Multi‐Scale Variability of Blue Carbon in Seagrass Sediments From Three California Estuaries

Abstract Seagrass meadows are a significant global carbon sink. Most of the carbon sequestered in these ecosystems is found as sedimentary organic carbon, which can vary spatially across seagrass meadows and can be derived from a variety of sources. Here, we investigate intra‐ and inter‐meadow spatial variability in sediment organic carbon storage and quantify potential organic matter sources in three Zostera marina seagrass meadows spanning three estuaries across California (USA). Seagrass sediment organic carbon storage varies as much intra‐meadow as it varies across regions. At the intra‐meadow scale we find significant correlations between organic carbon content and mud content (fraction of sediment grains <63 μm, %), C/N ratios and δ 13 C values in surface sediments across two meadows. At the regional scale greater organic carbon storage in the presence of seagrass compared to adjacent unvegetated sediments. Additionally, we find that mud content is the best predictor of sediment organic carbon content at the regional scale, indicating an enhanced role for mineral interaction in organic carbon preservation, but not consistently at the localized meadow scale. Approximately one fourth of organic carbon preserved in seagrass sediments is derived from seagrass. This research provides insights into the variability of organic carbon storage in seagrass meadows in California at multiple spatial scales. We demonstrate the need to account for small‐scale variation of organic carbon storage in seagrass meadows to better estimate the capacity for these ecosystems to serve as effective long‐term carbon sinks.