Oxygen- and carbon-14-based primary production estimates from 9-16 h in vitro incubations were compared in lakes Huron and Michigan. For surface mixing layer compansons, gross O2/14C photosynthetic quotients (gross PQ) averaged 2.2, and net O2/14C photosynthetic quotients (net PQ) averaged 1.4. The mean gross PQ is consistent with a theoretical P0 based on the CO2 and NO3 assimilation ratio. However, within the deep chlorophyll layer, gross PQ and net PQ averaged 4.9 and 2.8 respectively. These higher values were likely due to excess NO3 reduction at the expense of CO2 uptake. Thus, during short experiments under low light conditions, oxygen evolution and CO2 uptake may not be tightly coupled. In vitro and in situ O2 production estimates were compared in four diurnal (dawn to dusk) experiments in Lake Huron. In situ production estimates were determined by measuring water-mass oxygen changes and oxygen transfer across the air-water interface. In situ production estimates were approximately twice in vitro production estimates for both surface mixing layer and deep chlorophyll layer comparisons. The difference between estimates was attributable to containment effects manifest in 13-16 h bottle incubations. Short-term (1-2 h) in vitro production was also compared to diurnal in vitro production. Rates of short-term production were ̃1.6 times higher than rates of diurnal production, suggesting that short-term in vitro production experiments may provide reasonable estimates of in situ primary production.