Ocean Carbon Cycle Affected by Drought

Recent drought conditions in the North Pacific Ocean near Hawaii have caused a decrease in the strength of the carbon dioxide sink, according to a study published this week in the journal Nature. A team funded by the National Science Foundation (NSF) and led by scientists Dave Karl and Roger Lukas of the School of Ocean and Earth Science and Technology (SOEST) at the University of Hawaii used 15 years of time-series measurements to compare the precipitation, salinity and carbon dioxide (CO2) concentrations at Station ALOHA, located in the Pacific Ocean approximately 100 kilometers north of Oahu.

The study shows that a decrease in the tendency of the ocean to take up CO2 is due to an increase in the water’s salinity, which is a direct result of the drought seen in much of the North Pacific Ocean over the past five years.

“Our study can be considered an oceanic analogue of the long standing atmospheric measurement program at the Mauna Loa Observatory,” says Karl. “The results from this study were unexpected; we didn’t realize how much difference salinity can make when modeling the carbon cycle.”

According to John Dore, a SOEST researcher and lead author of the study, rainfall patterns and ocean CO2 are inexorably linked. “We all recognize the impacts of drought on land, but its effects on the biogeochemistry of the ocean have tended to go unnoticed,” says Dore.

The Hawaii Ocean Time-series (HOT) program is an ongoing field study designed to determine temporal variability in physical, chemical and biological processes in the North Pacific Subtropical Gyre (NPSG), one of Earth’s largest habitats. The program began in October 1988 with the establishment of the benchmark sampling site, Station ALOHA, at 22o45’N, 158oW. Nearly every month for the past 15 years, a team of interdisciplinary scientists with common research objectives have been making shipboard measurements, conducting experiments and testing a broad range of ecological hypotheses.

“This extended period of time-series measurements is very rare,” says Lukas. “Along with a sister station in Bermuda, Station ALOHA has the longest records of comprehensive biogeochemical and physical measurements anywhere in the world.”

The data have already revealed unexpected variability in habitat changes and in the response of the organisms living there. The present study is but one important example. “These interesting results are another example that shows the importance of longterm observations to ocean research,” says James Yoder, director of NSF’s division of ocean sciences, which funded the research. “Ocean observatories of the future will provide the capability to tease out important signals that are missed during the comparatively short duration of oceanographic expeditions.”

Scientists involved in the HOT project have recently received new funding (with other colleagues) from NSF to establish an autonomous ocean observatory at Station ALOHA, reusing an abandoned fiber optic telephone cable to extend the Internet from their offices to the seafloor three miles below the ocean surface. From there, the cable travels upward into the surface layer, providing the capability for real-time observations of ocean processes. “With this observatory we will move much of the sampling from the ship to our desktops,” says Lukas. “But, there will always be a need for us to go to sea, both to maintain the observatory, and to make measurements that require water sampling.”

During this next five-year observation period, the measurement program will move toward more autonomous detection of ocean characteristics by connecting instrumented moorings to the ALOHA observatory and using autonomous underwater vehicles and gliders to provide spatial context around the site.

The research was also funded by the State of Hawaii.