Nutrient dynamics in coastal and shelf oceans - sediments as a regulator of eutrophication feedbacks

Coastal and shelf systems are under increasing pressure from human activities. Many coastal systems currently suffer from excessive algae growth following increased nutrient input from land, a process called eutrophication. The algae eventually sink to the seafloor, where they are degraded, which consumes oxygen (O2). In severe cases, O2 can become depleted entirely, resulting in mass mortality of animals and substantial changes in the system’s chemical conditions. When the external nutrient input decreases, O2 conditions should improve. However, feedback mechanisms that strengthen eutrophication and O2-depletion can lead to non-linear recovery. Processes that occur in the seafloor account for a considerable part of this non-linearity, as the shallow water depth in coastal systems leads to a strong coupling between processes in the water column and the sediments. Although we know much about the processes that cause eutrophication, we lack detailed and mechanistic knowledge about many of the feedback mechanisms that keep systems in a eutrophic state. Methods to mitigate coastal eutrophication are being developed, but due to the non-linearity response of many systems, the results of restoration efforts are difficult to predict. A marine system that exhibits strong, non-linear eutrophication feedbacks is the Baltic Sea. Efforts to reduce the nutrient load to the Baltic Sea over the last decades have been successful, yet the system remains eutrophic and suffers from severe O2 depletion. The main basins of the Baltic Sea differ substantially in environmental conditions as well as input rates and sources of organic matter, nitrogen and phosphorus. This spatial variability makes the Baltic Sea ideal for assessing factors that affect organic matter and nutrient cycling in sediment, which can determine the recovery trajectory for eutrophic systems. In this thesis, I use the Baltic Sea as a study site to investigate the role of sedimentary feedback mechanisms that regulate coastal eutrophication. I show that the shuttling of sediment particles can redistribute organic matter from shallow to deep parts of basins. The reactivity of organic matter depends on the nature of the compound and its environment. Therefore, particle shuttling substantially affects the pattern of O2 consumption as well as degradation and burial of organic matter. I further show that natural oxygenation events can be insufficient to break the feedback between eutrophication and O2 depletion in long-term eutrophic systems where the sedimentary capacity to remove or retain nitrogen and phosphorus is decreased. Finally, I show that when placed in a suitable environment, mussel farms used for eutrophication mitigation only have a minor impact on the underlying sediment. However, in terms of eutrophication mitigation, other techniques may be more efficient than mussel farming. In conclusion, the results presented in this thesis inform us about sediment mechanisms that regulate eutrophication and can be used to find locally adapted solutions for systems around the world.