Large Phaeodaria in the twilight zone: Their role in the carbon cycle

Abstract

Advances in in situ imaging allow enumeration of abundant populations of large Rhizarians that compose a substantial proportion of total mesozooplankton biovolume. Using a quasi‐Lagrangian sampling scheme, we quantified the abundance, vertical distributions, and sinking‐related mortality of Aulosphaeridae, an abundant family of Phaeodaria in the California Current Ecosystem. Inter‐cruise variability was high, with average concentrations at the depth of maximum abundance ranging from < 10 to > 300 cells m−3, with seasonal and interannual variability associated with temperature‐preferences and regional shoaling of the 10°C isotherm. Vertical profiles showed that these organisms were consistently most abundant at 100–150 m depth. Average turnover times with respect to sinking were 4.7–10.9 d, equating to minimum in situ population growth rates of ~ 0.1–0.2 d−1. Using simultaneous measurements of sinking organic carbon, we find that these organisms could only meet their carbon demand if their carbon : volume ratio were ~ 1 μg C mm−3. This value is substantially lower than previously used in global estimates of rhizarian biomass, but is reasonable for organisms that use large siliceous tests to inflate their cross‐sectional area without a concomitant increase in biomass. We found that Aulosphaeridae alone can intercept > 20% of sinking particles produced in the euphotic zone before these particles reach a depth of 300 m. Our results suggest that the local (and likely global) carbon biomass of Aulosphaeridae, and probably the large Rhizaria overall, needs to be revised downwards, but that these organisms nevertheless play a major role in carbon flux attenuation in the twilight zone.

Phaeodaria: An Important Carrier of Particulate Organic Carbon in the Mesopelagic Twilight Zone of the North Pacific Ocean

“…The goal of this study is to quantify the magnitude of carbon export due to phaeodarians in the mesopelagic twilight zone. Recent field observations made with an Underwater Vision Profiler 5 (Picheral et al, ) and a short‐term drifting sediment trap have elucidated the contribution of phaeodarians to the marine carbon cycle (Biard et al, ; Stukel et al, ). Those estimates of the biomass and POC flux of phaeodarians were limited, however, to depths between the base of the euphotic zone and the upper mesopelagic zone at depths <500 m. The export flux and processes associated with sinking of phaeodarian POC in the lower mesopelagic zone (depths of 500–1,000 m) have not yet been elucidated.…”

“…In this study, we used a yearlong time series of sediment trap observations to clarify the contribution of phaeodarians to the POC flux in the lower mesopelagic zone for the first time. Biard et al () and Stukel et al () estimated the carbon content of phaeodarians based on their equivalent spherical diameter; in this study, we directly measured the organic carbon content of the whole phaeodarian community with a CHN analyzer. We propose a simple dissection method to quantify the export flux of organic carbon due to phaeodarians.…”

“…The malacoma contains the phaeodium and protoplasm such as the central capsule and ectoplasm (Nakamura & Suzuki, ). The organic carbon is presumed to be concentrated within the central capsule and phaeodium (Stukel et al, ). Therefore, POC derived from whole phaeodarian communities was determined by measuring the organic carbon contents of PHA samples.…”

“…In recent years, a worldwide in situ imaging survey of plankton larger than 600 μm has revealed that phaeodarians make a substantial contribution to zooplankton biomass at depths of 100–500 m in the global ocean (Biard et al, ), and their importance to biogeochemical cycles has become recognized (Biard et al, ; Guidi et al, ; Stukel et al, ). Phaeodaria represent a group of holoplanktonic marine protozoa with mostly fragile and delicate siliceous skeletons that belongs to the phylum Cercozoa of the infrakingdom Rhizaria (Nakamura & Suzuki, ).…”

“…Phaeodarians have been largely ignored in marine biology up to now because their delicate skeletons and hollow structure make them liable to collapse, and it has been difficult to collect and quantify specimens (Nakamura & Suzuki, ). Previous phaeodarian studies mainly focused on the <1‐mm fraction because this is where important phytoplankton components can be found (e.g., Ikenoue et al, , ; Okazaki et al, ; Takahashi & Honjo, ; Takahashi, , , ), and only a handful of studies have estimated the contribution to carbon and silica cycles of selected phaeodarian species (Biard et al, ; Gowing, ; Michaels et al, ; Stukel et al, ; Takahashi, , ; Takahashi et al, ). There has still been no quantitative estimate of the contribution of all phaeodarian species to vertical POC export in the mesopelagic zone.…”