An update from FIU graduate student Jenn Sweatman. Thanks Jenn!
In my post last September, I discussed a project I was conducting in the seagrass beds around Abaco. The primary focus of the project was to understand how the presence of propeller scars in seagrass bed affects the community of grazing amphipods, which are tiny shrimp-like animals that eat algae off of seagrasses. Sorting 120 seagrass samples to pick the amphipods out and identifying the amphipods takes some time, so you will have to stay tuned to see those results. I have run nutrient analyses on the seagrass tissues and have some interesting results!
Seagrasses and algae use nutrients, like nitrogen and phosphorus, to grow. These nutrients can accumulate in the tissues of seagrasses making them great indicators of how much nitrogen and phosphorus is available. In more polluted areas, for example, you would expect to see more nutrients in seagrass tissues than in a less polluted area. In my experiment, I measured carbon, nitrogen, and phosphorus in both continuous and scarred plots. Scarred plots were set up in a circular configuration (look for the ‘white’ circle in the seagrass bed photo above). Within scarred plots I measured nutrients from seagrasses located at edges (where the seagrass meets the propeller scar) and the center of the plot.
Seagrasses located near edges had less phosphorus than seagrasses in the center of scarred plots and seagrass in continuous plots. Because the seagrasses near the edges had neighboring seagrasses removed, they were receiving more light than seagrasses located in continuous plots and within the center of plots where seagrasses were very dense. When light availability increases, plants allocate carbon, nitrogen, and phosphorus to different places or work centers within the cell. In this case, growth rates didn’t change with increasing light availability but phosphorus decreased [possibly] meaning the seagrasses were busy storing carbon rather than growing or taking in phosphorus.
Animals that graze on seagrasses, such as sea turtles, prefer seagrasses with higher phosphorus in their tissues. These seagrasses are much more nutritious and delicious to grazing animals. In this experiment, seagrasses in continuous plots and in the center of plots are more nutritious for grazers because of their higher phosphorus content. This could potentially affect the diets of animals that directly consume seagrasses.
Updates to come: data on amphipod community responses to the presence of propeller scars.