Effects of sinking velocities and microbial respiration rates on the attenuation of particulate carbon fluxes through the mesopelagic zone

Abstract

The attenuation of sinking particle fluxes through the mesopelagic zone is an important process that controls the sequestration of carbon and the distribution of other elements throughout the oceans. Case studies at two contrasting sites, the oligotrophic regime of the Bermuda Atlantic Time-series Study (BATS) and the mesotrophic waters of the west Antarctic Peninsula (WAP) sector of the Southern Ocean, revealed large differences in the rates of particle-attached microbial respiration and the average sinking velocities of marine particles, two parameters that affect the transfer efficiency of particulate matter from the base of the euphotic zone into the deep ocean. Rapid average sinking velocities of 270 ± 150 m d À1 were observed along the WAP, whereas the average velocity was 49 ± 25 m d À1 at the BATS site. Respiration rates of particle-attached microbes were measured using novel RESPIRE (REspiration of Sinking Particles In the subsuRface ocEan) sediment traps that first intercepts sinking particles then incubates them in situ. RESPIRE experiments yielded flux-normalized respiration rates of 0.4 ± 0.1 day À1 at BATS when excluding an outlier of 1.52 day À1 , while these rates were undetectable along the WAP (0.01 ± 0.02 day À1 ). At BATS, flux-normalized respiration rates decreased exponentially with respect to depth below the euphotic zone with a 75% reduction between the 150 and 500 m depths. These findings provide quantitative and mechanistic insights into the processes that control the transfer efficiency of particle flux through the mesopelagic and its variability throughout the global oceans.

The remineralization of marine organic matter by diverse biological and abiotic processes

“…Iversen and Ploug (2010) reported a range of values for the carbon-specific respiration rate from 0.08 to 0.20 d -1 , based on a compilation of their own measurements and several from the literature. Using an in situ particle incubator in the Sargasso Sea (Bermuda Atlantic Time-series Study site), McDonnell et al (2015) calculated significantly faster particle "microbial remineralization rates" of 0.3 ± 0.1 to 1.7 ± 0.4 d -1 by normalizing oxygen consumption directly to trap fluxes; these exceeded our sole measurement from the Sargasso (0.084 ± 0.008, at station QL-2) by an order of magnitude.…”

“…Because our objective was to estimate generally the contribution to POC flux attenuation by particle-attached bacterial respiration compared to other possible processes, we chose to modify a simple, mechanistic, and frequently invoked exponential model of sinking particle flux attenuation that uses as primary inputs (1) sediment trap derived-particle fluxes and (2) rates of specific respiration on sinking particles (k R ; day -1 ) (Boyd and Trull, 2007;Buesseler and Boyd, 2009;Lutz et al, 2002;McDonnell et al, 2015;Sarmiento and Gruber, 2006;Volk and Hoffert, 1985). In the model, the POC flux at a given depth z (F z , in mg C m -2 d -1 ) is proportional to the flux observed at an overlying depth (F 0 ), which is attenuated according to first-order kinetics over a characteristic remineralization length scale (L remin ):…”

“…The majority of this export occurs as a rain of sinking particles that are progressively remineralized by heterotrophic bacteria or zooplankton as they sink through the mesopelagic ocean (Buesseler and Boyd, 2009;Buesseler et al, 2007b;Steinberg et al, 2008b). Respiration by particle-associated heterotrophic communities has been invoked in several specific ecosystems as the primary sink for these particles, based on observations either in the field or in the laboratory (e.g., McDonnell et al, 2015;Ploug et al, 1999) or statistical relationships, for example, between the remineralization length scale and temperature (Marsay et al, 2015). However, modeling exercises, together with an increasing number of environmental studies, indicate that a substantial fraction of particles from the ocean's surface can be degraded during their downward transit by other processes that rival particle-attached microbial respiration in magnitude (Close et al, 2013;Giering et al, 2014;Kiørboe, 2001;Kiørboe and Jackson, 2001;Stemmann et al, 2004aStemmann et al, , 2004bTaylor and Karl, 1991).…”

The Ballast Effect in the Indian Ocean

“…(λ) refers to the POC specific 20 respiration rate, which is assumed to vary in a relative narrow range (0.106 ± 0.028 day -1 Ploug and Grossart, 2000). More recent studies based on in situ incubation experiments carried at water-depth < 500 m indicate respiration rates of 0.4 ± 0.1 and 0.01± 0.02 day -1 in the subtropical North Atlantic Ocean and the Southern Ocean, respectively (McDonnell et al, 2015). This supports results obtained from laboratory and sediment trap studies suggesting that respiration rates are temperature dependent and decrease with decreasing temperatures (Iversen and Ploug, 2013;25 Marsay et al, 2015).…”

Modulation of the vertical particles transfer efficiency in the Oxygen Minimum Zone off Peru

“…Conversely, the large amount of matter collected at 149 m for the high T eff range (>50%) might be explained by potentially less-degraded particles, as a consequence of a faster sinking velocity (McDonnell et al, 2015). In theory, the sinking velocities of biogenic particles depend on various intrinsic factors (such as their sizes, shapes, densities, porosities) which can be modified along their fall by complex bio-physical processes (e.g.…”