November 20, 2008
Helping the Bay Help Itself — Seagrass Beds May Amplify Own Growth
With funding from Maryland Sea Grant, the pair from the University of Maryland Center for Environmental Science Horn Point Laboratory is testing their theory with the hope of applying what they learn to the restoration of Chesapeake Bay’s underwater grasses.
The decline of underwater grasses (more accurately called submerged aquatic vegetation or SAV) in the Bay during the 1960s and 70s startled scientists and citizens alike. Loss of grass beds meant loss of key nursery habitat for fish and crabs and loss of rich food sources for waterfowl. Efforts to restore grasses over the last several decades have been met with mixed results — and failures seem to outnumber triumphs.
But there’s one seagrass bed at the mouth of Broad Creek in the Choptank River that Kemp and Gruber call a success story. Exceptional for its size and health, the bed is made up of sago pondweed, an SAV species that was at one time common in this region, but is no longer abundant. Kemp and Gruber hypothesize that the bed’s success may be due to a positive feedback loop — a system where the bed enhances its environment, which then enhances the bed.
The idea is that the presence of seagrass improves water quality which in turn encourages more seagrass growth, leading to further improvement in water quality and so on. Positive feedback processes abound in science and nature, but Gruber notes that they have not been widely explored with regard to seagrass ecology.
So Kemp and Gruber set out to do just that.
First the researchers studied how the presence of the grass bed alters the movement of water. They used current meters and pressure-sensing gauges at sites inside and outside the bed to measure changes in water velocity and wave height. They confirmed that inside the bed the grasses exert drag on the water, slowing it down and reducing waves. The effect was especially apparent in summer months when the grass canopy can reach about 5 feet tall.
How does water movement affect water clarity in the bed?
To find out Kemp and Gruber collected water samples inside and outside the bed at regular intervals and then analyzed them for chlorophyll content and total suspended solids, a measure of plankton, suspended sediment, and what Gruber calls “organic miscellaneous junk.” The results showed that there was much less suspended material inside the bed than outside. Inside, things like dirt and detritus settled to the bottom rather than remaining suspended in the water.
The benefit for the plants? Light. Fewer particles in the water means that more light can reach the seagrass. Light catalyzes photosynthesis, the foundation of plant growth and survival. Scientists point to the interception of light by algae and epiphytes – tiny plants that grow on SAV – as a primary culprit of the seagrass die-off.
Light meters deployed by Kemp and Gruber did indeed show that light penetrated more deeply within the bed than outside.
Kemp points out that in addition to allowing more light to pass through, the settling of algal cells and detritus has another benefit to the SAV. As these bits and pieces decompose, they release nutrients that the roots of the grasses can then absorb. “By trapping those particles, the plants are providing more nutrition for themselves,” Kemp says. “They’re fertilizing themselves.”
Gruber is quick to caution that these findings were from one bed, in one place, and of one specific size. If you want to restore seagrass and improve water quality, she says, you need to be able to say, “OK, so the grass will be able to clear the water, but only if the bed’s area is larger than “x” acres.”
Finding that unknown “x” is key to the restoration equation. Gruber pictures a dartboard where the center of the plant bed is the bulls-eye and rings of grass encircle it. She and Kemp want to know how thick these different rings, or zones, need to be in order for the bed to benefit water quality and support its own growth.
To do this, the researchers identified beds of different shapes and sizes and turned to Dataflow, a tool that helps form the backbone of the Department of Natural Resources Eyes on the Bay Program. As the boat zigzags across the bed, the Dataflow pumps water to a system onboard that measures water quality every few seconds. A GPS system tells the researchers exactly where the measurements were recorded, allowing them to create maps of water quality in relation to the beds.
But this summer the researchers encountered an unexpected — and worrisome — challenge. When they returned to the comparison beds they had surveyed the year before, they found that many of them had died. Kemp blames what he calls the “awful water quality and turbid conditions” of the past year.
“The embarrassing and somewhat sad, honest answer is that we really don’t know what’s happened,” Kemp laments. He’s been struggling, along with his colleagues Chuck Gallegos of the Smithsonian Environmental Research Center and Larry Sanford of Horn Point Laboratory, to figure out the mystery of the Bay’s worsening water quality.
Kemp points to “flocs” — clumps of very small particles loosely connected in a watery organic matrix — as a likely offender. The flocs are larger than their constituent particles, but their density is so low that they can stay in suspension with virtually no water movement, Kemp says. “So the seagrass beds are not very effective in trapping and settling out these loose flocculant particles.” And while scientists have some educated guesses, there’s no clear answer as to what’s causing the degraded water clarity and what role these flocs might play in this trend. (See Chesapeake Quarterly's in-depth look at this subject.)
Kemp says the problem has thrown a curveball not only to his research project, but to the entire restoration effort. “The science is unfortunately behind where it should be to understand these processes.” He cautions that these unknown factors are not included in the models used to make forecasts.
This uncertainty makes the work he and Gruber are doing all the more important. Understanding feedbacks and their role in restoring ecosystems has implications beyond seagrasses. And setting in motion cycles of natural “self-healing” could be the key to getting the Bay’s water clarity back on track.
-- Jessica Smits