Station ALOHA Particle Fluxes and Estimates of Export Production

D. Karl, D. Hebel, L Tupas, J. Dore and C. Winn

Abstract

The fraction of primary production that is removed from the euphotic zone by sinking particles has been termed the "export ratio." Under steady-state conditions, the export ratio can be used to estimate the f-ratio and ecosystem new production. However, when other export processes dominate (e.g., horizontal transport, downward diffusion, plant and animal migrations), the sediment trap-derived sinking flux can only be used to set a lower bound on new production.

Since 1988, we have measured the downward flux of particulate carbon (PC), nitrogen (PN) and phosphorus (PP) at three reference depths (150, 300 and 500 m) at Station ALOHA using a drifting sediment trap array. These are discontinuous collection devices that are in the ocean, on average, less than 10% of the time (approximately 3 days per month). In June 1992, we added a continuous bottom-moored time-series particle collection capability with 4 Parflux MK7-21 sediment traps deployed at 800, 1500, 2800 and 4000 m.

The flux of particulate carbon from the euphotic zone at Station ALOHA is a variable function of time. During the first two years of the HOT program we observed a large amplitude (3-4 fold variation) and regular (seasonal?) oscillation in particle flux with peaks in late winter and late summer. A closer analysis of these data indicates a pattern that we interpret to be a free-running cycle, apparently independent of the predictable solar cycle. The mean carbon flux during this period was approximately 3 mmol C m^-2 d^-1. During the "quiescent," high primary production period (1991-92), the mean flux decreased to approximately 2 mmol C m^-2 d^-1 and the amplitude of the annual flux variability was < 2 fold.

Contrary to previous cross-ecosystem analyses that revealed an increased fraction of primary production lost to sinking particles (i.e., increased export ratio) with increased primary production, our data from Station ALOHA indicate a decreasing trend in particle flux with increasing primary production. Furthermore, the absolute values for export production at Station ALOHA are consistently < 10% with values of -5% during the quiescent period. These export ratios are much lower than predicted by the existing empirical models for oceanic carbon flux suggesting a more efficient euphotic zone recycling than previously suspected. Alternatively, other removal processes might dominate the removal of C, N and P from the surface waters of the oligotrophic North Pacific Ocean.