Reduced water motion enhances organic carbon stocks in temperate eelgrass meadows

Organic carbon (OC) storage in coastal vegetated habitats (blue carbon) is increasingly being considered in carbon financing and ecosystem-based management. Seagrass meadows have potential to sequester and store significant amounts of carbon, primarily belowground in the sediments beneath them. However, existing estimates are primarily from tropical and sub-tropical regions. On the northwest coast of North America, the magnitude and variability of seagrass carbon stocks, as well as local drivers of variability remain rare. We collected sediment cores from six eelgrass (Zostera marina) meadows on the coast of British Columbia, Canada, to quantify sedimentary OC stocks and accumulation rates. The top 20cm of sediments exhibited a 30-fold difference in OC stocks across meadows (185 - 5545 g OC m-2). Stocks in meadow interiors (1392 ± 928 SD g OC m-2) were 1.23 times greater than those along meadow edges (1130 ± 698 SD g OC m-2) and 1.42 times greater than adjacent unvegetated sediments (977.10 ± 516 SD g OC m-2). The top 20cm of sediment represented 21 to 74 years of accumulation, and OC accumulation rates ranged from 13 to 50 g OC m-2 year-1. Isotopic analysis of sediments (δ13C = 19.43%0 ± 3.25 SD) revealed that OC is largely derived from non-seagrass sources (terrestrial, benthic microalgae and/or macroalgae). OC stocks in the top 5 cm were most strongly influenced by water motion (Relative Variable Importance RVI = 0.81), relative to seagrass structural complexity (RVI = 0.21), and sediment size (RVI= 0.22). Specifically, higher OC stocks were associated with lower water motion, which may facilitate greater deposition of organic carbon particles and reduce rates of erosion and resuspension. This study highlights variability in carbon stocks at local scales with profound implications for estimating variability in carbon stocks at regional and global scales, typically unaccounted for in seagrass blue carbon estimates. To help account for this variability, we demonstrate that reduced water motion can indicate high potential for blue carbon storage in temperate soft sediment habitats.