Filtration of submicrometer particles by pelagic tunicates

Author Posting. © The Author(s), 2010. This is the author's version of the work. It is posted here by permission of National Academy of Sciences for personal use, not for redistribution. The definitive version was published in Proceedings of the National Academy of Sciences of the United States of America 107 (2010): 15129-15134, doi:10.1073/pnas.1003599107.Salps are common in oceanic waters and have higher per individual filtration rates than any other zooplankton filter feeder. Though salps are centimeters in length, feeding via particle capture occurs on a fine, mucous mesh (fiber diameter d ~ 0.1 μm) at low velocity (U = 1.6 ± 0.6 cm s-1, mean ± SD) and is thus a low-Reynolds number (Re ~ 10-3) process. In contrast to the current view that particle encounter is dictated by simple sieving of particles larger than the mesh spacing, a low-Re mathematical model of encounter rates by the salp feeding apparatus for realistic oceanic particle size distributions shows that submicron particles, due to their higher abundances, are encountered at higher rates (particles per time) than larger particles. Data from feeding experiments with 0.5, 1 and 3 μm diameter polystyrene spheres corroborate these results. Though particles larger than 1 μm (e.g. flagellates, small diatoms) represent a larger carbon pool, smaller particles in the 0.1–1 μm range (e.g. bacteria, Prochlorococcus) may be more quickly digestible because they present more surface area, and we find that particles smaller than the mesh size (1.4 μm) can fully satisfy salp energetic needs. Furthermore, by packaging submicrometer particles into rapidly sinking fecal pellets, pelagic tunicates can substantially change particle size spectra and increase downward fluxes in the ocean.This work was supported by the National Science Foundation (OCE-0647723 to LPM and OCE-074464- CAREER to RS) and the WHOI Ocean Life Institute