Abstract
The scarcity of data on tidal marsh soil accumulation rates (SAR) and soil organic carbon accumulation rates (CAR) globally precludes a comprehensive assessment of the role of tidal marshes in climate change mitigation and adaptation. Particularly few data exist from the southern hemisphere and for Australia in particular, which contains ~24% of globally recognised tidal marsh extent. Here we estimate SAR and CAR over the last 70 years using 210Pb-based geochronologies in temperate estuarine tidal marsh ecosystems in southern Western Australia (WA). Specifically, we assessed tidal marsh ecosystems situated in two geomorphic settings (marine vs. fluvial deltas) within 10 wave-dominated, barrier estuaries. Overall, average SAR (1.1 ± 0.3 mm yr−1) and CAR (32 ± 9 g m−2 yr−1) estimates were 5-fold lower than global mean estimates. Furthermore, we showed that hotspots of soil organic carbon stocks are not indicative of current hotspots for CAR. The lack of significant differences (P > 0.05) in SAR, CAR, and excess 210Pb inventories between marine and fluvial settings can be explained by the high heterogeneity among and within estuaries throughout the region. The relative stability of recent and Holocene relative sea-levels in WA likely explains the limited CAR potential in tidal marshes under relatively stable sea-level conditions. However, further research exploring interactions among biotic and abiotic factors within estuaries is required to shed more light on the small spatial-scale variability in SAR and CAR across tidal marsh ecosystems in WA and elsewhere. This study provides baseline estimates for the inclusion of tidal marshes in national carbon inventories, identifies hotspots for the development of blue carbon projects, and supports the use of site-specific assessments opposed to regional means for estimating blue carbon resources.
Highlights
Tidal marsh ecosystems are globally important carbon sinks that can accumulate and preserve sedimentary carbon reservoirs over millennia (McLeod et al, 2011; Nellemann et al, 2009)
The mean Corg accumulation rates (CAR) measured in this study is 5-fold lower than recent global estimates (168 ± 7 g Corg m−2 yr−1; Wang et al, 2020), which are largely biased toward tidal marshes situated in the northern hemisphere
Australia's lower CAR may reflect the relatively low sediment accumulation rates (SAR) measured in temperate Western Australia (WA) tidal marshes (1.1 ± 0.3 mm yr−1; ranging from 0.36 to 2.55 mm yr−1; Table 1) and eastern Australian counterparts (2.09 mm yr−1; Macreadie et al, 2017b), compared to global SAR estimates from tidal marsh ecosystems in the northern hemisphere (6.73 mm yr−1; Duarte et al, 2013)
Summary
Tidal marsh ecosystems are globally important carbon sinks that can accumulate and preserve sedimentary carbon reservoirs over millennia (McLeod et al, 2011; Nellemann et al, 2009). The capacity of tidal marsh and adjacent vegetated coastal ecosystems (namely seagrasses and mangroves) to function as organic carbon (Corg) sinks has led to the development of blue carbon strategies aimed at mitigating climate change (Nellemann et al, 2009; Serrano et al, 2019). Such strategies focus on ecosystem conservation and/or restoration through the preservation of habitat functions (such as Corg sequestration) and the abatement of CO2 emissions from ecosystem disturbance (Kelleway et al, 2017b; Macreadie et al, 2017a; McLeod et al, 2011). We hypothesised that, owing to the higher soil Corg stocks sequestered in fluviallysituated tidal marsh throughout the relative stability of WA's Holocene sea-levels, tidal marsh positioned in fluvially-influenced geomorphic settings will support higher CAR compared to those situated in marine flood tidal deltas
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