Mass spectrometers are versatile sensor systems, owing to their high sensitivity and ability to simultaneously measure multiple chemical species. Over the last two decades, traditional laboratory-based membrane inlet mass spectrometers have been adapted for underwater use. Underwater mass spectrometry (UMS) has drastically improved our capability to monitor a broad suite of gaseous compounds (e.g., dissolved atmospheric gases, light hydrocarbons, and volatile organic compounds) in the aquatic environment. Here we provide an overview of the progress made in the field of UMS since its inception in the 1990s to the present. In particular, we discuss the approaches undertaken by various research groups in developing in situ mass spectrometers. We also provide examples to illustrate how underwater mass spectrometers have been used in the field. Finally, we present future trends in the field of in situ mass spectrometry. Most of these efforts are aimed at improving the quality and spatial and temporal scales of chemical measurements in the ocean. By providing up-to-date information on UMS, this review offers guidance for researchers interested in adapting this technology as well as goals for future progress in the field.

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Highly permeable sandy sediments dominate the productive continental shelves and likely play a key role in global biogeochemical cycles. Despite their prominence, we currently have a poor understanding of how sandy sediments function and how they will respond to climate change. This knowledge gap is largely due to the difficulty in accurately sampling sandy sediments, as no method yet exists for measuring a range of analytes while accounting for advective pore-water flow in these dynamic environments. To help address this, we developed a new pore-water sampler that, when coupled to a portable mass spectrometer, can measure a suite of dissolved gases (e.g., O 2 , N 2 , Ar, CO 2 , and CH 4 ) in sandy sediments. Here, we present a series of laboratory experiments to validate and calibrate the instrument, as well as proof-of-concept submersion and field tests. Our results show that with some design improvements, our system will be capable of sustained (hourly to diel) in situ measurements in sandy sediments. This new approach has the potential to provide a large volume of high-quality data in any aquatic system with sandy sediments and thus greatly advance our understanding of biogeochemical processes occurring in these environments.

Human activities release vast amounts of reactive nitrogen, profoundly influencing the functioning of marine ecosystems. On the inner shelf, where these effects are most pronounced, sediments have emerged as potential hotspots of reactive nitrogen removal-and they are therefore increasingly thought to play a key role in the ocean nitrogen cycle. Despite their potential importance, shelf sediments are still overlooked in global models of ocean nitrogen and carbon cycles, limiting model accuracy and predictive power. Through analysis of recent work, we identified unresolved questions and controversies, and propose a three-pronged approach to improve our mechanistic understanding and modeling of nitrogen removal on the continental shelf and in the global ocean.

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