Fragmentation of marine snow by swimming macrozooplankton: A new process impacting carbon cycling in the sea

Comparisons of the abundances and size distributions of marine snow (aggreg ated particles > 0.5 mm in diameter) in the upper 100 m of the water column at ten stations off Southern California in the late afternoon with those i n the same parcel of water the following morning, after nocturnal vertical migration by zooplankton had occurred, revealed the existence of a previous ly undescribed process affecting marine particle dynamics. Aggregate abunda nces increased overnight and changes were positively and significantly corr elated only with the abundance of the common euphausiid, Euphausia pacifica , and with no other biological or physical factor. Moreover, mean aggregate size decreased and aggregate size distributions shifted toward smaller siz e classes when euphausiids were abundant. The only conclusion consistent wi th these findings was that euphausiids were physically disaggregating marin e snow into smaller, more numerous aggregates through shear stresses genera ted while swimming. Video-recording of both tethered and free-swimming E. p acifica in the laboratory dramatically confirmed that aggregates passing wi thin 8-10 mm of the animal's abdomen were fragmented either by entrainment and direct impact with the beating pleopods or by eddies generated during s wimming. At the abundances observed in this study, swimming E. pacifica wou ld have sufficiently disturbed 3-33% of the water column each night to disr upt the aggregates contained therein. This is the first evidence for the fr agmentation of large particles by the swimming activities of zooplankton an d suggests that macrozooplankton and micronekton play a significant role in the particle dynamics of the water column regardless of whether they consu me particles or not. Disaggregation of marine snow by swimming and migratin g animals may alter the sizes of particles available to grazers and microbi al colonizers and reduce the flux of particulate carbon by generating small er particles, which potentially sink more slowly and reside longer in the w ater column. This newly discovered process reduces carbon Bur while simulta neously conserving carbon and provides a previously unconsidered link betwe en animal behavior and the biogeochemistry of the sea. It may help explain the exponential reduction in particle Aux with depth observed in parts of t he ocean and help balance oceanic carbon models. (C) 2000 Elsevier Science Ltd. All rights reserved.