Pontchartrain Estuary Atlas 2017
The 2017 Pontchartrain Estuary Atlas
Introduction & Synopsis
The thirty-six maps included in the 2017 Pontchartrain Estuary Map Atlas represent PC’s current understanding of the estuary for 2017. The Atlas was composed utilizing data and analyses from an array of ongoing projects, but most significantly PC’s 2017 Hydrocoast maps which are a suite of five maps produced biweekly that are intended to give continuous “snap shots” of the estuary. Given enough snap shots, this becomes a “movie”, and movie tells a narrative that begins to suggest estuarine trends and future conditions. The Atlas is intended to summarize the year of general conditions and indicate possible trends.
The two constructed structural closures in the MRGO shipping channel were completed in 2009, and so, the 2017 Atlas represents conditions eight years after closure. There are compelling data that demonstrate that the closure in 2009 has affected surface water salinity over approximately 1.2 million acres or about 20% of the Lake Pontchartrain Basin. The Atlas represents some conditions which may have now stabilized to a new baseline (such as surface water), but other conditions are still adjusting to the new hydrological and surface water conditions, such as soil salinity, wetland habitat, and fisheries.
The Pontchartrain Estuary Atlas area is 5,489 square miles and represents 58% of the total Lake Pontchartrain Basin. The geographic boundaries of the Pontchartrain Estuary are the Mississippi River, the Chandeleur Islands, the LA/MS state line in Mississippi Sound (an arbitrary jurisdictional boundary), and the upper limits of the coastal zone around Lakes Pontchartrain and Maurepas. The estuary can be subdivided hydrologically into two sub-basins. The estuary north of the MRGO functions hydrologically independent of the area south of the MRGO. The two sub-basins are defined by the MRGO levee and spoil bank which act as a hydrological barrier between the two areas. Fortuitously, the MRGO barrier effect emulates what would otherwise be the natural hydrological barrier of the historic ridges of Bayou la Loutre and Bayou Terre aux Bouefs, which are now compromised with canals through the natural ridges.
Wetland Land Change 2006 - 2016
Wetland land change (gain and loss) since 2006 has been modest. The areas of largest loss were southwest of Lake Lery which seems to have occurred during Hurricane Gustav (2008). Post-MRGO closure in 2009 and post- Hurricane Isaac (2012), the region appears to be stable, except for local land growth near river outlets such as the Caernarvon Freshwater Diversion, Mardi Gras Pass, and Baptiste Collette.
To learn more about the MRGO Closure, Caernarvon Freshwater Diversion, and Mardi Gras Pass, please see:
Surface Water Salinity
Based on surface water salinity, the estuary falls into the estuarine habitats listed below, but it should be noted that wetland vegetation types do not fully match this surface water salinity regime (see soil salinity and wetland habitat types discussions).
- Fresh, <0.5ppt : Lake Maurepas, western Lake Pontchartrain, and the surrounding wetlands; Caernarvon Freshwater Diversion vicinity, Mississippi River Bird’s Foot delta wetlands
- Oligohaline, 0.5 to 5 ppt: eastern Lake Pontchartrain and southern Lake Borgne; Lake Lery
- Mesohaline, 5 to 18 ppt: northern Lake Borgne, western Mississippi Sound, western Biloxi Marsh
- Polyhaline, 18 to 30 ppt: Eastern Mississippi Sound and Eastern Biloxi Marsh, Chandeleur Sound, Breton Sound
There are two major paths for salinity to move up-estuary:
A northerly route through Chandeleur Sound > Mississippi Sound > Lake Borgne > Lake Pontchartrain
A southerly route through Breton Sound > Breton Marsh.
These areas of saltwater movement result in larger surface water salinity deviations seen in 2017 (standard deviation ~ 7 ppt). The salinity incursions are seasonally moderated by either Pearl River discharge or by discharges from the Mississippi River.
To view biweekly snapshots of salinity, biological, water quality, habitat, and weather conditions of the Pontchartrain Basin, please see:
Soil Porewater Salinity
Soil salinity is only collected in wetland areas. It is not collected in water bottoms of lakes or sounds. Wetland soil salinity generally reflects an estuarine gradient similar to surface water but, in some locations, seems to have lingering effects of the prior higher surface water condition of the estuary when the MRGO was open from 1965 to 2009. The Maurepas Land Bridge has areas with soil salinity over 2.0 ppt even though surface water salinity has been less than 1.0 ppt for over two years. The west side of the Biloxi marsh has a residual high soil salinity halo. In both cases, soil salinity has decreased since 2016, and CRMS data show soil salinity decreasing since 2009 for much of the Pontchartrain Basin. Overall, mean surface water salinity is lower than mean soil salinity in 2017 for 90% of the Lake Pontchartrain Estuary.
For more information about soil porewater salinity, please email info@ScienceForOurCoast.org to obtain a copy of:
Porewater salinity analysis to determine the Mississippi River Gulf Outlet (MRGO) influence area using the Coastwide Reference Monitoring System (CRMS)
Hypoxia occurs annually in the spring and summer in Chandeleur Sound and Breton Sound. The hypoxia most likely develops due to different drivers in the two areas, and the hypoxic areas may converge at times. Both regions show stratification but often at different depths and with different salinity patterns. Chandeleur Sound hypoxia originates in the deep pass between Chandeleur Island and Ship Island. It then appears to expand westward into Mississippi Sound and southward into Chandeleur Sound. Breton Sound hypoxia seems to stem from the influx of Mississippi River water and associated nutrients in the vicinity of Ft. St. Philip, and spreads northward toward Chandeleur Sound.
For more information about hypoxia in Chandeleur and Breton Sounds, please see:
Oyster Habitat Suitability
Oyster suitability analysis (based on surface water salinity only), indicates, that post-MRGO closure, oyster propagation has improved in the Biloxi Marsh and declined south of the MRGO. Oyster fleet activity generally corresponds to the area of good oyster habitat. However, there is indication that good habitat may extend further eastward into Chandeleur Sound, an area of underutilized potential to propagate oysters or a viable target for future oyster reef restoration.
For more information about oyster habitat suitability, please email info@ScienceForOurCoast.org for a copy of:
Eastern oyster, Crassostrea virginica, habitat suitability analyses in the Pontchartrain Basin Estuary, Southeast Louisiana, in 2017
Brown & White Shrimp Fishery
The highest densities of the brown and white shrimp fleet are located on the outer fringes of the Biloxi Marsh where it meets Chandeleur Sound, outer fringes of the Breton Marsh just north of Pointe à la Hache, around the Mississippi River Delta, and in Lake Borgne. During the spring brown shrimp season (May – June) fleet activity was located in salinities between 0.5 - 16 ppt, with a majority of the activity located in waters with a salinity of 12 ppt or less. During the fall white shrimp season (August – November) the fleet activity was located in salinities between 1 - 21 ppt, with a majority of the activity in waters with a salinity 10 ppt or less. The Pontchartrain Estuary also experienced more fleet activity during the brown shrimp season.
Wetland Vegetation Change
Soil salinity isohalines from 2017 follow patterns of wetland types mapped in 2013 by the USGS. However, in 2017, several areas in the basin were exposed to soil salinity and surface water salinity that is fresher than would be expected for the wetland type that was mapped in 2013. In part, this may be explained by the time difference in data sets. Because of the MRGO closure, the soils and vegetation, which tend to change more slowly than surface water, are still transitioning towards the fresher conditions of the surface water. Along the Mississippi River, habitats are also freshening due to increase discharge from Ft. St. Philip and Mardi Gras Pass.
Swamp Restoration Suitability
Swamp restoration suitability analysis uses surface water and soil porewater salinity data from two consecutive years, and other features, to define areas that if restored through planting swamp species, the trees will likely survive long term. The Maurepas Land Bridge now has a large area that is “restoration ready”, and some small remaining areas, currently slated as areas of “future restoration” that seem to be shifting toward being suitable, indicated by decreasing soil and surface water salinities. PC and partners have successfully planted trees since 2013 in the restoration ready area here and the trees have 78% survival rate. One factor in the success of the tree plantings in Maurepas is the reduction in surface water salinity due to the MRGO closures in 2009. The other large area which is “restoration ready” is near the Caernarvon Freshwater Diversion. PC and partners have been planting trees here since 2010 and the trees have a 70% survival rate. Due to the vast areas now suitable for swamp restoration, PC is testing other restoration techniques such as aerial seeding.
For more information about swamp habitat suitability, please email info@ScienceForOurCoast.org for a copy of:
Swamp Restoration in the Vicinity of the Caernarvon Freshwater Diversion: Update 2010-2017
Swamp Restoration Planting and Monitoring on the Maurepas Land Bridge: 2013 – 2017 Update
Swamp Restoration Suitability Assessment for the Pontchartrain Basin