With each issue, Trib+Water brings you an interview with experts on water-related issues. Here is this week's subject:

Steve DiMarco is a professor of oceanography at Texas A&M University, where he researches oil spills, pollution and hypoxia, the "dead zones" in bodies of water that have low oxygen levels. That includes the Gulf of Mexico dead zone that spans about 15,000 square kilometers in Louisiana and Texas. DiMarco is currently measuring how the recent storms in Texas might be impacting the Gulf of Mexico dead zone. His research is funded by the National Oceanic and Atmospheric Administration.

Editor's note: This interview has been edited for length and clarity.

Trib+Water: What are dead zones, and how are they caused?

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Steve DiMarco: Dead zones are areas in the ocean where the oxygen concentration levels, the oxygen actually in the water, falls below some critical value, and that critical value has to do with how fish and other organisms that are in the water can breathe. They’re caused by a variety of reasons. On the continental shelf, so close to land, it’s usually because of highly stratified waters, which means there is freshwater sitting above salty water, and theirs are different densities. And because they have different densities, they have difficulty mixing.

So since they don’t mix, whatever processes that’s pulling out oxygen in the water — and there’s lots of different things that can pull that oxygen in the water; decay of dead things is one of the principal reasons. What happens is the oxygen gets pulled out of the water and because there’s no mixing, particularly at the low level where there is salty water, the oxygen levels start to drop. And when something happens where you can mix, like a big storm comes by or something like that, then you can put oxygen back down into the water.

Trib+Water: Where does the freshwater come from?

DiMarco: Usually freshwater is coming from land, so it’s from whatever process that leads to freshwater on land. So it’s typically rain. It rains inland, and then the rain collects into streams and creeks and flows into rivers, and the rivers eventually make it out to a large body of water like the ocean, the Gulf of Mexico. And that comes down to the Gulf of Mexico and stratifies the system.

Trib+Water: Regarding the Gulf of Mexico dead zone, how has it changed over recent years, or how has it fluctuated?

DiMarco: The Gulf of Mexico dead zone is one that’s been studied for a few decades now, and it first started to be monitored in 1985 and has been monitored every year. There’s a cruise that goes out in July by a Louisiana group. They measure the size of that. In the last 10 to 15 years, it’s been about 15,000 square kilometers. It’s big. That’s a big area, and that's what it’s averaged for the last 10 to 15 years.

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The Louisiana hypoxia is often associated with the nutrient runoff that’s coming down from the Midwest, that’s providing the fuel for growing things like algae phytoplankton. So once that river water makes it out to the Gulf of Mexico, all the nutrients in it is like food for the plankton, and then you have a big algae bloom. And because that algae bloom then is sitting there with all that biomass that lives and dies, and when it dies, it goes through that process of decay, which I said earlier. And that process of decay consumes oxygen. What happens is all this bacteria or microbial organisms, very small organisms, start eating that organic material and because microbes use oxygen and breathe in oxygen, they start utilizing that oxygen and then the levels start to drop.

Trib+Water: So how does this impact the economy and the fishing industry?

DiMarco: The thought is that it has the potential for mass fish kill because the low oxygen in the water could affect fish that are living there, and if it’s severe enough, you can have fish kill — and this happens in many places throughout the world. You get these dead zones that appear, and that can kill the fish. It’s usually a bottom type of process, though. So it often kills things that aren’t fish, anything that’s living near the bottom, as well. And you’ve got to worry that when it starts doing that, that it’s affecting the entire ecosystem. So once you start affecting the entire ecosystem, you start to worry about fisheries collapse.

The other thing that happens, too, is if you’re a shrimper — and shrimping is a big industry on the Texas and Louisiana continental shelf region — when the hypoxia or the low oxygen comes, the shrimp tend to move away, move further out offshore. And when that happens, the shrimpers have to travel further.

Trib+Water: Is this problem fixable or manageable, or how can we ease that?

DiMarco: What the main plan is, the Action Plan, that NOAA has put forth for the Louisiana side, anyway, is to mitigate the amount of nutrients that are coming down the river. And so they’ve established voluntary reductions for farmers in the Midwest to reduce the amount of nitrates that they put on their crops, the amount of fertilizer. The thought with that is if you reduce the amount of nitrates, then you’ll also limit the size of the algae that grows when it comes to the Gulf of Mexico, and if you limit the amount of algae, you then limit the amount of biomass that’s put into the Gulf, which in turn causes the hypoxia because it eventually just decays. So that's kind of the plan is to have reduction in nitrates coming down the river, and that should reduce the size of the dead zone on the Louisiana side.

The Texas side, it’s a bit more complicated because it’s not particularly associated with this nutrient runoff. So the idea here is to just manage it, so just know that when it floods, and there’s hypoxia, that you may need to go further offshore, you’ll know where it is, there may be potential for fish kill, and maybe if you are a commercial fisher, you may avoid the area, you would go someplace else. But the knowledge that you would get [is that] you can attribute it to, “This is a low year because of hypoxia,” maybe not because of some other process that you may think — because the water is warmer, so climate change, or could it be because of upwelling on the west side of the Gulf of Mexico, which is another process that can bring nutrients to the surface.

Trib+Water: You went out to look at the latest results last week. What have you been seeing now that you’ve been out there?

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DiMarco: We went out on Friday on the Texas A&M-Galveston ship called the Trident, and we planned to deploy two gliders. These are two ocean vehicles, they’re autonomous, they’re unmanned. You can think of them as drones that swim in the ocean. They don’t move like swimmers, though. They move like gliders in the air, but they’re in the water.

Unfortunately on Friday, the sea state was such that we were unable to deploy the gliders from that ship. So we’re going to try again [this week]. But what we did find was that there was some low-salinity water, which meant that there was definitely freshwater capping at the location we were at, and that freshwater capping most certainly would lead to some kind of drop in oxygen levels. We don’t have a measurement there.

There was another group earlier in the week from NOAA, a NOAA fisheries vessel, and they were reporting low oxygen at and just above hypoxic levels, that critical level that detrimentally affects marine organisms at that location off of Galveston.

Trib+Water: How long have you been studying this issue?

DiMarco: I’ve been funded through NOAA since 2003. And it’s a big project. There’s 14 Ph.D.-level scientists on that. I lead that group. And it’s led to dozens of peer-reviewed papers, master’s degrees, Ph.D.s, so there’s lots of student training, as well. In fact, for Texas, we’ve trained on these series of cruises more than 100 students over that time period, which to me is the next generation of ocean scientists that are sensitized to this type of issue.

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