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Dealing with Dead Zones: Hypoxia in the Ocean

NOAA Ocean Podcast: Episode 13

When water runs off of farmland and urban centers and flows into our streams and rivers, it is often chock-full of fertilizers and other nutrients. These massive loads of nutrients eventually end up in our coastal ocean, fueling a chain of events that can lead to hypoxic "dead zones" — areas along the sea floor where oxygen is so low it can no longer sustain marine life. In this episode, we're joined by NOAA scientist Alan Lewitus to explore why dead zones form, how the problem of hypoxia is growing worse, and what we're doing about it.

animation of nutrient runoff into Gulf of Mexico

At 2,116 square miles, the 2020 hypoxic zone in the Gulf of Mexico is the 3rd smallest ever measured in the 34-year record, measured from July 25 to August 1. Red area denotes 2 milligrams per liter of oxygen or lower, the level which is considered hypoxic, at the bottom of the seafloor. (Bottom panel) Long-term measured size of the hypoxic zone (green bars) measured during the ship surveys since 1985, including the target goal established by the Mississippi River/Gulf of Mexico Watershed Nutrient Task Force and the 5-year average measured size (black dashed lines). In 2020, Hurricane Hanna passed through the central and western Gulf days prior to the research cruise and mixed the water column, disrupting the hypoxic zone which forms in the coastal ocean west of the Mississippi River delta. While the size of the hypoxic zone fluctuates naturally throughout the summer, it usually forms again within days or weeks after the passage of storms. Due to the close proximity of the storm to the survey cruise, the hypoxia area was only able to partially reform before the end of the monitoring cruise, resulting in a patchy distribution across the Gulf. Graphic credit: Louisiana Universities Marine Consortium


HOST: This is the NOAA Ocean podcast. I’m Troy Kitch.

In 2010, scientists discovered multicellular animals that don’t require oxygen to survive buried deep in the sediment at the bottom of the Mediterranean. Aside from some types of very simple bacteria and single-celled organisms, these are the only other known lifeforms on our planet that can survive in a zero oxygen environment.

As with life on land, practically all ocean life is dependent on oxygen to survive. It’s the key ingredient that makes life in the ocean work. The diversity and productivity of ocean life, and the complex biochemical cycles that keep ocean life in balance all depend on oxygen. Now here’s the problem. The ocean isn’t getting enough of it. And this lack of oxygen is leading to a chronic condition called hypoxia. Areas in the ocean that experience these hypoxic conditions over long periods of time are often referred to as “dead zones,” for reasons that will become very clear later in this episode.

So what’s causing this problem? Why and how is it getting worse? And can we do anything about it?

We’re joined in this episode by NOAA scientist Alan Lewitus to get some answers.

Alan directs the Competitive Research Program for the National Centers for Coastal Ocean Science, part of the National Ocean Service. His job is to oversee NOAA grants awarded to researchers around the nation who study topics like hypoxia — research that targets improving the health of our coastal ecosystems.

ALAN LEWITUS: Hypoxia refers to water conditions where the concentration of oxygen is so low that it is detrimental to organisms and very few organisms can survive in those conditions. Scientists refer to hypoxic waters as those waters where oxygen concentrations are below two milligrams per liter. Now, organisms that can swim away from those conditions do, they flee, and so they avoid hypoxic waters. But not always. Sometimes they’re trapped in bayments and other areas, so you see many cases where hypoxia events are associated with large-scale fish kills. In larger systems, they can flee, but you have other problems. Hypoxia can affect the habitat of fish. There’s a loss of bottom fauna, which are important food sources. Other organisms that can’t move such as shellfish and worms and so forth, are trapped and often suffocate and die.

HOST: As an example of how hypoxia can affect habitats, Alan pointed to the brown shrimp, a huge commercial fishery in the Gulf of Mexico. The area where hypoxia occurs today in the Gulf used to be the prime place for fisherman to harvest these shrimp.

ALAN LEWITUS: The habitat of the brown shrimp, the optimal habitat, was reduced by 25 percent. So you’re taking away 25 percent of the habitat. There’s other things, too. Hypoxia, by affecting the bottom fauna, you’re taking away a food source for fish and crustaceans and other things and that has ripple effects through the food chain. It’s a cascading effect.

HOST: And, he added, there are also sub-lethal effects on fish, which are becoming better understood as research progresses.

ALAN LEWITUS: Sub-lethal effects mean that the fish don’t need to be affected by death, they can be affected by exposure to hypoxia, which has certain physiological effects on fish function. A couple of common ones that are being found with Gulf of Mexico studies are that the reproductive potential and growth potential of certain fish, especially bottom-dwelling fish can be affected by even intermittent exposure to hypoxia. You could imagine, if these are bottom-dwelling fish that they’re probably going to hang around the edges of the hypoxic zone, but they still maintain some exposure through foraging activities as well as escape activities from predators. So they are exposed, in and out, in the hypoxic zone. And just this intermittent exposure can lead to serious reproductive impairments, changes in sex, and other bizarre things.

HOST: He added that scientists are now working on models that forecast how these cumulative sub-lethal effects from fish exposed off and on to hypoxia from year to year may be leading to long-term reductions in populations.

ALAN LEWITUS: It’s complicated because the more common effect of hypoxia on fish is not through fish kills. It more commonly affects them through these sub-lethal effects and indirect effects. Effects that sort of cascade through the food chain, as well as sub-lethal effects of hypoxia exposure on reproductive impairments and reductions in growth potential. These are hard to get at. You need sophisticated models to try to separate those adverse effects on fish health from other factors that sort of interact at the same time with hypoxia. But we’re making some headway with those models.

HOST: Talking about this complexity brought us back to the brown shrimp in the Gulf of Mexico and a study that came out in 2017.

ALAN LEWITUS: It found that, in years where hypoxia is large, there was an effect on the size of shrimp that were sold at market. There was an increase in the proportion of smaller size shrimp that were sold. It may be that these growth impairments of hypoxia on shrimp are at play and are causing a reduction in the growth rate. Another factor is that when hypoxia forms, fish and shrimp aggregate around the edges. They want to avoid the hypoxia, but they tend to stay around the edges and there’s a lot of reasons: there’s an accumulation of food sources there. But the fishermen know this, so they know where to go when hypoxia forms. They go around the edges and they can target the fish and shrimp in that way. So the other factor is that they think that some of the small shrimp are fished out and less make it to larger sizes. The bottom line is that when there are large hypoxia years, there’s an adverse effect on the economic profits of local fishermen. And brown shrimp is the largest commercial market in the Gulf of Mexico, so that’s a significant finding.

HOST: Alan said that hypoxic conditions can occur naturally under certain conditions. Records indicate that past events — say, earlier than 1970 — were episodic and generally small. But today, regions of the ocean experiencing hypoxia can be massive. Take the Gulf of Mexico, where scientists funded by NOAA map the size of a “dead zone” that appears every year. In 2017, it was measured at 8,776 square miles, about the size of New Jersey. It was the largest ever recorded.

Why are dead zones larger today and what’s causing this? It’s all about human activity. The culprit is runoff of polluted water that’s carrying tons of excess nutrients from agriculture and developed land from our interior waterways out to the ocean. But nutrients are good things, right?

ALAN LEWITUS: Nutrients are an essential element for plants and algae. So nitrogen and phosphorus are examples of nutrients that are needed by plants. And so they are a good thing from a standpoint that you have to have them to grow crops, for instance. But the problem is when they’re supplied in excess. They can become a bad thing. If you over-fertilize a field, the crops can’t take up all that fertilizer, so a lot of it leaks into water systems.

HOST: And these water systems carrying all this extra fertilizer ultimately flow to the ocean. For the Mississippi, this watershed is the third largest in the world and includes about 40 percent of the continental United States. Too much of the fertilized water we put on crops in the breadbasket of the U.S. eventually ends up in the Gulf.

ALAN LEWITUS: You have an immense amount of fertilizer application for the corn crops and so forth. A lot of it is leaked. Corn is actually a very inefficient plant in terms of using fertilizer, so a lot of it leaks out if not applied in a strategic way, and the nutrients are carried down the river into the Gulf of Mexico, where they stimulate algal blooms. Algae depend on nutrients and it’s good from the standpoint of providing the base of the food chain in aquatic systems, but when you have excess nutrients, you have excess algal growth. They can form blooms.

HOST: So these nutrients that were intended to be used by crops on land wash away to the sea, where they can lead to an explosive blooms of algae. It’s a process called nutrient loading. I asked Alan to connect the dots on how these blooms can lead to hypoxia.

ALAN LEWITUS: What happens is nutrients lead to excessive algal growth, which leads to algal blooms. And the algal blooms, at some point, start degrading and sinking to the bottom, and bacteria work on these algae — they decompose the algae. And as they do that, they consume the oxygen from the water. So that leads to low oxygen water, or hypoxia.

HOST: So as the algae bloom dies off and sinks to the bottom, bacteria eat them up, consuming oxygen. What’s left behind is a low-oxygen dead zone on and near the seafloor. Now you might wonder why these conditions persist. After all, the ocean is always sloshing around and mixing, right? Alan said that’s because water layers of different temperature, salinity, and density don’t like to mix. So the fresher water coming in from, say, the Mississippi River, doesn’t mix well with the hypoxic water on the bottom. Alan said this layering of the water is called stratification.

ALAN LEWITUS: Stratification often occurs when fresh water is loaded into a system which creates a barrier for mixing, so the fresh water sits on top of the more saline water. So bottom waters are restricted from mixing from high-oxygenated surface waters. That combination of high stratification and high nutrient loading are the factors that, in combination, can lead to your most problematic hypoxic zones. The ones that are very large-scale as well as long lived, for a long period of time.

HOST: For the Gulf, dead zones start to form in the spring, because that’s when crops are getting fertilized heavily. Hypoxic conditions persist and peak sometime in the summer, because conditions are right to keep the water layers from mixing. Then the dead zone dissipates in the fall and winter when the flow of nutrients slow down and temperature and other conditions are more favorable for the water in the Gulf to more readily mix together. Then it starts all over again during the next spring.

But nutrients aren’t the only factor contributing to less oxygen in the ocean. There’s another big variable that complicates the hypoxia problem: climate change. I asked Alan how global warming factors in.

ALAN LEWITUS: There is a link between climate change and hypoxia and it all goes in the wrong direction (laughs). The factors that we think about in terms of climate change and the models are telling us that they’re all sort of leaning towards promoting more hypoxia in the future, if we don’t do anything about it. In the open ocean, you have global warming, which is causing a greater and greater rate of de-oxygenation of open ocean waters.

HOST: Alan said this is mainly due to three factors: oxygen is less soluble with higher temperatures so less of it dissolves into the ocean; marine life consumes more oxygen because higher temperatures contribute to higher metabolic rates; and higher temperatures lead to more stratification, meaning the more oxygenated surface water doesn’t mix well with more hypoxic bottom waters.

ALAN LEWITUS: So those are all working in the direction of reducing oxygen in the open ocean. In the coastal areas, those same factors are working in the same way. There’s not as great of an effect observed yet in the coastal areas, but models tell us that global warming is going to work in that general direction in terms of reducing oxygen levels and increasing hypoxia.

HOST: And, he said, climate change is also contributing to more nutrients entering our coastal waterways.

ALAN LEWITUS: The arrows are pointing to an increase of nutrient loading, things like greater frequency of storm events and greater precipitation in certain areas. And those lead to both higher nutrient loads off the land. In addition, they will lead to greater freshwater into coastal waters, which will increase the stratification. So these are the forecasts that our models are telling us right now.

HOST: And this leads back to what Alan says is the primary way we can help to reduce the growing problem of hypoxia: by reducing the amounts of nutrients flowing into our ocean.

ALAN LEWITUS: The best management strategy is to reduce nutrient loading from the watershed. It’s a huge challenge, especially in large watersheds. The classic example is the Gulf of Mexico hypoxic zone. Forty one states in the U.S. (contiguous) drain into the Gulf of Mexico, so you can imagine the management challenge. Now, there is an interagency Gulf hypoxia task force that has been around for a number of years. It’s composed of five federal agencies, twelve state agencies, and a tribal association. And they have, as their primary goal, to reduce the hypoxic zone in the Gulf of Mexico to a certain level by a certain year. In order to achieve that goal, they need to reduce nutrient loading in the watershed by a certain amount, and they have all this figured out quantitatively, actually through the models we develop to help inform them of that. However, it’s not an easy task. You have to get all the states, all the state agencies in agreement working in the same direction. A lot of effort, coordination, effort to do that. You have to have the resources, the money, to support different practices. So it’s a huge challenge. They’re making some progress, but it takes years and years and years to do that sort of thing.

HOST: While reducing the fertilizer and other nutrients that flow into the Gulf of Mexico is a work in progress, Alan said that there are dead zone reduction success stories. He called out Narragansett Bay, where the nutrient problem is mainly due to sewage and wastewater treatment plants.

ALAN LEWITUS: So that’s a much easier thing to regulate, and actually in response to a fish kill on the order of a decade ago, the state imposed regulations on sewage treatment plants to reduce nutrient loading by 50 percent. They achieved that goal and our studies show that hypoxia was reduced as a result. The Bay turned from a eutrophic Bay to an oligotrophic Bay — which means cleaner water, essentially, better water quality. They haven’t achieved the ultimate goal with respect to hypoxia yet. They might need to actually reduce nutrients a little more, but they’re going in a great direction. So that’s a real success story there. So hypoxia can be mitigated.

HOST: I wrapped up our talk by asking what drives Alan forward working on intractable coastal problems like hypoxia. He stressed that NOAA supports the research and provides information, tools, and training to coastal managers who make it happen. But he said he and his colleagues take some ownership when those successes occur.

ALAN LEWITUS: Hypoxia is a challenging field to work in. It’s a double-edged sword, because the pay-off is great. Having some influence on activities that will lead to reduction in such an important stressor and ultimately societal benefits from that, is what I’m working for. The other edge of the sword is it often is a long, long road with lots of fights along the way. It’s not like there are very frequent and numerous successes, but the successes when they do come are great, and that’s what keeps me going. And there have been some. The Gulf of Mexico is still ongoing, still a challenge, and we haven’t seen the benefits of that entirely yet, though we know we’re moving the needle.

HOST: Thanks to Alan Lewitus for joining us on the program. Alan is director of the Competitive Research Program for the National Centers for Coastal Ocean Science.

And thank you for listening to the NOAA Ocean Podcast. Head to oceanservice.noaa.gov to learn about what we do.