Seagrass as a Coastal Filter: Investigating the Role of Seagrass Meadows in Mediating Nitrogen Cycling in Shallow Coastal Lagoons

Seagrass meadows are highly productive ecosystems that are widely distributed in coastal waters throughout the world. In shallow coastal lagoons, seagrass meadows provide an important ecosystem function by acting as a nitrogen filter. Nitrogen (N) that enters the lagoon is temporarily retained in seagrass biomass and is removed from the system through burial and denitrification in sediments. This filter function contributes to the health of seagrass ecosystems by reducing nitrogen in the water column, slowing rapid N cycling through algal biomass, and reducing the total nitrogen load exported from the ecosystem. However, seagrass meadows are declining worldwide; with the loss of seagrass meadow area, the N filter function is also lost. Restoration may offer a pathway to restore this important ecosystem function, but to date the effectiveness of the nitrogen filter in restored seagrass meadows has not been evaluated.

In this dissertation, I assessed the return of the nitrogen filter function within a successful seagrass restoration project in the Virginia coastal bays. I measured a suite of N cycling processes in the seagrass meadow and in adjacent bare sediments to understand the effects of the restoration. To assess retention and removal of nitrogen, I measured denitrification, burial, and assimilation into seagrass biomass. To evaluate the importance of the N filter function, I compared retention and removal to inputs of nitrogen to the lagoon from external N loading (from atmospheric and terrestrial sources) and from N fixation within the meadow. Finally, I compared the magnitude of N removal in the seagrass meadow to predicted changes in N loading from future development scenarios in this ecosystem in order to put the restored filter function in context within the landscape.
My results showed that the N filtration processes were enhanced in the restored meadow compared to adjacent unvegetated sediments. Through the development of a novel in situ incubation method (the push-pull method), I measured denitrification rates that were 4x greater in the restored seagrass sediments compared to bare sediments. Rates measured using this push-pull method were also significantly greater than rates measured using a conventional core technique, indicating that further research is needed to understand denitrification in seagrass sediments. N burial in sediments was 10x greater in the seagrass meadow than in bare sediments, and assimilation of N into seagrass biomass was the largest measured flux of nitrogen. The restored meadow had therefore regained both the retention and removal components of the coastal N filter. N removal in the meadow was comparable to 37% of current N loading from watershed and atmospheric sources, showing the importance of this restored function at the ecosystem scale. Future development scenarios were predicted to lead to increased N loads, but continued expansion of the seagrass meadow will also increase N removal. The maximum predicted meadow extent could offset >68% of the enhanced N load from future development scenarios, potentially limiting harmful effects from higher N loading. Overall, the seagrass restoration has reestablished the N filter function and has increased the capacity of the lagoon to withstand anthropogenic perturbations to the coastal nitrogen cycle.