Research papersDrivers of CO2 along a mangrove-seagrass transect in a tropical bay: Delayed groundwater seepage and seagrass uptake
Introduction
The large net primary production of the world's coastal embayments are exported to coastal waters (Robertson et al., 1992) primarily through the interplay of tidal dynamics and seasonal river discharge. With a global area of ~45000 km2, intertidal areas of temperate embayments predominantly comprise of salt marsh habitats (Greenberg et al., 2006) occupying low-lying topographic zones (Scott et al., 2014) and high distributions of seagrass beds in subtidal zones (Short et al., 2007). In contrast, tropical coastal embayments typically consist of a continuum of fringing coral reefs, seagrass beds and mangrove forests (Torres-Pulliza et al., 2013). Near-shore tropical mangrove forests are the most carbon-rich forests on earth, storing and sequestering globally significant amounts of carbon in their soils (Donato et al., 2011). Occupying only 0.02% of global surface area, mangrove forests are responsible for approximately 11% of the total terrestrial organic carbon delivery to oceans (Jennerjahn and Ittekkot, 2002, Sippo et al., 2017). Mangroves are tightly connected with their adjacent habitats (Signa et al., 2017) and support marine biodiversity, regulate water quality and protect tropical coastlines against storms (Ganguly et al., 2017).
Tropical seagrass beds are located shoreward of coral reefs and seaward of mangrove forests in areas with high light availability and favourable water quality (Guannel et al., 2016) and have been reported to be largely net autotrophic (i.e. a net atmospheric CO2 sink) (Duarte and Cebrian, 1996). Coastal geomorphology is recognised as being important in seagrass abundance, distribution and diversity, as these habitats usually exist near fringing reefs in protected, shallow coastal lagoons (Torres-Pulliza et al., 2013). Combined, mangrove forests and seagrass beds play a major role in biological connectivity of coastal embayments, acting as coastal buffers by filtering sediment and nutrient loads to adjacent coral reefs (Hemminga and Duarte, 2000).
Indonesia, lying between latitudes 6°N and 11°S, has a coastline of more than 95,180 km, the second longest coastline in the world (Spalding et al., 1997) and 2.9 Mha of mangrove cover, larger than any continent on earth (Atwood et al., 2017). With such an extent and high carbon stocks, Indonesia's mangrove forests store on average 3.14 PgC (Murdiyarso et al., 2015). However, in three decades (1985–2005), Indonesia has lost 40% of its mangroves, mainly as a result of aquaculture development (Giri et al., 2011). This has resulted in potential global annual emissions of 0.07–0.21 Pg CO2 (Murdiyarso et al., 2015). Seagrass beds cover an estimated 30,000 km2 of Indonesian coastline (Green and Short, 2003), and combined with mangrove forests, account for approximately 3.4 Pg C (~17%) of the global blue carbon reservoir (Alongi et al., 2016).
Mangrove-seagrass connectivity research has usually centred on the exchange of dissolved organic carbon (DOC) and particulate organic carbon (POC) (Dittmar et al., 2009, Hemminga et al., 1994, Maher et al., 2013, Müller et al., 2015). Little is known about CO2 interactions between near-mangrove forest surrounding waters (described as mangrove forest water from here on) and adjacent seagrass beds. Stable isotope studies show that seagrasses close to mangroves have a more depleted δ13C value than those further away (Bouillon et al., 2008b, Hemminga et al., 1994), suggesting that seagrasses are fixing DIC sourced from mangrove respiration.
Mangrove groundwater and porewater exchange can be an important source of carbon to coastal waters (Bouillon et al., 2007, Maher et al., 2013, Maher et al., 2017, Sadat-Noori et al., 2016). A recent literature review demonstrates that groundwater fluxes in mangroves can be a major component of tropical coastal carbon budgets with fluxes on the same order of magnitude as rivers (Chen et al., 2018). Since mangrove forests usually coexist with seagrass beds and coral reefs (Fourqurean et al., 1992), and carbon exchange along this continuum supports cross-productivity (Unsworth et al., 2008), understanding the relationship between groundwater seepage, carbon dynamics and ecosystem connectivity in transition zones is important.
Here, we investigate the drivers of pCO2 dynamics along a mangrove-seagrass transect in Bali, Indonesia. We performed coupled, automated seasonal pCO2 and radon (222Rn; a natural groundwater tracer) investigations to assess whether CO2 is derived from groundwater or porewater pathways. We investigate temporal and spatial scales of pCO2 dynamics, hydrological drivers such as groundwater seepage, delayed antecedent rainfall, and interplay along the mangrove-seagrass continuum in a non-impacted embayment.
Section snippets
Area description
Gilimanuk Bay, a 3.7 km2 coastal embayment, is located in Jembrana Regency on the northwest coast of Bali, Indonesia. Including two small islands, Kalong Island and Burung Island, the area contains some of Bali's most pristine mangrove forests (Thoha, 2007) (Fig. 1). Oceanic upwelling and tidal exchange from the deep Java Strait supply nutrients to Gilimanuk Bay (Ningsih et al., 2013, Siswanto, 2008). The average depth of the embayment is ~2 m, with intertidal zones in the upper embayment
Seasonal spatial surveys
Water temperature was lowest at the ocean mouth increasing towards the shallow mangrove forest water endmember (max = 34.7 °C; Table 2) throughout the eight underway surveys. The lowest salinity was observed in the mangrove forest water endmember (26.2; Survey 1) however overall average salinity ranges were relatively close to seawater (31.5–33.0) reflecting the characteristics of an ocean dominated embayment. Dissolved oxygen (DO) was slightly undersaturated at the ocean entrance and increased
Discussion
Aquatic systems in Southeast Asia are recognised as significant sources of CO2 to the atmosphere but remain poorly represented in global databases (Müller et al., 2015). The few studies available focus on tropical river-dominated estuaries which produce significant CO2 fluxes (Borges and Abril, 2011, Müller et al., 2015). Global summaries of water-to-air CO2 fluxes are generally confined to human impacted river-dominated estuarine systems (Cai, 2011). A recent study in a temperate autotrophic
Conclusion
Our investigations across a coral reef-seagrass-mangrove continuum revealed a CO2 source in the mangrove dominated upper bay apparently associated with delayed groundwater inputs, and differing CO2 dynamics in the lower bay driven by the uptake of CO2 by seagrass. The bay mouth was a source of CO2 possibly due to production of CO2 during fringing coral reef calcification. The average CO2 water-to-air flux along the transect was 9.8 ± 6.0 mmol m−2 d−1. Antecedent rainfall and radon were the best
Acknowledgements
Paul Macklin was funded by the Australian Government Research Training Program Scholarship. We acknowledge Ceylena Holloway from the National Marine Science Centre for support with research instrumentation. The Ministry of Research, Technology and Higher Education of the Republic of Indonesia (RISTEKDIKTI); The Ministry of Home Affairs of the Republic of Indonesia (MoHA); the Direktorat Jenderal Sumber Daya Air and Governor of Bali I Made Mangku Pastika are acknowledged for access and
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