Research articleBeach-cast seagrass wrack contributes substantially to global greenhouse gas emissions
Graphical abstract
Introduction
Seagrass beds are highly productive coastal ecosystems, with annual seagrass production estimated at 1 kg DW m−2 per year (Duarte and Chiscano, 1999). Despite covering only 0.15% of global sea surface area, these ecosystems contribute approximately 1% of the net primary production of the global ocean (Duarte and Cebrian, 1996). Roughly half of this seagrass biomass is in the above-ground biomass, and is mainly composed of leaf and stem material (Duarte and Chiscano, 1999, Hemminga and Duarte, 2000). The lifespans of seagrass leaves are on average 90 days (Hemminga et al., 1999), while rhizome lifespans can range from days to years (Hemminga and Duarte, 2000). Duarte and Chiscano (1999) estimated seagrass above-ground biomass turnover rate to be 2.6% of total standing stock per day. As a result of this high turnover rate, detached seagrass biomass that is subsequently transported by wind and current dynamics (Mateo, 2010, Jiménez et al., 2017) may result in large amounts of seagrass wrack accumulation along coastlines globally (Colombini and Chelazzi, 2003, Cardona and García, 2008, Macreadie et al., 2017) (Fig. 1).
Seagrass wrack has many important ecological functions, providing food and habitat for sandy beach fauna (Ince et al., 2007, Lewis et al., 2007), nutrients for dune vegetation (Cardona and García, 2008, Del Vecchio et al., 2017), and protection for coastal dunes (Kirkman and Kendrick, 1997, Dugan et al., 2003). Nevertheless, seagrass wrack is often considered a nuisance to humans due to the production of unpleasant odors when wrack mattes decompose on the shoreline (Hansen, 1984, Kirkman and Kendrick, 1997). This decomposition process also coincides with the production of carbon (C) emission hotspots (greenhouse gas, GHG) in coastal habitats (Coupland et al., 2007). Coupland et al. (2007) reported that if left on the beach, seagrass wrack has an approximate emission rate of 6 μmol CO2 m−2 s−1. However, thus far the GHG emission production of the accumulated seagrass wrack has only been measured at a single time point, leaving a significant gap in what is known about how GHG emissions vary during the seagrass wrack decomposition process.
Seagrass wrack deposited along the coast is exposed to periods of wetting through tidal inundation, and can be redistributed locally by storm and wind action (Coupland et al., 2007) (Fig. 1). The seagrass wrack accumulating near the water's edge is more likely to retain moisture due to tidal inundation (Fig. 1c), while seagrass wrack accumulating away from water's edge is exposed to relatively dryer conditions (Mateo et al., 2006, Coupland et al., 2007, Cardona and García, 2008) (Fig. 1d). Moisture can accelerate the decomposition of plant wrack by facilitating the loss of soluble compounds through leaching and enhancing the activity of decomposers (Dick and Osunkoya, 2000, Nicastro et al., 2012), which ultimately results in elevated GHG flux (Sayer et al., 2011, Liu et al., 2017). For example, Coupland et al. (2007) found that the macrophyte wrack near the water's edge showed much higher CO2 fluxes (∼6–12 μmol m−2 s−1) than those further away from the water's edge (∼2–3 μmol m−2 s−1). However, empirical evidence of how moisture influences seagrass wrack decomposition and thus GHG emissions is otherwise rare.
Seagrasses globally are diverse in their inherent structure and chemistry, which could influence their decomposition. Trevathan-Tackett et al. (2017a) has found that morphologically larger taxa have the potential to contribute more refractory organic matter to vegetated coastal ecosystems, i.e., Posidoniaceae > Zosteraceae in temperate seagrass families. de los Santos et al., 2012, de los Santos et al., 2016 have reported that high lignocellulose content of seagrass tissues usually corresponding to low consumption and high breaking force, with important implications of low decomposition rate. Furthermore, the decomposition rate of rhizome and root tissues, always with more lignocellulose content than leaf tissues, are significantly lower than those of leaf tissues (Fourqurean and Schrlau, 2003, Vichkovitten and Holmer, 2004). For example, only 5% of the original mass of Thalassia testudinum leaves remained, compared to 49% of rhizome after 348 days of decomposition (Fourqurean and Schrlau, 2003). As such, decomposition rates of seagrass wrack are influenced by seagrass species chemistry and tissue proportions, and consequently may result in different GHG emission potential.
Here we performed a laboratory experiment to test how the inherent seagrass characteristics (i.e. morphology and chemistry) and position on the beach (moisture content) affect GHG emission rates. We chose the common beach-cast seagrass species Amphibolis antarctica and Zostera nigricaulis that have contrasting morphologies (Amphibolis spp. have long vertical rhizomes) (Lavery et al., 2013), and chemistries (Zostera spp. wrack generally has more labile organics and high decomposition rates) (Cebrián et al., 1997, Trevathan-Tackett et al., 2017a). We aimed to use the different species with distinct chemistry and tissue proportions to explore the contribution of beach-cast seagrass wrack to GHG emissions under wet and dry conditions by estimating the possible range of GHG emission potential. The results from this study will help estimate the possible GHG emissions from seagrass wrack, as well as provide a basis for creating useful solutions for reducing GHG emissions for resource managers.
Section snippets
Experimental design and sampling
In April 2017, we collected freshly deposited, green A. antarctica (from outside the Sommers yacht club, 38.39° S, 145.15° E, VIC, Australia) and Z. nigricaulis (from outside the Hastings yacht club, 38.30° S, 145.19° E, VIC, Australia) wrack, which was distributed near the water's edge. During collection, seagrass samples with no visual signs of decomposition (no discoloration, intact leaves and stems) were targeted to ensure seagrass tissues were fresh with minimal decomposition occurring
Characteristics of seagrass wrack
The mean elemental C and N content of Z. nigricaulis and A. antarctica wrack at the beginning of the incubation were 30.1% C and 1.85% N, and 30.1% C and 0.92% N, respectively. After 30 days of decomposition, seagrass biomass loss was significantly influenced by moisture (Table 1, p = 0.003) but not species (Table 1, p = 0.534), with the wet group losing ∼6% more biomass than the dry group (Table 2). The results also showed that the remaining C content of the wet group was significantly lower
Discussion
The elemental C and N content and C/N ratio of collected Z. nigricaulis and A. antarctica wrack were similar to values of living tissues (Walker and Mccomb, 1988, Pedersen et al., 1997, Hirst et al., 2016, Hirst and Jenkins, 2017), indicating the collected seagrass wrack was fresh prior to the laboratory incubation. After 30 days' incubation, the remaining biomass of Z. nigricaulis wrack was 83%, which was consistent with previous studies that found 60–90% of Zostera spp. biomass remained after
Conclusion
This study suggests that beach-cast seagrass wrack can be a globally-significant contributor to greenhouse gas emissions. Using wrack with contrasting chemistries and tissue components, we estimated that the annual global CO2-C flux of seagrass wrack is estimated between 1.31 and 19.04 Tg C yr−1, which is equivalent to annual emissions of 0.63–9.19 million Chinese citizens. As dry conditions can lead to a 72% decrease in CO2-C flux compared to wet conditions, we recommend (where practical) that
Author contributions
SL, STT, XH and PIM conceptualized and designed this study. SL, CJEL, QRO, STT and PM performed the experiments. SL, ZJ and XH performed data analysis. All authors contributed to the writing of the manuscript. All authors have read and approved the final version of the manuscript.
Acknowledgments
This research was supported by the National Natural Science Foundation of China (41730529, 41806147), the National Basic Research Program of China (2015CB452905), the Natural Science Fund of Guangdong (2018A030310043), the National Specialized Project of Science and Technology (2015FY110600), and an AMP Tomorrow Maker Award.
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