Sinking carbon, nitrogen, and pigment flux within and beneath the euphotic zone in the oligotrophic, open-ocean Gulf of Mexico

Abstract

Abstract During two cruises in the oligotrophic oceanic Gulf of Mexico, we deployed sediment traps at three depths: center of the euphotic zone (EZ) (60 m), base of the EZ (117–151 m), and in the twilight zone (231 m). Organic carbon export declined with depth from 6.4 to 4.6 to 2.4 mmol C m−2 d−1, suggesting that net particle production was concentrated in the upper EZ. Net primary production varied from 24 to 29 mmol C m−2 d−1, slightly more than half in the upper EZ. Export ratios varied from 11 to 25%. Trap measurements of chlorophyll and phaeopigments allowed us to quantify fluxes of fresh phytoplankton and herbivorous fecal pellets, respectively, which were both minor contributors to total flux, although their contributions varied with depth. Phytoplankton flux was more important from the upper to lower EZ; fecal pellets were more important at the EZ base and below. C:N elemental ratios and 13C and 15N isotope analyses indicated particle transformations within and beneath the EZ. 234Th-238U disequilibrium measurements varied, likely reflecting the mixing of water from multiple regions over the ~month-long time-scale of 234Th. Our results highlight the complexity of the biological carbon pump in oligotrophic regions.