Six states whose waters feed the lower Mississippi River agreed this week to work together to reduce the dead zone in the Gulf of Mexico. Fertilizers, sewage, and other nutrient-rich pollution flowing from 42 states into the Mississippi produce the annual dead zone at the mouth of the river — a stretch of water with oxygen content so low that it drives off or kills most sea life. Last year, the dead zone was bigger than the state of Massachusetts. Arkansas, Louisiana, Mississippi, Missouri, Tennessee, and Texas — which together contribute about 7 percent of the nutrient pollution — are planning to pilot a program to teach farmers how to reduce nitrogen runoff from their land. Doug Daigle of the enviro group Mississippi River Basin Alliance said a much larger federal-state plan to stem the nutrient flow into the river is 18 months behind schedule.