Award: OCE-1829911

Gases like carbon dioxide and oxygen constantly move between the atmosphere and the ocean, and it’s important to understand where and when this happens for many reasons, including to ensure that our climate models are as accurate as possible, to understand the implications of a changing climate for biodiversity, and to predict how the ocean will respond to human interventions. Most studies of air-sea gas exchange have been conducted in the open ocean, and we know far less about this process close to the coast. In deep water, gas exchange depends mainly on wind speed, but in shallow waters, it is also likely to depend on the depth and shape of the ocean bottom, currents, waves, and tides. All of these processes affect how water mixes and moves and how much water is exposed directly to the atmosphere. This experiment aimed to directly measure gas exchange in a shallow coastal environment - Kaneohe Bay on the island of Oahu in Hawaii - and to study how the gas exchange depends on the physical environment. 

Two three-week experiments were carried out in August 2019 and August 2022, based at the University of Hawaii’s Hawaii Institute of Marine Biology at Coconut Island. Water movement around the bay was monitored at five locations, using acoustical instruments (ADCPs and ADVs) to measure flow speed and turbulence. The gas exchange measurement was done by releasing tracers into the water and then monitoring their concentration and distribution over many days, which allowed us to calculate the amount of gas crossing the ocean surface.

Our results showed that the amount of air-sea gas exchange taking place over the reef in Kaneohe Bay was substantially higher than would be expected in the open ocean for a given wind speed. There was a weak correlation with the wind speed (more gas transfer occurs at higher wind speeds), but it is clear that the shallow nature of the bay and the turbulence generated as water flowed over the rough reef had a more important influence on the gas exchange. This finding confirms that it is inappropriate to use open ocean prediction methods for gas exchange in these shallow coastal regions. We need to develop prediction methods that consider the water flows and higher turbulence found in shallow water, and our results provide a foundation for that work.

 

Last Modified: 08/28/2024
Modified by: David T Ho