<span>Well, there are five layers in the Earth's Atmosphere:
(In order from the inside out)
1) Troposphere - This is where weather occurs.
2) Stratosphere - This is the layer where jets fly, this is where jet-streams are found
3) Mesosphere - Meso ("Middle") named because it is in the middle. This is where meteros burn up in the atmosphere.
4) Thermosphere - Thermo ("Heat") Names because it is the hottest layer in the atmosphere. This is where the aurora borealis, or northern lights occur.
5) Exosphere - Exo ("Outer") This is the outmost layer and it is where the atmosphere merges with outer space.</span>
I think it is because in the ocean the plankton produces most of the oxygen which is used by aquatic plants and plankton is a primary producer known as autotrops.
The only independent variable in this experiment is the amount of nitrogen Sarah uses. All of the other variables are dependent on other factor and processes, but Sarah controls the amount of nitrogen that she uses for each plant.
