WASHINGTON, (Reuters) – This year’s low-oxygen “dead  zone” in the Gulf of Mexico is one of the largest ever, about  the size of Massachusetts, and overlaps areas hit by oil from  BP’s broken Macondo well, Louisiana scientists report.

The area of hypoxia, or low levels of oxygen, covered 7,722  square miles (20,000 square kilometres) of the bottom of the  Gulf and extended far into Texas waters, researchers from the  Louisiana Universities Marine Consortium said in a statement  late on Sunday.

“This is the largest such area off the upper Texas coast  that we have found since we began this work in 1985,” said  Nancy Rabalais, the consortium’s executive director. “The total  area probably would have been the largest if we had had enough  time to completely map the western part (of the Gulf).”

The annual summer “dead zone” in the Gulf is fueled by farm  chemicals carried by the Mississippi and Atchafalaya rivers.  Nitrogen and phosphorus in agricultural runoff stimulates algae  growth in the Gulf.

When these tiny plants or fecal matter from animals that  eat them settles to the bottom waters, decomposition of this  organic material by bacteria consumes oxygen in the water, the  consortium said. The result, the researchers said, is oxygen depletion that  forces many types of fish, shrimp and crabs to leave the area  or suffocate. Animals that live in the sediments that can  survive with little oxygen will die if the oxygen level falls  toward zero. To be considered hypoxic, oxygen content in the bottom  waters of the Gulf must reach the level of 2 parts per million  or less. By late July, large patches of the northern Gulf had  reached that level, including one swath off Texas’s Galveston  Bay.