The Importance of Organic Exudates in the Measurement of Oligotrophic Ocean Primary Productivity

Dale Hebel, Luis Tupas and David Karl

School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, HI 96822

Abstract

The development and introduction of the ¹⁴C technique to measure photoautotrophic production in 1952 provided oceanographers with a relatively sensitive and reproducible method for use in oligotrophic ocean environments. However, the accuracy of the method remains unknown. Since 1952, this method has been used and abused, praised and criticized but several first-order questions remain unanswered. Foremost among them is whether the ¹⁴C-based estimates approach gross or net photoautotrophic production. A related matter concerns the quantitative importance of exudation and, specifically, "Do healthy cells do it?"

The standard HOT program ¹⁴C procedure uses trace metal-clean water sampling techniques, and a full day (dawn-to-dusk) in situ light and dark bottle incubation. During an assessment of the size distribution of chl a biomass, we conducted a comparison of GF/F (0.7 µm, nominal) vs. Nuclepore (0.2 µ) filters for measurements of photoautotrophic production. Contrary to expectation and logic, our results from over 1.5 yrs of data (289 incubations on 10 separate cruises) indicate that GF/F filters collect 30% more organic ¹⁴C than the smaller pore size Nuclepore filters. Subsequent experiments revealed that the GF/F filters adsorb "dissolved" organic matter that is apparently not removed by the more inert polycarbonate (Nuclepore) filters. This 30% figure is likely to be a minimum estimate of total ¹⁴C-DOC because not all compound classes are likely to absorb. Direct estimation of radiolabeled extracellular organic carbon has confirmed this, and experiments are now underway to characterize these compounds. Although the physiological mechanisms are not yet known, the ecological implications of this rapid and extensive cycling of DOM in the surface waters near Station ALOHA are profound.

Accurate modeling of ecosystem production at Station ALOHA will require knowledge of the pathway(s) of EOC accumulation and the fate of this recently synthesized dissolved organic matter. We hypothesize that this large EOC flux is a direct consequence of the high rates and regional importance of nitrogen fixation during the past several years in the subtropical North Pacific Ocean and may not reflect steady-state conditions. Evaluation of this hypothesis will require a more comprehensive assessment of these processes, including direct chemical characterization.