What is the major cause of phytoplankton blooms in the gulf of mexico?

higher levels of sunlight from global warming
high water temperatures from global warming
excessive nutrients from fertilizer runoff
toxic pollution from oil refineries
: )

Excessive nutrients from fertilizer run off.  

Explanation:

Scientists have suspected that phytoplankton blooms might be connected to agricultural run-off. They got some evidence from a recent study by comparing the timing of irrigation along the west coast of Mexico and phytoplankton blooms in Mexico’s Sea of Cortez. During the comparison, they had found a close correlation. Phytoplankton blooms occurred in the Sea of Cortez shortly after the irrigation. This happened because of carried fertilizers after irrigation and other agricultural run-offs into the sea.

The bloom detected in the Gulf of Mexico on December 13 might be similarly linked to agricultural run-off.