Nitrogen inputs into the euphotic zone by vertically migrating Rhizosolenia mats

Abstract

Rhizosolenia mats conduct extensive vertical migrations in the oligotrophic central North Pacific (cNP) gyre that permit these diatoms to acquire nitrate at depth and return to the surface for photosynthesis. The ultimate fate of this N within the ecosystem is unknown, but may include remineralization by grazing, lass to depth by sinking biamass, or Nexcretion by Rhizosolenia mats. Direct release of N by mats into the mixed layer would represent an upward biological pump that circumvents the diffusion barriers and nutrient sinks at the base of the oceanic euphotic zone. We examined Rhizosolenia mat N release along a transect (28-31° N) in the summer of 2002 (Hawaii to California) and 2003 (Hawaii to west of Midway Island) using sensitive fluorometric and chemiluminescence methods. Nitrate, NO2- and NH 4+ release was determined. Nitrate and NH4+ release by the mats occurred in both 2002 (22.84 ± 6.04 and 3.69 ± 1.74 nmol N μg-1 Chl a h-1, respectively) and 2003 (23.74 ± 3.54 and 3.60 ± 0.74 nmol N μg-1 Chl a h-1, respectively). Nitrite release only occurred in the 2003 summer period but occurred in both years when Fe chelators were added. Fv/Fm values decreased westward in 2003 suggesting a gradient of increasing physiological stress towards the west. The various physiological measures are consistent with concurrent Fe stress; however, other possibilities exist. Nitrate excretion was the dominant form of N release in both years and provided a substantial addition to the ambient nitrate pool in the mixed layer. Rhizosolenia mat nitrate release supplies at least 4-7% of the nitrate pool on daily basis, and possibly as much as 27%. Rhizosolenia mats are part of a large phytoplankton community that appears to migrate, and rates could be significantly higher. Literature reports suggest little or no nitrification in the upper euphotic zone, and thus biological transport and release of nitrate may be a major source to this region. This N release is uncoupled from upward CO2 transport and, like N2 fixation, provides a component of the N pool available for net carbon removal. © The Author 2005. Published by Oxford University Press. All rights reserved.