Water floats. We don’t normally think about it that way, but water itself floats. On water.

When marine scientist Rick Luettich arrived in Morehead City in 1987, his colleagues were studying things in the water: fish, nutrients, bacteria. They assumed the water itself was just more floating on top of the same.

Luettich figured it was, too. After all, most of North Carolina’s coastal waters are shallow and constantly getting mixed together by tides, storms, and powerful winds.

But our coastal water turned out not to be one thing—it’s two. One layer on top of another. You don’t need to know this when you’re going out on a boat, but if you’re a fish, it’s a matter of life and death. Luettich and his lab discovered how pervasive these layers are, and they’re finding out other things scientists miss when they don’t realize that a body of water comprises two different worlds.

In those early days at UNC’s Institute of Marine Sciences, Luettich just wanted to get a basic understanding of the water in the Neuse River Estuary—the area where the river flows into the sea. He wanted to know how physical processes, such as winds and tides, affect the water.

But water is hard to keep an eye on. It has so many properties that are constantly changing: temperature, depth, oxygen level, cloudiness, salinity. How do you know what all of these numbers are at any given time? You can hand-sample over the side of a boat, Luettich says, but doing that day after day, month after month, wastes time and money.

Click to read photo caption. Courtesy of IMS

Luettich is an engineer, so he started thinking about making a machine: a  profiler that could sit in the water on its own and collect data above and below the surface. The lab’s first creations were bulky frames 10 feet tall that had to be lifted over the side of a boat. The first one worked like a garage-door opening mechanism: a chain loop that could lower monitoring and sampling equipment down and back up to the surface. A later model was like a long screw: turn it in one direction to lower the equipment, and reverse the motion to bring it back up.

“They broke a lot,” Luettich admits. Often something in the machinery would snag. But even with all the troubleshooting, the researchers started to find that the coastal waters were more complex than they’d believed.

Freshwater floats on top of heavier salt water. Scientists knew this, Luettich says, but they didn’t think it was happening much in North Carolina’s estuaries—the places where freshwater rivers mingle with salty oceans. With the exception of the Cape Fear River Estuary, most of North Carolina’s coastal water is less than 25 feet deep, or even half that. It’s shallow enough that scientists thought the constant churning from river flow, tides, and storms kept the water from differentiating into fresh and salt layers.

Luettich’s samples, though, showed that the water often has two distinct layers. But the salt water doesn’t always stay down there, and when it comes to the surface, it disturbs the normally freshwater environment.

Click to read photo caption. Courtesy of IMS

Since the early 1990s, large fish kills have occurred in North Carolina rivers, especially in the Neuse and Pamlico rivers and estuaries. Some fish turn up with markers of disease or toxins, but others are simply dead. Luettich, who knew about the layer of water below, suspected how that might happen.

The freshwater layer of the rivers gets oxygen from contact with the air. The water below hasn’t seen the surface for a while, so it doesn’t have as much oxygen. That deoxygenated water isn’t normally a problem for fish living near the surface—but it can be if the wind hits the water the wrong way.

Luettich’s lab set up two water profilers near opposite shores in the Neuse River Estuary. “We found that when the wind blows across the estuary, it drags the water along with it,” he says. “When the water hits the far shore, it moves downward and pushes the deoxygenated water out of the way. This makes the deoxygenated water well up to the surface along the upwind shore.”

The two water profilers showed that this happens often—sometimes daily—and so quickly that in a matter of minutes an unlucky school of fish can be surrounded by deoxygenated water. By the time the dead fish are found and reported, Luettich says, the water has gone back to normal, leaving scientists to gather evidence, like the presence of toxins, that may have nothing to do with why the fish died.

That’s not to say that North Carolina’s fish kills—tens of thousands of fish found dead, often several times a year—are inevitable. Luettich, his colleague Hans Paerl, and other researchers have found that the lower layer of rivers like the Neuse is more deoxygenated than it once was because of algae that use up oxygen as they decompose. The algae grow on nutrients, such as nitrogen, that run off from the land into the water. That’s why fish kills have increased with development and the growth of agriculture, Luettich says.

The profilers the lab builds have gotten more sophisticated over the years. By the time they started investigating fish kills, the lab had figured out how to replace the snagging machinery with a more reliable hoist mechanism. Next they wanted a system that could be towed behind a boat, rather than heaved over the side, so Luettich started designing systems that float. Rather than just sitting on the ground in shallow water, these can be anchored in water as deep as 100 feet.

As Luettich started getting more regular data, he noticed something funny about the particulate matter in the Neuse: it traveled up and down. At midnight, it would be down at the bottom; during the day, it would be near the surface. Winds and currents couldn’t explain that, so his group started working with Hans Paerl’s lab to collect and analyze samples at different times of day.

The mystery matter turned out to be many small organisms: microalgae that swim. During the day they stay near the surface to get light for photosynthesis; at night, they retreat to the depths to feed on other algae and nutrients that sink to the bottom. No one had realized before how pervasive this daily elevator pattern of movement is, Luettich says. Knowing how the algae move through the water will help researchers learn how algae form the blooms that can ruin water for humans and fish.

In recent years, the oceanographic instrumentation community has developed some sophisticated monitoring equipment, Luettich says, such as mini-submarines and water gliders that swoop slowly to the bottom. They’re impressive, but also expensive, and they don’t work well in shallow systems like the N.C. coast.

The lab’s newest profiler is not only much more reliable than its past models, but also can easily be upgraded as new monitoring equipment becomes available. They’re starting to think seriously about selling the design, Luettich says. “There’s going to be a need for surface-based systems like this for a long time.”

Rick Luettich is the Sewell Family Term Distinguished Professor of Marine Sciences in the College of Arts and Sciences and director of UNC’s Institute of Marine Sciences.