Mysteries of the deep have fascinated mankind for thousands of years, from the enigmatic location of the legendary Atlantis to unsolved disappearances in the Bermuda Triangle. In Tampa Bay, scientists are working to unravel another unexplained phenomenon: why seagrasses aren't growing in areas where they should be thriving.

Overall, Tampa Bay has gained about 350 to 500 acres of seagrass a year since the late 1980s, but recovery has been inconsistent, with advances in acreage near the mouth of the bay but declines in some northern sectors.

"We don't believe it's water quality," says Robin Lewis, a longtime bay advocate who recently established the not-for-profit Coastal Resources Inc. "Roger Johansson (chief biologist for the City of Tampa's Bay Study Group) and I have been monitoring some of these areas for years to see if the seagrasses will naturally expand. In the Wolf Branch Creek area, for instance, there are several thousand acres of submerged lands where the water quality is okay and light penetration is good, but seagrasses still aren't growing."

Scientists have proposed a series of theories to explain the mysterious dearth of seagrass in some areas, from propeller scars to seagrass diseases, but no clear answer has emerged. They're hoping that a new study on longshore bars, which once rimmed the bay but have largely vanished in its northern sectors, will help resolve the puzzle.

	Photos courtesy of Costal Resources Inc
Historical photos clearly show the decline of longshore bars - and bayside seagrass meadows - over the past 60 years.

Longshore Bars Could Play Key Role

"It's a hypothesis that makes a lot of sense - now we need to gather some data and test it," said Lewis. "Nobody has really looked at longshore bars specifically, although some studies in Mississippi and Texas have looked at ship wakes and how they affect submerged aquatic grasses.

The bars, which breach the surface of the water at extreme low tides, help reduce wave action, which may minimize stress on seagrass meadows and encourage natural colonization, notes Holly Greening, senior scientist for the Tampa Bay Estuary Program.

 "Historically, seagrasses were growing both bayside and inside the bars, but we don't know what happened to the bars," Greening said. "They could have washed away when the seagrasses died off between 1960 and 1980, or they could have been impacted by changing circulation patterns or increased wave action caused by large ships."

Supporting the hypothesis, Lewis adds, is the fact that bars are still standing in southern parts of Tampa Bay where seagrass meadows are expanding and the main ship's channel is many miles from shore. Further north, as ships enter Hillsborough Bay heading to the Port of Tampa, the longshore bars break down and seagrass meadows are limited.

"In some areas, the seagrasses started to expand and then the growth was stymied. We're not sure why, but we need to find out," he said. Timing is critical because the U.S. Army Corps of Engineers is reviewing alternatives to expanding the ship's channel to accommodate larger vessels and increased traffic, Lewis notes.

The first phase of an investigation, funded with an initial $45,000, is underway now, focusing on background work that includes mapping of the current and historical extent of longshore bars and development of a conceptual model of a restored longshore bar system.

TBEP and Coastal Resources applied for grants to continue the projects including $125,000 from the Hillsborough County Environmental Protection Commission's Pollution Recovery Fund and $150,000 from the Gulf of Mexico Program (GMP). If awarded, the local funds would purchase meters that measure wave action to determine the difference between natural erosion and that caused by ship traffic, and the GMP funds would cover initial costs of design, permitting and construction.

The Corps of Engineers has already expressed its interest in participating with the reconstruction of longshore bars using dredged material, Greening said. "We're looking at a potential site at the southern end of MacDill (Air Force Base) but we'll need more input from engineers before making a final decision," she said. "We're probably three to five years away from a prototype bar, by the time we complete the engineering, permitting and funding."

While subject to revision, the initial concept calls for a system of longshore bars stretching up to 6700 feet. "It's not one continuous eyebrow across the bay," says Greening. "It's a system of linear mounds of sand that run parallel to the shore punched through at intervals for water to flow and probably located at varying distances from the shore."

Seagrasses will be planted on both sides to "kickstart" growth and help determine the viability of longshore bars in other locations.

The final design also will hinge upon results of the wave action study. "Ship wakes clearly have the ability to impact the sea bottom, but we need to take a scientific look at them and differentiate between natural wave action and ship wakes," Lewis said. "Seagrasses have evolved to deal with natural wave action, but there may be as many as 9,000 one-way trips by ships over 50 feet long each year."

Some reports indicate that smaller tugboats, moving slowly with great power, actually have greater impact than larger boats moving quickly, he adds.

Once funding is approved for the special wave-measuring meters, Lewis expects that the study will require people on station with the meters to document ship traffic including size and type of the vessel as well as current weather conditions. "The port tracks larger vessels but not the smaller tugs and we'll need to be there to track the weather - we can't just blame everything on the ships," he said.