Just Below the Surface: Determining Patterns of a Critical Underwater Resource

Kristin DeMarco  |  6/9/2017 3:39:11 PM

Ask any fisherman or duck hunter in coastal Louisiana, and they’ll tell you that some of the best habitat for fish and waterfowl are areas that are loaded with underwater grasses. In technical terms, these plants just under the water surface are known as submerged aquatic vegetation. Submerged aquatic vegetation grows in shallow waters off the northern Gulf of Mexico coast and is one of the most productive ecosystems in coastal landscape. However, because submerged aquatic vegetation is harder to see, it often takes a back seat in conservation efforts. The structure created by these underwater forests provides excellent fish habitat in areas that would otherwise be mud bottom and is a primary food source for many species of wintering waterfowl. Because submerged aquatic vegetation changes the physical structure of the habitat, it is considered an ecosystem engineer – supporting diverse populations of fish and wildlife species and performing valuable ecosystem services by improving water quality and clarity as it takes up nutrients and stabilizes sediment.

Across coastal landscapes, submerged aquatic vegetation communities can develop over a wide salinity range, and beds will form in freshwater interior marsh ponds, brackish marsh creeks and bayous, navigation canals, as well as saline protected bays. This ability for the beds to persist across the entire range of salinities in the coastal marshes is a critical characteristic for creating high-quality waterfowl habitat; many duck species require a variety of habitats to meet foraging and resting needs. Despite the known importance of submerged aquatic vegetation in coastal ecosystems, basic baseline data and maps describing its geographic coverage in the northern Gulf of Mexico, especially in shallow marsh ponds, are lacking. Additionally, how changes in salinity, water depth, water clarity and exposure to waves will affect the presence of this vegetation in shallow wetland waters is poorly understood.

To learn more about submerged aquatic vegetation as a coastal resource, LSU AgCenter researchers developed the first dataset describing this vegetation across the coastal landscape. They traveled from San Antonio Bay, Texas, to Mobile Bay, Alabama, during the summer growing seasons from 2013 to 2015. Field data from almost 400 randomly selected sites of submerged aquatic vegetation communities were collected, primarily from an airboat, across coastal marshes. Analyses from these data indicate that saline marsh waters are less likely to support submerged aquatic vegetation because of the interaction between salinity and waves. But when submerged aquatic vegetation in saline waters was able to colonize, it tended to form meadow-like beds and cover larger areas. Shallow waters in fresh, intermediate and brackish marshes supported higher and similar amounts of submerged aquatic vegetation, although species communities were distinctive among marsh zones. The most frequently occurring species encountered were Ruppia maritima, or wigeongrass, and Ceratophyllum demersum, or coontail, both highly desirable types of waterfowl forage.

To understand how submerged aquatic vegetation coverage might change with the seasons, researchers again went out to collect field data closer to home in Barataria Bay, Louisiana, where they sampled the vegetation habitats every six to eight weeks in 2015. Results indicate that submerged aquatic vegetation growth in fresh marsh shallow waters is significantly hindered in the summer season because of the presence of floating aquatic vegetation, especially from the exotic invasive water hyacinth (Eichhornia crassipes). Floating aquatic vegetation will often grow so thick on the water surface that submerged species will die from lack of light.

LSU AgCenter researchers have joined forces with state and federal agencies and are currently developing a spatial model that will predict the likelihood of submerged aquatic vegetation occurrence in Louisiana coastal waters based on environmental conditions. The maps associated with this model are the first to geographically represent the distribution of vital submerged aquatic vegetation resources in coastal Louisiana and will be a valuable research and management tool. These maps and models are necessary to plan for conservation and restoration of coastal habitats as restoring this vegetation is essential to protect fish and wildlife populations in a changing climate. Mapping submerged aquatic vegetation resource availability is a particularly pressing need in the northern Gulf of Mexico; sea-level rise is predicted to inundate much of the coastal zone, changing salinity zones inland and increasing shallow open-water areas, dramatically altering the areas potentially able to support submerged aquatic vegetation habitats. The data, modeling tools, maps and quantification of ecological processes collected in this work are much needed by coastal researchers and will lead to more informed coastal habitat management on a landscape scale.

Kristin DeMarco is a doctoral student in the School of Renewable Natural Resources.

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In order to categorize the submerged aquatic vegetation, researchers take soil cores of the plant roots and collect the vegetation to take back to the lab to determine below- and above-ground biomass. Kristin DeMarco holds the above-ground submerged aquatic vegetation collected at site in Brazoria County, Texas. Photo by Eva Hillman

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Kristin DeMarco shows the healthy roots of shoalgrass at a site in Grand Bay, Alabama. Photo by Eva Hillman

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Submerged aquatic vegetation study sites and marsh types in the northern Gulf of Mexico.

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