Shelf seas are highly productive and economically important regions of our ocean, and therefore play a significant role in the global ocean carbon cycle. While accounting for only 8% of the ocean surface area they support between 15 and 30% of global ocean primary productivity (Wollast 1998) and are responsible for up to 40% of global ocean carbon sequestration (Muller-Karger et al. 2005). Their productivity is thought to be sustained through dynamic physical mixing processes combined with the supply of both terrestrial and oceanic nutrients and internal regeneration of nutrients. However, the relative importance of these different nutrient pools is currently not clear. Nitrogen is often a key limiting nutrient for biological production within the open ocean and shelf seas and therefore has an important role in ocean biogeochemistry influencing the marine carbon and phosphorus cycles. The processes within the nitrogen cycle are complex. This study aims to identify the sources of nitrogen to the NW European shelf and to understand how nitrogen is cycled on the shelf. In this thesis, the nitrogen stable isotope composition the d15N and stable oxygen isotope (d18O) composition of dissolved nitrate has been used to quantitatively assess the relative magnitude and importance of the oceanic source of nitrate versus on shelf regeneration of nitrate in sustaining the on shelf nitrate pool that supports the high primary productivity in temperate shelf seas. In the Celtic Sea there was greater in-situ remineralisation of nitrate across the inner shelf during the summer (= 30 %) and autumn (= 60 %) in 2015 indicated by the decoupling of the d15NNO3 and d18ONO3 in bottom water with the introduction of isotopically light d18ONO3. The influence of riverine water was also greater at the inner shelf stations as evidenced by a mixing of riverine and oceanic isotope end-members. In contrast, the proportion of nitrate remineralised in-situ across the outer shelf region of the Celtic Sea during the summer and autumn period was lower (0 to 10 %), with an off shelf supply of nitrate being more important than previously thought. Across the Hebrides shelf a significant proportion (up to 94 %) of the nitrate was remineralised in-situ at the inner and middle shelf stations during autumn 2014. The shelf edge was influenced by the off shelf supply of nitrate and the inner shelf regions were influenced by the Scottish Coastal Current and greater in-situ remineralisation. The results of this study have provided the first assessment of the regeneration of nitrogen in the North West European shelf seas using the stable isotopes of nitrate. This has improved the knowledge of nitrogen cycling on the shelf and identified the magnitude of the sources of nitrogen to the shelf over an annual cycle. These results have significant implications for the efficiency of shelf sea carbon pump and the method of subsequent carbon export from shelf seas.