Agricultural runoff can contain large amounts of chemical fertilizers. In areas where agricultural runoff meets the ocean, high levels of nutrients can cause events called algal blooms.

During an algal bloom, microscopic organisms called phytoplankton reproduce quickly in the water. The phytoplankton consume more nutrients and oxygen in the water than usual. Once the nutrients is used up, the phytoplankton die in large numbers. When this happens, more oxygen is used up by bacteria that decompose the phytoplankton.

How do algal blooms cased by agricultural runoff affect the benefits that humans receive from ocean resources?
A.
They support humans because it is an additional source of food.
B.
They provide humans with a reliable energy resource in the form of biofuel.
C.
They increase tourism because the water becomes blue-green in color.
D.
They make it harder for people to catch fish that are used as food.

During an algal bloom, the chemical fertilizers in agricultural runoff cause excessive growths of phytoplankton. The phytoplankton quickly reproduce and use up oxygen in the water. Once the phytoplankton die, they are decomposed by bacteria, which further uses up oxygen. This creates "dead zones", which are areas of water that lack oxygen. These zones are unable to support life, including fish and other animals that humans depend on.

So, algal blooms cased by agricultural runoff affect humans because they make it harder for people to catch fish that are used as food.