Question

in the late 1940s, following the end of world war ii, u.s. farmers and industries began widespread use of synthetic fertilizers. the fertilizer supplies nitrogen for chlorophyll, the pigment that makes plants and phytoplankton green, and aids energy production through photosynthesis. water runoff from these fields carries nutrients to the mississippi river and then to the gulf of mexico.

1. which of the following could explain the big increases in dissolved oxygen observed in the first half of may? (sep 2.3; dci: ps3.d; ccc: 4.0)

dissolved oxygen (mg/l) in the gulf of mexico in may
chart showing dissolved oxygen levels over may
a fertilizer runoff causes dead zones where fish and shrimp suddenly die because fertilizer is toxic to them.
b fertilizer runoff causes phytoplankton population to decrease because too much fertilizer can harm plants.
c fertilizer runoff causes phytoplankton population to increase more than usual because in addition to more sunlight, the phytoplankton have more nutrients.

Explanation:

Phytoplankton produce oxygen through photosynthesis. When fertilizer runoff (which contains nutrients like nitrogen) enters the water, along with more sunlight (as in May), it can cause a phytoplankton bloom (increase in population). More phytoplankton means more photosynthesis, and thus more dissolved oxygen production. Option A is incorrect because dead zones are areas of low - oxygen (not high) and fertilizer is not directly toxic to fish and shrimp in the way described here (dead zones are due to oxygen depletion from decomposing organic matter from algal blooms). Option B is incorrect because fertilizer runoff usually causes phytoplankton population to increase (not decrease) as it provides nutrients.

Answer:

C. Fertilizer runoff causes phytoplankton population to increase more than usual because in addition to more sunlight, the phytoplankton have more nutrients.