Hypoxia and Medicine

Gulf of Mexico Dead Zone Question? Why oxygen can't enter the gulf when there is FreshWater on top?

I am currently studying the Gulf of Mexico Dead Zone. It says that " The freshwater from the Mississippi River floats on top of the saltwater on the Gulf of Mexico. Due to this, the oxygen in the air cannot get into the deep water. " I was just wondering why is it that oxygen can't enter the Gulf of Mexico when there is Fresh Water on top of it? Thanks = )

Public Comments

1. Good question, and it frankly makes little sense to say that just because fresh water floats on salt water, the oxygen is depleted. But, i found a good study of what's going on here http://oceanservice.noaa.gov/products/pubs_hypox.html Waaaaay down at the bottom, (look for "Update as of November 1, 2000") it explains "coastal waters are swamped with nutrients, particularly nitrogen, from sewage or fertilizer. The excess nitrogen allows algae populations to explode. Dead algae in turn feed bacteria, which gobble up most of the oxygen in the water." So, it's not the fresh water so much as it is the fertilizer in the fresh water.
2. This is a rather complex topic and there is enough information on it to fill volumes of books. Here is the deal in a slightly over simplified explanation. #### The Mississippi river carries a large amount of debris down stream to the ocean. As the river water enters the ocean its speed (rate of flow) decreases. As the water slows down it drops smaller and smaller debris to the water bottom. There is a certain area in the Mississippi river delta where the river slows down to a point that fertilizer, pesticides, and other substances suspended in the river water fall out to the bottom. Fertilizer that falls out here causes the river/ocean water there to have minimal oxygen content. Hurricane Katrina gave that area a good scrubbing as the water moved faster because of the high water in the river. I doubt anything can realistically be done about it. Channelizing the delta has already been done for years, and even small amounts of fertilizer can be seriously concentrated because of the enormous area drained by the Mississippi river.