Characterising the seasonal cycle of dissolved organic nitrogen using Cefas SmartBuoy high-resolution time-series samples from the southern North Sea

The Cefas SmartBuoy network provides a unique insight into the biogeochemical dynamics of the Northern European shelf seas, particularly the North Sea, through high-resolution automated offshore water sampling. We present total dissolved nitrogen and dissolved organic nitrogen (DON) from the Dowsing SmartBuoy site (53.531A degrees N, 1.053A degrees E) from January to October 2010, the first high resolution seasonal (winter-autumn) cycle of DON from the open North Sea. On top of a refractory background DON concentration of approximately 5 mu M, a rapid increase in DON of a further similar to 5 mu M is observed over the course of the spring bloom. This rapidly produced DON declines at an estimated net decay rate of between 0.6 and 1.8 mu M month(-1). The slow decay suggests that the majority of the additional DON produced during the spring bloom is of semi-labile nature and has a lifetime of weeks to months. The dataset allows us to tightly constrain the budget for water column nitrogen over the winter, spring and summer of 2010 and clearly demonstrates the 'sawtooth' nature of the seasonal cycle of DON in the open North Sea, which has been impossible to resolve with a more traditional ship-based mode of operation. This work highlights the importance of autonomous sampling approaches in better understanding shelf sea biogeochemistry in the future.