Chesapeake Quarterly
Shadow on the Chesapeake
JACK GREER
sediment plume
Plumes of sediment billow off Maryland's Eastern Shore. Credit: Michael W. Fincham.
graph - secchi depth over the years is getting smaller
graph - water clarity
Click on
each graph
to enlarge.

A COWNOSE RAY BREAKS INTO THE LIGHT, raising one dark wing. As soon as it dips, the ray disappears into a watery haze. Though Maryland's Choptank River gleams in the mid-summer sun, it's hard to see much beneath the surface. Only a foot or so down, the ray and everything else vanishes in a greenish blur.

The ray performs its disappearing act near the research laboratory at Horn Point, just down river from Cambridge, where scientists are puzzling over a nagging mystery. With every passing year, the water's summer haziness appears to get worse.

Fishermen find it harder to see what's on the end of their line. Boaters find it harder to see the bottom, even in shallow water. Every year, even in years with low flow, when fewer nutrients and less sediment wash into the estuary, resource managers watch measurements of water clarity worsen (see graphs at right). What's driving this decline in water clarity? What, they ask, is snuffing out the light in the Chesapeake?

The Dark of Summer

Sediment is one suspect, a prime suspect. For a quarter century, Larry Sanford has been on the trail of sediment, tracking its movements. He's one of the scientists working on the banks of the Choptank at the Horn Point Laboratory, part of the University of Maryland Center for Environmental Science (UMCES). He measures where sediment goes, how it moves, and he puzzles over the gathering cloudiness in the river — what scientists call turbidity.

Seated in his cluttered office, Sanford says that the study of turbidity in the Bay has a long way to go. When he began his academic career, most students in his field studied sand. Sanford, who has piercing blue eyes and still looks athletic in his fifties, started that way as well. But while others focused largely on preserving and rebuilding sandy beaches, he zeroed in on fine sediment transport — the movement of silt and clay.

After graduating magna cum laude from Brown University in Providence, Rhode Island, he earned advanced degrees in coastal engineering from the Massachusetts Institute of Technology (MIT) and the Woods Hole Oceanographic Institute (WHOI). Then in 1984 he came to the Chesapeake, to Horn Point. He had no idea just how complex "sediment transport" in estuaries could get.

Once on the Bay, he sharpened his focus on fine "cohesive" sediment — call it mud. In the Chesapeake, sediment-laden waters drain from a 64,000-square-mile watershed to meet a rising sea that erodes riverbanks and shorelines. Here, he says, especially in the middle Bay, "mud is where the action is." Unlike sand, which slides and sinks in relatively predictable ways, mud can be, in Sanford's words, "ungodly complicated." Muddy particles can hang in the water for hours, for days. They get resuspended. They move around a lot.

"I'd never studied estuaries before," he says. It was a steep learning curve.

The first thing he learned, a lesson he now preaches to others, is that the Bay needs sediment — to replenish its tidal marshes, for example. The Bay depends on seasonal runoff of minerals and particles, including bits of plant material scientists call detritus, to stoke the fires of its great protein factory and to keep up with sea level rise.

"A blanket statement that 'sediment is bad' is just wrong," he says.

Besides, the biggest sediment loads to the Bay occurred long ago, he points out, during the eighteenth and nineteenth centuries, when trees came down and mold-board plows cut the earth. This sediment filled harbors and smothered oyster bars, but it did not cause the kind of turbidity we see now. It did not rob enough light from the Bay to shade out large areas of underwater grass, which remained relatively abundant until the 1960s and 70s.

If sediment is not new to the Bay, then why is turbidity increasing?

"Something is different," Sanford says. "Something has changed."

He finds this change "really intriguing." And, he says, "really, really worrisome."

It's Not Just Sediment
Larry Sanford kneeling next to DIPSTIC
Larry Sanford holds an instrument used to measure the size and quantity of fine sediment particles in the Bay. Credit: Jack Greer.

When Sanford heads out on the river to track turbidity, he takes a boatload of high-tech tools that would impress any crime scene investigator.

On the back of his boat he carries a contraption that he's hauled all over the Bay. Called a DIPSTIC (for Digital Imaging Particle Settling Tube with In-situ Capture), it sports a metal frame large enough to hold the common household stove, and it's packed with monitoring equipment that he drops or drags or anchors in the water. One instrument called a LISST (Laser In-Situ Scattering and Transmissometry) shoots out red laser beams that measure the quantity and size of tiny sediment particles. Another tool sends out sound waves that detect water velocity and turbulence. Other instruments measure depth, temperature, and salinity, telling Sanford and his assistant Yong Hoon Kim the conditions surrounding their sediment samples.

The gangly frame also holds two tubes that point skyward — they look like torpedoes aimed at heaven. While Sanford's laser beams and acoustics can count and measure millions of tiny particles, the two torpedo-like tubes have a different payoff. They can produce motion pictures.

When Sanford and Kim lower the frame-load of instruments off the back of the boat, the two tubes submerge and swing forward, like torpedoes should. But instead of moving through the water, they let water move through them. Their end-caps pop open, allowing water and sediment to enter slowly, so sediment particles remain unaltered. When the end-caps click shut again, each tube captures a snapshot of suspended sediment in that part of the Bay.

Not All Sediment Is Created Equal
sediment

THE WORD "SEDIMENT" conjures up different things, especially in a muddy estuary like the Chesapeake. There's sand, silt, clay. There's organic matter. According to sediment expert Larry Sanford, most geologists focus primarily on "what's on the bottom," but more and more he's turning his attention toward suspended sediment — sediment so fine or so light that it drifts through the Bay and settles slowly. [more]

After the device is hauled out, the tubes tip back up and sediment begins to rain down toward the bottom — just as it would in the Bay. At the bottom of the tube sits a high-resolution video camera that captures a microscopic view of the particles. Sanford and his colleagues are able to record this rain of suspended solids and to measure each particle's precise behavior. Modern computers can easily analyze these graphical images, giving Sanford a large digital data set.

All these laser beams, sound waves, and torpedo cameras help Sanford and Kim see sediment clearly, even in waters where cownose rays would vanish in the gloom. The scientists can see the exact size of particles, how many there are, and how they behave.

One obvious finding: there is a lot of sediment at large in the Bay. A less obvious finding: many of these sediment particles are very, very small. Sanford says that one of his colleagues in Virginia determined that individual particles suspended above the bottom typically measure no more than 10 microns. That's about one-tenth the diameter of a human hair.

These small particles are major players in what Sanford calls background suspended sediment (BSS). In an estuary like the Chesapeake, a lot of particles get stirred up by wind and waves — during storms, for example — but even after the waves die down some particles remain suspended for a long time. These become background sediment, and Sanford suspects that there is more of this background sediment now in the Bay.

From his torpedo-tube tapes, Sanford can see that all these fine grains often glom together in clumps he calls "flocs." That's short for flocculants, aggregations made up of many thousands of smaller particles that behave quite differently from other forms of sediment. Some of these flocs are 97 percent water. They don't sink like sand but settle at different rates, depending on size, shape, and composition.

Surprisingly all these flocs can actually improve visibility by clumping particles together. It's easier to see through big fat rain drops, Sanford notes, than through millions of tiny droplets (fog). "It's the packaging that counts."

The case against sediment is not as straightforward as it once seemed. Suspended sediment, Sanford says, does not equal turbidity. It may be an accomplice, but something else is at work. In the end turbidity has to do with how light penetrates the water. It's a question of light.

His advice: "You need to talk to Chuck Gallegos."

A Question of Light
Charles Gallegos
Researcher Charles Gallegos lowers a light meter into the murky waters of the Rhode River. Credit: Skip Brown.

Perhaps no one has been more perplexed by the Bay's rising turbidity — its gathering haze — than Charles Gallegos. Gallegos works on the other side of the Bay from Sanford, on the Rhode River just south of Annapolis, at the Smithsonian Environmental Research Center (SERC). For years he's measured light as it filters through the Chesapeake Bay, and lately the data have puzzled him.

He says his puzzlement came to a head when the local riverkeeper, Bob Gallagher, issued a scorecard for the West and Rhode rivers. Riverkeepers are part of a national network of watchdogs appointed to look out for local water quality. And scorecards, like annual wade-ins, have become a popular way of characterizing local water quality — including water clarity.

Gallegos, who's thin and looks studious in silver-rimmed glasses, says he's a big believer in public outreach. He wanted to contribute to this effort. But as he struggled to summarize information about turbidity for the public, he soon ran into a snag. His two main sources of data on light didn't match.

When it comes to light, Gallegos has amassed a lot of data. Both algae and underwater grasses need light for photosynthesis, and to study this he's become a leading expert in the way light moves through water. He's lowered instruments called radiometers down near the bottom in many parts of the Bay to measure how much light gets through. He uses radiometers because they help him gauge precisely how much light an underwater grass plant can see.

Oddly, the data from the radiometers didn't track well with mainstream turbidity records, most of which come from Secchi disks. These are simple disks — sometimes white, sometimes black-and-white — that researchers lower into the water until they can't see them anymore. The Secchi disk method, both easy and cheap, has a long history (see Now You See It, Now You Don't). Turbidity charts presented by the Chesapeake Bay Program and others have generally relied on Secchi disk measurements.

Now You See It, Now You Don't

IT'S NOT MUCH MORE than a plastic circle attached to the end of a rope. But despite its humble structure (or perhaps because of it), the Secchi disk holds its own among oceanographic instruments. [more]

To show the difference between the measurements, he pulls out graphs of data he's gathered from around the Bay. One group of charts — measurements from radiometers — looks variable, swinging up and down, with no clear trend. Charts based on data from Secchi disks show a general trend downward, toward decreasing water clarity — consistent with Bay Program reports.

This discrepancy between his two ways of tracking light stymied him. How could he explain turbidity trends to the public, when his data were apparently giving him conflicting results?

What he wanted was a better sense of how these two different measurements related to each other. He decided to do some simple math that would give him a "dimensionless coefficient" — a number that would stand for the product of these different units taken together. It was a way to compare the apples of the Secchi disk with the oranges of the radiometers.

It was a simple calculation, but what emerged startled him.

He saw that the difference between these two measurements has been widening for almost two decades — almost since he began measuring with the radiometers, around 1990.

In other words, according to his data, the gap between what a Secchi disk shows and what a radiometer sees has grown steadily wider with every passing year.

"It's one of the most remarkable trends I've seen in the data," he says.

Why is this so important? Because it means that two main methods of measuring light — visually (with a Secchi disk) and by direct measurement (using radiometers) — seem to be saying different things. And year by year that difference has increased.

Why would these two ways of measuring light in the Bay be drifting farther and farther apart?

Scattering

To unravel the mystery of his conflicting data, Gallegos had to think harder about the behavior of light.

Light, physicists tell us, is both a particle and a wave. It's hard to imagine how something can be both a particle and wave, but there it is. When asked which way he pictures light as it bounces around, Gallegos says he thinks of it more like a wave — as in a wave pool, for example, where ripples bounce off everything. In the end, though, he says it's both.

We know that when light hits the water, a lot of it gets absorbed — especially at the red end of the spectrum. That's why clean, deep water looks blue. It's the blue light that's left. But when light hits the Bay, it also smacks into a sea of particles, especially in summer. Some of those particles, like algae, will absorb light — grabbing it and keeping most of it. That's not surprising, since algae contain chlorophyll, the substance that gathers light for photosynthesis. Other particles, like small specks of sediment, will not absorb much light but instead will bounce it around (see What's in the Water). This bouncing around is what Gallegos calls scattering.

What's in the Water Makes a Difference
sediment in left image

LIGHT HEADING FOR THE BOTTOM of the Bay gets lost in two ways, not counting shading. It gets scattered or it gets absorbed. [more]

Imagine a particle-wave of light landing in a pinball machine. It will ping around madly until it drops into a hole. (In the Bay that hole might be an algal cell.) This is what happens in water filled with suspended solids. In the turbid Chesapeake there is a lot of pinging around, a lot of scattering.

What does this have to do with a Secchi disk? According to Gallegos, scattering can quickly confuse the human eye. All that bouncing light reduces the contrast between bright shapes and dark shapes and creates a haze. In that haze the Secchi disk disappears quickly, along with everything else.

The same occurs in the atmosphere on a muggy summer day. Even when the sun is shining, buildings look hazy, indistinct, light scattered by countless water droplets in the humid air. An increase in scattering would explain why Secchi disk data would look worse than data from radiometers.

It would also explain why water clarity doesn't always track closely with algae blooms.

Everyone knows that algae blooms block light. Too much algae is a major problem in the Chesapeake Bay. It leads to low oxygen zones and shades out underwater grasses on the bottom. But trends in algae abundance (measured by chlorophyll a) don't necessarily equal changes in water clarity. If algae were the only cause of the Bay's murkiness, when chlorophyll levels dropped, visibility would improve. But monitoring shows that in many parts of the Bay, even where algae abundance has decreased, water clarity has not improved — in fact in some places it seems worse.

For Gallegos, here was an answer. Increased scattering of light would explain why Secchi disk readings would be worse, even where algae counts are down. Suspended solids — mostly inorganic particles, not algae — would create the haze in the water. But why would light be scattering more now than in the past?

Gallegos thinks that suspended particles in the Bay are getting smaller. The smaller the particle, the greater the surface area. That means more reflectance, more scattering.

According to this hypothesis, tiny particles are creating a haze in the Bay, just as humidity and ozone create a summer haze in the city.

Where are all these fine particles coming from? Both Gallegos and Sanford are eager to do more experiments to find out. At present, they say, there is not enough conclusive data to know for sure. There are, of course, some likely candidates. For one thing, there is the construction boom that's been underway in many parts of the Bay watershed since World War II. Construction practices often loosen fine "colloidal" particles that wash into streams and rivers — some so tiny that they pass right through sediment fences.

Others point to stormwater from existing developed areas. Channeled by gutters, culverts, and pipes, stormwater forces high velocity runoff into streams and scours them. Many of those streams already hold sediment gathered over many years, perhaps dating back to the clearing of land for agriculture. As new development reshapes hydrology and concentrates runoff, these old sediments may be blasted downstream and into the Bay. Experts at the Anne Arundel County Department of Public Works have documented precisely this pattern in the South River.

Sanford also notes that dams — like the Conowingo at the mouth of the Susquehanna — are more likely to trap heavier particles, allowing the lightest and finest particles to flow over and into the Bay.

"Right now there's a lot of guesswork going on," Sanford says.

Into the Soup

A fine haze of sediment. That's one change that seems to be plaguing the Bay, but there is more to the mystery.

The fine particles hanging in the Bay appear to behave in strange ways. Even though they are extremely small, these inorganic particles should sink more quickly to the bottom. Why are they hanging around so long? Why doesn't this haze clear out, the way the air clears after a summer storm?

The second clue to the Bay's haze lies in what Sanford calls "packaging." In Sanford's view, the flocs now floating in the Bay are not necessarily bigger but they are lighter.

Gallegos explains it this way. The Bay is so full of nutrients, he says, that it feeds all kinds of organic productivity. The result is a rich organic soup — by-products of broken cells, dead algae, pieces of jellyfish, and countless other biological castoffs from the Bay's protein factory.

All this material drifts through the water and acts like glue, sticking particles together. The result: clumps of both organic and inorganic particles. Since organic matter is full of water, these clumps weigh only a little more than water itself. They almost float. Even though most fine particles floating in the Bay are inorganic (think dust), Gallegos suspects that it's largely organic matter that buoys up the clumps and keeps them hanging in the water for a long time. And as the organic part keeps them drifting, the inorganic part scatters light.

Sanford cautions that it will take more research to know for sure. For now, he says, the most likely explanation is that the abundance of these organic-inorganic particles relates to a larger shift in the Bay. That shift came about when the Bay changed from an ecosystem rich with bottom (or benthic) life to one where much of the productivity occurs in the water column. Organisms on the bottom once processed and packaged many of these particles, but now large clouds of this material continue to hang in suspension for long periods.

Even when this organic-inorganic mix falls to the bottom, it causes problems. It's highly "fluidized," Gallegos says, full of water — and that makes it mobile. A canoe paddle will stir it, as will wind or waves.

Call it "fluff." That's what Gallegos calls it. Benthic fluff. He's found it all over the Bay. He thinks that most of this fluff is formed in the shallower parts of the estuary and, because it drifts so easily, it tumbles into the Bay's mainstem. That's why we see it even in the lower Chesapeake, he says, where sediments are sandier and settle out fast. This fluff drifts over sandy bottom and lies there.

Underwater grasses can't root in fluff — it's too unstable. In fact, it's probably not good for many things that live on the bottom, where most Bay creatures are adapted to sand or mud. There are, though, some organisms that might thrive in fluff. Gallegos thinks it may be good habitat for dinoflagellates, some of which are harmful or even toxic.

In the end, this organic-inorganic mix, this fluff, moves particles around that would normally settle out. It's like attaching tiny balloons to sediment. It's like tumbleweed. Or dust bunnies. Fluff keeps things floating around.

But didn't Sanford say that particles sticking together should cause better visibility? Wouldn't joining smaller particles into bigger clumps reduce the haze?

Normally, according to Sanford and Gallegos, it would. Sanford remembers seeing this effect while diving. Particles sticking together at a certain depth (flocculation) make the water clearer at that level. The problem is that the flocs and fluff now floating in the Bay are different. These clumps of fine particles easily break apart. Their connection, Sanford suggests, is "weak." They drift like fragile storm clouds that break up in wind and waves.

Sanford says that when these flocs break apart they can explode into "a million bits" — you see "nothing but haze."

Clouds of inorganic particles that scatter light and cause a widespread haze. Organic material that keeps them floating about. These are the answers that Sanford and Gallegos give to the riddle of the Bay's increasing turbidity.

Where Do We Go from Here?
Autumn on the West River - credit: Sandy Rodgers
Autumn on the West River. Credit: Sandy Rodgers.

As summer turns to fall, the Bay's water begins to clear. "We have the cycle down pretty well," Gallegos says. Relatively clear water in winter, a big algae bloom in spring, followed by a pause in early summer when waters may clear again slightly. After this lull, water clarity drops, usually hitting its worst levels in mid-summer.

Such is a year in the life of turbidity in the Chesapeake Bay.

"The bottom line," says Gallegos, "is eutrophication." The result of too many nutrients and too much organic matter piling up. He feels that the Bay has become "chronically eutrophic." He thinks this process is cumulative, and that's why the turbidity graph keeps looking worse year by year, even in periods of low flow.

This means that as bad as sediment may be as it washes off farm fields and construction sites, it's nutrients that are — in his view — making it worse. Sanford and Gallegos suggest that this interaction between inorganic and organic material may have pushed the Bay into a new phase of degradation. The Bay may have crossed another ecological threshold, they say, heading in the wrong direction.

Sanford and Gallegos both caution that much of this work is still in progress. There is a lot more to learn about precisely how suspended sediment is behaving in the Bay. A lot more to learn about ecological thresholds, and whether or not we have passed another one. "The truth is," Sanford says, "we don't know."

Is the Bay's clouded water here to stay?

Not necessarily, Gallegos says. Without an overabundance of nutrients, these processes would not occur — at least not to the degree that they do now. He points to work by UMCES researcher Walter Boynton and others that suggests that the Bay does not have a long nutrient memory. That means that if we can reduce inputs of nutrients to the Bay and its rivers, the estuary will respond.

And if we don't reduce the flow of nutrients into the Bay? If we don't bring an end to what Gallegos calls "chronic eutrophication"? Then the Bay's haze — its cloud of fine particles — will arrive next year and the year after that, a new summer ritual that no one wants to celebrate.

Comment on this article1  Email the author  
 
Contents
September 2008
vol. 7, no. 3
Related Links
Related Articles
Chesapeake Quarterly
[Maryland Sea Grant]
Stay Connected
 
Support Maryland Sea Grant
Comment on this article1  Email the author  
 
bottom
Chesapeake Quarterly is published by Maryland Sea Grant | Privacy Policy | © 2017 - Maryland Sea Grant