Mapping Intertidal Oyster Reef in Galveston Bay
This project was funded by the Texas Commission on Environmental Quality and the United States Environmental Protection Agency
Oyster Life Cycle Unknown author, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
What is an oyster?
Oysters are mollusks with a two part hard shell to protect them from predators. The species found in Galveston Bay are Crassostrea virginica , or the eastern oyster. Oysters are an incredibly important part of Galveston Bay. They are filter feeders, and eat by filtering algae from the water. This removes excess nutrients and improves water quality. Check out this video from the Cornell Cooperative Extension Program showing just how much a single oyster can do to clean water!
Oysters begin life as tiny larvae swimming through the water. They soon settle down and attach themselves to a solid surface (like rocks, old shell, piers...) where they will grow for the rest of their lives.
As more and more oysters grow in a location, they begin to form intricate reefs. These reefs are vital to Galveston Bay ecosystems, creating habitat for hundreds of other marine species.
Shorebirds like these Semipalmated Plover depend on intertidal reef habitat for food and roosting sites. (Alan Wilde)
What is an intertidal reef?
Intertidal oyster reefs are the dominant form of oyster reefs along the southeastern Atlantic coast of the United States, and in the Gulf of Mexico. During various parts of the day, these oysters may be underwater, or high and dry, depending on the tide . Ocean life benefits from increased oxygen levels and food sources brought in from deeper areas during high tides. High tides also bring fish searching for their prey as invertebrates emerge from the sand. Intertidal beaches supply food and habitat for both ocean and land animals.
Intertidal oyster reef provides vital habitat for multiple members of the estuarine food web. Reefs act as habitat for oyster and other shellfish, provide refuge for an immense variety of young invertebrates and fish species during development, provide wavebreak duties for nearby islands and coastlines, are a vital source of food for higher vertebrates, and act as predator free areas for roosting birds.
Men shucking oysters in a 1910 Galveston postcard. (Galveston and Texas History Center - Rosenberg Library)
Oysters in Galveston Bay
Oysters are a vital part of Galveston Bay’s ecosystem and economy. Shell middens around the bay suggest the importance of shellfish to the Native Americans that lived in the area prior to European settlement. Shell was mined from these middens and the bottom of the bay to build roads and railroad right of ways, as the coastal prairie had no rock and gravel for paving.
Local economies still depend on natural resources from Galveston Bay, with shrimp, fish and oyster catches providing jobs and area income.
How do pollutants get into the bay? A watershed is the area of land that drains to a common outlet (usually a stream or river). The watershed is usually named for the river that drains it. Pollutants can flow into major rivers from their watershed, eventually draining to Galveston Bay and the Gulf. What watershed do you live in? Click the map to expand and explore!
Oyster reef has decreased dramatically in Galveston Bay over the past several hundred years. Dredging for shell for construction and addition of navigational waterways like the Houston Ship Channel have altered the bay bottom and hydrology. Numerous chemical contaminants, including heavy metals (Hg, Pb, Ni, Cu, Zn), hydrocarbons, and pesticides, have been introduced to the water column through point and non-point pollution pathways, as well as atmospheric fallout into the bay since the early 1900s.
Hurricane Ike covered about 60 percent of the oyster reefs in Galveston Bay with sediment and completely changed the distribution of this organism. Reef restoration projects have been ongoing since the 1980s, and multiple agencies are working in the area to restore former oyster habitat.
Kori Lugar sampling invertebrates on a reef. (Alan Wilde)
Project Goals
- Map possible intertidal reef locations using aerial photos, then "groundtruth" or visit a subgroup of the possible locations to check our accuracy.
- Determine the relationships between bird use of intertidal reef and the invertebrates that live in a location.
- Examine the health of oysters at locations around the study area to predict how bay-wide populations are faring.
Project Results
Click through the maps below to explore our results! On any map you can zoom in and out, click on an item for more information, or grab and drag the map to move around.
First we used U.S. Geological Survey (USGS) Quarter Quadrants (QQs) to establish and divide up the project area. Most USGS maps divide the United States into quadrangles bounded by two lines of latitude and two lines of longitude. For example, a 7.5-minute map shows an area that spans 7.5 minutes of latitude and 7.5 minutes of longitude, and it is usually named after the most prominent feature in the quadrangle. Quadrangles can then be divided into quarters to cover more specific areas. Most GIS data is available in this form, so we chose QQs to define our study area. Click on each red rectangle to view the information for that QQ. Learn more about the USGS mapping system and order your own maps here.
Our study area ranged from Dickinson Bay down to Tiki Island, across all of West Bay, and into Christmas and Drum Bay. We found nearly 818,128 square meters (over 200 acres) of intertidal reef across the study area. This map shows the ground truthing sites we visited to check our predictions (red, orange, blue, and purple circles), as well as likely intertidal oyster reef (red and orange squiggly lines and dots). Red and orange circles mean that intertidal oyster reef was found (red = live oysters, orange = oyster shells). Blue and purple circles mean that intertidal oyster reef was not found (blue = other habitat type such as rock or mudflat, purple = underwater structure). Click on the map and zoom in to see where we went, and access a legend of the symbols by clicking the blue circle with dots and lines in the map. In what quarter quadrants did we not find any reef?
Ground truthing was performed at random locations for each quarter quadrant during low tide periods. Ten random points were provided for each quarter quadrant and loaded onto a field tablet. In a few cases, there was only a small portion of the quarter quadrant that contained bay water and those quarter quadrants were assigned less than 10 random points. We visited the points in order using a field tablet to guide us to each point until we reached one that was suitable for oyster sampling. Once we found a site suitable for oyster sampling, we did not visit the rest of the random points in that quarter quadrant. If we visited all ten points and found no sites suitable for oyster sampling, we moved on to the next quarter quadrant. For each point visited, we made notes as to what we found.
Shorebirds roosting (resting) on reef. (Alan Wilde)
During ground truthing, when we arrived at a random point location that had suitable oysters for sampling, we took an avian survey prior to landing on the reef on the downwind side. Birds were viewed through binoculars from the boat. Data taken included a count of individuals of each species and observed behavior of each individual. Behaviors included loafing, feeding, roosting, preening, bathing and other.
Sometimes in science, a hypothesis is not proven correct and researchers have to go back and design a new study. We found no correlation between bird diversity and benthic marcofauna (invertebrates living underwater on the reef) diversity, suggesting the birds are not mainly utilizing this habitat as foraging habitat. This was true for foraging species as well as species engaging in self-maintenance like roosting and preening. By design, most of the surveys for this study were performed when the tide was very low and the Gulf Coast Bird Observatory has found that birds do not utilize reef habitats much when the tide is this low. Our assumption is that when the tide is that low, there are additional habitats exposed such as mudflat that may provide better food resources.
We found few birds using the reef during low tides. We hypothesize that when the water is low, they forage over lower lying areas (like mudflats) that may regularly be underwater. (Alan Wilde)
The pH of Galveston Bay changes over time depending on how much fresh water is flowing into the bay, how much carbon dioxide is absorbed by the water, what types of pollutants are present, etc. The map above shows pH estimates from two different time periods generalized from water quality testing points. On the left, you can see the average pH from the years 1993-1995, and on the right is 2012-2014. Darker brown areas are more basic on the pH scale, and the lighter colors are more acidic. You can click on the map to see what exact pH values the color represents in a popup. These time periods most closely align with the reef maps pictured. Purple spots on the map show reef identified in 1995. Reef in red or orange shows reef mapped in this project. Slide the arrows back and forth to compare reef coverage from the 1990s to present day. How has it changed?
Oyster reef sampled in this project. Notice that some reef is above water, while much is below. (Amanda Hackney)
When we visited a location that had reef, we surveyed to determine reef structure, oyster population characteristics, and associated benthic macrofauna. We gathered data on percent cover of oysters, percentage of live oysters, reef height, and reef rugosity (a measure of surface complexity). Healthy reefs have lots of nooks and crannies, which shelter oyster larvae and other small animals from currents and predators, while generating turbulence that helps keep sediment off the reef surface.
Oysters in East Matagorda Bay. (Alan Wilde)
For many aquatic organisms, pH is an important water quality measure. When the Industrial Revolution began, humans started mining and burning fossil fuels. Burning fossil fuels like coal, oil, and gas releases more carbon dioxide (CO2) and other greenhouse gases into the atmosphere. Ocean acidification occurs as ocean water absorbs CO2 from the air and forms carbonic acid, causing pH to drop. This carbonic acid releases hydrogen ions (H+), which bond with other molecules in the water. These hydrogen ions bond with carbonate, an important substance needed by oysters, mussels, clams and other shelled organisms. A lack of carbonate makes it harder for these animals to grow protective shells, and in some cases, acidic conditions may dissolve existing shells. See how ocean water being more acidic affects oysters here.
Since the late 1700s, the pH of surface ocean waters has become more acidic by 0.1 pH units. Remember that the pH scale is logarithmic, meaning that for every step down the scale, the amount of hydrogen ions increases by a factor of 10. This change of 0.1 pH represents roughly a 30 percent increase in acidity. Learn more about ocean acidification here.
Organisms require different amounts of oxygen depending on their size and the environment they are adapted to. Oxygen levels are lower on the muddy bottom of the bay, so worms, clams and other invertebrates that live here need less oxygen than animals living in other zones. (Chesapeake Bay Program, www.chesapeakebay.net)
We can also look at dissolved oxygen in the bay. Dissolved Oxygen (DO) is the amount of gaseous oxygen (O2) dissolved in the water. Oxygen enters the water by absorption from the atmosphere, by movement of the water at the surface, or as a by-product of photosynthesis in aquatic plants. DO is measured in units of parts per million (ppm). PPM signifies how many parts of oxygen there are for every one million parts of water. DO is important for water quality and necessary to all forms of life. Oysters and most animals that live on oyster reefs need at least 2-3 ppm DO to survive, and larger fish need a minimum of 5 ppm. DO can be affected by temperature, seasonality, and the volume of water actively moving.
The map above shows dissolved oxygen (DO) estimates from two different time periods generalized from water quality testing points. On the left, you can see the average DO from the years 1993-1995, and on the right is 2012-2014. Darker brown areas have higher DO values, and the lighter colors are lower DO. These time periods most closely align with the reef maps pictured. You can click on the map to see what exact DO values the color represents in a popup. Are we seeing higher or lower DO levels recently? How might this affect oysters and the other animals living in the bay? Purple spots on the map show reef identified in 1995. Reef in red or orange shows reef mapped in this project. Slide the arrows back and forth to compare reef coverage from the 1990s to present day. How has it changed?
The results from this study indicated a large variance in the amount of intertidal reef across west Galveston Bay. Oyster densities on representative reefs and estimated areas of reef cover were used to extrapolate oyster abundances per quarter quadrant. Avian fauna appeared to be utilizing the reefs for maintenance, rather than foraging grounds. This study updates the spatial extent of intertidal reefs in west Galveston Bay and provides a current estimation of the standing stock of intertidal oysters in Galveston Bay.
We'd like to thank the Galveston Bay Estuary Program, the Texas Commission on Environmental Quality, and the United States Environmental Protection Agency for the funding that made this project possible.
Galveston Bay is Texas' largest estuary -a coastal body of water with a free connection with the open sea – and it starts in your backyard.
For more information on our ongoing projects, visit our websites for up to date news:
American Oystercatcher (Amanda Hackney)
References
Al Mukaimi, M. E., K. Kaiser, J. R. Williams, T. M. Dellapenna, P. Louchouarn, and P. H. Santschi. 2018. Centennial record of anthropogenic impacts in Galveston Bay: Evidence from trace metals (Hg, Pb, Ni, Zn) and lignin oxidation products. Environmental Pollution 237:887-899.
HARC (Ed.) 2020. State of the Bay: A Characterization of the Galveston Bay Ecosystem (4th Edition). Texas Commission on Environmental Quality, Houston, Texas.
NOAA National Geophysical Data Center. 2007: Galveston, Texas Coastal Digital Elevation Model. NOAA National Centers for Environmental Information. Accessed [9/11/2019].
NOAA National Geophysical Data Center. 2005: Matagorda Bay, TX (G280) Bathymetric Digital Elevation Model. NOAA National Centers for Environmental Information. Accessed [9/11/2019].
Lester, L., and L. Gonzalez, editors. 2011. The State of the Bay: A Characterization of the Galveston Bay Ecosystem, Third Edition. Texas Commission on Environmental Quality, Galveston Bay Estuary Program, Houston, Texas.
Powell, E. N., J. G. Song, M. S. Ellis, and E. A. Wilson Ormond (1995). The status and long-term trends of oyster reefs in Galveston Bay, Texas. Journal of Shellfish Research 14: 439-457
Texas Commission on Environmental Quality (2019). Surface Water Quality (Segments) Viewer, Surface Water Quality Data. https://tceq.maps.arcgis.com/apps/webappviewer/index.html?id=b0ab6bac411a49189106064b70bbe778
