Despite efforts to reduce nitrate levels in the Mississippi River Basin, nitrate levels did not consistently decline from 1980-2008, a new study has revealed.

Missouri River, upper Mississippi River and groundwater are the major sources of nitrate in the Gulf of Mexico, United States Geological Survey (USGS), which led the study, said in a statement on Wednesday.

“While conservation practices may have decreased nitrate levels in some portions of the basin, we aren't seeing widespread effects at larger scales,” said Lori Sprague, USGS hydrologist and lead author on the report published in the journal Environmental Science and Technology.

According to research results, the nitrate transfer to the Gulf of Mexico was 10 percent higher in 2008 than 1980. The result highlights the need to curb nitrate transport that aggravates the formations of dead zones affecting aquatic ecosystem in the Gulf of Mexico, researchers said.

“Excessive nutrients like nitrate in the Mississippi River Basin contribute to hypoxia, or dead zones, in the Gulf of Mexico. The dead zones are the result of too little oxygen to support most marine life in bottom and near-bottom water.”

Gulf of Mexico became the victim of the largest offshore oil spill in U.S. history on April 20, 2010, when an explosion on the Deepwater Horizon rig at BP's Macondo undersea well, off the coast of Louisiana, spewed nearly 5 million barrels of oil into the sea.

An expedition to the bottom of the Gulf of Mexico in February 2011 revealed that last April oil spill from the UK energy giant's ruptured well will take months or even years to clean up, according to marine scientists.

Nitrate transport from Mississippi River Basin into the Gulf of Mexico is adding to the dead zones formed from oil spill.

State and federal partners are striving to decrease nitrate transported to the Gulf to reduce the size of the hypoxic zone to less than 5,000 square kilometers by 2015, USGS affirmed.