Two prominent changes in coastal ecosystems have been nutrient enrichment and precipitous declines of seagrass habitat. Seagrasses stabilize sediment, provide food and remove nutrients. However, differences in palatability, blade width, canopy height and root penetration depth can influence the ecosystem services provided by seagrass meadows. The amount and type of seagrasses in an area can change as a result of increased water temperature or decreased water quality. In Chesapeake Bay, coverage of the historically dominant species, Zostera marina (eelgrass), has declined and growth of Ruppia maritima (widgeon grass), a smaller, more heat tolerant form of seagrass has increased. In the Gulf of Mexico, increases in temperature and nutrients have affected the coverage and distribution of Halodule wrightii (shoal grass), Syringodium filiforme (manatee grass) and Thalassia testudinum (turtle grass). These changes in seagrass species composition and abundance can have ecosystem level consequences if key ecosystem functions and services are lost. We measured fluxes of NOx-, NH4+and N2 from seagrasses in Chesapeake Bay and the Gulf of Mexico during the late summer. Fluxes from different seagrass species were compared to bare sediments to determine how changes in species affects the ability for seagrass meadows to mitigate nutrient pollution through denitrification.
Results/Conclusions
Seagrass meadows had higher rates of denitrification (net N2 production) than bare sediments in both regions. However, the magnitude of this increase varied by species. In Chesapeake Bay, bare sediments were net nitrogen fixing, while sediments from Ruppia maritima and Zostera marina meadows were net denitrifying. Thus, restoration of seagrass meadows may help remove nitrogen in the region. Yet, the type of seagrass species is important as denitrification was lower in Ruppia maritima sediments than Zostera marina sediments. In the Gulf of Mexico, Syringodium filiforme and Thalassia testudinum increased denitrification over bare sediment by 101% and 30%, respectively. Denitrification in Halodule wrightii sediments was slightly lower than in bare sediments. Syringodium filiforme sediments removed more nitrogen through denitrification than either Thalassia testudinum or Halodule wrightii sediments. Collectively, these results suggest that changes in seagrass species composition impacts the effectiveness of seagrass meadows to filter and remove nutrients. The physical structure of the seagrass as well as variation in salinity and temperature of the regions contributed to the observed differences in rate processes. Because of the link between seagrass species and denitrification, changes in seagrass species composition will have consequences for both ecosystem functions and the delivery of ecosystem services.