Ocean dead zones
When the algae induced to grow by fertilization die, they suck up all the oxygen in the area during their decomposition process and make the area unlivable for a wide range of other species.
The notorious dead zone in the Gulf of Mexico, for example, where algae fertilization is caused inadvertently by fertilizer runoff from agricultural lands, was measured in 2006 to cover over 6000 square miles
Source:
BBC