The Significance of Giant Phaeodarians (Rhizaria) to Biogenic Silica Export in the California Current Ecosystem

In marine ecosystems, many planktonic organisms precipitate biogenic silica (bSiO2) to build silicified skeletons. Among them, giant siliceous rhizarians (>500 μm), including Radiolaria and Phaeodaria, are important contributors to oceanic carbon pools but little is known about their contribution to the marine silica cycle. We report the first analyses of giant phaeodarians to bSiO2export in the California Current Ecosystem. We measured the silica content of single rhizarian cells ranging in size from 470 to 3,920 μm and developed allometric equations to predict silica content (0.37–43.42 μg Si/cell) from morphometric measurements. Using sediment traps to measure phaeodarian fluxes from the euphotic zone on four cruises, we calculated bSiO2export produced by two families, the Aulosphaeridae and Castanellidae. Biogenic silica export ranged from <0.01 to 0.63 mmol Si · m−2 · day−1. These two families alone contributed on average 10% (range 0–80%) of total bSiO2export from the euphotic zone. Their proportional contributions increased substantially in more oligotrophic regions with lower bSiO2fluxes. Using the in situ Underwater Vision Profiler 5, we characterized vertical distributions of the giant phaeodarian family Aulosphaeridae to a depth of 500 m and inferred their contribution to bSiO2export in deeper waters. We found a significant increase of Aulosphaeridae export (<0.01 to 2.82 mmol Si · m−2 · day−1) when extended to mesopelagic depths. Using a global data set of in situ profiles, we estimated the significance of Aulosphaeridae to bSiO2export and revealed that they can act as major exporters of bSiO2to the mesopelagic zone in various regions.