Carbon accumulation and vertical accretion in a restored versus historic salt marsh in southern Puget Sound, Washington, United States

Abstract

Few comparisons exist between vertical accretion (VA) and carbon accumulation rates (CARs) in restored versus historic (i.e. reference) marshes. Here, we compare these processes in a formerly diked, sparsely vegetated, restored salt marsh (Six Gill Slough, SG), whose surface is subsided relative to the tidal frame, to an adjacent, relatively pristine, historic salt marsh (Animal Slough, AS). Six sediment cores were collected at both AS and SG approximately 6 years after restoration. Cores were analyzed for bulk density (BD), % loss of ignition, % organic carbon, and 210Pb. We found that sharp changes in BD in surface layers of SG cores were highly reliable markers for the onset of restoration. The mean VA since restoration at SG (0.79 [SD = 0.29] cm/year) was approximately twice that of AS (0.41 [SD = 0.16] cm/year). In comparison, the VA at AS over 50 years was 0.30 (SD = 0.09) cm/year. VA consisted almost entirely of inorganic sediment at SG whereas at AS it was approximately 55%. Mean CARs at SG were somewhat greater than at AS, but the difference was not significant due to high variability (SG: 81–210 g C m−2 year−1; AS: 115–168 g C m−2 year−1). The mean CAR at AS over the past 50 years was 118 (SD = 23) g C m−2 year−1. This study demonstrates that a sparsely vegetated, restored salt marsh can quickly begin to accumulate carbon and that historic and restored marshes can have similar CARs despite highly divergent formation processes.

Carbon Sources in the Sediments of a Restoring vs. Historically Unaltered Salt Marsh

“…At the NRD, previous research has shown that the restoring, subsided, and sparsely vegetated salt marsh (Six Gill Slough, hereafter the Restoring Marsh), accreted sediment at approximately twice the rate of the nearby, historical reference salt marsh (Animal Slough, hereafter the Reference Marsh) since restoration in 2009 (Restoring Marsh: 0.79 ± 0.29 (SD) cm year −1 ; Reference Marsh: 0.41 ± 0.16 cm year −1 ; Drexler et al 2019). In addition, vertical accretion was found to consist of5 5% inorganic matter at the Reference Marsh in contrast to 95% inorganic matter at the Restoring Marsh (Drexler et al 2019). Such data suggest that a greater amount of allochthonous material is being deposited in the Restoring Marsh vs. the Reference Marsh.…”

“…The relative contributions of carbon sources determined by the Bayesian SIMM analysis for the original model were multiplied by the mean carbon accumulation rates at the Reference Marsh and Restoring Marsh to estimate the carbon accumulation rates of individual source components at both sites. Mean carbon accumulation rates in sediments~6 years after restoration at the Restoring Marsh and Reference Marsh were approximately 164 ± 54 and 134 ± 19 g C m −2 year −1 , respectively (Drexler et al 2019).…”

“…Uncertainties in 210 Pb dating were calculated following Van Metre and Fuller (2009). Further details on dating cores and estimating carbon accumulation rates for each study location can be found in Drexler et al (2019).…”

“…4 and Online Resource 1: Table S5), with much of this carbon likely originating from root material, which has a slower decomposition rate relative to aboveground biomass (Hackney and de la Cruz 1980;Craft 2001). Out of the total mean carbon accumulation rate of 134 ± 19 g C m −2 at the Reference Marsh (Drexler et al 2019), Marsh C3 Plants contributed approximately 98 and 119 g C m −2 year −1 at Reference-Inland and Reference-Seaward, respectively (Fig. 6).…”

“…As salt marsh restoration becomes increasingly intertwined with the carbon market (Crooks et al 2019;Vanderklift et al 2019), it becomes important to consider whether restoring salt marshes have the capacity to store carbon in their sediments, a function typically provided by mature "blue carbon" ecosystems (coastal ecosystems including salt marshes, mangroves, and seagrasses with atmospherically significant and manageable carbon stocks and fluxes; McLeod et al 2011;Duarte et al 2013;Windham-Myers et al 2019). However, little is currently known about the carbon storage potential of such novel transitional ecosystems (but see Drexler et al 2019), raising several important questions. For example, how can a restoring salt marsh store organic carbon if it is largely unvegetated?…”