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Habitat Focus: Seagrasses

Wednesday, March 23rd, 2016

Keep off the seagrass! Have you ever seen this sign boating? And then wondered what exactly is a seagrass and why you should protect it?

Seagrasses are flowering plants that grow under the water in shallow, saline and brackish coastal waters across the globe. Similar to terrestrial plants, seagrasses have leaves, stems, flowers and root systems and require sunlight to create their own food and oxygen through photosynthesis. They also require nutrients, like nitrogen and phosphorus, which they can absorb through their roots and store in their rhizomes (i.e. horizontal stem). In contrast, seaweeds or macroalgae, like kelp or red algae, are much simpler plant-like organisms, as they do not have a true root system. Seagrasses are also known as submerged aquatic vegetation (SAV), which includes any vascular, submerged aquatic plant with roots that grows in fresh, coastal and estuarine waters.

Turtlegrass meadow
View looking down at a turtlegrass bed. White square is a 0.5m x 0.5m PVC quadrat used for surveying seagrass benthic cover. Photo credit: D. Byron

There are over 50 species of seagrass worldwide, and the northern Gulf of Mexico is home to six of those species. Depending on where you are along the coast, you can find turtlegrass (Thalassia testudinum), widgeon grass (Ruppia maritima), shoalgrass (Halodule wrightii), and manatee grass (Syringodium filiforme) as discrete patches or meadows with one or many species occurring in an area. There have even been reports of two of the smallest species of seagrass, star grass (Halophila engelmannii) and paddle grass (Halophila decipiens) found within our coastal waters. In the Chandeleur Islands where ACER is conducting their field work, seagrass diversity is high with 5 of the species listed above present. These species have different morphology in their sizes and leaf shapes as well as their tolerance for salinity, light and nutrients, but all are known to provide important ecosystem services.

seagrass habitat
Seagrass beds are highly productive habitats that provide numerous ecosystem services, including providing refuge for juvenile fish and invertebrates. This shoalgrass meadow near Robinson Island, AL is home to numerous small fish. Photo credit: D. Byron

Seagrass beds are among the most productive ecosystems in the world and provide several important ecological functions: nutrient uptake, sediment stabilization, decreased wave energy and food and shelter for small fauna. For example, the root and rhizome system of seagrasses trap and stabilize sediment, which helps improve water clarity and prevent erosion. Seagrasses are also considered nursery habitats for small juvenile fish and invertebrates, like blue crab and penaeid shrimp, as well as a permanent home for species such as pipe fish and seahorses. Some recreationally important species like red drum also use seagrass beds for food and shelter and many threatened or endangered species such as manatees, dugongs and green sea turtles rely on seagrasses to make up the majority of their diet.

Seagrasses, however, are declining globally due to numerous human-related and natural causes. Large physical disturbances such as hurricanes may bury seagrasses and animals, such as manatees or skates and rays, may eat or destroy beds in search of food. Human impacts include eutrophication, boating damage and dredging. Boat propellers and anchors can tear and remove seagrasses, leaving bare scars in the seagrass bed, fragmenting once continuous habitat. Many Gulf Coast states have established aquatic preserves to help protect seagrass beds and are working to increase awareness of the benefits of seagrasses by boaters, recreational divers, coastal homeowners and tourists.

Given their ecological functions and proximity to shorelines that experienced oil exposure, seagrass habitats are a focus of ACER’s research. ACER scientists are investigating the impact of the oil spill on seagrass habitats by comparing oiled to non-oiled areas and measuring associated animals and the microbial community.