The Long Shadow of Nitrogen Legacies in the Gulf of Mexico

Gulf of Mexico in 3D perspective. (Credit: By NOAA (Source (High-Res: 3000x2451)) [Public domain])

New research from a University of Waterloo team indicates that achieving Gulf of Mexico water quality goals may still take decades rather than years. This suggests that current policy goals may be too ambitious—but not that management and restoration efforts aren’t working.

To place this into context, though, it helps to look into the past.

The water quality of the northern Gulf of Mexico has become increasingly impaired over time. Since the 1950s in particular, both widespread use of commercial fertilizers and intensive livestock production across the Mississippi River Basin have meant more nitrogen running into the Atchafalaya and Mississippi Rivers, and from there into the Gulf.

Every year starting in late spring, a dead zone forms in the northern Gulf of Mexico. It expands throughout the summer, and finally ends in the autumn. Scientists began to notice this phenomenon in the 1980s, and they began conducting annual surveys of the dead zone in 1985. The zone forms when subsurface waters become so depleted of dissolved oxygen that they can no longer support most forms of aquatic life—even life that normally thrives in the area.

The zone stretches from west of the Mississippi Delta, off the coast of Texas, and over the continental shelf off the Louisiana coast. As oxygen depletion starts in late spring, the oxygen-depleted subsurface waters which are thick with nutrient-rich discharge from the Atchafalaya and Mississippi Rivers generate algal blooms. As the blooms die, bacteria biodegrade them, further depleting the oxygen in the subsurface water. Many organisms that cannot escape the area die of hypoxia.

Since measurements began in 1985, scientists have noted the expansion of this dead zone. By 2002, the dead zone was approximately the size of New Jersey, over 20,000 square kilometers. Last year is spanned more than 22,000 square kilometers.

Dead zone in the Gulf of Mexico. (Credit: By NASA NOAA (NASA NOAA) [Public domain])

“I am from Iowa, where Corn is king; I grew up feeling like row-crop agriculture was a part of the natural landscape!” explains Van Meter. “Nandita [Basu] started out her career as an Assistant Professor in Civil and Environmental Engineering at the University of Iowa. Iowa is a place that struggles with water quality, as nitrate runs off of the fields and gets into groundwater and nearby streams, eventually making its way to the Gulf. It’s also a place where people are very concerned about finding ways to improve agricultural management practices and to reduce the negative environmental impacts of farming. In our work, quantifying time lags between implementation of conservation measures and seeing real, measurable improvements in water quality, we hope to improve our understanding of how what we do today will impact our ability to achieve environmental change.”

It’s a fair point. In places like Iowa, large investments into fixing the dead zone in the Gulf have been made, yet to many it feels like there’s little progress—or even like things are getting worse. The team analyzed more than 200 years of agricultural data to model the ways that nitrogen accumulates in soil and groundwater to more accurately predict how it will travel to the coast in future decades.

“The excess nitrogen that is applied to crops or that leaches off of animal feedlots today may take many years to actually make it to nearby rivers,” states Basu. “Nitrogen can accumulate within the soil layer or in groundwater. In many cases, subsurface travel times for nitrogen are on the order of decades.”

The result is extensive lags in time between when conservation measures are implemented by farmers and visible improvements in water quality. While most scientists are used to taking this kind of a long-term view, for policymakers, it’s not always so easy.

“The Nutrient Task Force developed a plan to reduce the average area of the summer hypoxic zone to less than 5,000 km2,” details Van Meter. “The target year for achieving this goal has now been set at 2035. Other scientists have shown that a 60% reduction in nitrogen loading from the Mississippi River would be necessary to achieve this goal.”

The team’s modeling results reveal that even with the immediate adoption and implementation of very effective conservation measures, it will take approximately 30 years for the excess nitrogen that’s already accumulated in the agricultural land to deplete.

“In our model, we have coupled simulation of soil nitrogen dynamics with a travel time-based model of groundwater transport,” Van Meter describes. “In the travel time model, we simulate the subsurface as a large distribution of travel pathways, with a distribution of travel times to the catchment outlet. Some of these pathways may have very short travel times (a year or less), while others may have travel times of more than decades. This modeling approach allows us to realistically estimate how long it will take excess nitrogen to cycle through the landscape, from the point of application at the land surface to the coast.”

Of course, this problem is not solely in the lap of agricultural lands bordering the rivers that drain into the Gulf.

“The dead zone in the Gulf is understood to be driven by nitrogen coming down the Mississippi River,” remarks Basu. “The majority of that nitrogen comes from what we call nonpoint sources, meaning that nitrogen from corn fields in Iowa or Illinois, or anywhere across the Mississippi basin can make it’s way to small streams and eventually to the Gulf. In other words, local actions, far from the Coast, are having major impacts on downstream waters.”

Furthermore, when a dead zone gets as large as this one is, there are additional complications inherent to the recovery process.

“Once we actually achieve the reductions in loading from the watershed, it has been estimated that it might take at least another 5 years for the Gulf to fully respond,” adds Van Meter. “Nutrients like nitrogen and phosphorus accumulate in sediments and can then be released again sometime in the future, meaning that even if nutrients are buried, they can still negatively impact water quality. This ‘internal loading’ effect can increase time lags to achieving the improvements we hope to see in water quality.”

The ELEMeNT model accurately predicts long-term trends in Mississippi nitrate loading, as demonstrated by the close relationship between the modeled annual loads (green line) and depth-varying chloropigment concentrations (purple and orange) within a sediment core obtained from the northern Gulf of Mexico. (Credit: Van Meter et al.)

Now, the research team is expanding their analysis to include phosphorus, another important driver of dead zones.

“Both nitrogen and phosphorus are known to be major contributors to hypoxia and the development of dead zones in coastal areas,” details Basu. “While nitrogen is generally understood to be a key player in marine systems like the Gulf, phosphorus is more frequently the major driver of eutrophication in freshwater systems—think of inland lakes and wetlands. Lake Erie, for example, has been experiencing record-setting algal blooms in recent years, largely driven by nonpoint source phosphorus pollution from agricultural watersheds. The preliminary results from our models suggest that time lags for phosphorus are even longer than those for nitrogen. The implications of these findings in the Great Lakes region are significant, as the Great Lakes Commission as well as state and provincial governments have set a goal to reduce phosphorus loads to Lake Erie’s Western Basin by 40%.”

However, although there’s a long road ahead for the Gulf of Mexico’s recovery, the team stresses that this isn’t bad news, per se. In fact, the restoration efforts are working—and everyone involved should know that.

“We think it’s important to understand that this is not a negative or ‘depressing’ message,” adds Van Meter. “In fact, it’s just the opposite. Farmers and watershed managers currently feel frustrated when they make changes but don’t see any real changes in water quality. What we are saying is that many of the things we are doing right now are working, but it takes time to see the results. We believe that we have already made important first steps on our way to improving water quality in the Gulf.”