Answer:
Excessive nutrients from fertilizer run off.
Step-by-step 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.