Humans have learned how to manipulate and harness the elements that sustain life on Earth (Carbon – C; nitrogen – N and phosphorus – P). Indeed, we have become so skilled at this that we have practically doubled the amount of fixed nitrogen (N) available to us to grow crops and, with current farming practices, we simply couldn’t sustain the human population without it. This harnessing of N has come at a considerable cost to the environment, however, particularly rivers, estuaries and coastal seas, where it affects their quality and value as ecosystems. Riverbeds can naturally reduce these high N loads and thus provide an important “ecosystem service” globally, not only for the rivers – but also for the estuaries and coastal seas into which they drain. Consequently, riverbeds are recognized hot-spots of N cycling, converting ~40% of N-runoff back to inert, atmospheric nitrogen gas (N2). This ecosystem service is largely thought to be regulated by either a, nitrate in the river b, organic matter or c, the residence time of the water. We can now demonstrate, however, that the fraction of bio-available nitrogen that is either conserved or lost from the ecosystem appears to be dependent on phosphorus (P). The focus here for this PhD is to understand how P actually affects the cycling and availability of N in river ecosystems. Ultimately, such understanding could be translated into more efficient wastewater treatment processes and the development of operational best practice for better process control and general management of water resources.