In temperate shelf seas, a deep chlorophyll maximum (DCM) persists within the thermocline during summer stratification. This study explores the significance of primary production (PP) within the DCM and provides a detailed investigation into the mechanisms of phytoplankton adaptation to this tidally dynamic and dimly-lit environment. Comparison is drawn to the DCM in a range of hydrographic regimes in the open ocean. Data are presented from two cruises in the Celtic Sea during summer (2003 and 2005) and one from the Atlantic Ocean (AMT15, 2004). Phytoplankton physiological measurements were obtained from 14C Photosynthesis vs. Irradiance (P vs. E) experiments and a bench-top Fast Repetition Rate Fluorometer (FRRF). Water-column profiles of PP were empirically modelled using P vs. E parameters and measurements of spectral in situ irradiance. Across the Celtic Sea shelf the DCM was located towards the base of the thermocline and was tightly coupled to the nitracline. The thermocline is presented as a 3-layer system, the top and bottom layers dominated by physical mixing and a mid-layer which is relatively stable and occurs at the top of the nitracline. Nitrate flux into the thermocline from the BML was maximal at spring tides and could support almost all the PP in the thermocline. It appears that nitrate supply and utilisation is roughly balanced over spring - neap timescales. Within the shelf sea thermocline, layering of phytoplankton taxonomy was observed, with the maxima of cell concentrations decoupled from those of cellular pigment concentrations. The FRRF-derived effective absorption cross section of photosystem II (σPSII) and photosynthetic efficiency (Fv/Fm) co-varied with pigment composition, and did not appear to contribute to photo-acclimation. Data indicated that the mechanism of photo-acclimation was by the number, rather than size, of PSII reaction centres. In contrast to the surface mixed layer, phytoplankton at the DCM remained light limited and PP in the thermocline was more sensitive to incident irradiance than in the surface. In the Celtic Sea, the potential range in water-column integrated PP between a cloudy and sunny day was greater than the spatial range in PP across the region. PP within the thermocline contributed 28 – 84 % of vertically-integrated daily PP. The mechanism of photoacclimation appeared to be consistent between the shelf sea and open ocean environments. Throughout the Atlantic Ocean PP in the DCM contributed between 15 – 80 % of total water column production. It is shown that physical – biological coupling is a major determinant on phytoplankton taxonomy, physiology and productivity in the DCM.