Using Mass Spectrometry to Measure Dissolved Gases in Sandy Sediment Porewater

Coastal ecosystems are key sites of important nutrient cycling processes such as photosynthesis and respiration. While some of these processes occur in the water column, an often overlooked component is the sandy sediments blanketing the coastal seafloor. These sediments are alive with marine microorganisms which mediate nutrient recycling, producing and consuming gases such as O₂, CO₂, N₂, and CH₄ in the process. Quantifying dissolved gases in the porewater of sandy sediments traces these processes and reveals how these sediments function. In practice, sandy sediments are notoriously difficult to sample – due to their low cohesiveness, traditional core extraction methods routinely used for muddy sediments cannot be employed, necessitating in situ measurements. To address this formidable task, we are developing a flow injection system, coupled to a membrane inlet mass spectrometer, which can measure multiple environmentally-important gases simultaneously in shallow coastal sediments. Recent initial tests of the porewater sampling system occurred in a series of experiments, ranging from small-scale manipulations in the laboratory, to two deployments on sandy beaches in Florida (United States) and Nova Scotia (Canada). For the field deployments, both an underwater mass spectrometer, as well as a benchtop version adapted for field use, were employed. Details about the use of flow injection mass spectrometry for porewater dissolved gas analysis, as well as results from the preliminary lab and field tests, will be presented. Ultimately, this system is envisioned to be routinely deployed in sandy coastal sediments, providing a powerful tool for investigating their role in key environmental processes.