| Large, rapidly sinking particles, known as marine snow, form a crucial link in the ocean's carbon cycle. The vast majority of these particles are produced in surface waters, yet their abundance drops off exponentially with depth. The ability of zooplankton to consume particles of marine snow and thus to reduce their abundance was investigated using a variety of techniques. Both Euphausia pacifica and Calanus pacificus, common macrozooplankton off the coast of California, consumed natural marine snow at high rates. Most categories of marine snow tested were consumed, including diatom flocs, dinoflagellate aggregates, and abandoned larvacean houses. Euphausiid feeding rates on marine snow were 9-15 {dollar}mu{dollar}g C{dollar}cdot{dollar}euphausiid{dollar}rmsp{lcub}-1{rcub}cdot hrsp{lcub}-1{rcub},{dollar} similar to feeding rates on single diatom cells. Assimilation efficiencies for E. pacifica grazing on marine snow ranged from 64-83%. To test whether euphausiids would feed on marine snow in the presence of dispersed phytoplankton, feeding was traced by adding stable isotopes of silicon to each food source. When both diatom aggregates and dispersed diatoms were available, E. pacifica consumed both food sources. However, E. pacifica may feed on aggregates preferentially when the abundance of dispersed cells is low. Evidence from field studies suggests that in addition to consuming aggregates, euphausiids may be able to break them apart while swimming and feeding. Fragments of marine snow will sink more slowly, and may be subject to transformative processes such as microbial decomposition and grazing for a longer period of time. |
