Seagrass Bald Spots Cause Head-Scratching

Susan M. Green, The Tampa Tribune
TBO.com 30 Dec 07;

APOLLO BEACH - Standing ankle-deep in clear water, Robin Lewis doesn't really need his snorkel and mask to see what he came to see.

Under his wading boots, gleaming between the ripples of sunlight on the water's surface, the sand looks as white and barren as a sun-bathed beach temporarily flooded at high tide.

A half-century ago, a visitor likely would have been ankle-deep in seagrass.

"This whole area," Lewis said, swinging his arms wide, "everything from here to the edge, everything was covered with seagrass."

Scientists are pretty sure they know what killed the grass: untreated sewage and other polluted runoff into Tampa Bay in the 1960s and '70s. But much of that was cleaned up in the 1980s and '90s, and seagrass beds across the Bay have been making a dramatic comeback.

Still, some big patches, including a spot where Wolf Branch Creek empties into the Bay, stubbornly remain bald or sport a few sprigs where lush meadows used to be.

Lewis, a wetland scientist who has studied Tampa Bay for more than 40 years, is among experts trying to figure out why and come up with a formula to promote seagrass growth.

One theory: There used to be a symbiotic relationship between seagrass and underwater sand bars, including a long mound that stretched maybe two miles along the shoreline from Apollo Beach to Ruskin.

"We know the bars in Tampa Bay that were here decreased by about 50 percent," Lewis says. "We know the seagrass decreased by about 80 percent."

The question is: Which came first, the sand bars or the seagrass?

Aerial photographs dating to the 1930s and '50s show seagrass on both sides of the underwater linear mounds. Could the vegetation on the outer edge of the bars have anchored the sand and kept it from washing away? Or did the bars buffer the seagrass beds that nestled in the crevice between sand and shore?

Could wave action resulting from storms or shipping wakes be affecting seagrass growth? What other factors might affect seagrass recovery?

As president of the nonprofit organization Coastal Resources Group Inc., Lewis is seeking up to $100,000 in grant money to try to answer the chicken-and-egg question and spur more seagrass recovery. At Wolf Branch, he will be working alongside scientists from the Florida Fish and Wildlife Research Institute who are looking into whether the foraging habits of rays and manatees could be pinching off seagrass regeneration before it hits full swing.

Lewis hopes to start transplanting seagrass to the Wolf Branch site in the spring. A second part of his study involves installing artificial sand bars off the shoreline of MacDill Air Force Base.

He is seeking state environmental permits to begin a 10-year plan that will target up to 10 sites in the Bay, including a site near Gibsonton known as The Kitchen.

Lewis' efforts are among several projects examining the role of longshore bars in seagrass recovery being coordinated by the Tampa Bay Estuary Program. Lindsay Cross, an environmental scientist overseeing the longshore bar projects for the estuary program, said about $700,000 in state, federal and local grants has been allocated for related studies.

Bedeviled By Rays

Meanwhile, also at the Wolf Branch site, the Florida Fish and Wildlife Research Institute in St. Petersburg just completed a two-year study of the effects of rays and manatees on seagrass beds. The institute plans to apply for a federal grant to extend and modify the study to dovetail with Lewis' efforts.

The area near Wolf Branch is Tampa Bay's ray central, especially in winter, said Paul Carlson, a research scientist with the institute. Particularly prevalent are cow nose rays, but other kinds of rays also occur in higher densities - up to twice as many per acre - than elsewhere in the Bay, he said.

Manatees feed on seagrass, while rays rip up the beds in search of their favorite food: shellfish.

"This time of year and in the fall, you can walk across the seagrass area and the sand glistens from bivalve shells after the rays eat them and expel them," Carlson said.

Some people think the rays are drawn to that part of the Bay by the warm-water discharge of Tampa Electric Co.'s Big Bend power plant.

"Cow nose rays have adopted the same behavior as manatees," Carlson said. "They go into the TECO discharge canal in cold weather and stay there and then forage in the seagrass nearby."

The canal lures thousands of rays to the area in winter, he said.

But other factors may influence their numbers, including depletion of the rays' No. 1 enemy: sharks.

"Either there's a lot of food for rays there ... or a lot less predation on rays from sharks in Tampa Bay," Carlson said.

To gauge whether the rays are affecting seagrass recovery, the institute conducted a two-year study that involved fencing off sections of a seagrass bed near Wolf Branch. Researchers checked aerial photography of the sites four times a year.

Preliminary results indicated the fencing protected seagrass from animals in winter but hindered growth in the spring by collecting debris and reducing water circulation, Carlson said.

The new proposal is to devise a large protective cage, about 700 square feet, to keep out foraging critters. The fenced off bay-bed will be divided into sections that will include bare sediment ripe for vegetation, as well as transplants and naturally occurring seagrass, so that scientists can determine whether rays and manatees pose a significant obstacle to seagrass proliferation.

Rx For Rejuvenation

Lewis' prescription for seagrass growth is twofold: jump-start revegetation with transplanted plugs from healthy parts of the Bay and introduce artificial sand bars at a site where transplanting already has proved successful.

In July 2006, Coastal Resources teamed up with volunteers from Tampa Bay Watch to hand-plant about 1,200 seagrass roots in six measured plots near MacDill Air Force Base.

"The great thing is they're taking off like gangbusters," Lewis said, adding that about half the transplanted seagrass plugs survived and spread.

He wants to follow a similar strategy at Wolf Branch. Currently, the site not only doesn't have the seagrass coverage it had 50 years ago, but it also hosts mostly shallow-rooted shoal grass that dies off and comes back. Historical information indicates several kinds of seagrass once blanketed the Bay bottom there year-round, Lewis said.

He proposes introducing a type of seagrass known as manatee grass. He knows the project faces challenges, including potential damage from propellers in an increasingly popular boating area and foraging from manatees and rays. But he thinks the seagrass can survive.

"It's really a good site," Lewis said. "It's like Mother Nature is saying, 'Give me a little help.'"

At the MacDill site, he is proposing installing four types of artificial underwater bars made of sand and shell. Two would include fill encased in protective synthetic material to make up what's called a Geotube, which resembles a larger version of crop rows enshrouded in plastic mulch in farm fields. He also wants to install one bar made of rock rubble and another of sand fill without the protective wrap.

Seagrass will be planted on the Bay side of the artificial bars to test the theory that aquatic vegetation helps anchor the fill and protect it from violently sloshing water.

"Our aim is to create a wave shield, an energy shock absorber," Lewis said.

Lewis said he expects to wait at least a year to see if seagrass growth takes off at Wolf Branch to determine whether artificial sand bars should be installed.

SIGNIFICANCE OF SEAGRASS

Scientists say seagrass provides critical habitat for fish and other aquatic organisms and offers a reliable indicator of water quality. The Tampa Bay Estuary Program has focused on restoring seagrass to its prevalence in the 1950s, when the Bay had an estimated 40,400 acres of underwater meadows. Estimates from 2006 indicate about 28,300 acres of Bay bottom hosts seagrass, up from a low of 21,650 in 1982.

Source: Tampa Bay Estuary Program