Award: OCE-1635403

This project developed a quantitative understanding of the controls and drivers of carbon cycling in temperate and tropical seagrass meadows that will increase our fundamental knowledge of carbon cycling in shallow coastal environments and increase our ability to quantify their potential as blue carbon sinks. We combined new methods for evaluating oxygen and carbon exchange with traditional measurements of biomass, sedimentary, and water column chemistry to test numerical model predictions of carbon cycle and sequestration dynamics in seagrass meadows. This comparative analysis across latitudinal and geochemical gradients provided information on the relative contributions of different seagrass species and geochemical processes that control carbon sequestration in shallow marine environments. Our results indicate that tropical Bahamian seagrass meadows (mostly Thalassia testudinum) allocate 2-4x more organic carbon to roots and rhizomes buried below-ground than do turtlegrass in Northern Florida/Gulf of Mexico, and nearly 10x more organic carbon than temperate eelgrass (Zostera marina) for equivalent above ground biomass. Integrating to 30 cm depth produced a range of sediment carbon between 300 and 1200 mg C cm-2, and represented about 0.1% of the standing seagrass biomass. Allocation of organic carbon to below-ground roots and rhizomes was strongly correlated to water transparency, as measured by the diffuse attenuation coefficient. The strength of this non-linear relationship across different taxa and habitats underscores the importance of light in driving the carbon sequestration potential of seagrasses. Further, we found that dissolution of carbonate sediments and precipitation of iron sulfide particles can trap much of the CO2 generated by organic carbon remineralization in the bicarbonate pool of the ocean where it can remain out of circulation with the atmosphere for a considerable time. Our broadening of the definition of "blue carbon sequestration" to include alkalinity-driven CO2 sequestration resulting from carbonate dissolution & FeS precipitation improves our understanding of the role played by these coastal ecosystems in the ocean carbon cycle. The models developed from this effort directly link seagrass metabolic processes by individual shoots to sediment diagenetic processes controlling carbon cycling that can be scaled to the submarine landscape and a variety of sediments (siliciclastic to carbonate), biogeographic regions (temperate to tropical North Atlantic), and species with different growth morphologies and life-history characteristics. Our model and data products will also provide a visual key for coastal managers to understand the impact of environmental controls on coastal seagrass ecosystems by projecting the model across the submarine landscape. These products will include maps of seagrass density, carbon burial and sediment geochemistry using present-day, as well as future projections of ocean temperature and CO2 levels. Descriptions of our new instrumentation (sediment O2 profiling system) and inexpensive radiometers are being disseminated through public presentations and manuscripts in preparation. We are in discussion with outside interests regarding commercialization of the radiometer systems. Last Modified: 12/27/2021 Submitted by: Richard C Zimmerman