Multiyear decline in surface ocean carbon dioxide at Station ALOHA: Has the efficiency of the biological carbon pump increased?

J. E. Dore¹, R. Lukas¹, M. J. Church¹, D. W. Sadler¹, R. M. Letelier², D. M. Karl¹

¹Department of Oceanography, University of Hawaii, 1000 Pope Road, Honolulu, HI 96822

²College of Oceanic and Atmospheric Sciences, Oregon State University, 104 Oceanography Administration Building, Corvallis, OR 97331-5503

Abstract

The Hawaii Ocean Time-series (HOT) Station ALOHA was established in October 1988 as a benchmark site for monitoring and understanding the physical, chemical and biological processes that shape the oligotrophic central North Pacific Ocean environment. High-precision measurements of carbon dioxide (CO₂) parameters have been carried out at this location at nearly monthly intervals, and have revealed substantial seasonal and interannual variability in the carbonic acid system, which are influenced by both physical and biological mechanisms. It was anticipated that mixed layer dissolved inorganic carbon (DIC) would rise with time at a rate consistent with the increasing CO₂ concentration of the atmosphere above. From 1989-1999, despite considerable interannual variability caused mainly by changes in salinity, the overall mixed-layer DIC trend was positive. From 2000-2004, however, the mixed layer DIC concentration has exhibited a decreasing trend. We consider four possible contributing factors to this DIC decline: (1) a decrease in the net air-sea flux of CO₂, (2) a decrease in the diffusion and/or entrainment of DIC from depth, (3) a decrease in the net importation of DIC via lateral advection, and (4) an increase in biologically-mediated carbon export. We examine each of these possibilities using depth distributions of DIC within and beneath the euphotlc zone and other relevant physical, chemical and biological data. A rise in the DIC content and a parallel drop in the oxygen content of the mesopelagic zone suggest a possible increase in the rate of organic carbon remineralization at depth during this five-year period. We suggest that the declining trend in surface DIC may be at least in part driven by an increase in the efficiency with which the biological pump is removing organic carbon from the euphotic zone to the mesopelagic zone. We discuss evidence of possible changes In biological community structure that may explain the inferred increase in export efficiency.