Mobile County Health Department
Coastal Texas Water Resources
Rooting Out the Problem: Mesquite Trees' Effect on Estuary Health

>> Ryan: The Laguna Madre of the Padre Island National Seashore is located on the southern coast of Texas between Corpus Christi and the border of Mexico. It is the only hypersaline estuary in North America; this means that its water is saltier than the ocean.However, the lagoon has not alway been this way.The Coastal Texas Water Resources team at DEVELOP’s Mobile County Health Department node partnered with the National Park Service to investigate this change.

>> Joe Meiman: My name's Joe Meiman. For the past twenty-five years, I've been a hydrologist for the National Park Service.

>> Georgina: The sustainability of the Laguna Madre ecosystem is important from both an ecological and economic standpoint.The estuary is an important spawning ground and habitat for many fish, hundreds of native birds, and thousands of migratory birds. In addition, 650,000 people visit the park annually for recreation. The Laguna is also   one of the most productive fisheries in Texas. Our partner, Joe Meiman, explains some of the effects of the increased salinity in the lagoon.

>> Joe Meiman: Without the seagrasses and the water quality necessary to support healthy seagrass growth, the entire  functionality of the estuary will be vastly diminished.

>> Elaina: It is hypothesized that the increase in salinity is due to an increase in the occurrence of the native Prosopis glandulosa, commonly known as honey mesquite tree. The long tap roots of these trees, commonly extending 12 meters in depth, can reach groundwater, while the shallower roots of nearby grasses cannot, as our partner describes.

>> Joe Meiman: So imagine, tens, hundreds of thousands of mesquite trees acting as individual wells all tapping into this surficial groundwater.

>> Elaina: Because of their longer taproots, the mesquites are also able to evapotranspire during droughts.

>> Joe Meiman: So in times of drought the grasses shut down and mesquite trees continue to tap into the groundwater, continue to withdraw that groundwater, and thus reduce any potential inflows of groundwater into the Laguna Madre. 

>> Rodrigo: To examine the relationship between the mesquite trees and the lagoon, the team analyzed land cover, water, and thermal data.  A land cover analysis was done using Landsat 5, 7, and 8 data to examine mesquite tree coverage. Land cover maps were created with fall data every four years from 2002 to 2014. Our analysis showed that the number of trees increased, primarily replacing grassland.The Normalized Difference Infrared Index (NDII) was calculated to assess the stress levels of the vegetation by mapping the leaves’ water content. The Mesquite trees appear less stressed than the other vegetation. Lower stress is shown in green, and matches where the trees were found. 

>> Elaina: To analyze the relationship between precipitation and root zone soil moisture content (RZSMC) the team looked at monthly data derived from the TRMM  and GRACE satellites. RZSMC is the water in the soil which is available to plant roots. Monthly averages and anomalies from 2003 through 2014 were compared.The correlation coefficient between the monthly anomalies of these datasets was moderately strong.

>> Tyler: Thermal data were obtained from  Landsat 5, 7, and 8 for winter months between 2000 and 2015 to identify ‘hot spots’ of warm, fresh groundwater inflow into the cool, salty lagoon. To do this thermal bands were converted to  surface temperature.The raw data were first converted to Radiance, atmospherically corrected. The data were then converted to Kelvins and finally to Celsius. Although no “hot spots” were clearly identified, the team is continuing to analyze the inflow of groundwater into the lagoon.

>> Georgina: NASA Earth observations can help create a systematic method for studying the relationship between the mesquite trees and the salinity of the lagoon.

>> Joe Meiman: NASA Earth observations, and particularly the DEVELOP team I'm working with, have opened my eyes to a whole new realm of possibilities.

>> Ryan: By better understanding the effects of mesquite trees on the lagoon, this research can inform land management practices to help preserve estuary health.