Carbon Flux and Blue Carbon Potential of Seagrass Ecosystems in the Anthropocene

Richard Carl Zimmerman, Old Dominion University, Ocean, Earth, & Atmospheic Sciences, Norfolk, VA, United States, Matthew Herman Long, Woods Hole Oceanographic Institution, Marine Chemistry and Geochemistry, Woods Hole, United States, David Burdige, Old Dominion University, Norfolk, VA, United States, Victoria J Hill, Old Dominion University, Ocean, Earth, & Atmospheric Science, Norfolk, VA, United States, Brian Collister, NASA Langley Research Center, Hampton, VA, United States, Kazi Aminul Islam, Old Dominion University, Electrical & Computer Engineering, Norfolk, United States, Jiang Li, Old Dominion University, Electrical & Computer Engineering, Norfolk, VA, United States, Megan Coffer, Oak Ridge Institute for Science and Education, Oak Ridge, United States and Blake A Schaeffer, Environmental Protection Agency, NERL Exposure Methods and Measurement Division, Research Triangle Park, NC, United States
Seagrass ecosystems are thought to be important for carbon sequestration in shallow coastal environments. Distributed worldwide from the tropics to polar climes, and on all continents except Antarctica, seagrasses grow in siliclastic and carbonate sediments and exhibit a wide range of morphologies. Despite this high degree of variation, seagrass ecosystems are often viewed as a single entity in terms of their contribution to global carbon flux. We measured short term metabolic fluxes, below-ground carbon allocation, organic carbon burial and diagenetic processes in temperate, subtropical and tropical seagrass habitats distributed along the Atlantic and Gulf of Mexico coasts of North America in order to compare their potential for Blue Carbon sequestration. Although above-ground leaf biomass and short-term metabolic rates were similar across all habitats, subtropical and tropical meadows dominated by turtlegrass (Thalassia testudinum) allocate much more biomass to below-ground structures than did eelgrass (Zostera marina) that dominates temperate N. Atlantic seagrass meadows. Further, the allocation to below-ground structures responds positively to light availability and ocean acidification in both species, indicating important linkages between whole plant photosynthesis and Blue Carbon sequestration. In addition to direct burial via allocation to below-ground roots and rhizomes, seagrass metabolism influences rates of sulfide precipitation and carbonate dissolution that increase seawater alkalinity and enhance the potential of seagrasses, especially those growing in carbonate sediments to serve as a Blue Carbon sinks for atmospheric CO2. Differences in Blue Carbon potential for temperate, subtropical and tropical seagrass meadows are now being quantified using algorithms we developed for analysis of high resolution multispectral imagery obtained by commercial satellites. Results to date indicate that Blue Carbon burial is on the order of 1 Mg C ha-1 in temperate meadows dominated by eelgrass (Zostera marina), almost 5 Mg C ha-1 in subtropical meadows from the Gulf of Mexico dominated by turtlegrass (Thalassia testudinum) and at least 10 Mg C ha-1 in tropical turtlegrass meadows growing in the carbonate sediments of the Great Bahama Bank.