What forecasting dead zones teaches us about Chesapeake Bay

August 30, 2017
The process that creates low oxygen levels in an estuary. Diagram from "Global Warming and the Free State." Illustration by Jane Thomas, Integration and Application Network, University of Maryland Center for Environmental Science

Scientists forecast a larger-than-average dead zone—an area of low to no oxygen that can kill fish and aquatic life—in Chesapeake Bay during summer 2017. Spring rainfall amounts in New York and Pennsylvania contributed to an above average Susquehanna River nitrogen load (81.4 million pounds) and hinted at a higher than usual impact on the Bay.

Summer hypoxia reports released through the Maryland Department of Natural Resources showed dissolved oxygen conditions were better than average, suggesting the forecast overpredicted. What happened and why can teach us a lot about the Bay, so we turned to Jeremy Testa, a hypoxia expert with the University of Maryland Center for Environmental Science's Chesapeake Biological Laboratory, who helps with the forecast, to learn more about what makes a dead zone, why the forecast might have missed the mark, and what the general public can do to help.

Story transcript

Maryland summers are made for baseball games at the Yard and crab dinners on the patio. The Chesapeake Bay has its own longstanding summer tradition, and it’s unlike any other. In some areas of the bay, oxygen levels drop, forcing most aquatic life to evacuate or die. Scientists dub these low-oxygen areas as hypoxic, but you might know them by another name: Dead zones.

For about a decade, our experts have studied the spring weather to make a summer hypoxia forecast. This spring, all signs pointed to a larger-than-average dead zone, but as the last days of August ticked by, it became clear this year’s forecast missed the mark. Instead, the summer months averaged out to be, well, about average. So what does that mean? Is the Bay healthier now? What made our experts overestimate? And why should anyone who isn’t a scientist care? We posed those questions and more to one of our hypoxia experts at the University of Maryland Center for Environmental Science.

JEREMY TESTA: My name is Jeremy Testa and I’m an assistant professor at Chesapeake Biological Laboratory, and I’m a marine ecologist who focuses on aspects of how the chemical and biological and physical environment interact in estuaries.

UMCES: So how did you get into that?

TESTA: When I was younger, I sort of knew that I wanted to do something related with the environment, but I also knew I wanted to do something that was scientific or quantitative, so I ended up getting into environmental science. Because I grew up in upstate New York, I was sort of attracted to fresh water and forests because that’s where I spent most of my time, but when I was an undergrad I was introduced to these problems of pollution and dynamics that were happening in coastal regions and it really intrigued me because it was a way to marry my interest in science and the environment with these issues that happen on the coast that really allow science to be applied to societal questions because humans and the environment are so intertwined on the coast.

UMCES: So something you study a lot is hypoxia. Can you talk about what that is and what makes that?

TESTA: Sure, so hypoxia is a word we use to describe, simply, the condition of having low oxygen levels in the water. The condition of low oxygen is something that is of interest from a societal perspective because there’s a lot of organisms that simply can’t tolerate these low oxygen conditions, so their potential for habitat and their potential for growth is reduced when you have low oxygen.

Like us, most marine organisms require oxygen for their metabolism, so when oxygen disappears, those organisms can’t live there. Most organisms will leave, so they’re not really dying in the dead zone, but they can’t be there. But if fish, for example, get trapped in a place where there’s no oxygen, then there could be potentially be mortality. 

It’s a scientifically interesting topic because oxygen is this gas that’s dissolved in water and as a result of that, it is influenced by hydrodynamics—how estuaries move, by the biology in the estuary—how plants grow and how organisms move around, and then the chemistry of the estuary—how different reactions happen and what controls those reactions. And oxygen just happens to be a very strong determinant on how different reactions happen in the environment, especially reactions that are related to nitrogen and phosphorous, which are the key elements we think of in terms of the eutrophication process.

Eutrophication is a type of pollution that takes place when an excessive amount of nutrients, such as nitrogen or phosphorous, access a body of water. Those nutrients can come from sewage treatment plants, agricultural fields, and even our own over-fertilized lawns. Too many nutrients in the water can help create dead zones either through dense growth of plant life, such as algal blooms, which grow at the water’s surface and deprive organisms below from sunlight and oxygen, or hypoxia, which happens when that algae dies.

TESTA: Once they sink, they end up in bottom water and the processes that break down that material consume oxygen. The other thing you need to get a hypoxic zone is you need to isolate water from other sources of oxygen. If you thought of the Bay as a bathtub and at one end you have freshwater coming in and that water’s going to move toward the ocean in the surface, and so at the same time, you have ocean water coming in the bottom and moving in the opposite direction up the Bay. 

When you have relatively high fresh water coming down its less dense, it sits on top of the denser saltwater, you kind of have two layers of the water. The one on the surface has oxygen in it because it’s getting it from the air and it’s getting it from photosynthesis and the bottom water tends to not have oxygen in it. There’s no photosynthesis because there’s generally no light. The stronger the separation of those two layers is, the more likely the deeper water is going to lose its oxygen.

UMCES: So when you get a storm and that water mixes, does that improve the situation or make it worse?

TESTA: If the storm is rainy and it creates a lot of new freshwater flows into the system that enhances this layering of this water and sort of cuts off the deeper water from the oxygenated surface water, but if the storm is a wind storm, usually very strong winds have the effect of mixing the water and taking that oxygen rich water near the surface and mixing it down to the bottom and that usually improves the situation. So wetness tends to make hypoxia worse, windiness tends to make hypoxia better.

That summary is pretty important considering predictions for climate change in the Chesapeake Bay region, but before we get more to that, we talked to Testa about the role rain and wind play in making the forecast and sometimes, in breaking it.

UMCES: We were anticipating a larger-than-average dead zone this summer. When you’re forecasting that, that’s something you do every year, can you talk about the process of putting together that forecast?

TESTA: There’s basically a fairly simple statistical model that makes a prediction of the summer volume of water that’s hypoxic—or the size of the dead zone—based on the amount of nitrogen loading that comes into the system during the winter and spring from the Susquehanna and Potomac rivers.

So the way that works is just about the time when the loads are able to be computed, that is we’ve gotten into May and we sort of know what the January to May nitrogen load is, we interact with our colleagues at the United States Geological Survey, they generate estimates of the nitrogen loading that’s measured from the Susquehanna and the Potomac from their monitoring stations and take our statistical models and use that input data to make a prediction of what we expect in the summer. 

This spring the Susquehanna River flow was above average and there is a fair amount of nitrogen loading associated with that, so we predicted an above average hypoxic zone and also anoxic zone. So in the forecasts we predict hypoxia which we define as less than 2 milligrams per liter, so that’s on a scale where oxygen pretty much varies between 0 and 14 in Chesapeake Bay. We call anoxia basically when there’s absolutely no oxygen in the water. Operationally we define it as just a little bit of oxygen, but it’s basically no oxygen. So we make a forecast of hypoxia and anoxia.

And the Department of Natural Resources and Environmental Protection Agency have a collaboration that results in the monitoring of the oxygen conditions all summer, and although we predicted higher-than-average hypoxia, the earlier part of the summer in June was about average and the estimates that came out in July and August indicated below average conditions. So it turns out, our forecasts over-predicted what’s happening this summer.

Now, that’s not unusual. We can only expect to forecast so well because we have to make that forecast based on what we know has happened in the spring, but anything can happen once the summer comes and we don’t have a great way of predicting what that will be.

So one of the main factors that kind of determines whether or not our spring-based forecasts will be realized is what the wind conditions are like in the summer. I think it would be fair to characterize it in this way: The amount of nutrient that comes into any given system, so for Chesapeake Bay, the amount of nutrient and the rain that pushes that nutrient into the Bay, that sets the amount of hypoxia we could realize and in some ways, that’s what we’re forecasting. Then the wind that happens in the summer decides how much of that potential is realized. 

Recently, we did a review of the forecasts that have been done since 2007 and that’s where we realized that under a situation where there weren’t really strong storms—so we’re talking about an event that lasts for a day or two, that’s normally associated with a strong tropical system that’s just has really high wind speeds—in the case that that happens, we can sort of expect that there would be a lot less hypoxia than we forecast.

After each summer, we usually go back and see how the forecast did and we’ll often look at the wind direction as well. So if the winds are primarily from the west, we’re likely to under predict the hypoxic volume and if the winds are strongly from the south, we’re likely to over predict. The south winds tend to mix, the west winds tend to allow for the waters to be layered or stratified.

UMCES: So this year’s was an over prediction. Is there any good news in over predicting?

TESTA: Well, there can be, and although we don’t have a long enough record of prediction to understand that. One thing we have noticed recently is sort of the size of the hypoxic zones and primarily the size of the anoxic zones that we see in the later part of the summer have actually been trickling down over the past 30 years or so. This year when we look at the early August, it was lower than we predicted and that’s sort of consistent with this overall pattern.

In that way, the Chesapeake Bay’s summer was more typical than it seemed. In this year’s forecast, Dr. Testa said the predicted summer dead zone of about 1.89 cubic miles—which equal in volume to about 3.2 million Olympic-size swimming pools—would “buck a recent trend toward lower anoxic volumes in the Bay.” What will future forecasts look like then?

UMCES: Does climate change have any impact on dead zones, are they more likely, are they less likely?

TESTA: I would say it’s completely up in the air. There’s a number of things that we might except to happen with climate change that could affect the dead zone. The main thing we think about with climate change is temperature. People have pointed out as temperature increases, we should expect more hypoxic zones. That’s because the warmer the water is, the less oxygen it can hold under normal conditions, so the solubility of the gas is just lower at high temperature, and also high temperature tends to speed up the rates of reaction that consume oxygen.

The problem is that temperature will have an unknown effect on how much oxygen consuming material like the microscopic plants will generate. In a warmer climate it’s not clear if there will be more production of this material that consumes oxygen by these plants, or less.

Then there’s a host of other issues. With climate change we might expect more precipitation in some seasons in this region of the world. So if there’s more precipitation, there’s more water rushing off the land bringing nutrients into the bay and then setting up this layering that reduces the mixing of oxygen. That would probably make it worse.

The winds, we really don’t know. The winds, we don’t really know what to expect looking decades down the road.

And then finally, there’s sea-level rise that we expect to happen. So with sea-level rise people have speculated will there be more salt coming into the Bay so maybe this layering will be more intense, and you’ll be able to favor more hypoxia. But at the same time, if you’re elevating sea-level rise, you’re just getting more relatively oxygenated ocean water coming into the Bay, potentially stronger tidal mixing, so more sea-level rise might actually mean less hypoxia for the Bay.

There’s a number of different compensating potential changes that could happen in Chesapeake Bay that makes our predictions of what will happen in the future challenging and that really speaks to how complex and interesting the whole problem is, that there’s a number of different changes we could see that could either work together or work against each other to change the volume of hypoxia.

Essentially, scientists make reasonable forecasts each spring, but the weather 1 to 2 months later can have a big influence on the accuracy of those forecasts. That doesn’t mean we’re helpless.

UMCES: What’s the most important thing that people should know?

TESTA: There’s probably two take-home messages. The first is that the thing that we can do societally to limit the size and how long these hypoxic zones stick around for, the knob that we can turn to control that is the amount of nutrients that we put on land and then get put in the estuary. So while wind matters, while sea level matters, the thing that we can do is reduce the amount of nutrients that get into the Bay.

And the second thing is just to appreciate that this volume of low-oxygen water is something that we pay a lot of attention to and we focus on because it’s such an important process that determines how much of the Bay can be inhabited by a lot of the organisms that we care about, like striped bass and crabs.