Deep-Sea Research I 47 (2000) 367}395 Diel variations of marine snow concentration in surface waters and implications for particle #ux in the sea William M. Graham*, Sally MacIntyre, Alice L. Alldredge Marine Environmental Sciences Consortium, Dauphin Island Sea Lab., 101 Bienville Blvd., Dauphin Island, Alabama 36528, USA Marine Science Institute, University of California, Santa Barbara, CA 93106, USA Department of Ecology, Evolution and Marine Biology, University of California, Santa Barbara, CA 93106, USA Received 9 September 1998; received in revised form 11 May 1999; accepted 18 May 1999 Abstract Successive measurements of the size distribution and abundance of marine snow in the upper 100 m of the Santa Barbara Channel, California, with an in situ still camera system following 11 tagged water masses revealed a consistent pattern of nighttime decreases in the abundance of large particles. A net nocturnal reduction in particulate #ux from the upper 100 m as calculated from camera pro"les occurred in 75% of the day}night comparisons, and nighttime aggregate carbon losses resulted in a 38% average reduction in camera-derived aggregate #ux. Intensive investigation of three stations for 24}48 h each indicated that nighttime decreases in aggregate concentrations and derived aggregate #ux could be registered throughout the observed water column. Nocturnal decreases in marine snow concentration are unlikely to result from diel variations in the production of marine snow either as feeding webs of zooplankton or through variations in aggregation rates of smaller particles. Moreover, measured diel variations in the intensity of surface mixing and convective overturn during one of the 24 h deployments were not intense enough to produce aggregate fragmentation and reduced aggregate #ux. Nighttime increases in large crustacean zooplankton (i.e., euphausiids and the large copepod Calanus pacixcus) could explain some or all of the reduction in aggregate abundance at most stations. Fragmentation and consumption of marine snow by migrating macrozooplankton could produce our observed synchronous diel cycles in marine snow concentration. This is the "rst empirical evidence of a diel pattern in the concentration and calculated particulate #ux of large sinking particles in near-surface waters. The data presented here are consistent with the only other existing diel study, which also reported decreases in marine snow abundance at night at 270 m depths in the oceanic north Atlantic. Diel variations in the sizes and concentrations of * Corresponding author. 0967-0637/00/$ - see front matter  2000 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 3 7 ( 9 9 ) 0 0 0 6 3 - 1