Why would my empty (of kelp sample) control bottles be producing oxygen?

Running a series of light and dark bottle experiments in order to make P vs I curves for two species of kelp and as part of each run I have two control bottles which are filled with low oxygen, filtered sea water (same as the rest with kelp samples). In theory the oxygen level after incubation should be the same as the intial reading beforehand but I have been getting an increase, which also happens to increase with each run of the experiment. Apart from human error or error in the methodology what else could be responsible?