One year after the deepwater horizon oil spill, the biggest threat to the ocean ecosystem near the spill was from:

One year after the deep water horizon oil spill, the biggest threat to the ocean ecosystem near the spill was from the residual oil that reached to the ocean floors in a while after the oil spill incidence.

Explanation:

Once there is an occurrence of an oil spill in the ocean, it is practically impossible to completely clear the spilled oil from the ocean using the technologies available in today's date. The spilled oil gradually reacts with the saline water and breaks down into other chemicals. These chemicals reach the seafloor and make a long-lasting impact on the ecosystem.

Ansarabs took control;