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Question: What did the recent hurricanes do to the "dead zone" in the Gulf of Mexico? Did the massive winds and tides do anything to help oxygenate the areas?

If you're from the Southern United States, come spring, there's almost always a sudden explosion of news articles that talk about this dead zone in the Gulf of Mexico. Scientists and agencies like the NOAA(National Oceanic and Atmospheric Administration) have a penchant for predicting how big it's going to be this year, and very often you'll find a who's-who list of states or operations that are to blame (at least partly) for the dead zones.

For over two decades, we’ve seen dead zones grow and grow, compounded by other catastrophic environmental disasters like the Deepwater Horizon oil spill.

Now it's hard not to be curious about something that makes it to the newspapers with such alarming regularity every year, which brings us to the question asked by one curious reader of the column.

To answer that shortly: yes, the hurricanes did affect the dead zone. But that's a very "deep" question (HA! Get it? No?!), which requires at least a cursory understanding of what a dead zone is, how it's formed and how it affects the body of water.

Let's start with what dead zones are. We all know that they're probably dangerous and deadly, hence the spooky name. But why?

The dead zone is a very large region of water that has an extremely low concentration of oxygen and, well, since you need oxygen to live, this place is pretty dead. Without the oxygen, fish either die or swim as far away as they can from the “hypoxic,” meaning oxygen-deficient, water.

How many dead zones are there? There are quite a few; it's just that the one near us happens to be the second largest. The largest one covers almost 5% of the Arabian Sea and spans nearly the entire Gulf of Oman (that's roughly a whopping 70,000 square miles).

The dead zone in the Gulf of Mexico is usually around 6,000 square Miles in size (give or take 1,000 square miles or so) in case some of you wanted to compare. For the sake of this column and yours truly, we'll try and focus on just this one.

So what causes this, or any other, dead zone? You see, the ocean has microscopic algae called Phytoplankton, which happen to be really small plant-like microorganisms that float about in the top layer of the ocean using sunlight to make their food. Like all living things, they need food to survive, which consists of compounds with nitrogen, phosphorus and sulfur.

Luckily for these little critters, we humans serve them these nutrients on metaphorical platter. Every year, excess nutrients through anthropogenic sources like fertilizer, manure, industrial waste and inadequately treated wastewater from cities and farms in the upland Mississippi River watershed drain into the Gulf.

And boy do the phytoplankton eat, gorging themselves relentlessly. Microbes don’t exactly consciously go on diets. They just eat or don’t. They eat until they cause what we call an "algal bloom," where the ocean turns green with so much algae on the surface that you can actually see it from space! This whole process is called Eutrophication.

But surely these algae produce oxygen, so how does that lead to hypoxia? It’s all due to imbalance in nutrients, and it is brutal. The algae, in their fit of gorging on all the food, cannot sustain their growing numbers and eventually start to die.

Algae’s natural predator? Zooplankton, tiny animals that wander aimlessly in the ocean, eat up the Phytoplankton and need oxygen. Then the phytoplankton run out of food because they’ve also grown in numbers, which produces a massive amount of animal waste and uh, corpses — plankton corpses. Yeah, tons of little corpses, literally raining down on the oceanic floor.

Then those get eaten by good ol' decomposing bacteria, munching on the bodies and using up whatever oxygen is left in the column of water. Then the fish don’t have any oxygen, so they die or leave. And bam! You get a dead zone.

Now, remember that bit about NOAA forecasting how big the dead zone is going to be? This year they predicted it to be around 7,800 square miles which is smaller than the largest observed in 2017 (8,776 square miles). However, the actual size of the dead zone this year is 6,952 square miles, which makes it the eighth largest in the 33-year record.

So what happened to make it smaller than the forecast despite the Mississippi watershed having one of the wettest-ever springs?

According to Steve Thurr, director at NOAA, "Hurricane Barry, which passed over the hypoxic zone just before the cruise, likely played a major role in mixing the water column and disrupting the hypoxia that already formed. Hypoxia typically takes some time to reform after an event like Barry."

This is not the first time it has happened, though, with similar findings being reported back in 2011. However, this shrinking of the dead zone due to Hurricane Barry was only temporary.

Nancy Rabalais, a scientist from LSU who has measured the dead zone since 1985, said that in a week following the NOAA cruise that measured the hypoxic zone, it could be "larger than it is right now."

So yes, Hurricanes do mix up the water column, but how does that help to get rid of the dead zone? Freshwater that flows in from the Mississippi watershed into the Gulf is less dense than the ocean's saltwater and, as such, floats on top of it. This layering means that the freshwater and saltwater do not mix and, as such, do not allow the transportation of oxygen.

Some of the key factors that therefore affect the size of the dead zone, and this mixing, are the amount of sunshine, temperature, rainfall and wind.

Predicting what will happen to the dead zone and trying to control it is a difficult thing to do with the dynamic natural systems involved and the sheer scale of the problem. Nevertheless, agencies like the NOAA and the Mississippi River/Gulf of Mexico Hypoxia Task Force are working to improve the forecasting ability and understand the impact of hypoxia on marine resources.

So, the next time hurricane season comes huffing and puffing, you can be sure that the dead zone will shrink, but it won't be for long!

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