Can macroalgae contribute to blue carbon? An Australian perspective

Abstract

Macroalgal communities in Australia and around the world store vast quantities of carbon in their living biomass, but their prevalence of growing on hard substrata means that they have limited capacity to act as long-term carbon sinks. Unlike other coastal blue carbon habitats such as seagrasses, saltmarshes and mangroves, they do not develop their own organic-rich sediments, but may instead act as a rich carbon source and make significant contributions in the form of detritus to sedimentary habitats by acting as a "carbon donor" to "receiver sites" where organic material accumulates. The potential for storage of this donated carbon however, is dependent on the decay rate during transport and the burial efficiency at receiver sites. To better understand the potential contribution of macroalgal communities to coastal blue carbon budgets, a comprehensive literature search was conducted using key words, including carbon sequestration, macroalgal distribution, abundance and productivity to provide an estimation of the total amount of carbon stored in temperate Australian macroalgae. Our most conservative calculations estimate 109.9 Tg C is stored in living macroalgal biomass of temperate Australia, using a coastal area covering 249,697 km 2 . Estimates derived for tropical and subtropical regions contributed an additional 23.2 Tg C. By extending the search to include global studies we provide a broader context and rationale for the study, contributing to the global aspects of the review. In addition, we discuss the potential role of calcium carbonate-containing macroalgae, consider the dynamic nature of macroalgal populations in the context of climate change, and identify the knowledge gaps that once addressed will enable robust quantification of macroalgae in marine biogeochemical cycling of carbon. We conclude that macroalgal communities have the potential to make ecologically meaningful contributions toward global blue carbon sequestration, as donors, but given that the fate of detached macroalgal biomass remains unclear, further research is needed to quantify this contribution.

Carbon dioxide mitigation potential of seaweed aquaculture beds (SABs)

“…For example, a total of 5774 km 2 of blue carbon ecosystems (estimated at the Abu Dhabi workshop; seagrass, algal mat, mangrove and saltmarsh) potentially stored 3.9 × 10 6 t C (144.8 × 10 6 t CO 2 e) (AGEDI 2013). When compared to wild seaweed beds, 49,939-124,849 km 2 of Australian temperate wild seaweed beds could store up to 109.9 Tg C (Hill et al 2015) and 2012 km 2 of algal and seagrass beds along the coasts of Japan could store 2.7 × 10 6 t C (Muraoka 2004). It is important to include SABs in C emission schemes as they are increasing in terms of volume of production and cultivation area, whilst other important blue carbon coastal habitats (mangrove, seagrass, and saltmarsh areas) have decreased by 340,000-980,000 ha annually as a result of human pressures on coastal ecosystems (Murray et al 2011).…”

“…Both wild seaweed communities and SABs are important habitats that can also be considered as short-term blue carbon sinks and significant donors to long-term carbon sequestration along the coasts of all continents (Hill et al 2015;TrevathanTackett et al 2015). Countries with extensive shallow waters suitable for seaweed cultivation should be further explored for their contribution to mitigation efforts to reduce GHG emissions and existing wild seaweed beds/forests should be the focus of protection and restoration for their carbon sink mitigation potential.…”

“…Seaweeds and SABs can potentially make effective contributions to CO 2 mitigation because some seaweeds have cell wall structures and composition that can store carbon over the long-term by becoming a carbon donor to other ecosystems (Hill et al 2015;Trevathan-Tackett et al 2015) and by converting the biomass into a range of bioenergy products from biogas to liquid and solid biofuels. We discuss these roles in more detail below.…”

“…The three-dimensional structure of natural seaweed beds/forests provides shelter to many organisms, and these beds also serve as feeding and nursery grounds for many commercially important species (Olafsson et al 1995;Paddack and Estes 2000;Eklöf et al 2006;Christie et al 2009;Wattage 2011;Walsh and Watson 2011;Eklöf et al 2012;Valderrama 2012). Natural seaweed beds/forests play very important roles in facilitating recruitment of marine organisms (Okuda 2008), absorbing excess nutrients (Fei 2004;Yang et al 2006;Huo et al 2012), dampening waves (Jackson 1984;Anderson et al 1996;Lovas and Totum 2001), buffering against ocean acidification (Gao and Zheng 2010), and potentially in serving as a carbon sink for anthropogenic CO 2 (Hill et al 2015).…”

“…This potential role of SABs, however, has not been seriously evaluated. It should be far easier to quantify the amount of carbon sequestration by SABs than that by natural seaweed beds as the latter are spatially and temporally more variable (Fei 2004;Hill et al 2015). Furthermore, being excellent nutrient removers, farmed seaweeds like Pyropia (formerly Porphyra) can be integrated into cultivation operations for fish that produce high nutrient loadings (Chopin et al 1999;He et al 2008).…”