Diel variations of marine snow concentration in surface waters and implications for particle flux in the sea

Successive measurements of the size distribution and abundance of marine sn ow 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 consis tent pattern of nighttime decreases in the abundance of large particles. A net nocturnal reduction in particulate flux from the upper 100 m as calcula ted from camera profiles occurred in 75% of the day-night comparisons, and nighttime aggregate carbon losses resulted in a 38% average reduction in ca mera-derived aggregate Bur. Intensive investigation of three stations for 2 4-48 h each indicated that nighttime decreases in aggregate concentrations and derived aggregate flux could be registered throughout the observed wate r column. Nocturnal decreases in marine snow concentration are unlikely to result from diel variations in the production of marine snow either as feed ing webs of zooplankton or through variations in aggregation rates of small er particles. Moreover, measured diel variations in the intensity of surfac e mixing and convective overturn during one of the 24 h deployments were no t intense enough to produce aggregate fragmentation and reduced aggregate B ur. Nighttime increases in large crustacean zooplankton (i.e,, euphausiids and the large copepod Calanus pacificus) could explain some or all of the r eduction in aggregate abundance at most stations. Fragmentation and consump tion of marine snow by migrating macrozooplankton could produce our observe d synchronous diel cycles in marine snow concentration. This is the first e mpirical evidence of a diel pattern in the concentration and calculated par ticulate flux of large sinking particles in near-surface waters. The data p resented 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 concentrati ons ofmarine snow may impact water column processes dependent upon particle availability and size, such as grazing and remineralization, and may gener ate a diel cycle of food availability to the benthos. (C) 2000 Elsevier Sci ence Ltd. All rights reserved.