Describe a method to determine net and gross primary productivity in a freshwater pond over a 24-hour period.

To determine the the net and gross primary productivity in a freshwater pond over a 24 hour period do the following:
Measure the dissolved oxygen content and the carbon dioxide usage of the pond using oxygen meter. The initial measurement should be taken at first hour and the final measurement at the 24th hour. To find the gross productivity, add oxygen production and carbon dioxide usage over the 24 hour period.

The answer would be sand and gravel