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| Gulf Dead Zone 2017 from EPA |
The 2019 NOAA forecast calls for an above average dead zone in the Gulf of Mexico this summer. Scientists are predicting that the dead zone will cover a 7,829 square miles much larger than the 5-year average measured size of 5,770 square miles, but less than the 2017 record of 8,776 square miles. The prediction model uses the U.S. Geological Survey (USGS) river flow and nutrient data.
Dead zones are a yearly occurrence in the Gulf of Mexico and other estuaries. Dead zones form in summers when higher temperatures reduce the oxygen holding capacity of the water, the air is still and especially in years of heavy rains that carry excess nutrient pollution from cities and farms. The excess nutrient pollution combined with mild weather encourages the explosive growth of phytoplankton, which is a single-celled algae. While the phytoplankton produces oxygen during photosynthesis, when there is excessive growth of algae the light is chocked out and the algae die and fall from the warmer fresh water into the colder sea water. The phytoplankton is decomposed by bacteria, which consumes the already depleted oxygen in the lower salt level, leaving dead marine life in their wake.
A major factor contributing to the large dead zone this year is the abnormally high amount of spring rainfall in many parts of the Mississippi River watershed, which led to flooding that carried soil and nutrients flowing to the Gulf of Mexico. This past May, the USGS reported the flow from the Mississippi and Atchafalaya rivers was about 67% above the long-term average between 1980 and 2018. USGS estimates that this larger-than average river discharge carried 156,000 metric tons of nitrate and 25,300 metric tons of phosphorus into the Gulf of Mexico in May alone. These nitrate loads were about 18% above the long-term average, and phosphorus loads were about 49% above the long-term average. The USGS operates more than 3,000 real-time stream gauges, 50 real-time nitrate sensors, and 35 long-term monitoring sites throughout the Mississippi-Atchafalaya watershed, which drains all rivers and streams in parts or all of 31 states and 2 Canadian provinces into the Gulf of Mexico.
While nutrient inputs to the Gulf of Mexico vary from year to year because of natural swings in precipitation and runoff, USGS also tracks longer-term gradual changes in nitrate and phosphorus loading into the Gulf of Mexico from the Mississippi River. “Long-term monitoring of the country's streams and rivers by the USGS has shown that while nitrogen loading into some other coastal estuaries has been decreasing, that is not the case in the Gulf of Mexico," said Don Cline, associate director for the USGS Water Resources Mission Area. “The Mississippi River/Gulf of Mexico Watershed Nutrient Task Force, an EPA group working to reduce the Gulf dead zone through nutrient reductions within the Mississippi River watershed, has set a 5-year average measured size target of 1,900 square miles.
NOAA issues a dead zone forecast each year; the forecast assumes typical coastal weather conditions, but the measured dead zone size could be very different. The measured size of the Dead Zone in August could be disrupted by major wind events, hurricanes and tropical storms which mix ocean waters. This happened in 2018 in the Gulf of Mexico and our own Chesapeake Bay. A NOAA-supported monitoring survey will confirm the size of the 2019 Gulf dead zone in early August.
Dead zones are a yearly occurrence in the Gulf of Mexico and other estuaries. Dead zones form in summers when higher temperatures reduce the oxygen holding capacity of the water, the air is still and especially in years of heavy rains that carry excess nutrient pollution from cities and farms. The excess nutrient pollution combined with mild weather encourages the explosive growth of phytoplankton, which is a single-celled algae. While the phytoplankton produces oxygen during photosynthesis, when there is excessive growth of algae the light is chocked out and the algae die and fall from the warmer fresh water into the colder sea water. The phytoplankton is decomposed by bacteria, which consumes the already depleted oxygen in the lower salt level, leaving dead marine life in their wake.
A major factor contributing to the large dead zone this year is the abnormally high amount of spring rainfall in many parts of the Mississippi River watershed, which led to flooding that carried soil and nutrients flowing to the Gulf of Mexico. This past May, the USGS reported the flow from the Mississippi and Atchafalaya rivers was about 67% above the long-term average between 1980 and 2018. USGS estimates that this larger-than average river discharge carried 156,000 metric tons of nitrate and 25,300 metric tons of phosphorus into the Gulf of Mexico in May alone. These nitrate loads were about 18% above the long-term average, and phosphorus loads were about 49% above the long-term average. The USGS operates more than 3,000 real-time stream gauges, 50 real-time nitrate sensors, and 35 long-term monitoring sites throughout the Mississippi-Atchafalaya watershed, which drains all rivers and streams in parts or all of 31 states and 2 Canadian provinces into the Gulf of Mexico.
While nutrient inputs to the Gulf of Mexico vary from year to year because of natural swings in precipitation and runoff, USGS also tracks longer-term gradual changes in nitrate and phosphorus loading into the Gulf of Mexico from the Mississippi River. “Long-term monitoring of the country's streams and rivers by the USGS has shown that while nitrogen loading into some other coastal estuaries has been decreasing, that is not the case in the Gulf of Mexico," said Don Cline, associate director for the USGS Water Resources Mission Area. “The Mississippi River/Gulf of Mexico Watershed Nutrient Task Force, an EPA group working to reduce the Gulf dead zone through nutrient reductions within the Mississippi River watershed, has set a 5-year average measured size target of 1,900 square miles.
NOAA issues a dead zone forecast each year; the forecast assumes typical coastal weather conditions, but the measured dead zone size could be very different. The measured size of the Dead Zone in August could be disrupted by major wind events, hurricanes and tropical storms which mix ocean waters. This happened in 2018 in the Gulf of Mexico and our own Chesapeake Bay. A NOAA-supported monitoring survey will confirm the size of the 2019 Gulf dead zone in early August.
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