In August 2017, the National Oceanic and Atmospheric Administration announced that the hypoxic dead zone in the Gulf of Mexico, just off the coast of Louisiana, had grown to approximately the size of New Jersey—the largest area of hypoxia ever measured. The agency determined that nutrient pollution (eutrophication), largely from the Mississippi River, is the primary cause for the zone’s expansion. Anna Michalak, faculty member of the Department of Global Ecology at the Carnegie Institute of Science, recently co-authored a National Science Foundation-funded study about the effect of climate change on eutrophication that was published in the July 28, 2017, issue of Science. WQP Associate Editor Michael Meyer spoke with Michalak about eutrophication and what can be done to combat its harmful effects.
Michael Meyer: What is eutrophication and how does it affect waterways?
Anna Michalak: Very broadly, eutrophication refers to the delivery of nutrients into waterways, and so at least in principle, eutrophication is neither good nor bad. Some bodies of water naturally have more nutrients flowing into them, and some have less. But when we talk about eutrophication in the context of water quality impacts, what we’re talking about is the delivery of an excessive amount of nitrogen and/or phosphorus into streams or lakes, or into the coast. When we have too many nutrients arriving into the water, it tends to stimulate growth of certain types of phytoplankton, and that can lead to water quality impacts, like hypoxic dead zones or harmful algal blooms.
Meyer: Why is eutrophication on the rise?
Michalak: Based on the research that we just published, it’s on the rise for two reasons at a high level. One is, in the U.S. over the past several decades, the way in which we manage land has intensified, and so we’re using much more fertilizer in agriculture. Some of those nutrients from the fertilizer, rather than ending up in the crops, end up in waterways, and then ultimately end up in lakes and along the U.S. coast.
One of the other big controls on how much of the nutrients gets flushed into the water has to do with precipitation. We had shown in an earlier study that the way in which precipitation varies from year to year actually has a big impact on how nutrient loading varies from year to year. What we showed in the current study is that if we look at the increases both in total precipitation and in [the] extreme precipitation that we’re expecting as a result of climate change in the coming decades, those changes are large enough to have a substantial impact on how much nitrogen will be flushing off of the soil and into the coast as the century progresses.
Meyer: What can be done to address this trend?
Michalak: At a high level, we can do two things. The one that current management strategies are focusing on is to either reduce the amount of nitrogen that is being added to the soil or to add it in ways that are less likely to have it flush off of the soil if you do have precipitation events. That can have to do with the way the fertilizer is applied, when it’s applied, where the water goes once it washes off of farm fields, and so on.
But to me, one of the main takeaways from the study is that we need to realize that water quality is not just about human action at the local scale or the regional scale; the water quality impacts we’re seeing locally are also the result of human action at the global scale. Through our burning of fossil fuels, we’re changing precipitation patterns, and those are then changing the amount of nutrients flowing into the coast. To address that, we need to do all the same things that we need to do to address climate change more broadly, and so we need to reduce our reliance on fossil fuels as a source of energy.
Some of the nitrogen that’s ending up in the system isn’t just from agriculture—it’s actually nitrogen that gets put into the atmosphere as a result of burning of fossil fuels. When it rains, that nitrogen essentially gets scrubbed out of the air and deposits onto soils as well. If we manage to reduce fossil fuel emissions, we’re not just slowing down climate change, but also reducing the amount of nitrogen that’s being deposited onto soils and that could eventually end up on the coast.