Journal
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Volume 107, Issue 34, Pages 15129-15134Publisher
NATL ACAD SCIENCES
DOI: 10.1073/pnas.1003599107
Keywords
biofiltration; low Reynolds number; salps; colloids; carbon cycle
Categories
Funding
- National Science Foundation [OCE-0647723, OCE-074464-CAREER]
- Woods Hole Oceanographic Institution Ocean Life Institute
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Salps are common in oceanic waters and have higher per-individual filtration rates than any other zooplankton filter feeder. Although salps are centimeters in length, feeding via particle capture occurs on a fine, mucous mesh (fiber diameter d similar to 0.1 mu m) at low velocity (U = 1.6 +/- 0.6 cm.s(-1), mean +/- SD) and is thus a low Reynolds-number (Re similar to 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-mu m diameter polystyrene spheres corroborate these findings. Although particles larger than 1 mu m(e. g., flagellates, small diatoms) represent a larger carbon pool, smaller particles in the 0.1- to 1-mu m 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 mu m) 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.
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