Light and dark bottles under lights in an aquatic primary productivity study. The dissolved oxygen (D.O.) of the bottles was measured initially and will be measured the next day. Changes in D.O. will reveal the extent of photosynthesic production of organic miolecules (primary productivity) by the algae in the water. The screens on the bottles mimic the attenuation of light with increasing depth in a water column. The experiment is part of a high school biology lab.