Nitrogen cycling in a changing ocean: overlooked pathways, key ecosystems and large-scale implications

Abstract

Nitrogen (N) regulates ocean primary production and drives the production and consumption of the potent greenhouse gas nitrous oxide (N2O). Excess anthropogenic N inputs have driven widespread coastal eutrophication, leading to oxygen depletion and disruptions to marine N cycling. Investigation of the controls on marine N loss, particularly nitrate (NO3–) reduction via denitrification, anammox, and sediment N burial, is critical for combatting coastal eutrophication. However, estimates of marine N loss vary widely (130–480 Tg N year-1) due to limited empirical data, particularly for overlooked processes such as N burial and N2O emissions across diverse ecosystems. In this thesis, I quantified NO3– reduction processes in sediments. My first study focused on oligotrophic New Zealand continental shelf. A reassessement of global shelf denitrification in revealed that this pathway accounts for 20±2% of marine N loss. Previous estimates might have been overestimated due to sampling bias toward more eutrophic waters. In the second study, I discovered that sediment N burial – rather than denitrification as commonly assumed – is the dominant N loss pathway in global fjords. In the third study, I quantified N loss via N2O emissions in the anthropogenically perturbated Baltic Sea lagoons, underscoring NO3– input is a significant driver of N2O emissions in hypertrophic lagoons. In the fourth study, estimates of N2O emissions across the entire Baltic Sea showed that low-oxygen conditions and NO3– enrichment promote N2O oversaturation. This climate impact of N2O exceeds that of methane over a 30-year timescale, making N2O the dominant gas offsetting the net climate balance in the Baltic Sea. Overall, this thesis examines key marine N loss processes and identifies the critical controls on N cycling, providing insights to better manage ecosystem responses and associated climate feedbacks in an anthropogenically impacted ocean.