Port Aransas Transect

The Port Aransas (PA) transect is located on the northeast part of Mustang Island (Mustang map, PA map). This transect extends 2.4 km from the gulf beach to the modified and frequently flooded wetlands several hundred meters inland from Corpus Christi Bay. We surveyed vegetation and measured apparent conductivity at 122 locations along this transect (PA transect, plant list) and obtained elevations at these locations from a DEM (PA 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

We used a geographic information system to extract transect locations that are within units mapped on the 1992 NWI (NWI classification, PA NWI stats) to compare vegetation, elevation, and conductivity data with existing habitat data. In contrast with the relatively undisturbed MISP transect, segments of the PA transect have been altered by human activities such as residential development, sewage disposal, channel dredging and modification, and dredge material disposal. More than half (66 of 122) of the locations on the PA transect are classified in the 1992 NWI as estuarine (E1UBL, E2AB1P, E2USN, E2USP, E2EM1N, or E2EM1P); slightly less than half (51) are classified as upland (U). The remaining few locations are mapped either as marine (M2USN or M2USP at three locations near the gulf shoreline) or palustrine (PEM1C at two locations bayward of the dunes).

Because NWI maps are largely based on aerial photographic interpretation, extents of units depicted on the NWI maps correspond reasonably well to tonal boundaries on the aerial photograph of the PA transect area (PA NWI map). Elevation changes depicted on the detailed digital elevation model constructed from the lidar data also correlate well with NWI boundaries, but there are significant elevation changes in areas where a single NWI unit is mapped (PA NWI DEM). Habitat boundaries are commonly clearly expressed and readily mapped on aerial photographs, but the habitat type may be difficult to distinguish from aerial photographic expression alone.

Field observations allow greater detail and accuracy in establishing the appropriate coastal environment for a given location. As was also true for the MISP transect, comparing mapped NWI habitats and field-determined coastal environments shows that NWI units commonly include several coastal environments. For example, ground-based observations reveal that 32 out of 51 locations mapped within the 1992 NWI upland unit (U) are classified as VBF and 14 locations are classified as dune environments, both of which are common in uplands. The interpreted coastal environments at the remaining 5 transect locations within the upland unit are beach, low and high fresh marsh, and high salt marsh. At a more detailed mapping scale, these environments would not be classified as upland. The second-most common NWI unit along the PA transect is the irregularly flooded estuarine intertidal wetland (E2EM1P). The most common coastal environment observed within this mapped unit is VBF (17 out of 30 locations), which is generally considered to be an upland environment. Other environments interpreted within the 1992 E2EM1P habitat on the PA transect are high salt marsh (10 locations) and wind-tidal flat (3 locations).

Elevation and Vegetation Units

Lidar-derived elevations at the 122 locations along the PA transect range from -0.1 to 7.5 m NAVD (PA elevation, PA DEM). Highest elevations (2 m or more) were measured across the fore-island dunes within about 300 m of the gulf shoreline. Elevations within about 1000 m of the gulf shoreline are generally above 1 m. Farther bayward, elevations are below 1 m except along short segments where elevation exceeds 1 m at distances of about 1.2, 1.7, and 2 km from the gulf shoreline.

The height of massed vegetation represents potential land-surface elevation error if the lidar pulse does not penetrate the dense vegetation. Measured height of dense vegetation averaged 0.5 m, ranging from 0.1 to 1 m at 72 locations along the PA transect. If we assume that the lidar-derived elevation represents the top of dense vegetation at these sites, we can subtract the vegetation height to produce a corrected land-surface elevation (PA elevation). The largest corrections occur at high elevations (upland, dune, and VBF habitats) where plant stature and density is commonly greater than in lower environments.

The PA transect elevation profile (uncorrected for vegetation height) correlates reasonably well with 1992 NWI units (PA elevation NWIa, PA elevation NWIb). Highest elevations on the profile coincide with areas mapped as upland (U). Marine NWI units are found at low elevations at the gulf shoreline. Locations with the lowest elevations are within an area mapped as irregularly flooded estuarine intertidal aquatic bed (E2AB1P) at a mid-island topographic low between about 1.3 and 1.5 m from the gulf shore.

Locations within the upland unit (U) have the highest average elevation (2.1 m) of all NWI units mapped along the PA transect (PA NWI stats). These 51 locations range widely from 0.4 to 7.5 m in elevation, a range that overlaps at the low end with several palustrine, estuarine, and marine wetland units. Next highest is the seasonally flooded palustrine unit (PEM1C) at 1.3 m, followed by the irregularly flooded estuarine unit E2EM1P and irregularly flooded marine unit M2USP at about 0.9 m and the regularly flooded marine unit M2USN at 0.6 m. Average elevations for the remaining estuarine units are 0.3 m or lower. Among locations within similar NWI units (E2USP and E2USN, for example), increasing flooding frequency correlates with decreasing average elevation.

Coastal environments along the PA transect identified during the field investigation have a level of detail that is greater than that shown on NWI maps and more closely matches topographic detail obtained from lidar (PA elevation env a, PA elevation env b). A strong spatial correlation between elevation and coastal environment is evident. Dunes and VBFs are found where elevations are relatively high, VBFs and fresh marshes occur at intermediate elevations, and salt marshes and wind-tidal flats occupy the lowest island elevations. Average elevation at locations classified as dune is 3.0 m, the highest of all coastal environments along the transect (PA env stats). Elevation for the one low fresh marsh location is 2.0 m, significantly higher than the 1.1 m average elevation of the three high fresh marsh locations. Measured vegetation heights of about 2 m at the low fresh marsh location suggest that the lidar elevation does not represent the ground surface. Average elevation at high salt marsh locations is 0.5 m, distinctly higher than the low salt marsh average of 0.1 m and the wind-tidal flat averages of 0.1 m for high and 0.0 m for low wind-tidal flats. Elevation ranges overlap significantly for the low salt marsh and wind-tidal flat environments. High salt marshes have a distinct elevation range with little overlap with higher or lower environments. Fresh-marsh elevations can overlap with elevations measured for VBF and dune environments.

Conductivity and Vegetation Units

Apparent ground conductivities measured along the PA transect vary over more than three orders of magnitude, from nonconductive values near 1 mS/m to relatively conductive values of more than 2,000 mS/m (PA conductivity). Values measured in the shallower-exploring horizontal dipole orientation are very similar to values measured in the deeper-exploring vertical dipole orientation, but are slightly lower at most locations. Highest apparent conductivities were measured within 100 m of the gulf shoreline and along a broad zone extending 1.4 km toward the gulf from the bayward end of the transect. Relatively low conductivities (about 100 mS/m or less) were measured between 100 and 1000 m from the gulf shore and along three short segments at about 1.2, 1.7, and 2 km inland from the gulf shoreline.

With a notable exception, general trends on the conductivity profile along the PA transect correlate reasonably well with 1992 NWI units (PA conductivity NWIa, PA conductivity NWIb). At several locations, small-scale NWI map units encompass zones having systematic variations in apparent conductivity that suggest more than one habitat type may be present. Lowest conductivities (200 mS/m or less) are measured within the mapped upland (U) unit; elevated conductivities coincide with higher salinity estuarine and marine NWI units. A small palustrine unit about 500 m from the gulf shoreline coincides with measured conductivities higher than those at locations within the adjacent upland unit. Between about 800 and 1000 m from the gulf shoreline, however, the mapped estuarine unit E2EM1P contains transect locations with relatively low conductivities that would better match those of upland units. The elevations (1 to 2 m, PA elevation env a), conductivities (PA conductivity env a) and coastal environment (VBF) identified along this stretch suggest that this segment should be mapped as an upland rather than an estuarine unit.

The greater resolution of field-based coastal environment classification more closely matches the lateral resolution achievable with ground-based conductivity measurements (PA conductivity env a, PA conductivity env b). Further, there are few if any discrepancies between identified coastal environment and measured conductivity. Elevated conductivities coincide with beach, wind-tidal flat, and salt marsh environments; low conductivities coincide with dune, VBF, and fresh marsh environments. Lowest conductivities are measured at transect locations on high fore-island dunes and elevated VBF environments. Less well-developed dunes at lower elevations have conductivities that approach, but are lower than, those of surrounding salt marshes and wind-tidal flats.

Statistically, conductivities measured for the saline environments (salt marsh, wind-tidal flat, and forebeach) are very high and distinctly higher than those measured for dune, VBF, and fresh marsh environments (PA env stats). Highest average conductivities were measured on wind-tidal flats; high wind-tidal flats have a slightly higher average conductivity (1762 mS/m in the vertical dipole orientation) than do low wind-tidal flats (1643 mS/m), perhaps due to greater evaporative concentration of dissolved minerals at slightly higher elevation. Average conductivities in salt marshes remain very high (1285 mS/m in low salt marshes and 797 in high salt marshes), but are below those measured in wind-tidal flats. Average conductivities in dunes, VBF, and fresh marshes are about 100 mS/m or less, significantly below those measured in more saline environments. There is some overlap in conductivities at the upper end of the salt marsh range and the lower end of the wind-tidal flat range, as well as significant overlap among conductivity measurements in the dune, VBF, and fresh marsh environments.

Elevation, Conductivity, and Vegetation Units

There is a strong inverse correlation between elevation and apparent conductivity measured along the PA transect (PA elev cond). Highest conductivities are measured where elevations are low near the gulf shoreline and in the back-island environments. Lowest conductivities are measured where higher ground is rarely flooded by saline waters, including upland (dune and VBF) environments. Relatively minor elevation increases in dominantly saline environments are accompanied by strong local decreases in conductivity.

Comparisons of elevation and conductivity measurements made for NWI units mapped at a small scale (low detail) and more detailed coastal barrier environments show that many of the habitats are statistically distinct but have ranges that overlap to varying extents. Taken together, elevation and conductivity measurements for a given location can be used to better discriminate among wetland habitat and coastal environments. For the PA transect, for example, apparent conductivities below about 15 mS/m are indicative of environments mapped as upland (U) on the 1992 NWI, as are elevations above 2 m (PA elev cond NWI). NWI upland and estuarine (E2EM1P) units have similar elevation and conductivity values at lower elevations and higher conductivities, but field investigations suggest that many of the locations classified as E2EM1P are actually within the VBF environment and might be more accurately classified as upland (PA elev cond NWI, PA elev cond env). NWI units with the lowest elevations (less than 0.1 m) and highest conductivities (greater than 1000 mS/m) along the PA transect include the estuarine units E1UBL, E2AB1P, and E2USN (fig. 37a and table 4). Other marine (M2USN and M2USP) and estuarine (E2USP and E2EM1N) NWI units are slightly higher and less conductive.

Among the coastal environments identified along the PA transect, the dune and VBF environments have relatively large ranges in elevation and the lowest conductivities (PA elev cond env, PA env stats). Low elevations, limited elevation ranges, and very high conductivities are measured in the most saline environments such as low salt marsh and low and high wind-tidal flat. Slightly higher elevations and slightly lower conductivities are typical of high salt marshes. Beach and berm environments have elevations and conductivities that are similar to those of lower VBFs.