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Sun-bleached manatee grass washes up on Sanibel, Captiva beaches

By Staff | Jun 3, 2009

Last weekend, unusually large quantities of fragmented and sun-bleached manatee grass (Syringodium filiforme) washed up on Southwest Florida beaches, from Collier County to Sarasota County, including Sanibel, Captiva, Fort Myers Beach, North Naples, Charlotte County, Manasota Key and Venice Inlet. It has been seen also three to eight miles offshore of Sarasota, but has not yet washed up.

SCCF Marine Lab scientists have been in touch with other groups, including the City of Sanibel, the Town of Fort Myers Beach and Mote Marine Laboratory, which reported the Charlotte County and Venice sightings and is checking Sarasota beaches.

The manatee grass washing up does not pose a nutrient issue because the loss of nitrogen and other organic content already occurred when it decomposed in the Gulf. The live shoots are green, but after they die and float to the surface in the Gulf, they bleach out and become clear to white. The cylindrical shape of the blades and the internal cells probably help keep the blades from completely breaking up as other seagrass blades probably do.

Under the summer sun, the manatee grass that is now on Sanibel and Captiva beaches will dry out and flatten, and look like straw wrappers. Once dry, it is very light and will probably be blown away by the wind, or washed back out into the Gulf on a high tide.

Manatee grass is unlike most seagrasses because the shoots are cylindrical, rather than flat. Manatee grass usually grows in association with other seagrasses, and its leaves can grow to about 20 inches long. It has a thick root and rhizome system and can often form dense underwater mats on both muddy and sandy bottoms.

While the manatee grass is harmless on the beach, anytime that seagrasses in this volume wash up, it’s a concern and SCCF will be working with colleagues up and down the coast to look at potential causes.

For more information, contact Drs. Loren Coen or Rick Bartleson at SCCF’s Marine Lab by calling 395-4617.

Marine Lab monitoring Caloosahatchee seagrasses

The SCCF Marine Lab is continuing to monitor tapegrass and ruppia growing in the estuarine portion of the Caloosahatchee River.

“Tapegrass seedlings are sprouting east of Beautiful Island in the river,” reported Rick Bartleson, Ph.D., Lab Research Scientist.

The drought which began in 2006 wiped out the tapegrass when salinities (salt content) became too high; the seedlings are from the seedbank remaining from the earlier plants.

Since tapegrass cannot grow in high salinities, it serves as an important indicator of the impact of salinity levels on the ecosystem.

The Lab is providing data on the presence and survival of tapegrass to the South Florida Water Management District. The District uses the data as a factor in determining whether freshwater should be released from the Franklin Lock. Readings from SCCF’s RECON sensor at Shell Point are also used in making decisions about releases.

In the 1990s, tapegrass grew as far downriver as the Midpoint Bridge but the westernmost range now is at the head of the Caloosahatchee (by Beautiful Island), extending up to the Franklin Lock and then occurring patchily east to the lake.

Ruppia is more tolerant of salinity, and Bartleson began planting ruppia in exclosures in the lower estuary last fall, when the water cleared after the summer rainy season. Ruppia in the exclosures at sites 2, 3 and 4 have long, dense blades and they are flowering.

Ruppia outside the exclosures is subject to grazing; the shoots are kept too short by grazing to provide the plant with enough energy to send out reproductive shoots.

To learn more about the SCCF Marine Lab’s seagrass restoration and other projects, visit www.sccf.org and click on “Marine Lab.”