While true seagrasses (submerged marine angiosperms) disappeared from the mainstem Galveston Bay system by the late 1970's (Pulich and White 1991), a remnant population has survived in the secondary bay system, Christmas and Drum Bays, in the extreme southwestern bay region adjacent to Follet's Island. Because of its very sensitive seagrass habitat, the Christmas Bay system has been a priority conservation site for State resource management programs. In 1988, this area was designated an official State Coastal Preserve and State Scientific Area by the Texas General Land Office (TGLO) and Texas Parks and Wildlife Department (TPWD), the two State agencies with primary coastal management authority for coastal wetlands (TPWD 1996). The Galveston Bay Estuary Program (GBEP) also has worked through these agencies to develop protective strategies focused on the pristine Christmas Bay system. Water quality protection and monitoring of discharges of pollutants and wastewater, shoreline erosion control, and status and trends inventories of critical biological resources have been proposed (GBNEP 1995) for the system in the Galveston Bay Plan. This report summarizes a recent mapping project to determine the current status of Christmas Bay seagrasses that was undertaken as part of an EPA wetlands management grant, X986174-01-2 to GBEP.Methods.
The mapping work followed methods described by Pulich et al. (1997) and the NOAA Coastal Services Center (NOAA CSC, 2001). The protocol used was based on high resolution, large format (9” x 9”), color aerial photography (Ferguson et al. 1993) taken in late fall at the end of the seagrass growing season in Texas. TPWD staff in the Resource Protection Division, Coastal Studies Program, Austin, supervised and coordinated all phases of the project, and performed the photography analysis and thematic mapping.
A commercial aerial photography company, Aerial Viewpoint, Inc. of Houston, TX was contracted by the GBEP to fly the mission for Christmas/Drum Bays. Although the mission was planned for late October – early November 1998, a very rainy fall season with lots of fronts and tropical weather systems prevented the mission from being flown as scheduled. High resolution aerial photography (using true color Kodak SO358 film, flight scale 1:10,000) was finally acquired on December 15, 1998, a clear, cool day after a cold front had pushed through Texas. Groundtruthing surveys supported by differential GPS technology were performed the next summer (Sept.) and fall (Oct.) of 1999 by TPWD, Coastal Studies Program, with assistance of GBEP staff.
Analysis of the photography included scan-digitizing 9” x 9” positive transparencies using a flat-bed scanner at a resolution of 1 meter per pixel; and georegistering the raster images to 1:24,000 scale USGS Digital Ortho Quads of the area at 1m resolution. Seagrass polygon boundaries were then photointerpreted and delineated off the images in ArcView Image AnalystTM software, using “on-screen” digitization techniques, and the datasets entered into the GIS (Pulich 2000, unpubl.). Generic seagrass polygon distributions only were classified except for discrete Thalassia patches. Approximately 75 GPS point samples were taken during field surveys to provide species identification and other field data. The original film photography is archived at GBEP headquarters in Clear Lake.Mapping Results.
Figure 1 shows the 1998-99 seagrass distribution overlaid onto the mosaicked, digitized photoimages of the Christmas Bay system. Total seagrass coverage in Christmas and Drum Bays amounted to 172 ha (424 acres), consisting predominately of mixed shoalgrass (Halodule wrightii) and clovergrass (Halophila engelmanni) beds, with widgeongrass (Ruppia maritima) abundant during spring and fall. However these shoalgrass-dominated beds are also interspersed with about 20 patches of Thalassia testudinum along the south shore, totaling 1.6 acres. This represents the northernmost location for Thalassia on the Texas coast, somewhat of a distribution anomaly, since the next-closest known population is 150 miles to the south in Aransas Bay near Rockport. No attempt has previously been made to quantify acreage of Thalassia in this system.
Previous mapping from 1975 and 1987 color infrared aerial photography by Pulich and White (1991) resulted in photodelineation of generic seagrass coverage of 97 ha (247 acres) in 1975 and 77 ha (190 acres) in 1987. White et al. (1993) later examined some 1950's black and white photography and estimated about 130 ha (300 acres) in Christmas Bay in 1956. This seems to suggest that Christmas Bay grassbeds also may have declined during the 1960's through the 80's period, although not as precipitously as in West Galveston Bay. Based on these recent 1998 mapping results (172 ha of seagrass), this trend appears to be reversing.
However, the size of the increase in seagrass acreage between 1988 and 1999 (as well as the decrease between 1975 and 1987) is probably misleading because reexamination of the 1987 photography reveals that some seagrass was probably missed during this previous mapping due to turbid, high-tide water and lack of resolution in the high altitude 1987 photos. The study by White et al. (1993) corroborates this idea by examination of 1989 color-IR aerial photography. They re-mapped Christmas Bay seagrass from this photography and determined about 156 ha (385 acres) of seagrass were present then. The 1989 photographs were taken at lower tide (December) than the 1987 photographs (November) from two years earlier, and seagrasses were probably more clearly visible.Conclusions.
These seagrass trends appear consistent with the relative protection of Christmas Bay from anthropogenic factors (e.g. absence of major dredging and waterfront development in the bay; lack of point source discharges; and generally low NPS runoff). Along with recent seagrass increases in the West Galveston Bay area, the status of Christmas Bay seagrasses may reflect the benefits of renewed efforts to manage and protect these sensitive estuarine resources under State and federal coastal zone programs. Regular field monitoring, combined with aerial photography surveys repeated every 2 – 5 years, will provide the necessary data to assess seagrass management and restoration programs in this coastal preserve system. Directed conservation efforts should also be taken to maintain, and even expand, the limited amount of Thalassia by protecting these patches from physical disturbances (e.g. boat propeller scarring and trampling by wade fishermen) using markers and signage.
Ferguson, R. L., L. L. Wood, and D. B. Graham. 1993. Monitoring spatial change in seagrass habitat with aerial photography. Photogrammetric Engineering & Remote Sensing 59(6)1033-1038.
Galveston Bay National Estuary Program. 1995. The Galveston Bay Plan. Publication GBNEP-49. Galveston Bay National Estuary Program, Webster, Texas.
U.S. NOAA Coastal Services Center. 2001. Guidance for Benthic Habitat Mapping: An Aerial Photographic Approach (by Mark Finkbeiner, Bill Stevenson, and Renee Seaman, NOAA-CSC, Charleston, SC). NOAA/CSC/20117-PUB. 73 pp.
Pulich, W., Jr. and W. White. 1991. Decline of submerged vegetation in the Galveston Bay system: chronology and relationship to physical processes. Journal of Coastal Research 7 (4): 1125-1138.
Pulich, W., Jr., C. Blair, and W.A. White. 1997. Current status and historical trends of seagrass in the Corpus Christi Bay National Estuary Program study area. Publication CCBNEP-20 ( Texas Natural Resource Conservation Commission, Austin , TX). 131 pp.
White, W.A., T. A. Trembley,, E. J. Wermund, Jr., and L. R. Handley. 1993. Trends and status of wetland and aquatic habitats in the Galveston Bay System, Texas. Publication GBNEP- 31. Galveston Bay National Estuary Program, Clear Lake, Texas. 135 pp. + appendices
Texas Parks and Wildlife Department. 1996. Management Plan for Christmas Bay Coastal Preserve. Resource Protection Division, Texas Parks and Wildlife Dept., Austin, Texas. 42 p.