Quantification of marine snow fragmentation by swimming euphausiids

Sinking of marine snow is a major mechanism of particulate carbon transport from surface waters to the seafloor. Any process altering the abundance or size of marine snow influences carbon flux and food availability to pelagic and benthic organisms. We explored whether zooplankton can alter carbon transport by a new mechanism-physical fragmentation of marine snow. The fluid stress created around the appendages of swimming Euphausia pacifica is capable of fragmenting a single aggregate into multiple, smaller aggregates. The reduced size and slower sinking rate of the daughter aggregates may increase their residence time in the water column, promoting decomposition and decreasing particle flux to depth. To determine the importance of fragmentation, tethered and free-swimming euphausiids were videotaped in the presence of marine snow representing a range of aggregate strengths, sizes, and ages. Image analysis was used to characterize the particles prior to and following fragmentation and to determine the area of influence around a single euphausiid. Tethered euphausiids pulled in particles from an average of 6.7 mm away, and most aggregates were fragmented in either the region of the rapidly beating pleopods or in the high-velocity jet that forms off the tail. Euphausiids were capable of fragmenting all aggregate types and produced an average of 7.3 daughter particles, with 60% of these daughter particles remaining within the marine snow size class ( > 0.5 mm). Thus, physical fragmentation by swimming euphausiids increases the abundance of marine snow while decreasing overall marine snow mass. This novel process, which alters particle size structure without loss of total particulate organic carbon (POC), will be important in upwelling regions where euphausiids are the dominant macrozooplankton migrators.