The influence of a mature cyclonic eddy on particle export in the lee of Hawaii

Mesoscale eddies may enhance primary production (PP) in the open ocean by bringing nutrient-rich deep waters into the euphotic zone, potentially leading to increased transport of particles to depth. This hypothesis remains controversial, however, due to a paucity of direct particle export measurements. In this study, we investigated particle dynamics using Th-234-U-238 disequilibria within a mesoscale cold-core eddy, Cyclone Opal, which formed in the lee of the Hawaiian Islands. Th-234 samples were collected along two transects across Cyclone Opal as well as during a time-series within the eddy core during a decaying diatom bloom. Particulate carbon (PC), particulate nitrogen (PN) and biogenic silica (bSiO(2)) fluxes at 150 m varied spatially and temporally within the eddy and strongly depended on the Th-234 model formulation used (e.g., steady state versus non-steady state, inclusion of upwelling, etc.). Particle fluxes estimated from a steady state model assuming an upwelling rate of 2 m day(-1) yielded the best fit to sediment-trap data. These Th-234-derived particle fluxes ranged from 332 +/- 14 to 1719 +/- 53 mu mol Cm-2 day(-1), 27 +/- 3 to 114 +/- 12 mu mol Nm(-2) day(-1), and 33 +/- 20 to 309 +/- 73 mu mol Si m(-2) day(-1). Although PP rates within Cyclone Opal were elevated by a factor of 2-3, PC and PN fluxes were the same, within error, inside and outside of Cyclone Opal. The ratio of PC export to PP remained surprisingly low at <0.03 and similar to those measured in surrounding waters. In contrast, bSiO(2) fluxes within the eddy core were three times higher. Detailed analyses of Th-234 depth profiles consistently showed excess Th-234 at 100-175 m, associated with the remineralization and possible accumulation of suspended and dissolved organic matter from the surface. We suggest that strong microzooplankton grazing facilitated particulate organic matter recycling and resulted in the export of empty diatom frustules. Thus, while eddies may increase PP, they do not necessarily increase PC and PN export to deep waters. This may be a general characteristic of wind-driven cyclonic eddies of the North Pacific Subtropical Gyre and suggests that eddies may preferentially act as a silica pump, thereby playing an important role in promoting silicic-acid limitation in the region. (C) 2008 Elsevier Ltd. All rights reserved.