Impacts of land reclamation on tidal marsh ‘blue carbon’ stocks
Graphical abstract
Introduction
Land use change is one of the greatest sources of greenhouse gases to the atmosphere, contributing approximately 30% of global emissions and accelerating climate change (Houghton, 2003). Deforestation alone is the second greatest source of emissions, next to fossil fuel combustion (van der Werf et al., 2009). Loss of vegetated coastal wetlands, such as mangroves, tidal marshes, and seagrass meadows, also results in greenhouse gas emissions to the atmosphere, as well as the loss of many valuable ecosystem services (Himes-Cornell et al., 2018), but have largely been ignored in greenhouse gas inventories until fairly recently (Kennedy et al., 2013), likely due to their relatively small distribution, which is about 1% or less for each of them in comparison to terrestrial ecosystems such as grasslands and forests (Pidgeon, 2009). However, carbon (C) stocks in vegetated coastal wetlands, termed ‘blue carbon’, are comparable in magnitude to those of terrestrial ecosystems, with C stocks of mangroves comparable to those of terrestrial forest, and C stocks of seagrass meadows comparable to those of croplands (Pidgeon, 2009; Mcleod et al., 2011; Nellemann et al., 2009).
The most noteworthy difference between terrestrial ecosystems and blue C ecosystems is the capacity of blue C ecosystems for long-term C sequestration in their soils and sediments (hereafter referred to as sediments; Pidgeon, 2009). Pools of organic C in blue C ecosystems include living above ground biomass, living belowground biomass, leaf litter and detritus, and that found in sediments. The largest C pool resides in the sediments where it is stored due to a combination of potentially continuous accretion and conditions favorable for long-term organic C storage (Donato et al., 2011; Fourqurean et al., 2012). The sediment C pool is composed of both autochthonous C produced within the ecosystem itself (e.g. leaf litter) and allochthonous C that has been imported (e.g. from land via freshwater inputs, or the ocean; Mcleod et al., 2011).
Disturbance to any of these organic C pools may result in blue C sinks becoming sources of greenhouse gas emissions. In a review of global blue C emissions caused by disturbance, Pendleton et al. (2012) estimated an annual release of 0.15–1.02 Pg (billion tons) of CO2 to the atmosphere, or 3–19% of that caused by deforestation. However, Pendleton et al. (2012) acknowledged that these estimates need refining through improvements to estimates of global distribution and land use conversion rates, as well as research on the fate of C stocks with disturbance. Lovelock et al. (2017) developed a qualitative framework to help assess the risk of emissions from blue C ecosystems, and emphasized a need for more direct evidence for the impacts of specific disturbance types on blue C stocks to turn it into a quantitative framework.
Tidal marshes are particularly vulnerable to human disturbance in that they have been targeted for settlement, livestock grazing, and fishing as far back as the Neolithic Era (Gedan et al., 2009). Land claim – or ‘reclamation’ – is one of the most prevalent causes of intertidal wetland decline globally. Estimates of coastal habitat loss due to reclamation include 88,820 ha from English estuaries since Roman times (Healy and Hickey, 2002), 6500 ha from the Shannon estuary, Ireland, since the 19th century (Healy and Hickey, 2002), and ~755,000 ha from China between 1985 and 2010 (Tian et al., 2016), reflected in a 59% loss of tidal marshes in China between the 1980s and 2010s (Gu et al., 2018) and >80% of wetlands developed in some areas of China (Wu et al., 2018). There are no direct measurements available for the area of tidal marsh reclaimed in Australia, but it is well known as one of the, if not the greatest, cause of tidal marsh loss on the continent over the past two centuries (Laegdsgaard, 2006; Macreadie et al., 2017; Saintilan and Rogers, 2013).
In this study, we investigated the impacts of land reclamation on tidal marsh sediment C stocks in Westernport Bay, Victoria, Australia, an area home to a substantial proportion of the region's blue C stocks (Ewers Lewis et al., 2018). This study is among the first to assess the long-term impacts of reclamation on blue C stocks (see also Ma et al., 2019; Yim et al., 2018), and to the best of our knowledge, the only study to assess the impacts of reclamation empirically across the time scale of centuries using paleoreconstruction. The aim of our study was to assess the impacts of reclamation on: 1) sediment organic C stocks sequestered prior to reclamation; 2) ongoing C sequestration and storage after reclamation; and 3) C quality (i.e. the intrinsic chemical stability of the C). First, we collected one-meter deep sediment cores in adjacent tidal marsh (n = 3) and reclaimed (~150 years ago; n = 3) ecosystems. We next used paleoanalytical techniques to identify the time of reclamation in the sediment horizons, as well as a comparable starting time point towards the base of each core. With these temporal markers, we were able to quantify and qualify C stocks before and after reclamation to assess the impacts of reclamation on tidal marsh sediment C storage.
Section snippets
Study site
Koo Wee Rup (meaning “Great Swamp”), Victoria lies ~75 km southeast of Melbourne on the northern shore of Westernport Bay in southeast Australia. The Bay today is fringed with mangrove and tidal marsh ecosystems, but the region has a history of drainage and land-use conversion following European settlement that led to the destruction of freshwater and tidal wetlands. This included the conversion of the expansive Koo Wee Rup Swamp (around the year 1900) to agriculture that the region is known
Radiometric dating and Itrax-XRF scans
Unsupported 210Pb activities in the TM3 core exhibited a decay profile with depth, enabling 210Pb dating calculations (Fig. S1, Table S1). The 210Pb dating shows the top 25 cm of sediment were accreted in the past 90 ± 11 (CIC model) to 92 ± 10 (CRS model) years at an average mass accumulation rate of 0.052 ± 0.0045 g cm−2 yr−1(CRS model only), equivalent to an average sediment accretion rate of 2.66 (CRS model) to 2.89 (CIC model) mm yr−1 (Fig. 3).
Unsupported 210Pb activities in the RECL1
Identification of historical time points using Itrax and radiometric dating
Based on the radiometric dating and Itrax patterns in our cores, we have identified two major ecologically important time periods in the tidal marsh cores and three in the reclaimed cores (Fig. 3, Fig. 4, S3–S6).
Sediments in the base of the reclaimed core date back to around the time of the last interglacial period (5–6000 years ago). Shifts in Itrax parameters (at >247 cal AD) suggest a shift from a subtidal marine environment (the “before tidal marsh colonization” period) to progradation of
Acknowledgments
We thank the Australian Institute of Nuclear Science and Engineering (AINSE) for grant ALNGRA 15530 that funded the Itrax, 210Pb, and 14C dating. We acknowledge financial support from the Australian Government for the Centre for Accelerator Science at the Australian Nuclear Science and Technology Organisation (ANSTO) through the National Collaborative Research Infrastructure Strategy (NCRIS). JB, JS, and PM thank the support of the CSIRO Carbon Cluster. PM thanks the support of an Australian
Conflict of interest statement
We state there were no financial or other conflicts of interest for any author. Supporting data for this study can be found in the Supplementary materials associated with this article.
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