Lake Borgne - Ecology

Ecology

The Mississippi River levees significantly limit the input of fresh water, sediment, and nutrients into the Pontchartrain basin. This reduction in input plays a part in the major critical problem in the Pontchartrain Basin — increased salinity. Construction of the Mississippi River-Gulf Outlet Canal (MRGO), which breaches the natural barrier of the Bayou La Loutre ridge and the Borgne land bridge, allowed saline waters to push farther into the basin. Relative sea level rise of up to 0.96 feet per century (3 mm per year) gives saltier waters greater access to basin wetlands. Mean monthly salinities have increased since the construction of the MRGO and other canals. However, these mean increases are less than the overall variability in salinity. In recent years, salinities have stabilized. The heightened salinity, caused mainly by subsidence, stressed wetlands, especially freshwater marshes and swamps.

The third critical problem is the potential loss of the Borgne and the Maurepas land bridges where wetland soils are especially vulnerable to erosion. Since 1932, approximately 24% of the Borgne Land Bridge has been lost to estuarine processes such as severe shoreline retreat and rapid tidal fluctuations, and the loss rate is increasing. During the same time, 17% of the Maurepas Land Bridge marshes disappeared due to subsidence and spikes in lake salinity. In addition, from 1968 to 1988, 32% of the cypress swamp on this land bridge either converted to marsh or became open water. These land bridges prevent estuarine processes, such as increased salinities and tidal scour, from pushing further into the middle and upper basins. If these buffers are not preserved, the land loss rates around Lakes Pontchartrain and Maurepas will increase dramatically.

The fourth critical problem is that several marshes in the basin are vulnerable to rapid loss if adequate protection is not provided soon. Examples of theses areas are: marshes adjacent to lakes and bays where if the narrow rim of shore is lost, interior erosion will increase dramatically; the perched fresh marsh on the MRGO disposal area which will drain and revegetate with shrub unless the back levee dikes are repaired; and near Bayou St. Malo, where unless canals are plugged, rapid water level fluctuations and salinity intrusion into adjacent marshes will continue.

Site specific problems of shoreline erosion, poor drainage, salinity stress, and herbivory are apparent throughout the basin. Solving these problems is important, but less urgent than solving the four critical problems described above.