CADICA: Continuous Automated Dissolved Inorganic Carbon Analyzer with application to aquatic carbon cycle science

Abstract

Dissolved inorganic carbon (DIC) represents a significant component of both the terrestrial and oceanic carbon cycles. An instrument for the continuous and automated analysis of dissolved inorganic carbon in the field is described. The analyzer is designed to allow collection of high-resolution (sub hourly) DIC data over prolonged field deployments without the need for labor-intensive maintenance and monitoring. The instrument sequentially acidifies an aliquot of water in gas permeable tubing through which CO2 diffuses into a gas tight chamber and is quantified by an infrared gas analyzer. The instrument is accurate to ± 0.04 mM with a detection limit of 0.01 mM. Comparisons with conventional head space equilibration techniques using mass spectrometry yielded a good correlation differing by 0.016 ± 0.014 mM. Deployment in a tropical estuary and freshwater rainforest catchment revealed the dynamic nature of inorganic carbon cycling in these aquatic systems. DIC concentration was found to track the mixing of fresh and salt water well in the tropical estuary (as indicated by conductivity measurements), and comprised a significant component of dissolved carbon export from the estuary catchment. DIC totalled 24 ± 6% of the total carbon export from the rainforest catchment and varied between 0.04 and 0.13 mM over a 4-d period. The dynamic nature of DIC in the two environments illustrates the pressing need for the continuous and high-resolution data our instrument provides.