Practice question

In the light-and-dark bottle method, a dark bottle blocks photosynthetically active radiation while retaining plankton and microbes from the water sample. Without light, photosynthetic oxygen production stops, but cellular respiration continues and consumes dissolved oxygen. The decline in oxygen during incubation estimates community respiration. A transparent light bottle permits both photosynthesis and respiration, so its oxygen change estimates net community production. Combining the two changes gives an estimate of gross primary production: oxygen produced overall equals the net increase in light plus the respiratory loss inferred from darkness. An initial bottle or immediate measurement provides the starting oxygen concentration. Controls are important for temperature, incubation depth, bottle effects, and gas exchange. The method assumes comparable respiration in light and darkness, an assumption that is not always exact, and prolonged incubation can alter community composition. Nevertheless, exclusion of light is the crucial mechanism. It isolates oxygen consumption from oxygen-generating photosynthesis, which is why the dark bottle records respiration alone within the standard assay.