“Drains to River”…And then what? The Connection between Stormwater and the Gulf Dead Zone

It was recently reported that the dead zone in the Gulf of Mexico is projected to be “large-to-very-large” this year. It might come as a surprise that the main contributing factor to this projection is…stormwater! Perhaps you’ve heard the phrase “only rain down the drain,” or “we all live downstream” (if you’re a stormwater professional, they are your mantra!), and we KNOW you’ve seen markers like this one on local storm drains:

We all know rain washes pollutants into local streams, rivers, etc., but sometimes we forget that’s not where the story ends. Where do all of those streams and rivers end up? In the United States, most of them end up in the Gulf of Mexico. The Mississippi River watershed is the largest watershed in the United States, and the fourth largest watershed in the WORLD! This means that all the pollutants that reach streams and rivers in this zone, which encompasses nearly 40% of the continental United States, eventually gets funneled into the same place, the Gulf of Mexico.

Image: AmericanRivers.org


So what exactly causes the dead zone? Of all the pollutants washed into the water, the largest contributors to the dead zone are nutrients, particularly nitrogen. This one confuses people sometimes because we do not think of nitrogen as “toxic.” In fact, we all NEED nitrogen to survive, so you could say this is a case of “too much of a good thing.”  Nitrogen helps things grow; this is why it is a main ingredient in the fertilizers used by farmers and people around their homes. Fertilizer that stays in place helps the plants grow is one thing. Fertilizer that washes away? Well, that’s a different story altogether.

Once fertilizer reaches a waterway, it does not suddenly become toxic; it STILL spurs growth. Micro-organisms feed on the nutrients in the fertilizer. It’s this process that can get out of control, causing large algae blooms. Once the nutrients have all been used up, those micro-organisms can no longer be supported, so they die and fall to the bottom. As they decompose, they “suck up” oxygen in the water, sometimes to the point where there is no oxygen left, a situation called hypoxia. If this process affects a large geographic zone, it is termed a dead zone. Dead zones can happen at any time, but they tend to be more severe in the summer.

There are two main reasons for this. First, algae prefer to grow in warmer water, so the large blooms that lead to dead zones are more likely during the warm summer months. Second, warm water cannot hold as much oxygen as cold water (think about how warm soda goes flat! It can’t hold as much gas at a higher temperature). So, in the summer we have a situation where more algae are blooming in water with ALREADY reduced oxygen…not good.

Image: NOAA

To recap, a dead zone of some size is always expected in the Gulf of Mexico; the dead zone is the result of stormwater washing excess nutrients into local waters that eventually empty into the Gulf, and the dead zone reaches its peak during the summer months because warm temperatures encourage algae growth while lowering oxygen levels.

But why is it projected to be worse than normal this year? Again…the answer is STORMWATER! Winter and Spring 2019 saw record-setting rainfalls over much of the Mississippi watershed. This NOAA map of one of the most severe months on record, February 2019, shows many regions within the Mississippi watershed receiving 150-600% the amount of rain that is typical; and remember, all of this rain that could not soak into the ground “drains to river”:

Now that temperatures are beginning to warm, the stage is set for one of the largest dead zones we have seen in recent years. The estimated size of the dead zone this year is 8,000 square miles! For reference, that is roughly the size of Massachusetts. NOAA will conduct a monitoring study in August to assess their projections and refine their model for future projections.

Image: The Weather Channel

The Mississippi River/Gulf of Mexico Task Force has set a goal to reduce the annual average Gulf dead zone to be 1,900 square miles, about a quarter of this year’s projection. As part of this directive, they have set up a monitoring program to understand why nitrogen levels are not decreasing in the Mississippi watershed at the same rate they are in other coastal zones. Because the Mississippi watershed overlaps with America’s breadbasket, agricultural sources of nutrients are arguably the largest.

While farmers cannot ditch fertilizers all together, they can certainly get smarter about it. For example, a recent study at Michigan State University utilized satellite imagery to help farmers assess small scale variability in their crop yields so they could more effectively apply fertilizer. This led to reduced fertilizer wash-off and increased crop yields, both of which save farmers’ money (!) while also mitigating environmental damage. Win-win! Meanwhile, the #1 thing residents can do is just say NO to fertilizers! Realizing that institutional change is required to address a problem this large, many coastal regions, even outside of the Mississippi watershed, have begun implementing summertime fertilizer bans in effort to block the harmful chain of events caused when excess nutrients hit local waters. Those bans are often paired with campaigns aimed at environmentally friendly landscaping to use plants that can survive without excessive watering and fertilizers.

While these efforts are promising, much work remains to be done, and we do not have time to waste! A NOAA study published in March 2019 found that the Great Lakes region was warming at a faster rate than surrounding areas, which could result in more intense rains over the Mississippi watershed, feeding back into the cycle that intensifies the Gulf dead zone.

Want to learn more about the Gulf dead zone and what’s being done? Check out Nancy Rabalais’ TED talk on the subject:

About the author

Suzie Housley

Suzie has over a decade of experience in the Stormwater industry including both government and academic work. She leans on her experience to meaningfully interpret scientific studies and government policies to communicate a practical message. Suzie lives just outside Nashville, TN and gets outside whenever she can to explore nature with her husband and two small children.