Low Tidal Flow
Lack of Tidal Flow Killing Mangroves Around the World - Bay Soundings
From the shore, the mangrove forest at the mouth of the Alafia River looks lush and healthy. Beyond that edge, however, aerial images show a different picture. Deep inside the forest, where tidal flows have been blocked for decades, an area of dead trees is slowly but surely spreading. And it’s not just the mangroves
“Lack of Tidal Flow Killing Mangroves Around the World”
From the shore, the mangrove forest at the mouth of the Alafia River looks lush and healthy. Beyond that edge, however, aerial images show a different picture. Deep inside the forest, where tidal flows have been blocked for decades, an area of dead trees is slowly but surely spreading.
And it’s not just the mangroves in Tampa Bay, it’s happening around the world, says Robin Lewis, president of Lewis Environmental Services and the not-for-profit Coastal Resource Group. Ditches dug decades ago to drain land or control mosquitoes are filling in and choking mangroves out. In other places, roads or seawalls built without culverts to allow tidal flows have cut the lifeblood from nearby mangrove forests.
While the Giant’s Camp restoration is one of the first in Tampa Bay, it’s expected to be a prototype as managers look past the healthy fringe of mangrove forests bordering many parts of the bay. “Robin made a compelling argument that mangroves are dying because they’re getting little or no tidal flushing,” said Tom Ash, assistant director of the water management division of the Environmental Protection Commission of Hillsborough County and a member of the board of trustees that approved the restoration. “There’s no telling how many other areas are dying back because they’re not getting the tidal flush they need, but there are probably quite a few.”
And if the restoration is as successful as it is expected to be, it could open up other opportunities at other sites, adds Sean Meehan, habitat restoration specialist with the National Oceanic and Atmospheric Administration, who also served as a trustee. “So much of Tampa Bay’s shoreline is hardened that it is nearly impossible to create new mangrove habitat. It makes a lot of sense to restore these mangroves. They’re almost impounded with low flushing and low oxygenation – they’re just rotting from the inside out.”
While the concept of restoring mangroves by opening tidal flows is relatively new in Tampa Bay, Lewis has been successful in other parts of Florida. At Rookery Bay National Estuarine Reserve south of Naples, mangroves began dying off after Hurricane Andrew came through in 1992. “Nobody knew why they were dying off because a heavy accumulation of rainfall shouldn’t have killed them,” he said.
After a series of mangrove plantings failed to survive, Lewis persuaded managers to try opening up tidal flows, placing a series of culverts under a road and opening choked channels. While the construction just began in 2012, results of water quality and fish monitoring already show improvements.
Contrary to popular belief, mangroves don’t thrive in standing water, and they need both salt and fresh water to thrive, Lewis said. “Mangroves are easy to drown – they want to spend 30% of the time or less in standing water.”
And there’s no reason to plant mangroves if the hydrology is right, he adds. “Ninety-nine percent of planted mangroves die, but they’ll move in if you provide the right conditions. Mother Nature plants mangroves much better than you and I.”