Autonomous observations of in vivo 
fluorescence and particle backscattering
in an oceanic oxygen minimum zone

Abstract

The eastern South Pacific (ESP) oxygen minimum zone (OMZ) is a permanent hydrographic feature located directly off the coasts of northern Chile and Peru. The ESP OMZ reaches from coastal waters out to thousands of kilometers offshore, and can extend from the near surface to depths greater than 700 m. Oxygen minimum zones support unique microbial assemblages and play an important role in marine elemental cycles. We present results from two autonomous profiling floats that provide nine months of time-series data on temperature, salinity, dissolved oxygen, chlorophyll a, and particulate backscattering in the ESP OMZ. We observed consistently elevated backscattering signals within low-oxygen waters, which appear to be the result of enhanced microbial biomass in the OMZ intermediate waters. We also observed secondary chlorophyll a fluorescence maxima within low-oxygen waters when the upper limit of the OMZ penetrated the base of the photic zone. We suggest that autonomous profiling floats are useful tools for monitoring physical dynamics of OMZs and the microbial response to perturbations in these areas.

Size-fraction partitioning of community gene transcription and nitrogen metabolism in a marine oxygen minimum zone

“…Bulk seawater counts declined from~6 × 10 5 per ml in the primary chlorophyll maximum and upper oxycline to 1.3 × 10 5 at the oxic-nitrite interface (90 m), before increasing to a maximum of 1.4 × 10 6 at the OMZ core. This pattern agrees with trends from other OMZ sites, where particulate backscattering and bacterial load show local maxima in the photic zone and again in the OMZ (Spinrad et al, 1989;Naqvi et al, 1993;Whitmire et al, 2009). At all depths, gene numbers were substantially higher in the 0.2-1.6 μm (FL) fraction compared with the 1.6-30 μm (PA) fraction, with FL/PA ratios from 8 to 15 (Figure 1f).…”

Particle size distribution and estimated carbon flux across the Arabian Sea oxygen minimum zone

“…Intermediate depth particles maxima (revealed by light transmission) have been observed earlier in OMZ off the Peruvian coast (Pak et al, 1980;Whitmire et al, 2009), off Mexico (Garfield et al, 1983), in the Cariaco basin of the Gulf of Mexico (Taylor et al, 2001), and in the Indian Ocean Shailaja, 2001). However, these studies could not unambiguously identify the nature and sources of these INL (local production vs. advective transport).…”

“…In order to understand the fate of particles through the OMZ, backscattering sensors have been extensively used to study microscopic particle spatial patterns, revealing large and permanent intermediate nepheloid layers (INL;Whitmire et al, 2009). However, to our knowledge, no information exists on the vertical distribution of large (> 100 µm to a few mm) particles that are the main vector of carbon flux to depth (Alldredge and Silver, 1988;Stemmann et al, 2002;Guidi et al, 2008).…”

Metagenomic analysis of size-fractionated picoplankton in a marine oxygen minimum zone

“…However, the patterns by which microbial physiological traits are distributed between particleassociated and free-living communities are not well understood for many ocean regions. Characterizing particle-associated microbes may be especially important in OMZs, where local particle maxima have been positively related to both oxygen depletion (Garfield et al, 1983;Whitmire et al, 2009) and microbial metabolic activity (Naqvi et al, 1993).…”

“…Although taxonomic surveys have compared free-living and particle-attached communities in a variety of marine ecosystems, the differences in functional gene composition that distinguish free-living from surface-attached life histories have been explored only sparingly. For OMZs in particular, the microscale partitioning of microbial communities and metabolisms has not been explored, despite a potentially significant role for particle-associated microhabitats in these zones (Whitmire et al, 2009).…”