Phytoplankton Energetics in a Sewage-Treatment Lagoon

Metabolic rates in a sewage-treatment lagoon system near Deshler, Ohio, were investigated between March 21 and June 10, 1964. The ponds were well mixed vertically, exhibiting daytime average eddy diffusivities of about 1 cm² /sec. The waters had a high titratable base, ranging from 3 to 10 meq/1 during the period of study. Low oxygen levels were encountered for only a short period in the second stage of the two-pond system. Light absorption averaged about 99%/m. The phytoplankton communities, dominated by Scenedesmus, rose to values near 50 (mu)1/1. Rates of CO_2-uptake in the order of one mole-m⁽ -3) day⁽ -1) were observed, but O_2 production averaged only one-fifth as high. A graph of CO₂ -uptake vs. light intensity revealed a curve closely similar in shape to the typical photosynthesis vs. light curve, but a separate set of data, based on the ⁽ 14)C uptake, showed anomalously high values at surface light intensities. CO_2 uptake by a photosynthetic metabolic process that does not produce oxygen is postulated to explain the imbalance between CO_2 and O_2 budgets.