Quantifying and modelling the carbon sequestration capacity of seagrass meadows – A critical assessment
Graphical abstract
Introduction
Reducing carbon (C) emissions is a necessary step in the fight against climate change. In addition, because greenhouse gases will linger in our atmosphere for another hundred years, there is also a need to find ways to remove C from the atmosphere. Biosequestration is one promising option that capitalises on natural CO2 capture and storage by photosynthetic organisms and soil microbes. Ironically, it is the same process that created fossil fuels (i.e. the carboniferous forests, which produced the coal measures, and the rich deposits of microalgae which gave rise to oil-rich strata). Although much of the attention on biosequestration has centred on terrestrial forests, the world’s greatest C storage potential may be in our coastal oceans.
Recent data estimates that seagrasses, together with saltmarshes and mangroves, are responsible for capturing up to 70% of the organic C in the marine realm (Nellemann et al., 2009), making them one of the most intense C sinks on the planet. Seagrass meadows bury C at a rate that is 35× faster than tropical rainforests, and their sediments never become saturated (McLeod et al., 2011). Furthermore, while terrestrial forests bind C for decades, seagrasses meadows can bind C for millennia (Macreadie et al., 2012, Mateo et al., 1997, Serrano et al., 2012). In a comprehensive survey of seagrass C stocks collected from almost 1000 meadows, Fourqurean et al. (2012) estimated that seagrasses can store 4.2–8.4 Pg C, 26 times higher than earlier estimates (Duarte and Chiscano, 1999). However, the significant capacity of coastal seagrasses to sequester C has gone unrecognised in models of global C transfer, and greenhouse gas abatement schemes. This is a major problem since the role of seagrasses as global C sinks continues to be threatened by coastal development and climate change.
Already 29% of the world’s seagrasses have been destroyed (Waycott et al., 2009), heralding the loss of an important long-term C sink, and raising concern that degraded seagrass meadows could leak vast amounts of ancient C back out into the atmosphere, thus shifting seagrasses from C sinks to C sources, and potentially accelerating climate change. Recent estimates suggest that continued seagrass loss could release up to 299 Tg C into the atmosphere each year, which equates to 10% of all CO2 emissions attributed to anthropogenic changes in land use (Fourqurean et al., 2012). The economic cost of this seagrass loss in terms of C emissions, at a C price of US$ 41 per ton of CO2, is estimated to be between US$ 1.9 and 13.7 billion yr−1 (Pendleton et al., 2012). Thus, the potential emissions from continued loss of seagrass meadows is likely to have globally significant economic consequences, not to mention costs associated with loss of other ecosystem services provided by seagrasses, such as: shoreline stabilization (Bos et al., 2007); nutrient cycling (Costanza et al., 1997); and provision of habitat for fish, bird, and invertebrate species (Heck et al., 2003, Hughes et al., 2009).
A current limitation to the inclusion of seagrasses in global greenhouse gas (GHG) abatement schemes (e.g. REDD+) is a paucity of data on C budgets from seagrass meadows covering a range of species and conditions. Those seagrass budgets that have attracted global interest are derived from a few pristine habitats and are not globally representative. Furthermore, the techniques used to generate these data are considered rudimentary and outdated by terrestrial standards. It is therefore necessary to conduct a comprehensive and rigorous assessment of seagrass C budgets using the latest technologies, and to use this information to model the sequestration capacity for different species and conditions.
The aim of this paper is to: (1) provide an update on policy development concerning inclusion of seagrasses (and other ‘Blue Carbon’ habitats; salt marshes and mangroves) within global C accounting frameworks; (2) highlight complexities and challenges in developing accurate C budgets; (3) review and critique key techniques and methodologies that can be used in research towards developing C budgets; (4) describe a process-based data assimilation model for studying C cycling within seagrass ecosystems; and (5) provide a practical list of research priorities that will lead to policy change concerning the development of effective measures to protect vulnerable seagrass C stocks, as well as restore and improve the C sequestration capacity of seagrass ecosystems.
Section snippets
Policy status: protecting C stocks and the sequestration capacity of seagrasses
In 1988, the Intergovernmental Panel on Climate Change (IPCC) was established as the world authority to assess the state of knowledge on climate change. The expert opinion of the IPCC influenced the Kyoto Protocol that was established by the United Nations Framework Convention on Climate Change (UNFCCC). Commissioned by the UNFCC, The International Blue Carbon Scientific Working Group has been tasked with determining the role of coastal wetlands (seagrasses, as well as saltmarshes and
Developing a seagrass C budget: components, challenges, and complexities
The overall C budget of an ecosystem is defined by the amount of C stored (C stock), which is altered by the accumulation or release of C from this stock (=C flux). Simply measuring the C stock in isolation, without taking into consideration the rate of change or flux of a C stock, is not sufficient to assess whether the stock is accumulating, stable, or declining. Depending on their health, seagrasses can either behave as C sinks by sequestering C and burying it in the sediment, or as C
Techniques and technologies for measuring C flux and stocks in seagrass meadows
During the past several decades, numerous methods have been used to measure C flux and stocks in seagrass meadows, each having a set of benefits and limitations based on cost, accuracy, ease of use and autonomy (Table 1). Below, several of the most common methods will be briefly reviewed for the purposes of highlighting their advantages and disadvantages. By no means is this section intended an exhaustive review; rather, this section is intended to provide an overview of the challenges and
Process-based models and the potential of data assimilation schemes to improve C budgets
As outlined in the Section 2, approaches to generating C budgets commonly involve measuring C fluxes directly, or measuring changes in C stocks through time. Both approaches have advantages and disadvantages, and we argue that it is necessary to measure both flux and stock in order to develop robust models of C dynamics in seagrass ecosystems. However, given the current state of knowledge and complexities of C cycling dynamics within seagrass ecosystems, it is not possible to include all
Better estimates of global seagrass area
There is major uncertainly in the estimates of global seagrass area and biomass as there is for most submarine populations. This necessarily results from the vastness of the seas and the limited resources of human societies to devote to these estimates. Nevertheless there is a need to define the problems involved. Of the over 35 species of seagrass, the Halophila species are not significant in terms of biomass and can be ignored in the first analysis (Hillman et al., 1989). The remaining genera
Conclusions
The most effective approach for coastal management policy to promote C sequestration is to aim to maintain water quality conditions that encourage seagrass health (Laffoley and Grimsditch, 2009). Furthermore, it should be noted that there is equal or greater value in retaining the biodiversity, a range of ecosystem services, and healthy coastal wetland as a GHG sink rather than a source. For seagrasses to be included in IPCC frameworks, the first priority is to develop global seagrass C
Acknowledgements
We acknowledge the support of the CSIRO Coastal Carbon Cluster (P.M., M.B., A.L., P.R.), an Australian Research Council Discovery Early Career Researcher Award DE130101084 (P.M.), and an American Australian Association Dow Chemical Company Fellowship (P.M.).
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current address: Commonwealth Scientific Industrial Research Organisation, Marine and Atmospheric Research, GPO Box 1538 Hobart 7001 Australia.