The programme ‘A thermocline nutrient pump’ dataset comprises hydrographic, biogeochemical, biological and meteorological data. Hydrographic profiles, towed and underway measurements and point sources provided information on free-fall turbulence data, current velocities and acoustic backscatter, water column structure including temperature and salinity, the underwater light field, fluorescence and dissolved oxygen. A comprehensive biogeochemical water sampling programme provided details on nutrients, primary productivity, dissolved organic matter and phytoplankton pigments. Biological samples such as zooplankton were obtained from the water column using nets, and from the sea-bed using grabs. Bathymetry and meteorological parameters were measured across the study area. Data collection was undertaken in the Clyde Estuary, the Irish Sea and St Georges Channel and the Celtic Sea, including the territorial waters of the UK and the Republic of Ireland. The data were collected during the period 02 – 26 June 2010 during RRS Discovery cruise D352. Measurements were taken using a variety of instrumentation, including conductivity-temperature-depth (CTD) profilers with attached auxiliary sensors, bathymetric echosounders, water bottle samplers, nets, acoustic Doppler current profilers (ADCPs), remote access water samplers, towed undulators, free-fall turbulence profilers, temperature loggers, grabs and ship flow-through and meteorological packages. The project focused on the effects of wind stress at the sea surface, creating inertial oscillations and shear and mixing across the thermocline. The project also aimed to investigate the mixing in the context of supplying nutrients to the sub-surface layer of phytoplankton commonly seen within the shelf seas in summer, and to assess any possible shifts in the phytoplankton ecology as a response to the supply of nutrients.  The programme ‘A thermocline nutrient pump’ was undertaken jointly by the School of Ocean Sciences, University of Wales Bangor and the Proudman Oceanographic Laboratory (POL), now the National Oceanography Centre, Liverpool (NOCL). CTD and Sea-Soar data from the programme are held at the British Oceanographic Data Centre. Other data have not yet been supplied.