A high precision technique for measuring dissolved oxygen was used in conjunction with the coulometric back titration for total inorganic carbon in seawater, to make observations of the photosynthetic and respiratory quotients in a culture and an open ocean environment. The relationship between the PQ and the nitrogen source utilised was assessed by the concomitant measurement of nitrogen assimilation by the 15-N mass spectrometric technique, with the aim of resolving some of the uncertainty surrounding the calculated and observed PQ values. For the most part the PQ values were found to lie within the constraints set by theory. A complete resolution of the exact relationship between the PQ and the nitrogen source was limited by the analysis of the ambient NH3 concentrations. Measurements of community production and respiration were made in the N.E.Atlantic as part of the BOFS programme. concomitant 14-C rate measurements were made, enabling a comparison of the methodologies. The comparison of the oxygen and 14-C technique was found to be affected by the magnitude of the respiration rate, resulting in elevated PQ values. The in vitro techniques employed in this study were compared to in situ techniques for the analysis of the fluxes of oxygen and carbon dioxide. It was confirmed that the in vitro techniques were not a serious or overriding cause for concern. The productivity rate measurements from the oxygen technique were used to assess the critical depths which are consistent with the development of the phytoplankton bloom. A relationship was observed between the critical depth and the net community production rate.