New Discovery from Station Aloha: Carbon Cycle Affected by Drought

UH Manoa researchers' findings published in the journal Nature

University of Hawaiʻi at Mānoa
Dave Karl, (808) 956-8984
School of Ocean & Earth Science & Technology
Roger Lukas, (808) 956-7896
School of Ocean & Earth Science & Technology
Posted: Aug 14, 2003

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 led by 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 approximately 100 km north of Oahu.

The study shows that a decrease in the tendency for CO2 to be taken up by the ocean is due to an increase in the salinity of the ocean, which is a direct result of the drought seen in much of the North Pacific Ocean over the last 5 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 we got from this study were unexpected; we didn‘t realize how much salinity can make a difference 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 program designed to determine temporal variability in physical, chemical and biological processes in the North Pacific Subtropical Gyre (NPSG), one of Earth's largest biomes. The program began in October 1988 with the establishment of the benchmark sampling site, Station ALOHA, at 22o45'N, 158oW. At approximately monthly intervals 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 emergent data sets have already revealed unexpected variability in habitat forcing and in the response of the biological assemblages. The present study on climate-induced changes in the inventories and fluxes of carbon dioxide in the near surface waters is but one important example.

This research, funded by the National Science Foundation and by the State of Hawaii, has just been awarded another 5-year grant to extend the period of observation to at least two decades.

PIs for the HOT project have also recently received new awards (with other colleagues) from the NSF for establishment of an autonomous ocean observatory at Station ALOHA, reusing an abandoned fiber optic telephone cable to extend the Internet from their offices at UH-Manoa to the seafloor 3 miles below the ocean surface, and then upward into the surface layer, providing the capability for real-time observations of ocean processes. "With this observatory we will evolve much of the sampling from the ship to our desktops," says Lukas. "However, 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 5-year observation period, the measurement program will move towards more autonomous detection of oceanic phenomena of interest by incorporating fixed point instrumented moorings connected to this ALOHA observatory, and through the use of autonomous underwater vehicles and gliders to provide spatial context around this site.

All HOT program data are freely accessible for use by others involved in ocean research and education.

The success of the HOT program, to date, is a result of the coordinated, dedicated efforts of a large team of scientists, students and technical staff led by Dave Karl and Roger Lukas of the UH Department of Oceanography.

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