Video plankton recorder reveals high abundances of colonial Radiolaria in surface waters of the central North Pacific

Abstract

Colonial spumellarian Radiolaria are heterotrophic protists that form large (up to several meters in length), gelatinous structures in the surface waters of all tropical and subtropical oceanic ecosystems. These species are morphologically and trophically complex and some, but not all, produce silica skeletal structures of considerable paleontological significance. Skeletonless species of Radiolaria are poorly sampled by plankton nets, which can severely damage these delicate organisms. Therefore, abundances of colonial Radiolaria typically have been underestimated in quantitative studies of zooplankton abundance and biomass. Here we document the abundances of colonial Radiolaria in the central North Pacific based on analysis of video images from a miniaturized video plankton recorder. We observed abundances of radiolarian cells in colonies that exceeded previous reports of total Radiolaria by more than ten-fold, and counts of skeleton-bearing Radiolaria by more than two to three orders of magnitude. Biomass (carbon) within these colonies was similar to or greater than the total radiolarian biomass (i.e. including all solitary species) previously reported for the Pacific. Symbiont productivity within colonial Radiolaria was estimated to constitute a modest but significant fraction of total primary productivity (up to ≈9%) in the upper 150 m. These findings indicate an important contribution of skeletonless spumellaria to food webs and biogeochemical cycles of these communities.