Tuesday, July 10, 2012

Mangrove Trees Showing up on Horn Island May Indicate Climate Change

Scientists from the Dauphin Island Sea Lab, the U.S. Fish & Wildlife Service, and the National Park Service are studying this black mangrove bush, which is growing farther north than any other mangroves along the Gulf of Mexico. A wasp can be seen in the center of the photo, about to alight on a mangrove flower. (Press-Register/Ben Raines)

A low shrub sitting at the mouth of the big lagoon on Horn Island provides another piece of evidence that our climate may be changing. 

The bush, covered in small, butter-yellow flowers and thick green leaves, looks out of place amidst the surrounding sea of black needlerush marsh grass. It is a black mangrove, a plant typically associated with the coastlines of south Florida and the Caribbean. This particular bush is believed to be growing farther north than any other black mangrove on the Gulf Coast. 

Scientists from the Dauphin Island Sea Lab, the U.S. Fish & Wildlife Service, and the National Park Service are studying the bush and three smaller black mangroves growing nearby. The other bushes are all a few hundred yards farther south, but still many miles north of the next closest group of mangroves, which grow on the Chandeleur Islands off Louisiana. The mangroves there are relatively recent arrivals as well, apparently gaining a foothold after warmer winters in recent years. 

Each month, the scientists visit the trees to measure their growth and study the plant and animal colonies surrounding them. Last week, blue crabs, hermit crabs, and stone crabs all were visible beneath the mangrove's canopy. Sediment cores drilled out of the swamp muck around the bushes revealed a variety of marine worms, known collectively as polychaetes, and a host of tiny creatures seldom seen by humans. A gas generator floated into the marsh on pontoons was used to power a suction pump that vacuumed up shrimp, minnows and other creatures swimming in the shallow marsh. 

In their native habitats, mangroves support a diverse ecosystem, but one that is different than the typical salt marsh systems of the northern Gulf Coast. By studying the creatures around the plants, the scientists hope to understand what it will mean if warmer winters allow the species to proliferate along Alabama and Mississippi's shores.

A light dusting of salt can be seen on the tops of these mangrove leaves. Black mangroves are able to survive with their roots in the salty gulf waters due to the ability to expel salt through their leaves. (Press-Register/Ben Raines)

A light dusting of salt can be seen on the tops of the leaves, evidence of the mangroves' unique ability to expel salt sucked up through the roots. Scientists say the plants excrete salt at night and on cloudy days, which allows them to live in places traditional plants cannot. 

"They really change the area. They can change the sediment composition in the area around them," said Whitney Scheffel, with the Sea Lab. Scheffel said the lab is studying how the plants affect nutrient levels in the sand and mud of the marsh, and is investigating any changes in the creatures using the adjacent marsh. 

"They've just started showing up in this area. We hope it won't have a negative consequence, but right now we don't have the data to say one way or the other," Scheffel said. "We want to see if having these black mangroves here will actually change the growth rates of the spartina and the juncus, the marsh grasses we have here. That could be important." 

Matt Johnson, a marine ecologist with the Gulf Islands National Seashore, said mangroves have colonized the area in the past, but ultimately were wiped out in particularly cold winters. 

"This is right at the edge of their range. This population is the northernmost example we have," Johnson said. 

The plant at the edge of the lagoon has died back a couple of times, but managed to rebound. 

"We call it 'dead but not dead.' The top dies off, but the roots don't and the plant comes back," Johnson said. He said he didn't think the arrival of the plants in the area represented an ecological threat. 

"You can't really say (marsh or mangroves) are a better habitat. They both hold the land in place, they both support a lot of life, so as long as you've got one, that's good."


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