Seasonal and interannual variability in the biological pump at Station ALOHA

D. M. Karl, J. E. Dore, T. A. Houlihan, D. V. W. Hebel, L. A. Fujieki

University of Hawaii, Department of Oceanography, Honolulu, HI 96822

Abstract

Since Oct 1988, measurements have been made of the rates of primary production (in situ ¹⁴C method) and particulate matter export from the euphotic zone (free drifting sediment traps at 150 m) at Station ALOHA (22 45 N, 158 00 W). Contrary to a priori expectation, these two processes were not well correlated at this open ocean site. The export ratio (e-ratio), defined as particulate C flux / primary production, varied nearly an order of magnitude from 0.02 to 0.15 with overall mean and standard deviation values of 0.062 and 0.026, respectively (n=89). The e-ratio displayed temporal variability with sustained higher than average e-ratios for a two-year period from 1989 to 1990 and again in 1997, and sustained lower than average e-ratios during 1991-1992 and 1994-1996. These variations in e-ratio were caused almost exclusively by changes in C export; annually integrated rates of primary production were nearly invariant over the 11-year observation period. From Oct 1988 to May 1995, free-drifting sediment traps were also deployed at 300 and 500 m to measure the net lost of biogenic materials in the upper mesopelagic zone. While the average particulate matter attrition between 150 and 500 m during the 7-year observation period was not significantly different from that predicted by the Martin et al. C flux model (1987, Deep-Sea Res. 34: 267) with an average 60-65% loss over this depth interval, there were also significant deviations from the mean. Changes in the remineralization length scale imply variations in particulate matter composition affecting the lability of the exported organic matter, variations in sinking rates or other non steady-state mesopelagic zone processes. Bottom-moored sequencing sediment traps deployed at reference depths of 2800 and 4000 m confirmed these dynamical patterns and also revealed a sustained decade-long increase in organic C export to depth. Because the patterns and dynamics of C export from the euphotic zone (150 m) were different from those observed in the dark abyss, it is likely that processes in the "twilight zone" are ultimately responsible for controlling the overall efficiency of the biological C pump and, therefore, the C sequestration potential of the intermediate and deep water reservoirs.