August 6 – In The Zone

Today’s factismal: Scientists have been measuring the Gulf of Mexico’s “Dead Zone” for thirty years.

Last year, we had some good news: the 2014 “dead zone” in the Gulf of Mexico started out just half as big as the one in 2013; unfortunately, before the year ended, the dead zone had grown to be almost as large as the previous year’s. And the 2015 dead zone looks to continue the trend; it will cover an area the size of Connecticut (roughly 5,500 square miles) and will wreaks massive havoc on fish stocks, crawfish, and shrimp. The relatively steady size of the dead zone indicates that the Mississippi basin is relatively stable.

But why should what happens in Ohio, Illinois, Arkansas, and 28 other states have an effect on the Gulf of Mexico? It all happens because of the fertilizer. The Mississippi and its tributaries gathers run off from farms in 31 states; quite often, that run off includes fertilizer and top soil. Those two nutrients wash all the way down into the Gulf of Mexico where it forms a thin lens over the denser, saltier ocean water. This had the effect of ringing the dinner bell for algae and phytoplankton (those autotrophs we were just discussing) and they respond by gorging themselves on the nutrients and running off to make little baby algae and phytoplankton; it becomes what a biologist calls a “bloom” and swimmers call “yucky”.

Water from the Mississippi and other rivers carries nutrients into the Gulf, creating dead zones (red) (Image courtesy MMM)

Water from the Mississippi and other rivers carries nutrients into the Gulf, creating dead zones (red)
(Image courtesy LUMC)

The process doesn’t stop there. All that blooming phytoplankton brings in hungry little critters like krill and copepods. These critters feast on the phytoplankton and algae and excrete used autotroph that drops to the ocean flooor where it feeds colonies of bacteria (sorry, Sponge Bob!). And that’s where the trouble starts, because the bacteria use up so much oxygen when they eat the slimy scat that nothing else can live in the ocean column above them; they create a “dead zone” devoid of oxygen.

How to create a dead zone (Image courtesy Louisiana Universities Marine Consortium)

How to create a dead zone
(Image courtesy Louisiana Universities Marine Consortium)

This isn’t a new problem; according to sediment cores, there’s been a summertime dead zone in the Gulf of Mexico for more than a century. And scientists have been sailing the Gulf of Mexico to measure the size of the dead zone fro thirty years now. And in those thirty years, they’ve discovered some interesting things. The size of the dead zone changes year by year. During drought years, the dead zone shrinks because less fertilizer is washed to the ocean. And during wet years, the dead zone gets bigger. For example, they estimate that the Mississippi and Atchafalaya rivers pushed 104,000 metric tons of nitrate and 19,300 metric tons of phosphorus into the Gulf of Mexico in one month alone! The good news is that the level of nitrogen was 21% below average. The bad news is that hte phosphorus was 16% above average.

To understand what effect these sorts fo changes have, the scientists need more data data than they can collect on one cruise – and that’s where you come in! The marine biologists at the National Centers for Coastal Ocean Science would love to have you help them monitor the number and types of phytoplankton in the water near where you live. They’ve set up the Phytoplankton Monitoring Project, where you can volunteer to do a plankton tow (more fun than it sounds) and enter your results. They are particularly interested in getting groups of Girl Scouts or Boy Scouts to adopt an area and take samples twice a month. To learn more, float on over to:
http://products.coastalscience.noaa.gov/pmn/default.aspx

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