Blue carbon stocks and exchanges along the California coast

Melissa A Ward,Kristen Elsmore,Aurora M Ricart,Tessa M Hill,Walter C Oechel,Lena R Capece,Chelsey Souza,Kathryn M Beheshti,Tessa Filipczyk,Brady C O'Donnell,Sarah Merolla

doi:10.5194/bg-18-4717-2021

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.

Highlights

As carbon dioxide (CO2) concentrations in the oceans and atmosphere continue to rise, interest in measuring the relative quantities of carbon stored within natural ecosystems has increased
While all sampled seagrass meadows were dominated by a single seagrass species (Zostera marina), salt marshes contained a mixed community of halophytes, predominantly composed of pickleweed (Sarcocornia pacifica), and to a lesser extent salt grass (Distichlis spicata) and marsh jaumea (Jaumea carnosa)
Significant differences between organic carbon (OC) stocks emerged when cores were compared between habitats, with salt marshes containing significantly more carbon than both bare sediment and seagrass meadows (Fig. 3b; simple linear models (SLMs), p < 0.01, F statistic = 13.3, DF = 3)

Summary

Introduction

As carbon dioxide (CO2) concentrations in the oceans and atmosphere continue to rise, interest in measuring the relative quantities of carbon stored within natural ecosystems has increased. Salt marshes, and mangroves have earned the moniker “blue carbon” habitats for their ability to store and sequester disproportionally high levels of organic carbon (OC) in their sediments relative to other habitat types, thereby potentially serving in a management context to provide carbon mitigation (Lovelock and Duarte, 2019; McLeod et al, 2011). This can be largely attributed to the tendency for these habitats to exhibit high sediment accretion rates and low decomposition rates (Peck et al, 2020; Serrano et al, 2019). Salt marshes and seagrass meadows cover extensive portions of North America’s west coast, yet their carbon stocks have been relatively understudied compared to other habitats in North America and other blue carbon habitats in different regions of the world (Ouyang and Lee, 2014; Postlethwaite et al, 2018)

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Discussion

Conclusion