It’s Earth Day 2009 and MRGO is closed to navigation, but it’s still there, still conveying salt water into the intermediate and fresh wetlands of the Pontchartrain Basin. Until it is truly closed and filled in, it is not really gone.
We have already seen the damage it caused to Bayou Bienvenue in the the immediate vicinity of New Orleans, where it wiped out a cypress forest that protected the city from storm surges.
Along Bayou La Loutre at Alluvial City, the evidence of salt water intrusion was almost immediate.
Way down on the southern end of the Central Wetlands along Bayou la Loutre around Alluvial City, stressed bottomland hardwoods still anchored its ridge in the Fall of 1995. A dozen years later the forest had thinned, the trees were broken and short. In 1932 the Bayou la Loutre supported a mix of bottomland hardwoods on it ridge and a cypress swamp in its backswamp. The tale-tell evidence of cypress canals as far along Bayou la Loutre as Baker’s Canal suggests that loggers cut trees from fresh habitat that extended well east of the cut through the Bayou la Loutre ridge when engineers dredged MRGO.
The Corps of Engineers made monthly samples of the salinity of Bayou la Loutre at Alluvial City before and after the construction of MRGO. Salinity rose an average of 4.5 parts per thousand after the construction of MRGO, from levels as low as 6.3 parts per thousand in March before construction to levels as high as 16.1 parts per thousand in July after. Generally, saline levels, both before and after MRGO carried saltwater into Bayou la Loutre, were lowest during the wet winter and spring months and highest during the dry summer and fall months. Before MRGO, salinity in Bayou la Loutre varied by a little more than two points spring to summer, after by as much six points. And, in July the saline levels in the bayou doubled after the completion of the channel.
Researchers at the Baruch Forest Science Institute of Clemson University and the Savannah River Ecology Laboratory at the University of Georgia, wanting to see the affect of sea level rise on bottomland hardwoods, studied the growth and physiological responses for four species of bottomland hardwood seedlings to flooding and salinity. All four–green ash, tupelo, cypress, and Chinese tallow–withstood thirteen weeks of freshwater flooding. Cypress and tupelo did best, followed by Chinese tallow, and then green ash, but their trunks grew shorter and wider and their roots smaller.
They all survived light doses of salinity, but became stressed. Their tips died, they lost leaves, and the diameter of the trunks on the green ash, the Chinese tallow, and the tupelo did not enlarge, but the cypress did. The cypress and the tallow did best when flooded with water with salinity levels of two parts per thousand; the green ash and tupelo did not. Up the dosage to ten parts per thousand and all four species died within six weeks. Simulate a hurricane surge at thirty-two parts per thousand and all species experienced stress within eight hours. A third of the cypress survived until the following spring. Chinese tallow, an unfortunate invasive species, had the highest survival rate.
Researchers from the National Wetlands Research Center of the U.S. Geological Survey collected and germinated 2,700 cypress seeds from fresh and brackish waters in Louisiana, Alabama, and Mississippi. They wanted to measure the seedlings tolerance to salinity and to identify and develop salt-tolerant species. The range of salinity of the seeds collected was from 0.1 parts per thousand, fresh, to 7.5 parts per thousand, brackish. They grew them in solutions of zero, two, four, six, and eight parts per thousand. They measured the seedlings for their height, diameter, and physiology. And, the scientists studied the parents in the field. They found that seedlings from trees growing in brackish water carried their parents’ tolerance for salinity. The scientists hoped from these studies they could develop strains of cypress that tolerated increased levels of salinity and flooding, cypress that could be used to restore forests to places like the Central Wetlands and the Falgout Marshes a victim of the Houva Navigation Channel in Terrebonne Parish.
Lake Pontchartrain Basin Foundation, Comprehensive Habitat Management Plan for the Lake Pontchartrain Basin, February 28, 2006, 79, http://www.saveourlake.org/pdfs/JL/CHMP_final_%2022706.pdf ; Tate, J.N., A.R. Carrillo, R.C. Berger, and B.J. Thibodeaux, Salinity Changes in Pontchartrain Basin Estuary, Louisiana, Resulting from Mississippi River-Gulf Outlet Partial Closure Plans with Width Reduction, Vicksburg and New Orleans: U.S. Army Corps of Engineers, Coastal and Hydraulics Laboratory, August 2002, 5,-6, 13, http://mrgo.swg.usace.army.mil/Documents/2002%20August%20Salinity%20Changes%20in%20Pontchartrain.pdf; McLeod, Ken, and Will Conner, “Rising sea level making conditions tougher for survival of forested coatal wetlands,” News Release, Savannah River Ecology Laboratory, University of Georgia, March 19, 1996, http://www.uga.edu/srel/searise.htm; U.S. Geological Survey, National Wetlands Center, “Salt Tolerance of Southern Baldcypress,” Lafayette, Louisiana, June 1997, http://www.nwrc.usgs.gov/climate/fs92_97.pdf.