Ecological response of forested wetlands with and without Large-Scale Mississippi River input: Implications for management

We investigated two adjacent wetlands in the Lake Pontchartrain basin, one of which receives periodic input of Mississippi River water and one which does not, to gain insight into how isolation from river input impacts wetland loss in the Mississippi delta. The LaBranche (LB) wetlands bordering Lake Pontchartrain are severely degraded due to saltwater intrusion, subsidence, leveeing of the river, and hydrologic alterations including partial impoundment. Directly adjacent is the Bonnet Carré (BC) spillway, a geomorphically similar area that contains healthy baldcypress swamp. The spillway carries river water to the lake during high discharge years and has been opened eleven times in 80 years, with flows as high as 9000 m3 s−1. The primary hydrologic difference between the two areas is the regular input of River water to the BC wetlands while the LB wetlands are isolated from the river. The interior of the LB wetlands is also isolated from sediment originating from Lake Pontchartrain. Long-term accretion, tree growth, and elevation were measured in these two wetland areas to determine impacts of riverine input. 137Cs accretion rates in the BC wetlands were 2.6–2.7 cm yr−1, compared to 0.43 and 1.4 cm yr−1, respectively, in the LB wetlands in areas without and with sediment input from Lake Pontchartrain. Baldypress growth in the BC averaged about 2.3 mm ring width yr−1, compared to 1.4 mm yr−1 in LB. Trees are of relatively the same age due to lack of recruitment and widespread logging. Tree height, an indicator of site quality, is about 20% less at the LB sites compared to BC, even though the trees are approximately the same ages. The average wetland elevation in the BC wetlands was about one meter with some areas higher than two meters, and was significantly higher than elevations in the LB (average sea level and 0.3 m, respectively, in areas with and without input from Lake Pontchartrain).