The Fingerprint of Nitrogen

The Neuse River, which was recently identified as one of American’s twenty most threatened rivers by the environmental group American Rivers, regularly experiences nuisance algae blooms that create water-quality problems, including toxic conditions for marine life. Hans Paerl, William R. Kenan, Jr. Professor of Marine Sciences, has determined that a key culprit in the Neuse River estuary is excessive nitrogen loading. As every gardener knows, nitrogen makes plants grow; and in the Neuse River, certain species of algae are getting too much of a good thing.

Think of nitrogen as the currency of primary production, or the substance that limits the ability of organisms at the base of the food web to produce. Some kinds of algae are opportunistic, able to exploit excess nitrogen as it is introduced into the estuarine system. “Not unlike financial matters and monetary situations, if you get too much currency flowing, you get corruption in the system,” Paerl explains with a chuckle.

A Whole New Breed of Scientist

Housed in a modest, 60’s-era institutional building right next door to the North Carolina Division of Marine Fisheries, UNC-CH’s Institute of Marine Sciences (IMS), part of UNC-CH’s Marine Sciences Program, has for years been quietly fulfilling its mission of service to the state through research. But while IMS’s mission of serving the state through research has been consistent over the years, the strategies have changed dramatically.

Twenty-five years ago, the Institute’s research was what Director John Wells calls “narrowly focused, descriptive ecology,” often intended simply to describe things that live in the ocean. Now, IMS researchers strive to understand how a broad range of marine systems actually work.

“We can’t solve the world’s problems simply by describing what lives in the water,” Wells says. “It’s not static-‘Here’s something that’s living and here’s what it looks like’-but rather, ‘These things are moving around, they’re extremely dynamic, they’re changing, and they’re interacting with everything else that’s around them, including what’s in the water.’ We no longer just describe variability and change-we look at what it is that results in change. There’s a whole new breed of scientist, there’s a whole new dazzling array of technology out there, and they allow us to start looking at processes and complex interactions rather than just responses.”

—Kelly Malone

This process of corruption is called “eutrophication,” and its effect on estuarine ecology can be dramatic. If the rapidly growing, opportunistic algae that exploit excess nitrogen are not effectively utilized by higher organisms in the food web, they run rampant until either they run out of nutrients or growth conditions become less favorable. Then, says Paerl, they suddenly die or “crash,” creating a huge mass of decaying organic material that consumes a tremendous amount of oxygen. When estuarine water loses oxygen, or goes anoxic, the results can include fish kills and loss of habitat for fish and shellfish, not to mention foul-smelling water.

Some of Paerl’s earlier studies have revealed that nitrogen is introduced into the Neuse River estuary from a variety of sources. About a quarter of all excess nitrogen in the river is put there by natural processes such as plant decay. People put in the other three-fourths. Fertilizers coming from farm land can make their way to the river in either groundwater or surface runoff. Most recently, Carolina’s booming hog industry has created real challenges: hog waste is stored in containment lagoons that can leak into groundwater, and worse, may sometimes overflow or rupture. Point sources, such as pipelines from factory and industry or waste-water-treatment plants, generate a third of the nitrogen introduced into the Neuse by people.

But surprisingly, a quarter to nearly a half of the man-made nitrogen that ends up in the Neuse River falls from the sky. Fossil fuels and agricultural emissions, such as the ammonia from swine and poultry waste stored in lagoons and applied to the land as fertilizer, accumulate in rain and dryfall, tiny particles that settle to earth in dry weather. “Ten years ago, atmospheric deposition was never considered in our assessment of how man is impacting the coastal ocean,” observes Paerl, “so this is a very important sort of new twist to the eutrophication story.”

Paerl and colleagues at North Carolina State University and at the Carnegie Institution of Washington are now working on ways to trace nitrogen to its source by “fingerprinting” it. It turns out that nitrogen from a particular source has an innate ratio of the two different isotopes, 14n and 15n. Using a mass spectrometer, an instrument originally designed to analyze for sample purity in the chemical and biotechnology industries, Paerl analyzes samples for their isotopic signatures.

“We’re trying to apply that technology to back-track where a rainstorm may have obtained its nitrogen-whether it got it from industrial pollution, which emits nitrogen oxide, or from agricultural sources, like effluent from a chicken or hog operation, or from automobile exhaust. Those all have different nitrogen isotope signatures,” Paerl explains. Being able to “fingerprint” nitrogen will make regulating nitrogen-producing sources easier and more effective, Paerl says.

Helping Oysters Survive Off the North Carolina Coast

Traditionally, fisheries management has been commodity-based, looking at the individual resource, or species of fish, without trying to understand how it related to other components of the ecosystem. That’s changing now, says Professor Charles Peterson, who brings his own expertise on ecosytems management, which places fisheries in the context of marine ecosystems in general, to his work on North Carolina’s Marine Fisheries Commission.

Peterson, who is chair of the Environmental Management Commission’s Water Quality Committee, believes that water quality issues are an important cause of record oyster loss in the Pamlico Sound. In recent years, the annual oyster harvest has been about 40,000 bushels. That’s a tiny fraction of the harvests of a few decades ago, which totalled well over a million bushels of oysters per year.

After a brief, free-floating larval stage, oysters affix themselves for life to reefs composed of oyster shells and other oysters. Once there, they depend on currents to bring them food and oxygen. Swiftly moving currents deliver more food and oxygen to oysters at the tops of the reefs, increasing their growth rates.

Naturally disadvantaged oysters at the bottom of the reefs are especially vulnerable to water-quality problems. “In the deeper waters of the Pamlico Sound and the rivers that serve it, we get summertime anoxia at depth,” Peterson explains. “But while anoxia occurs there naturally, we are doubtless enhancing its duration, its spatial coverage, and perhaps its frequency and intensity, by the eutrophication of coastal waters.”

“What we would really like to know,” he adds, “is how many nutrients put in through run-off and sewage waste-water-treatment plants result in how much anoxia, and therefore how much oyster habitat loss downstream. That’s the relationship that is elusive.”

Poor water quality is just one of many factors that contribute to the decline in the oyster population. In response to the drastic depletion of oysters in North Carolina waters, the Blue Ribbon Oyster Panel was established in 1994 by the North Carolina General Assembly to examine why the state’s oyster resources had declined so dramatically and what might be done to restore them. Peterson, whose involvement in the Panel is intense, has determined that a primary cause of oyster loss has been the degradation of oyster reefs.

“To catch an oyster,” he points out, “you remove it as well as the back of the shell that it’s growing on, and so you effectively mine the habitat that oysters need to propagate and continue.” In particular, mechanical oyster dredges used in some parts of the state flatten reefs. Gentler methods of harvesting may help protect the oyster habitat, Peterson says-oyster loss is less dramatic in parts of the state where hand-harvesting is used in lieu of dredges. The higher the reef, the better the flow of water over it-and the less vulnerable the oysters are to anoxia, especially in the summer months, when oxygen is depleted from the water more rapidly. In addition, Peterson is doing his part to restore the habitats: since 1993, he’s been involved in a project funded by the General Assembly that includes designing, building, and seeding artificial reefs using oyster shell on which young oysters can settle.

While his research clearly benefits North Carolina’s struggling oyster industry, far more is at stake than oysters alone, says Peterson. “The oyster reef has an entire ecosystem that develops around it that has importance beyond the oyster itself. It serves, for example, to cleanse the water, because oysters filter so intensively. It serves as habitat for small fishes and bigger fishes, including many of sports and commercial value. The removal of the oyster habitat not only affects the oyster, but has broader ecosystem impacts.” Cleaner water, for instance, allows more sunlight to reach seagrass, which provides habitat for other estuarine animals.

A Marine Ecology Encyclopedia

In the past, Professor Mark Hay’s research in the tropics has shown that when reefs were overfished for species that eat sea urchins the sea urchin population would explode. These urchins would then strip the reefs of certain kinds of algae that other species of fish needed for food. The result was that the reefs would become depleted, not just of the overfished species, but also of the species that depended on the edible alga.

Since few species of fish in the North Carolina waters eat seaweed directly, Hay assumed that seaweed wouldn’t be a very important part of the equation of North Carolina reef ecology. He was wrong. As it turns out, the seaweed sargassum plays a crucial role in the survival of juvenile fish-not as food but as habitat.

In a recent experiment, Hay’s divers “weeded” rocky reefs about 20 miles off North Carolina’s coast. Each reef was divided into three sections. In the first section, divers removed a low-growing alga called zonaria. In the second, they removed the tall, sinuous sargassum. The third section was left alone as a control.

Hay discovered that juvenile fish, uninterested in the low-growing zonaria, were more than twice as likely to recruit to the patches that contained sargassum, where they could hide from predators. “When we removed

similar amounts of other species from those other areas it didn’t make any difference,” Hay observes. “So it really looks like sargassum, among all the common things out there, is particularly important in terms of juvenile habitat.

“Reefs with this kind of seaweed are critical habitats that are important to protect,” Hay concludes. “We don’t know that they’re threatened at present, but we didn’t know they were important until a few months ago.”

Hay’s research helps to fill a niche traditionally neglected by people involved in fisheries management. “Where people have looked at offshore fish,” he points out, “they’ve looked at what people catch, the size and numbers of fish being caught. They’ve looked at what’s being taken out of the population.” Almost no one, he adds, has examined the factors that might make reefs more attractive to juvenile fish in the first place.

Twenty miles from the coast, North Carolina’s rocky reefs remain relatively untouched by the pollution that plagues the Neuse River Estuary and the Pamlico Sound. But Hay is planning ahead: “Part of what we as academic researchers do is that we try to figure out how systems work from a basic level to serve as encyclopedias when something starts going wrong somewhere,” says Hay. “People will say, well, what are the possibilities? What could be causing the problem here? Is it overfishing? Is it septic tank leakage? Is it changing global climate?

“Hopefully, we know enough to go in and start addressing those complex questions. If we can’t answer them, at least we know where to start looking.”

Stormy Weather

Click to read photo caption. Photo by Dan Sears.

Animated weather maps detailing the movement of fronts and storms across the United States are a commonplace of the evening news. Associate professor Rick Luettich has adapted the same technology used by meteorologists to create animations that predict how the ocean responds to forces that make it move. Meteorologists describe the way the atmosphere moves using a mesh connected by nodes representing complex weather equations. This square mesh works well enough for the atmosphere and the open ocean, where the movement of elements is unimpeded by dry land. It was far less useful, Luettich discovered, in detailing what happens closer to the shoreline-especially an irregular shoreline, like North Carolina’s, broken up by barrier islands.

“The problem with squares is that there’s no way to change the size of the square as you get near to land,” Luettich points out. “How can you get away from the use of squares and get a more flexible mesh?” By using a mesh based on triangles rather than squares, Luettich is able to narrow the spacing of the nodes to get more detail where it’s most needed: by the shoreline, where people live. “What’s critical about the triangular mesh is the ability to zoom in,” he adds. In the middle of the ocean, where fewer calculations are necessary to tell Luettich what he wants to know about the ocean’s movement, nodes can be spaced about a hundred kilometers apart. In contrast, nodes describing the ocean around the Morehead City area are spaced about a hundred meters apart.

Luettich’s finished map of the North Carolina waters will contain about thirty thousand nodes. At each node, about 50 calculations for such variables as water speed, water direction, and surface elevation are performed every minute. These numbers are then plugged into animated computer simulations of storms that color-code changing water levels off the coast of North Carolina, showing which areas are liable to experience flooding given the mix of large-scale weather features and the storm being tracked.

Luettich’s storm-surge models have proved uncannily accurate in tests comparing his animation’s predictions with actual statistics. A computer animation of Emily shows a yellow bulge of water beneath the hooked Capes; bright blue indicates negative flooding along Atlantic Beach, where Emily sucked coastal water out to sea. “Were the shore of North Carolina straight,” says Luettich, “it would be dramatically different. You’d get flooding along the whole thing.”

As Emily passes the North Carolina coast in Luettich’s simulation, shades of red moving across the Pamlico Sound show flood water sloshing toward the mainland and then out again toward the barrier islands. In almost no time at all, the barrier islands were flooded from the force of sound water moving back out toward the ocean. Much to most people’s surprise, barrier island flooding typically comes from inside the sound, not the open ocean, says Luettich. “Even though the average depth of the sounds is only about ten feet, there’s an awful lot of water that can be piled up on one end or the other, and it just can’t get out through the inlands fast enough to keep its levels down.” During Emily, Luettich’s model predicted flooding in the Buxton area of about 6 feet. Trash lines six feet up on the inside of Atlantic Beach dunes confirmed the accuracy of Luettich’s model.

Meteorologists who make hurricane flood predictions commonly anticipate a hurricane’s path in isolation from other major weather features, such as storms and fronts, that can drastically affect where it reaches land. Last fall, for instance, the flooding that resulted from Hurricane Gilbert took everyone by surprise because a large front blocking Gilbert in the north caused it to stall off the North Carolina coast. In addition to providing essential detail about flooding patterns along the irregular North Carolina coast, Luettich’s model also views the impact of a storm in the context of other major weather features that might affect its course. This is something that simply hasn’t been done until now, says Luettich.

Luettich’s after-the-fact runs on major storms have reflected actual storm conditions with such accuracy that he is now ready to test his model using predictive winds. This fills a gap left by the National Weather Service, which does very little predictive oceanography. Since the National Weather Service is currently responsible for making flooding predictions on which evacuation plans are based, Luettich’s more accurate model could very well save the lives of coastal North Carolinians.

Kelly Malone was a student who formerly contributed to Endeavors.