Most life on Earth needs oxygen to survive - even those living in our oceans.  So when oxygen levels in our coastal waters are severely reduced, (hypoxia), and worse still reduced to almost zero levels (anoxia), the impacts can be deadly. These 'dead zones' (as they are commonly known) occur naturally, but there has been increasing concern that human activities - primarily our agricultural practices - are causing more, larger, and longer lasting dead zones in our coastal waters.  Essentially, excess nutrients from the land run off (or are removed) either directly into coastal waters, or down rivers which end up in the coast.  Algae loves these extra nutrients - so much so that they 'bloom' to extraordinarily numbers - forming algal blooms at the surface.  Eventually they die off, sink down to the ocean floor where microbes get to work decomposing the remains.   This is quite an oxygen-intensive process and eventually oxygen in these lower levels becomes fatally low for anything that cannot move away from the area.

The Gulf of Mexico is home to arguably the most famous of these man-made dead zones, and is subject to regular monitoring.  In fact, researchers are even forecasting dead zones.  And 2013, they said, would be a biggie.  Researchers from Texas A&M University have been busy keeping an eye on the oxygen levels in the lower waters from the Gulf.  Turns out, the predictions were right on target.  And they are predicting as the summer temperatures warm up, it will get worse.

Want to explain the dead zone to your kids?  Check out this interactive look at how dead zones in the Gulf of Mexico form, courtesy of the Science Museum of Minnesota.