Drivers of Spatial Change in Seagrass Carbon Stock Amid Shifting Species Dominance

Abstract Seagrass meadows are recognized for their significant role in blue carbon sequestration and storage. However, these ecosystems are being altered by climate change, and consequently so too are the ecosystem services they provide. Here, we evaluate drivers of spatial variation in carbon storage among seagrass meadows in the Lower Chesapeake Bay, which underwent a temperature-driven shift in species dominance from Zostera marina to Ruppia maritima in the last decade. We collected 140 cores from meadows of varying historic vegetated cover, depth, and dominant species identity and measured belowground carbon stock, grain size, and associated meadow characteristics. We then used a structural equation model to develop a single causal network to evaluate the drivers of spatial variation in sediment carbon storage across seagrass meadows. We show that site and both seagrass species and characteristics influence carbon stocks, but surprisingly, meadow age did not have explanatory power. In the Chesapeake Bay, Ruppia meadows have less carbon storage potential compared to Zostera when at similar depths ( Ruppia : 25.0 mg C cm −2 , Zostera : 31.4 mg C cm −2 ). However, due to climate-driven temperature increases, Ruppia occupies more of the optimal near-shore “real estate” for carbon storage than it did historically. Thus, despite an estimated 25% increase in dense seagrass cover from 2010 to 2020, the estimated sediment carbon only increased 2.6%. Our study highlights the need to consider regional and interspecific variability when assessing seagrass meadow carbon storage.