The area of temporary hypoxic bottom water that occurs most summers off the coast of Louisiana in the Gulf of Mexico is the largest recurring hypoxic zone in the United States. It occurs only during the summer months of the year due to summer warming, regional circulation, wind mixing and high freshwater discharge.  The Mississippi River, which is the drainage area for 41% of the continental United States, dumps high-nutrient runoff such as nitrates and phosphorus into the Gulf of Mexico. According to a 2009 fact sheet created by NOAA, "seventy percent of nutrient loads that cause hypoxia are a result of this vast drainage basin". which includes the heart of U.S. agribusiness, the Midwest. The discharge of treated sewage from urban areas (pop. c 12 million in 2009) combined with agricultural runoff deliver c. 1.7 million tons of phosphorus and nitrogen into the Gulf of Mexico every year. Nitrogen is indeed needed to increase crop yields, but plants are inefficient at taking it up, and often more fertilizers are used than plants actually need.  Therefore, only a percentage of applied nitrogen ends up in the crops; and in some areas that number is less than 20%.   Even though Iowa occupies less than 5% of  the Mississippi River drainage basin, average annual nitrate discharge from surface water in Iowa is about 204,000 to 222,000 metric tonnes, or 25% of all the nitrate which the Mississippi River delivers to the Gulf of Mexico. Export from the Raccoon River Watershed is among the highest in the United States with annual yields at 26.1 kg/ha/year which ranked as the highest loss of nitrate out of 42 Mississippi subwatersheds evaluated for a Gulf of Mexico hypoxia report.  In 2012, Iowa introduced the Iowa Nutrient Reduction Strategy, which "is a science and technology-based framework to assess and reduce nutrients to Iowa waters and the Gulf of Mexico. It is designed to direct efforts to reduce nutrients in surface water from both point and nonpoint sources in a scientific, reasonable and cost effective manner."  The strategy continues to evolve, using voluntary methods to reduce Iowas negative contributions through outreach, research, and implementation of nutrient holding practices. In order to help reduce agricultural runoff into the Mississippi Basin, Minnesota passed [https://www.revisor.mn.gov/statutes/cite/103F.48 MN Statute 103F.48] in 2015, also known as the "Buffer Law", which was designed to implement mandatory riparian buffers between farmland and public waterways across the State of Minnesota. The [http://bwsr.state.mn.us Minnesota Board of Water and Soil Resources] (BWSR) issued a January 2019 report stating that compliance with the Buffer Law' has reached 99%.