Zooplankton grazing and egestion shifts particle size distribution in natural communities

Marine plankton communities can be viewed in terms of their size structure rather than their taxonomic composition, revealing how allometric relationships affect the functioning of the community. Oceanic particle size spectra can be used to explain and predict variability in carbon export efficiency, because large particles generally sink faster than small particles. Since plankton trophic interactions impact particle size in the surface ocean, this size-structured view is a useful simplification for connecting plankton ecology with biogeochemistry. We conducted a series of grazing experiments to test the hypothesis that mesozooplankton shift particle size spectra toward larger particles, in a predictable manner that reflects their community size structure, through grazing and egestion of fecal pellets. These experiments were carried out over several months, and used natural communities of mesozooplankton and their microplankton prey collected in the coastal Gulf of Maine. After incubation, we analyzed the samples to determine size distribution and taxonomic information. Our results show that mesozooplankton grazing impacts microplankton in proportion to their abundance. Size relationships between plankton predators and prey that have been established at the individual level do not linearly translate to the community level. Further, while grazing itself does not significantly alter the particle size spectrum, repackaging of prey into mesozooplankton fecal pellets shifts particle size spectra toward larger particles.