Oxygen microprobes were used to estimate Respiration (R), Net Production (NP) and Production (P) in coastal sea water samples. Using this highly stable and reproducible technique to measure oxygen change during alternating dark and light periods, we show that respiration in the light could represent up to 640% of respiration in the dark. This light enhanced dark respiration can remain elevated for several hours following a 12 h period of illumination. The non inclusion of R_light into calculations of production leads to an underestimation of P, which can reach up to 650% in net heterotrophic systems. The P:R ratio is in turn affected by the higher respiration rates and by the underestimation of P. While the integration of R_light in to the calculation of P:R ratio does not change the metabolic balance of the system, it decreases the observed tendency, thus net autotrophic systems become less autotrophic and net heterotrophic systems become less heterotrophic. As a consequence, we propose that efforts have to be focused on the estimation and the integration of R_light into the determination of P and R for a better understanding of the aquatic carbon cycle.