Cruise datasets generated for project 'Physical-Biological Control of New Production within the Seasonal Thermocline' in the Celtic Sea, July 2003 - July 2005 dataset https://portal.medin.org.uk/portal/start.php?tpc=006_7af3bbbbf52d5a0dabf402fd14faf6ae This dataset comprises measurements of primary productivity, nutrient, optics and water column structure data (including turbulence information) collected from the Celtic Sea and nearby shelf-edge during 25 July to 14 August 2003 and 15 July to 06 August 2005. Compatible, high-resolution vertical profiles of physical, chemical, and biological parameters were obtained. These were collected by deployments of a conductivity-temperature-depth (CTD) sensor package, a Fast Light Yo-yo (FLY) turbulence profiler and SeaSoar profiler sections, thus allowing quantification of vertical fluxes and primary production on tidal and internal wave time scales. During the CTD profiles, water samples were taken and analysed for nutrients, chlorophyll, primary productivity, sediment concentration, coloured dissolved organic matter (CDOM), trace metal concentrations, dissolved oxygen and salinity. Optical profiles were also taken during 2005 using two radiometers; a Biospherical Instruments PRR-600 and a Satlantic MicroPro. In addition, acoustic Doppler current profiler (ADCP) moorings were deployed around the Celtic Sea to measure current and temperature profiles. The main aim was to investigate the generation and dissipation of turbulence in the thermocline, and to quantify how the resulting mixing (supplying nutrients and controlling the light experienced by the algae) affects the growth of phytoplankton within the sub-surface chlorophyll maximum. The data were collected by the Proudman Oceanographic Laboratory (now the National Oceanography Centre) and by Bangor University. Data management was provided by the British Oceanographic Data Centre. 20180518 20110517