Primary production in lakes Huron and Michigan: In vitro and in situ comparisons

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.