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24 Apr 07
Seagrasses On Rebound
By Mike Salinero
TAMPA - Seagrasses were once an endangered species in Tampa Bay. The aquatic plants that shelter juvenile fish and shellfish were practically snuffed out during the 1960s and '70s by poorly treated sewage, industrial waste and dredge-and-fill operations.
Seagrasses have revived, however, and now cover more Bay bottom than at any time since 1950. The latest map completed by the Southwest Florida Water Management District shows seagrasses increased by 1,275 acres between 2004 and 2006, a gain of nearly 5 percent.
Seagrasses now cover nearly 28,300 acres across the 400-square-mile Bay, the most since 40,400 acres were identified in aerial photographs taken in 1950.
Holly Greening, lead scientist at the Tampa Bay Estuary Program, said the increasing seagrass acreage is due to clearer water. Like most other plants, seagrasses need sunlight to grow. Algae blooms, fueled by nutrients such as nitrogen, can block the sunlight, killing the grasses.
Greening said scientists measure water clarity two ways. One is the amount of chlorophyll A, an indicator of algae, in the water. The other is measuring how deep light penetrates through the water.
The estuary program has targets for both of these measures that indicate ideal conditions for seagrass growth.
In 2006, all segments of the Bay met both targets, the first time that's happened since 1975. "We saw very clear water last year," Greening said. "We met both of those targets, and that tells us how we're doing as far as meeting the light requirement for seagrasses."
Seagrasses started regrowing after state legislation in the late 1980s forced sewer plant upgrades in West Central Florida. Since then the estuary program and local governments across the Bay have signed an interlocal agreement to reduce nitrogen pollution into the Bay.
Cities and counties have begun cleaning stormwater outflows, and the water management district and other agencies are restoring shoreline to provide natural filtration of runoff.
"Certainly 2006 was a good year," Greening said. "It indicates the projects that are being conducted throughout the Bay by public and private entities seem to be keeping up with the growth and the nitrogen associated with growth."
The water district is now working on its 2008 seagrass map. Digital photography has given scientists a good idea of the extent of seagrasses, but the photographs must be confirmed by on-the-scene mapping. That was Kris Kaufman and B.J. Grant's assignment Wednesday.
The two district scientists jumped into the chilly, but clear, water off Weedon Island with snorkeling gear and a measuring tape. They swam and walked along the edge of the seagrass, measuring its extent and comparing it with the digital maps.
Kaufman said there are sure to be differences between the photographic map and their on-site charting. For one thing, there is a built-in error factor of 30 feet to 60 feet because of the scale of the map - one meter on the map equals 12,000 meters on the ground.
Also, dark patches on the photographs could be seagrasses or they could be algae, oysters or mud. "If they have questions, we have to come out here," Kaufman said. "If it wasn't seagrass, we pull it off the map."
The scientists measured one stretch of seagrass that extended out from the shore about 300 feet farther than what the map showed. Kaufman said the increase could be due to timing: The photographs were taken last winter when the seagrass was dying. Now, the grasses are in growth mode.
The finished maps are the definitive measures of seagrass growth in Tampa Bay and are widely used by the estuary program, the state Fish and Wildlife Conservation Commission and other agencies doing research on aquatic habitats.
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