More seagrasses, more critters in Sarasota Bay
Call it a huge success story for Sarasota Bay.
Mark Alderson, executive director of the Sarasota Bay Estuary Program (SBEP), told Sarasota City commissioners Sept. 21 that bay water quality appeared great in his annual "state of the bay" report.
"Seagrass bed increase is up 30 percent," he said, "which is perhaps unprecedented in the United States."
"Sarasota Bay" in this article includes, south to north, Blackburn, Little Sarasota, Roberts, Sarasota and Palma Sola bays.
The state of the bays is pretty good as far as seagrass beds are concerned, generally an indicator of vitality in estuarine systems.
Seagrass meadows are obvious at any extreme low tide: seagrass beds poking green-and-brown leaves out of the tidal muck.
Seagrasses are what most people think about when they consider what’s under the surface of our bays. They’re a critical element of the marine food chain, providing food and homes for thousands of varieties of plankton, crabs and fish.
And the underwater grasses are starting to flourish – again – after years of declining productivity.
Blackburn Bay had the greatest increase in the five bay segments measured by the Sarasota Bay Program at a 39 percent increase.
Little Sarasota Bay with what used to be Midnight Pass as an inlet focal point scored the lowest of the region, with a decline of 21 percent.
By the numbers, upper Sarasota Bay saw an increase in seagrasses of 1,844 acres. The southern portion of Sarasota Bay had an 850-acre increase; Little Sarasota Bay had a 1,970-acre increase, while Roberts Bay and Blackburn Bay had declines of 25 acres and 79 acres, respectively, according to biologists with the Southwest Florida Water Management District (Swiftmud).
Swiftmud conducts aerial surveys of the bays throughout its region and compares the photos with previous shots to determine seagrass coverage.
For comparison, Tampa Bay saw a 5-percent increase in seagrasses. Charlotte Harbor had a 5-percent decrease.
Why the increase hither and yon?
Officials with the SBEP said a reduction in the amount of treated effluent from sewage treatment plants flowing into the bays had a direct correlation with the added seagrass beds.
The City of Sarasota has dramatically curtailed treated effluent discharges into the bay at Whitaker Bayou, causing an increase in seagrass beds there. In northern Sarasota Bay near Tidy Island, Manatee County has added treatment processes at the treatment plant near there, plus a deep-well injection process for even more treated sewage.
The "beds of gold" for the bay environmental system should reap added harvests of fish, crabs and other marine life in the years to come. More important, though, is the indication that the enhanced seagrass beds have reversed the declines in productivity for Sarasota Bay.
Aerial photos show seagrass bed in the vicinity of Midnight Pass in 1948 (l) and 2009 (r). Beds have declined in that area by 21%.
Seagrass 101
"Benthos is a Greek word that refers to the depths or bottom of the sea," writes Jim Culter in the technical assessment of Sarasota Bay titled "Framework For Action." Culter is a benthic scientist at Mote Marine Laboratory.
"The science of benthic ecology is the study of the plants and animals that inhabit the benthos. Benthic organisms are generally not highly mobile; the majority of individuals in the benthic community will live their adult lives within a relatively small area. It is due to this restricted mobility that benthic organisms are considered excellent indicators of habitat conditions and, in some cases, environmental quality."
In the Sarasota Bay area, including the near-shore Gulf of Mexico waters, Culter estimates there are 500-1,000 different species of tiny benthic creatures.
Culter discovered five different types of bay bottom in Sarasota Bay. From most- to least-common, Sarasota Bay is divided into:
• Unvegetated subtidal sand, muddy/sand,
• Intertidal and subtidal seagrass meadows,
• Artificial hard bottom (reefs, seawalls, bridge pilings),
• Natural rock outcroppings (including the nearshore gulf), and
• Oyster-bar habitat.
There are seven different species of seagrasses in Florida. Sarasota Bay has five of them: Thalassia testudinum (turtle grass), Syringodium filiforme (manatee grass), Halodule wrightii (shoal grass), Ruppia maritima (widgeon grass) and Halophila engelmannii (star grass).
Generally, Culter found that the closer the seagrass beds are to an inlet with regular exchange of water with the gulf, the healthier the seagrass.
"In Sarasota Bay, barring direct physical impacts, seagrass meadows decline in diversity and abundance at an increasing distance from open gulf waters," Culter said.
So what’s so important about seagrasses?
"Through their complex physical structure, seagrass meadows serve as habitat by providing shelter for both adult and juvenile crabs, shrimp and fish," Culter said.
"Seagrass blades act as a substrate for an entire community of marine organisms, including microalgae, microinvertebrates, protozoa and diatoms. Herbivores that graze directly on the seagrass blades include fishes, manatees, sea turtles and sea urchins, while the attached epiphytes provide a valuable food source for a host of other marine life. Dead leaves and epiphytic growth falling to the bay bottom make up the majority of the material in the detrital food web.
"Seagrass leaves slow the water current and promote the deposit of organic and inorganic particles in the water column; their presence also inhibits the re-suspension of sediments. Roots, runners and rhizomes form an interlocking grid that tends to lock in the accumulated sediments and retard erosion of the bay bottom. The physical energy of waves and currents tends to be dissipated by the presence of seagrass leaves, helping to protect adjoining shorelines from erosion. With their photosynthetic ability, seagrasses are a major contributor of dissolved oxygen to the water column.
"Finally, seagrasses play an important role in the nutrient cycle. Seagrass leaves take up dissolved nutrients from the water column, while the roots take up and store nutrients from the bottom sediments. The removal of excess nutrients improves water quality with respect to light penetration."
In Sarasota Bay, seagrasses are threatened by added nitrogen found in treated sewage effluent and in increased levels of contaminants found in stormwater runoff. Pesticides, excess fertilizer, and petroleum products from automobile exhausts run from yards and streets into the bay, polluting the water and destroying seagrass beds.
More seagrasses means more fish, and more fish means more potential catch for fishers. A good thing all around.
October 1st 2009 - 2:25AM