Harnessing autonomous technologies and discrete water sampling to investigate the role of high pCO2 river inputs on coastal acidification: A case study from the Belize River and Mesoamerican Barrier Reef.

The Mesoamerican Barrier Reef, the second largest barrier reef in the world, is vitally important to the ecology and economy of Belize and neighbouring countries. Coral reefs are inherently vulnerable to ocean acidification and those exposed to significant riverine input may be under enhanced threat. In tropical rivers pCO₂ levels may be linked to land use in their catchment, with conversion of pristine forest to agricultural land potentially enhancing carbon flux to the coastal ocean. Through a multidisciplinary approach, we investigated the effect the Belize River may have on the carbonate chemistry of surrounding coastal seas. Samples were collected for total alkalinity, dissolved inorganic carbon and δ¹³C_DIC alongside using pH and pCO₂ sensors mounted on an autonomous surface vehicle, C-worker 4. Sampling was conducted from river source, downstream and over the barrier reef. We measured pCO₂ >1000-µatm at the mouth of the Belize River suggesting high levels of respiration. Using salinity as a tracer we note both CO₂ degassing near the river mouth and transport of high pCO₂ water towards the reef, indicating the importance of tropical rivers to both atmospheric CO₂ budgets and coastal acidification. δ¹³C_DIC samples were taken to identify terrestrial DIC signatures and were used in combination with sensor data to identify potential controls on coastal pH. Our data allows us to shed light on the complex processes controlling coastal ocean acidification not only for a country heavily dependent on their barrier reef but also as a case study for similar regions.