Blue carbon stocks and exchanges along the California coast

Abstract

Abstract. Salt marshes and seagrass meadows can sequester and store high quantities of organic carbon (OC) in their sediments relative to other marine and terrestrial habitats. Assessing carbon stocks, carbon sources, and the transfer of carbon between habitats within coastal seascapes are each integral in identifying the role of blue carbon habitats in coastal carbon cycling. Here, we quantified carbon stocks, sources, and exchanges in seagrass meadows, salt marshes, and unvegetated sediments in six bays along the California coast. In the top 20 cm of sediment, the salt marshes contained approximately twice as much OC as seagrass meadows did, 4.92 ± 0.36 kg OC m−2 compared to 2.20 ± 0.24 kg OC m−2, respectively. Both salt marsh and seagrass sediment carbon stocks were higher than previous estimates from this region but lower than global and US-wide averages, respectively. Seagrass-derived carbon was deposited annually into adjacent marshes during fall seagrass senescence. However, isotope mixing models estimate that negligible amounts of this seagrass material were ultimately buried in underlying sediment. Rather, the vast majority of OC in sediment across sites was likely derived from planktonic/benthic diatoms and/or C3 salt marsh plants.

Rapid enhancement of multiple ecosystem services following the restoration of a coastal foundation species

“…This is likely due to a change in grain size moving upstream, from predominantly sandy to silty (Ward et al. 2021 ), and shows that in Elkhorn Slough, the ability of eelgrass to shift sediment properties is context dependent (Appendix S1 : Fig. S9).…”

“…To convert percent TOM to percent organic carbon (OC), we used a power model () derived using regional core data (Ward et al. 2021). Carbon storage was calculated by multiplying percent OC and the bulk density of each interval, reported as kg OC/m 3 .…”

“…Last, we expected restored and reference plots to have greater OC than unvegetated plots, and instead observed no detectable differences among habitats but a near doubling of OC with increased distance from mouth. This is likely due to a change in grain size moving upstream, from predominantly sandy to silty (Ward et al 2021), and shows that in Elkhorn Slough, the ability of eelgrass to shift sediment properties is context dependent (Appendix S1: Fig. S9).…”

Beyond habitat boundaries: Organic matter cycling requires a system‐wide approach for accurate blue carbon accounting

“…Generally, autochthonous sources are expected to dominate sedimentary OM in salt marshes (Kelleway et al 2017a), but deposition of imported OM in form of Ulva spp. (Watson et al 2015) and seagrass wrack (Ward et al 2021) is known to occur in Eastern Pacific salt marshes. The incorporation of seagrass detritus into marsh surface sediments is not widely reported and previous studies suggest both negative (Macreadie et al 2013) and positive (Chapman and Roberts 2004) impacts of seagrass wrack on salt marsh productivity.…”

High nutrient loads amplify carbon cycling across California and New York coastal wetlands but with ambiguous effects on marsh integrity and sustainability

“…Depending on soil characteristics, this can drive differences in SOM stocks apparent between east and west coast marshes. For instance, recent studies reported carbon densities four times greater in Northeastern than Californian salt marsh habitats [111,112]. This study found soil carbon inventories that were six times higher in NY than in CA (CA mean = 2500 g m -2 vs. NY mean = 16,000 g m -2 for the top 20 cm).…”