Mustang Island State Park Transect

The Mustang Island State Park (MISP) transect is located on the southwest part of Mustang Island (Mustang map, MISP map). This transect extends 2.2 km from the gulf beach to the Corpus Christi Bay shore. We surveyed vegetation and measured apparent conductivity at 112 locations along this transect (MISP transect data, plant list) and obtained elevations at these locations from a DEM (MISP DEM) constructed from lidar data acquired in 2003 along a swath about 350 m wide that was centered on the transect.

Wetland Units and Coastal Environments

To compare our vegetation, elevation, and conductivity results with existing habitat data, we used a geographic information system to extract transect locations that occur within units mapped on the 1992 NWI (NWI classification, MISP NWI stats). More than half of the transect locations are within upland (U) or palustrine (PEM1A or PEM1C) mapped units; the remainder are within either estuarine- (E2EM1P, E2EM1N, or E2USP) or marine-influenced (M2USN) units.

Boundaries between units on the NWI maps correspond reasonably well to tonal boundaries on the aerial photograph (MISP NWI map) and elevation changes as depicted on the DEM (MISP NWI DEM), but the units may or may not be classified accurately. Direct field observations made during this project allow greater detail and accuracy in establishing the appropriate coastal environment for a given location than is achievable on the smaller-scale, aerial photograph-based NWI maps. Comparisons between the two habitat assignments show that mapped NWI units may encompass several distinct coastal environments. For example, the palustrine unit PEM1A, classified as temporarily flooded emergent persistent wetland, is mapped where ground surveys identified dunes, VBF, standing fresh water, high and low fresh marsh, and high salt marsh (MISP PEM1A). The estuarine unit E2USP, classified as irregularly flooded intertidal unconsolidated shore, is mapped where ground surveys identified low dunes and high and low wind-tidal flats (MISP E2USP). The non-wetland U (upland) category includes locations categorized as dune, VBF, low fresh marsh, beach, and standing fresh water in the ground-based survey (MISP U).

Elevation and Vegetation Units

Lidar-derived elevations at the 112 locations along the MISP transect range from 0.01 to 5.5 m NAVD (MISP DEM, MISP elevation, MISP transect). Highest elevations (2 m or more) were measured across the fore-island dunes within about 300 m of the gulf shoreline and mid-island dunes between about 800 and 1500 m from the gulf shoreline. Lowest elevations (0.3 m or less) were found bayward of the mid-island dunes to the bay shoreline.

At a third of the locations (38 of 112), vegetation was sufficiently dense to question whether the lidar-derived elevation represented the ground surface or the top of the vegetation mass. At these locations, measured height of massed vegetation averaged 0.5 m, ranging from 0.1 to 1.4 m (MISP transect). If known, these heights can be subtracted from the lidar-derived elevation profile to produce a corrected ground-surface elevation profile (MISP elevation), assuming lidar was unable to penetrate the vegetation at these locations. In densely vegetated areas, vegetation mass heights might cause significant overestimation of land-surface elevation and potential misclassification of environments.

Regardless of the accuracy of the 1992 NWI maps in correctly identifying habitats, transect locations with the highest elevations generally correlated with upland or high palustrine units and locations with the lowest elevations generally coincided with estuarine units (MISP elevation NWIa, MISP elevation NWIb). Average elevation was highest (2.6 m) for the 25 locations classified as U (MISP NWI stats), but elevation for this unit ranged from 0.5 to 5.5 and overlapped with elevation ranges for other mapped units. Unit PEM1A, the topographically highest of the mapped palustrine units, had the next highest average elevation (1.1 m). Unit PEM1C, considered topographically lower than PEM1A, had a slightly lower average elevation (0.9 m). Estuarine units E2EM1P, E2EM1N, and E2USP have similar average elevations (0.19 to 0.26 m) that are considerably lower than those for the upland and palustrine units. Upper and lower elevation limits for the mapped upland and palustrine units overlap, as do ranges for the estuarine units. There is a distinct difference in average elevation (and little overlap in elevation range) between the palustrine and estuarine units.

During the ground-based survey along the MISP transect, we classified each of the 112 locations into one of nine coastal environments based on vegetation (MISP env stats). Most common were dune, VBF, and low and high wind-tidal flat, which together account for 85 of the 112 locations. These ground-based surveys produced greater vegetation classification detail than that shown on the NWI maps, as well as one that is more representative of the variability evident from the topographic profile (MISP elevation env a, MISP elevation env b). The dune environment has the highest average elevation (2.6 m) as well as the largest elevation range (0.8 to 5.5 m), overlapping at the low end with the VBF, fresh marsh, and beach environments. Relatively high elevation averages are associated with VBF (1.3 m), high fresh marsh (0.86 m), low fresh marsh (0.77 m) and beach (0.79 m) environments, which all have some degree of overlap in elevation ranges. Distinctly lower elevation averages are associated with high (0.29 m) and low (0.17 m) salt marsh and high (0.23 m) and low (0.2 m) wind-tidal flat environments. Elevation ranges for these environments overlap with each other, but not with fresh marsh, VBF, or dune environments.

Conductivity and Vegetation Units

Apparent ground conductivities measured along the MISP transect vary over more than three orders of magnitude, ranging from very resistive ground at a few mS/m to relatively conductive ground at more than 2,000 mS/m (MISP conductivity). Conductivities measured at the 112 locations along the transect in the shallow-exploring hd orientation are similar to, but generally lower than, conductivities measured in the deeper-exploring vd orientation. Highest apparent conductivities (greater than 100 mS/m) were measured within a few tens of meters of the gulf shoreline, along two mid-island segments about 750 and 1000 m from the gulf shoreline, and along a long segment from the bay shoreline to a point about 700 m inland. Lowest apparent conductivities (about 10 mS/m or less) were measured between 50 and 250 m inland from the gulf shoreline and along two mid-island segments about 850 and 1200 m inland from the gulf shoreline.

Measured apparent conductivities correlate reasonably well spatially with mapped NWI units (MISP conductivity NWIa, MISP conductivity NWIb). Upland (U) and high palustrine (PEM1A) units tend to occur where apparent conductivities are low (less than about 100 mS/m), whereas lower palustrine (PEM1C), estuarine (E2EM1P, E2EM1N, and E2USP), and marine (M2USN) units have been mapped where apparent conductivities are relatively high (greater than 100 mS/m). Among the more conductive NWI units, average apparent conductivities measured in the vd mode (MISP NWI stats) are highest for the topographically lowest estuarine unit (1467 mS/m for E2USP), decrease slightly for the next lowest estuarine unit (1318 mS/m for E2EM1N), and decrease again for the highest of the mapped estuarine units (1254 mS/m for E2EM1P). There is considerable overlap in measured apparent conductivities for these units. The marine-influenced unit (M2USN) averaged 515 mS/m, followed by the lowest palustrine unit (PEM1C) at 266 mS/m. There is no overlap between conductivities measured for these units and those measured for the more conductive estuarine units. Among the relatively nonconductive NWI units, the lowest average conductivity (26 mS/m) is associated with locations within areas mapped as upland (U). Slightly higher average conductivity (94 mS/m) is associated with the highest palustrine unit (PEM1A). The conductivity range measured for locations within U units overlapped with ranges measured for locations within palustrine units, but not with marine or estuarine units.

Coastal environments surveyed along the MISP transect also correlate well with measured apparent conductivity (MISP env stats). Highest apparent conductivities measured in the vd orientation occur in beach, low fresh marsh, low and high salt marsh, and low and high wind-tidal flat environments (MISP conductivity env a, MISP conductivity env b). Lowest apparent conductivities occur in dune, VBF, and low and high fresh marsh environments. Locations within dune environments have the lowest average conductivity (59 mS/m, vd orientation), but their measured range extends above the average values observed for low (242 mS/m) and high (145 mS/m) fresh marshes. Low average conductivities (76 mS/m, vd orientation) are also found in VBF environments. Gulf beach and bay berm environments have higher average apparent conductivities (578 mS/m) than are found in dune and fresh marsh environments. Salt marsh and wind-tidal flats have the highest apparent conductivities; each environment averages more than 1000 mS/m (vd orientation). There is an increase in average apparent conductivity from high (1175 mS/m, vd orientation) to low (1223 mS/m) salt marsh and from low (1397 mS/m) to high wind-tidal flat (1489 mS/m). Ranges of measured conductivities overlap for the salt marsh and wind-tidal flats and for the dunes, VBFs, and fresh marshes, but there is little or no overlap in observed conductivity range between these two groups of relatively saline and non-saline environments.

Elevation, Conductivity, and Vegetation Units

In general, elevation and apparent conductivity vary inversely along the MISP profile (MISP elev cond), reflecting the strong inverse correlation between elevation and salinity in coastal environments. As elevation decreases, the frequency of flooding by saline water increases. At higher elevations, infrequent saline flooding, infiltrating fresh precipitation, and relatively dry soil combine to produce less electrically conductive soil. Conductivity values show a greater range of variation than do elevations, but both types of data vary significantly across the island.

By combining elevation and apparent conductivity, we can attempt to better discriminate NWI and coastal environment units that may have overlapping elevation or conductivity ranges. For example, locations within the upland (U) NWI unit generally have both low apparent conductivities and high elevations, whereas the highest palustrine unit (PEM1A) generally has lower elevations and higher conductivities (MISP elev cond NWI). High and low palustrine units PEM1A and PEM1C have minor differences in elevation but more distinct differences in apparent conductivity. All estuarine and marine units have both very low elevations and very high apparent conductivities.

Similarly, dune environments have high and highly variable elevations, but have low conductivities that vary over a relatively small range (MISP elev cond env). VBF environments generally have lower elevations than dune environments and higher and more variable conductivity values. High fresh marshes have elevations that are indistinguishable from VBF environments, but have apparent conductivities that tend to be higher than those observed in VBF environments. Salt marsh and wind-tidal flat environments all have very low elevations and very high apparent conductivities.