(
2010-07)
Sutherland, Kelly R.; Madin, Laurence P.; Stocker, Roman
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.
