Nobody cared about the extensive stands of roseau cane at the mouth of the Mississippi River until 2016 when numerous stands appeared to be senescencing three times a year. In the spring of 2017, Dr. Andy Nyman began collaborating with entomologist Dr. Rodrigo Diaz to study the phenomena. Within a few years, the dieback had spread west into Texas and east into Mississippi, and Diaz was working with dozens of collaborators from LSU and across the country.

The most likely cause is an exotic insect new to North America, but transient salinity stress and flooding stress may also be relevant. Initial fears focused on the possibility that dieback would spread to agricultural grasses, such as sugarcane and rice, or important marsh grasses, such as Spartina spp. and Schoenoplectus spp., but greenhouse experiments suggested such possibilities were low. Unlike Phragmites in much of North America, most of the Phragmites in Louisiana is not the invasive European variety that dominates North American research on the species. The European variety of Phragmites did not reach Louisiana until the 1990s or 2000s. Phragmites at the mouth of the river may be some of the most diverse globally because it contains four varieties: Gulf, Delta, European and Greeny.

Nyman initially worked with graduate students Aimee Beaudette (M.S., 2020) and Madeline Gill. Beaudette’s thesis using Landsat images indicated that dieback probably began in 2009. Before then, the area of Phragmites and the area of all vegetation varied similarly from year to year, but after 2009 the area of Phragmites and the area of all vegetation varied differently. Gill’s field research compared vegetative response to disturbance via mowing and found that Phragmites at higher elevations was more likely to be replaced by other species such as elephant ear (Colocasia esculenta) but Phragmites at lower elevations was more likely to be replaced by open water.

Nyman and collaborators at Tulane University explored consequences of Phragmites dieback on water and sediment discharge in the navigation channel. Dredging there exceeds $100 million annually; thus, even small declines in water velocity increase dredging costs. Numerical modeling indicated that a single channel most limited sedimentation, but the second-best configuration was one in which any secondary outlets were long and bifurcating. Thus, Phragmites dieback probably is increasing the costs of dredging by making all other outlets more hydraulically efficient.

Nyman now is collaborating with LSU and Tulane researchers to collect field data that will allow numerical modeling of the mouth of the Mississippi River to more realistically account for effects of vegetation so that managers can better evaluate the effects of restoration options on navigation dredging and shallow water wetlands that provide for fish and wildlife.

The mouth of the Mississippi River is internationally important because it contains a deep draft navigation channel, shown by the yellow line, that allows ocean-going vessels to reach ports as far inland as Baton Rouge Louisiana. The mouth of the river also is important internationally because it contains coastal wetlands including the Delta National Wildlife Refuge (48,000 acres) and the Pass A Loutre Wildlife Management Area (115,000 acres). These wetlands provide habitat for numerous species of migratory birds and recreationally and commercially important fish, shrimp and crabs.

Examples of hydraulically inefficient, top, and efficient outlets from the Mississippi River. The left shows robust vegetation on the banks of bifurcating channels and the associated shallow water wetlands that obstruct flow, which reduces the costs of navigation dredging by forcing more water down the navigation channel. The image on the bottom is from Neptune Pass, which is too young to have developed bifurcating channels and associated shallow-water wetlands. Its hydraulically efficient channel tasks much flow in the navigation channel and thus increases dredging costs.

On higher elevations such as natural levees, Phragmites is being replaced by less robust species, such as elephant ear, which lose all aboveground biomass during winter and thus makes the channels leakier to lateral flow. At lower elevations, Phragmites is being replaced by open water, which makes channels shorter and thus also more hydraulically efficient.