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. 

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

  1. Map possible intertidal reef locations using aerial photos, then "groundtruth" or visit a subgroup of the possible locations to check our accuracy.
  2. Determine the relationships between bird use of intertidal reef and the invertebrates that live in a location.
  3. 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.

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)

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 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

Mapping Intertidal Reef

Black Cat GIS and Biological

Oyster Life Cycle Unknown author, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons

Shorebirds like these Semipalmated Plover depend on intertidal reef habitat for food and roosting sites. (Alan Wilde)

Men shucking oysters in a 1910 Galveston postcard. (Galveston and Texas History Center - Rosenberg Library)

Kori Lugar sampling invertebrates on a reef. (Alan Wilde)

Shorebirds roosting (resting) on reef. (Alan Wilde)

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)

Oyster reef sampled in this project. Notice that some reef is above water, while much is below. (Amanda Hackney)

Oysters in East Matagorda Bay. (Alan Wilde)

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) 

American Oystercatcher (Amanda Hackney)