Drivers of CO₂ along a mangrove-seagrass transect in a tropical bay: Delayed groundwater seepage and seagrass uptake

Highlights

• An aquatic CO₂ source in the mangrove dominated upper embayment waters was associated with delayed groundwater inputs.

• A shifting CO₂ source-sink in the lower embayment bay was driven by the uptake of CO₂ by seagrass and mixing with oceanic waters.

• This more pristine system differs to modified landscapes, where potential uptake of CO₂ is weakened due to the degradation of seagrass beds, or emissions are increased due to drainage of coastal wetlands.

• Antecedent rainfall and radon were found to be the best predictors of CO₂ dynamics, with no clear seasonality observed, in contrast to better studied seasonal temperate systems.

• Extended delays of groundwater-derived CO₂ in the coastal waters compared to mangrove forest waters highlight the importance of comparative studies with more modified systems which are known to have more rapid delivery of carbon to coastal waters.

• This study may assist when determining anthropogenic modification buffer zones in mangrove forest embayments.

Abstract

Water-to-air carbon dioxide fluxes from tropical coastal waters are an important but understudied component of the marine carbon budget. Here, we investigate drivers of carbon dioxide partial pressure (pCO₂) in a relatively pristine mangrove-seagrass embayment on a tropical island (Bali, Indonesia). Observations were performed over eight underway seasonal surveys and a fixed location time series for 55 h. There was a large spatial variability of pCO₂ across the continuum of mangrove forests, seagrass meadows and the coastal ocean. Overall, the embayment waters surrounded by mangroves released CO₂ to the atmosphere with a net flux rate of 18.1 ± 5.8 mmol m⁻² d⁻¹. Seagrass beds produced an overall CO₂ net flux rate of 2.5 ± 3.4 mmol m⁻² d⁻¹, although 2 out of 8 surveys revealed a sink of CO₂ in the seagrass area. The mouth of the bay where coral calcification occurs was a minor source of CO₂ (0.3 ± 0.4 mmol m⁻² d⁻¹). The overall average CO₂ flux to the atmosphere along the transect was 9.8 ± 6.0 mmol m⁻² d⁻¹, or 3.6 × 10³ mol d⁻¹ CO₂ when upscaled to the entire embayment area. There were no clear seasonal patterns in contrast to better studied temperate systems. pCO₂ significantly correlated with antecedent rainfall and the natural groundwater tracer radon (²²²Rn) during each survey. We suggest that the CO₂ source in the mangrove dominated upper bay was associated with delayed groundwater inputs, and a shifting CO₂ source-sink in the lower bay was driven by the uptake of CO₂ by seagrass and mixing with oceanic waters. This differs from modified landscapes where potential uptake of CO₂ is weakened due to the degradation of seagrass beds, or emissions are increased due to drainage of coastal wetlands.

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.