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Water Today Title December 4, 2020

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Update 2018/8/6
Blue-green algae


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By Michelle Moore

National Oceanic and Atmospheric Administration (NOAA) supported scientists revealed this week that the dead zone in the Gulf of Mexico is less than half the size of what had been predicted for the summer.

The Gulf's dead zone, an area of water depleted of oxygen that cannot support life, is the fourth smallest zone since 1985. In June, the NOAA forecasted a size of roughly 5 780 square miles for this summer, but scientists have determined it's size is closer to 2 720 miles or 7 040 square kilometres.

The NOAA uses modelling as well as information from the United States Geological Survey to estimate the size of the dead zone as well as determine it's contributing factors. For instance models have helped show the significant role the Mississippi River plays in terms of nutrient runoff.

Nutrient runoff from manure and fertilizers are thought to be the biggest cause of eutrophication, which causes harmful algal growth to accumulate and deplete waters of oxygen.

Blue green algae or cyanobacteria tend to bloom in these conditions covering the top of the water in a green slime. Cyanobacteria is known to produce several toxins such as neurotoxins, and hepatotoxins which affect the liver.

In addition to producing toxins, these algae hide other plants from the sun which causes them to die, decompose and deplete the oxygen further still, which can result in fish kills.

Louisiana State University and the Louisiana Universities Marine Consortium were tasked with measuring the size of the dead zone during a research cruise from July 24 to July 28.

Leader of the survey mission Dr. Nancy Rabalais said "persistent winds from the west and northwest in the few weeks preceding the cruise likely pushed the low oxygen water mass to the east and piled it towards the central shelf and towards Grand Isle."

Rabalais added that winds west of the Mississippi River delta off the Barataria Pass could have mixed the oxygen depleted waters with other water therefore increasing the oxygen levels.

Rabalais said "this resulted in an unexpectedly small dead zone, though similar conditions and smaller areas of bottom-water hypoxia were documented in 1998 and 2009."

The largest dead zone was measured in 2017 at a size of 8 776 square miles, or 22 730 square kilometres. The NOAA explains that the size of the zone can be made smaller by strong winds and storms as was witnessed this year.

Cyanobacteria is known to proliferate in sustained warm temperatures and shallow, brackish water, but it is also impacted by other conditions that are not yet as well understood.

For instance, the NOAA hopes to gain a better understanding of how wind patterns and ocean conditions can affect the dead zone through ongoing monitoring.

New programs like Runoff Risk are working with farmers and using moisture modelling to determine moisture levels in soil while forecasting rain conditions as well as snow melt to help determine the best time to fertilize crops.

By helping farmers learn more about optimum nutrient application times, fertilizer is used more effectively and the amount of nutrient runoff decreases dramatically, improving the health of lakes, streams and rivers.


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