Lateral exchange of mangrove sediment porewater dissolved inorganic carbon to coral reef ecosystems

Ocean acidification caused by anthropogenic CO2 is a major threat to coral reef ecosystems. However, natural acidification can also occur from upwelled CO2 rich waters or from lateral exchange of CO2-rich mangrove sediment porewater. This study explores how natural acidification through respiration in mangrove sediments affects nearby coral reefs. Our goal is to determine if tidal pumping of mangrove sediment exports water with lower pH and thus exposes corals to water with lower aragonite saturation state (Ω). Water samples were taken from various ecosystems around Isla Colon, Panama (Bocas del Toro province) including mangrove sediment porewaters, a creek draining a mangrove forest, two mangrove-proximal reefs and one mangrove-distal reef. We also sampled water from rivers draining karst terrain and waters draining a rainforest on silicate bedrock as well as waters farther from the coast. Mangrove porewaters had high pCO2 (as high as 56,000 ppm) and high alkalinity (TA, up to 8.5 mmol/Kg). The adjacent creek also had high, but variable pCO2 over a week of sampling (1650-7100 ppm), which increased with increasing precipitation, but had highest concentrations during the night (~4000 ppm) compared to the day (~850 ppm). Total alkalinity remained somewhat constant (2.5±0.1 mmol/Kg), but was higher at night (3.3±0.2 mmol/Kg) compared to day (2.4±0.1 mmol/Kg). The mangrove-proximal reefs generally had higher pCO2, TA and Ω (pCO2 ~ 550 ppm; TA ~ 2.25 mmol/Kg; Ω ~ 2.7) compared to mangrove-distal reefs (pCO2 ~ 400 ppm; TA ~ 2.1 mmol/Kg; Ω ~ 2.4). The higher saturation state is likely from the dissolution of carbonate bedrock below the mangrove sediments. Moreover, the rivers draining this island are also high in alkalinity compared to those on silicate bedrock (~2.0 vs. 0.5 mmol/Kg). Nevertheless, continual monitoring (every 15 minutes) of one mangrove-proximal reef illustrated that pH can drop to 7.7 and dissolved O2 to 40% saturation during periods of high precipitation coupled with outgoing tides. These data illustrate that although mangrove sediments are a major source of H+, it is buffered by carbonate rock weathering, which in turn keeps saturation states high. However, there are occasions when the reefs are exposed to lower pH waters