In order to estimate the gross primary productivity (GPP) of a pond containing phytoplankton and zooplankton, a student collected water samples from the pond in two bottles labelled X and Y . The dissolved oxygen (DO) of the samples immediately after collection was 8 mg ml ⁻¹ . The DO content when measured after 24 hours of incubation was as follows: Bottle X: 2 mg ml ⁻¹ after incubation in dark Bottle Y: 18 mg ml ⁻¹