Macroscopic ocean aggregates, generically categorized as marine snow, have significance in the ocean as chemically and biologically distinct microhabitats and serve as the primary transporter of surface-derived organic matter to the ocean interior. 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 was adequate to fragment a single aggregate into multiple, smaller aggregates producing an average of 7 daughter particles, 60% of which remained within the marine snow size class (>0.5 mm). Thus, physical fragmentation by swimming euphausiids increased the abundance of marine snow while decreasing overall marine snow mass and conserving particulate organic carbon (POC). Fragmentation events also resulted in the release of nutrients (dissolved organic carbon, nitrate, phosphate) from aggregate interstices, making them available to free-living biota. Fragmentation increased the aggregate surface area available for bacterial colonization but did not appear to result in any increased removal of POC relative to whole, intact aggregates. Thus, the most important effects of marine snow fragmentation were the release of nutrients to surrounding seawater and the slower daughter particle sinking rates, potentially increasing aggregate residence time and reducing carbon flux to depth. Because euphausiids are diel vertical migrators swimming-induced fragmentation could produce daily patterns in marine snow concentration and flux. However, in a mesocosm experiment we observed a diel cycle in marine snow in the absence of macrozooplankton, although fragmentation did appear to be important following euphausiid addition. In a field study in the Santa Barbara Channel, CA, a similar diel pattern was observed with daytime maxima in marine snow concentration followed by nighttime lows. In this case, particulate flux to a sediment trap accounted for little of the nighttime loss of marine snow while grazing and fragmentation by migrating macrozooplankton easily accounted for the nighttime reductions. Thus, the distribution of euphausiids could be important in structuring the pelagic environment by creating spatial and temporal patchiness of particulate material, aggregate-associated biota, and remineralization products.