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.
Highlights
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
Water temperature was lowest at the ocean mouth increasing towards the shallow mangrove forest water endmember throughout the eight underway surveys
The lowest salinity was observed in the mangrove forest water endmember (26.2; Survey 1) overall average salinity ranges were relatively close to seawater (31.5 - 33.0) reflecting the characteristics of an ocean dominated embayment
Summary
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 km, 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). Tropical coastal embayments typically consist of a continuum of fringing coral reefs, seagrass beds and mangrove forests (Torres-Pulliza et al, 2013). 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 & 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)
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