization framework is developed within this context to predict both net (oxygen, ammonium and nitrate) and gross (ammonium and nitrate) benthic reaction rates based on sedimentary predictor variables. The findings indicate that mud content and phaeophytin-a concentrations are among the most suitable proxies for assessing sediment reactivity. The following chapter (chapter 3) builds on a previously established pore water advection model and the parameterization framework introduced in chapter 2 to develop a process-driven biogeochemical model for permeable sands. This model utilizes the previously identified proxies to predict benthic fluxes of oxygen, ammonium, nitrate and dinitrogen. It examines how the vertical distribution of substrates within the sediment affects the magnitude of these fluxes, revealing that this vertical substrate distribution has minimal impact on the modeled benthic fluxes in permeable advection controlled sands. The fourth chapter extends the application of the benthic biogeochemical model to calculate total fluxes within the Natura 2000 MPAs of the German Bight. This forms the basis for integrating the sediment into a digital twin, enabling the exploration of "what-if" scenarios, such as the effects of excluding mobile bottom-contact fishing on the sedimentary organic matter inventory and sediment reactivity. The results show that excluding bottom trawling results in only slight increases in sedimentary total organic carbon content, leading to minor changes in the benthic fluxes of oxygen and nitrogen. Overall, this study provides a foundational understanding of sediment reactivity in coastal systems and provides crucial insights for refining management strategies to protect and sustain the southern North Sea.