Loss of ‘Blue Carbon’ from Coastal Salt Marshes Following Habitat Disturbance

Citation: Macreadie PI, Hughes AR, Kimbro DL ( Loss of 'Blue Carbon' from Coastal Salt Marshes Following Habitat Disturbance Peter I. Macreadie 0 A. Randall Hughes 0 David L. Kimbro 0 0 1 Plant Functional Biology and Climate Change Cluster (C3), School of the Environment, University of Technology , Sydney (UTS), New South Wales , Australia , 2 Marine Science Center, Northeastern University (NU) , Boston, Massachusetts , United States of America Increased recognition of the global importance of salt marshes as 'blue carbon' (C) sinks has led to concern that salt marshes could release large amounts of stored C into the atmosphere (as CO2) if they continue undergoing disturbance, thereby accelerating climate change. Empirical evidence of C release following salt marsh habitat loss due to disturbance is rare, yet such information is essential for inclusion of salt marshes in greenhouse gas emission reduction and offset schemes. Here we investigated the stability of salt marsh (Spartina alterniflora) sediment C levels following seagrass (Thallasia testudinum) wrack accumulation; a form of disturbance common throughout the world that removes large areas of plant biomass in salt marshes. At our study site (St Joseph Bay, Florida, USA), we recorded 296 patches (7.5 2.3 m2 mean area SE) of vegetation loss (aged 3-12 months) in a salt marsh meadow the size of a soccer field (7 275 m2). Within these disturbed patches, levels of organic C in the subsurface zone (1-5 cm depth) were ~30% lower than the surrounding undisturbed meadow. Subsequent analyses showed that the decline in subsurface C levels in disturbed patches was due to loss of below-ground plant (salt marsh) biomass, which otherwise forms the main component of the long-term 'refractory' C stock. We conclude that disturbance to salt marsh habitat due to wrack accumulation can cause significant release of below-ground C; which could shift salt marshes from C sinks to C sources, depending on the intensity and scale of disturbance. This mechanism of C release is likely to increase in the future due to sea level rise; which could increase wrack production due to increasing storminess, and will facilitate delivery of wrack into salt marsh zones due to higher and more frequent inundation. - Funding: This work was supported by an Australian Research Council DECRA grant DE130101084 (to PIM), an American Australian Association Dow Chemical Company Fellowship (to PIM), a University of Technology Sydney International Researcher Development Program Grant (to PIM), National Science Foundation Division of Environmental Biology grant 0928279 (to ARH), and National Science Foundation Division of Biological Oceanography grant 0961633 (to DLK). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing interests: The authors have declared that no competing interests exist. Salt marshes are one of the most powerful carbon (C) sinks on the planet. They bury at a rate ~55 times faster than tropical rainforests (regarded as one of the most significant terrestrial C sinks), and their global carbon burial (up to 87.2 9.6 Tg C yr-1 based on preliminary assessments) appears to exceed that of tropical rainforests (53 9.6 Tg C yr-1). These rates are particularly staggering given that salt marshes occupy only a small fraction (0.1-2%) of the total land area of tropical rainforests [1]. Furthermore, salt marshes can store C for millennia [2,3], whereas rainforests usually only store C for decades. Despite their value as C sinks, salt marshes have undergone rapid global decline [25% since the 1800s; 4,5], particularly due to landscape conversion for housing and farming [6,7]. This raises concerns that society is losing an important C sink, and that large amounts of ancient buried C are being released into the atmosphere as CO2 and contributing to global warming. Disturbance is likely to affect the C sink capacity of salt marshes in four main ways. First, salt marshes filter and capture laterally-imported allochthonous C that contributes to the below-ground sediment C stock; therefore, loss of salt marsh plant material following disturbance could reduce this particulate C trapping capacity, thereby causing a reduction in C stock accumulation [8,9]. Second, disturbance of salt marshes could reduce the overall plant biomass contributing to C capture via photosynthesis [10]; this loss of photosynthetic capacity could also reduce the total amount of C captured by salt marshes [11]. Third, disturbance of salt marsh could cause loss of C stored in plant material itself (structural C) due to plant die off [12], which could be exported and lost from salt marsh ecosystems if it does not make its way into the salt marsh sediment C stock [13]. Fourth, and perhaps most importantly, disturbance of salt marsh could result in the release of buried ancient sedimentary C via erosion, leaching, and microbial mineralization [14]. There is relatively little empirical evidence for C loss from salt marshes following disturbance, yet such information is essential for inclusion of salt marshes in greenhouse gas emission reduction and offset schemes (e.g. the United Nations programme on Reducing Emissions from Deforestation and Forest Degradation - UN-REDD+). Most studies that have reported loss of C from salt marshes have been done in systems that have been broadly classified as wetlands. Although these may include salt marsh, they typically consist of mixed habitats, complicating attempts to ascertain the responses of salt marsh responses per se. Furthermore, these studies are usually based on landscapescale conversions (e.g. conversion into farmland), and there appears to be no data on the effects of smaller-scale (withinhabitat) disturbances, which are increasingly common and ecologically relevant [15,16,17]. Nevertheless, we do know that landscape-scale disturbances of wetlands can cause significant depletion of organic C (e.g. 96% [18]) and weakening of the C sequestration capacity [2,19]. The purpose of this study was to quantify the effects of small-scale (within-habitat) disturbance and concomitant habitat loss on the C sink capacity of salt marsh. Specifically, through analyses of sediment cores, we tested the hypothesis that disturbed areas of salt marsh dominated by the marsh grass Spartina alterniflora (hereafter referred to as salt marsh) would have significantly lower amounts of organic C than adjacent undisturbed salt marsh. We then explored which C pools (below-ground plant C biomass vs. sedimentary organic C) are vulnerable to disturbance, and the ecological consequences of these losses. In our study, we capitilised on a natural disturbance event that is common along the Gulf Coast of the USA (and elsewhere around the world): die-off of salt marsh due to seagrass (Thalassia testudinum) wrack accumulation, which occurs when senesced or stormfragmented seagrass leaves are (...truncated)