PLEASE PARAPHRASE THIS:
One way to determine net and gross primary productivity in a freshwater ecosystem is to first take equal-volume samples of pond water. Determine the dissolved-oxygen content of each. Expose one sample to light and one sample to zero light. Leave the samples alone for 24 hours; measure the dissolved-oxygen content again, and determine the differences in dissolved-oxygen content. The first difference is gross productivity; the first minus the second is net productivity.

PLEASE JUST PARAPHRASE IT BUT KEEP THE CONTENT THE SAME