SEA GRASSES IN CHOCTAWHATCHEE BAY
What types of grasses occur within Choctawhatchee Bay?
Choctawhatchee Bay has two species of grasses: shoal grass (Halodule wrightii) and widgeon grass (Ruppia maritime). Unlike the estuaries to the East and West, Choctawhatchee Bay doesn’t have turtle grass (Thalassia testudinum). Choctawhatchee Bay is a relatively new estuary. In 1929, a pass was cut to the Gulf of Mexico at what is now referred to as “East Pass” in Destin and maintained in 1930 by the Army Core of Engineers. So, prior to the cut, the area functioned like a big coastal dune lake prior to 1929—opening intermittently to the Gulf of Mexico and closing with sediment transport along the beaches.
Why are Sea Grasses important?
Biologically Important to Choctawhatchee Bay
- Sea Grasses provide important nursery/habitat during some or all stages in the life cycle of marine fish & invertebrates
- Relatively high rate of primary production drives detritus-based food chains
- Directly consumed by birds, mammals and turtles
- Biological indicators of deteriorating water quality
Physically Important to Choctawhatchee Bay
- Reduce current velocity
- Increased water clarity
- Dampen wave intensity
- Reduced shoreline erosion
- Stabilize marine sediments
Economically Important to Choctawhatchee Bay
- $2.9 billion local sales annually
- Over 36,000 jobs
- Tourists spend $1.6 billion annually
So for all of these important reasons, the factors influencing sea grass growth and abundance in Choctawhatchee Bay needs to be fully understood.
What do we know about Sea Grass distribution and abundance within Choctawhatchee Bay?
Sea Grass distribution and abundance has been sampled sporadically over the 40 years by many different groups (see the table below).
|McNulty et al.||1972||1,237|
The abundance of sea grasses within Choctawhatchee Bay varies from as little as 750 hectares in 1976 to as much as 1,726 hectares in 1992. Each group has different methods of evaluating sea grasses in Choctawhatchee Bay causing changes from each survey difficult to understand. Analyses of distribution maps for various studies also are problematic. The 1972 study show sea grasses along the northern shore and in the eastern side of Choctawhatchee near the river. The 1992 study, while it shows a net increase in sea grass hectares, the map documents the loss of all grasses along the northern shore and near the river in the eastern portion of the bay. In 2003, there was an almost 40% decrease in total sea grass hectares but the distribution map shows the loss of sea grass beds in Hogtown Bayou (an area with significant growth of sea grasses).
The length of time between each survey, the different groups and methodologies used to complete the surveys lead CBA to develop a sampling methodology and protocol that can be done at least annually (if not more frequently) to evaluate the distribution and abundance of sea grasses within Choctawhatchee Bay.
Sea Grass Sampling
In 2009, CBA staff and interns sampled 108 sites within Choctawhatchee Bay for Sea Grasses. Sites were 30 meters by 30 meters and various parameters were sampled. Each site sampled had to have greater than 30% density for sea grass. In 2010, CBA staff and interns sampled 150 sites within Choctawhatchee Bay. Each 30 meter by 30 meter site was identified as either a sea grass site (having greater than 30% density of sea grass) or identified as a sediment site (sediment characteristics were identified).
Then, working with University of Florida graduate student (PhD Student), Jenney Lazzarino, and using Landsat 5 TM Imagery, a 2009 and 2010 estimate of Sea Grass abundance and a distribution map for each year was produced. Mrs. Lazzarino estimated that in July 2009, there were 1,439 hectares and in 2010 1,385 hectares of sea grasses in Choctawhatchee Bay. Using the reference points that CBA provided, her maps have an accuracy ready of 70%. To read Mrs. Lazzarino's brief summary of CBA's Seagrass Sampling Methods and Results. To view the 2009 and 2010 CBA Sea Grass Distribution Maps click here.
With various analyses in the water quality data demonstrating seasonal changes and significant spatial differences, it become evident that at least annual sampling of sea grasses is needed in Choctawhatchee Bay. The Choctawhatchee Bay has two rainy seasons within a given year and the length of these seasons and amount of rainfall can significantly affect the distribution of sea grasses in the bay. Therefore, CBA will attempt to complete annual sea grass assessments throughout the bay.
This provides a cost efficient way to routinely sample sea grass abundance and distribution in Choctawhatchee Bay. These data can serve as baseline reference points in between the larger more intense efforts conducted by USGS and others every 5 to 10 years.