Variability in the Carbon Storage of Seagrass Habitats and Its Implications for Global Estimates of Blue Carbon Ecosystem Service

Abstract

The recent focus on carbon trading has intensified interest in ‘Blue Carbon’–carbon sequestered by coastal vegetated ecosystems, particularly seagrasses. Most information on seagrass carbon storage is derived from studies of a single species, Posidonia oceanica, from the Mediterranean Sea. We surveyed 17 Australian seagrass habitats to assess the variability in their sedimentary organic carbon (Corg) stocks. The habitats encompassed 10 species, in mono-specific or mixed meadows, depositional to exposed habitats and temperate to tropical habitats. There was an 18-fold difference in the Corg stock (1.09–20.14 mg Corg cm−3 for a temperate Posidonia sinuosa and a temperate, estuarine P. australis meadow, respectively). Integrated over the top 25 cm of sediment, this equated to an areal stock of 262–4833 g Corg m−2. For some species, there was an effect of water depth on the Corg stocks, with greater stocks in deeper sites; no differences were found among sub-tidal and inter-tidal habitats. The estimated carbon storage in Australian seagrass ecosystems, taking into account inter-habitat variability, was 155 Mt. At a 2014–15 fixed carbon price of A$25.40 t−1 and an estimated market price of $35 t−1 in 2020, the Corg stock in the top 25 cm of seagrass habitats has a potential value of $AUD 3.9–5.4 bill. The estimates of annual Corg accumulation by Australian seagrasses ranged from 0.093 to 6.15 Mt, with a most probable estimate of 0.93 Mt y−1 (10.1 t. km−2 y−1). These estimates, while large, were one-third of those that would be calculated if inter-habitat variability in carbon stocks were not taken into account. We conclude that there is an urgent need for more information on the variability in seagrass carbon stock and accumulation rates, and the factors driving this variability, in order to improve global estimates of seagrass Blue Carbon storage.

Effect of environmental factors (wave exposure and depth) and anthropogenic pressure in the C sink capacity of Posidonia oceanica meadows

“…The tourism industry is the main economic sector in the region and has led to intensive urbanization during the last century (Pons et al 2014), particularly in the coastal zone (Murray et al 2008). Tourism in the Balearic Islands started booming around the 1960s (Rullan 1998), leading to a high influx of tourists during the summer period and an increase in population density in the coastal tourist areas, due to the arrival of rural islanders and migrants (Pons et al 2014). As a result, in only 50 yr, the coastline has been rapidly transformed through intensive construction of tourist resorts and the development of urban areas (Pons et al 2014).…”

“…Tourism in the Balearic Islands started booming around the 1960s (Rullan 1998), leading to a high influx of tourists during the summer period and an increase in population density in the coastal tourist areas, due to the arrival of rural islanders and migrants (Pons et al 2014). As a result, in only 50 yr, the coastline has been rapidly transformed through intensive construction of tourist resorts and the development of urban areas (Pons et al 2014). The Balearic Islands are also a globally significant spot for recreational boating (Balaguer et al 2011), which delivers additional pressure to P. oceanica meadows through intensive anchoring (Ceccherelli et al 2007;Montefalcone et al 2008) and through the input of organic matter and nutrients through wastewater discharges (Matthew Leon and Warnken 2008) from boats and harbors.…”

“…In addition, the increasing urbanization and usage of the coast may have affected the carbon burial capacity of P. oceanica meadows over time by altering the patterns of sedimentation and the inputs of autochthonous and allochthonous carbon into meadows sediments. High human activity usually enhances nutrient inputs to coastal areas (Bowen and Valiela 2001) that, when moderate, may favor seagrass productivity (Powell et al 1989). On the contrary, an excess input of nutrients may lead to eutrophication, reducing water quality and seagrass production (Short and Burdick 1996) while increasing the load of allochthonous carbon such as phytoplankton and organic detritus (Borum 1985;Duarte et al 1995) that may get trapped and buried in seagrass sediments.…”

Valuing and Evaluating Marine Ecosystem Services: Putting the Right Price on Marine Environments?

“…A remarkably similar seagrass value to that of Unsworth and colleagues (2010) is cited by Samonte-Tan et al (2007), but the latter study purports to measure a much wider range of economic values and ecosystem services than those evaluated by the former. Refl ecting the increasing interest in carbon sequestration and storage by marine habitats, the seagrass literature now includes estimates of their regulating services based on carbon valuation in the order of A$3.9-5.4 billion (approximately US$3.6-5 billion) (Lavery et al 2013). …”

“…Th ese extensive intertidal, subtidal and deepwater habitats in the tropics are important for their ecological functions and ecosystem services, such as their role in food web dynamics, seascape interactions and biogeochemical processes (Duarte 2002;Moberg and Ronnback 2003). While the economic value of seagrasses for cycling nutrients has been long established (Costanza et al 1997), there is now increasing recognition of how seagrass meadows store and sequester globally significant amounts of carbon (Fourqurean et al 2012;Lavery et al 2013) and buff er seawater acidity (Unsworth et al 2012;Hendriks et al 2013). Seagrass meadows and their fauna also directly and indirectly provide food security and livelihoods globally (Jackson et al 2001;.…”