What dissolved oxygen concentrations classify a water body as "hypoxic"? b. What are the two primary nutrients that cause hypoxia? c. By what mechanism does the addition of these nutrients cause hypoxia? List each step beginning with the addition of nutrients and ending with a decrease in dissolved oxygen concentrations. d. List the four sources of nutrients that contribute to the hypoxic zone in the Gulf of Mexico.

Basically when the concentration of oxygen drops below 2-3mg/L , then the  water body is said to be hypoxic. That is the oxygen concentration of the water body is low.

B. The two primary nutrients that are responsible for this are Nitrogen and Phosphorus. Theses two elements are components of NPK fertilizers which are washed(run off) from the  the farms closed aquatic bodies( lakes) through leaching.

C, This is Eutrophication, from nutrients accumulation, leading to algae boom, and therefore consumption of oxygen in the water bodies,

D, The addition of nutrients leads to the increase in the nutrients supply to the water body.

this ]leads to  algae population growth-algae boom.  

the  rapids algae population decline , because of exhaustion of nutrients and available oxygen.

therefore decomposing bacteria used the available oxygen to decompose the algae further reducing the oxygen concentration (increasing the biological oxygen demand of the aquatic body)

Eventually, the aquatic organism die from hypoxic effects from shortage of oxygen.

D.

fertilizers from agriculture usage of fertilizers. also from golf courses to improve lush green lawn growth,

from erosion of soils rich with  high  nutrients,

from  nitrogen deposited from atmospheric nitrogen- nitrogen cycle.

High nutrients discharge from sewage treatment plants.

Explanation:

the solute can move "downhill," from regions of higher to lower concentration, relying on the specificity of the protein carrier to pass through the membrane. this process is called passive transport or facilitated diffusion, and does not require energy.