Describe how the relationship between producers and consumers in the lake would cause the co2 concentration to fluctuate between day and night?

In an aquatic ecosystem, the plants are present both on the subsurface and bottom bed rock of the aquatic body. These are the producers of the ecosystem because they provide food to the living organism in the lake.  In day plants growing on the surface of the water takes up carbon dioxide from the atmosphere which is being absorbed in the water, to conduct the process of photosynthesis. These plants release oxygen, which supports the life of aquatic animals. In this way plants limit the abundance of carbon dioxide in water. But in night plants undergo a process of respiration, which releases carbon dioxide. The abundance of carbon dioxide increases in night along with the carbon dioxide being absorbed from the atmosphere. Therefore, concentration of carbon dioxide increases considerably in night as compared to day.

"do a testcross with a homozygous recessive plant."