Video that shows a clear line separating bright blue water from murky green water is rumored to show that water from the Gulf of Mexico and Mississippi River don’t mix.
The area, which is called a “dead zone,” was located off the shore of Louisiana and Texas in the summer of 2015. It was caused by high levels of nutrient runoff into the Mississippi River that left the water emptying into the Gulf of Mexico with high levels of nitrogen.
Because freshwater from the river is lighter than the salty seawater, it settles as a distinct layer above the seawater.
This phenomenon is called stratification. The freshwater layer acts like a blanket to prevent the exchange of gases, oxygen for example, between the seawater and the atmosphere.