The abundance, vertical flux, and still-water and apparent sinking rates of marine snow in a shallow coastal water column

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In Cape Lookout Bight, NC, measurements were made of marine snow (e.g., aggregates >0.5 mm in diameter) abundance and size, the vertical flux of material through the water column, the vertical flux of marine snow, and the measured still-water sinking rate of marine snow. The abundance and size of marine snow was determined from photographs taken by SCUBA divers. Sampled marine snow were used to determine aggregate dry weight. Marine snow flux and measured still-water sinking rate were made using a time-lapse camera that photographed a sediment trap mounted on a drogue. Flow over the drogue mounted sediment trap was <3 cm s −1. Knowing the flux of marine snow into the trap and the abundance of marine snow in the water column, the sinking rate of marine snow sinking freely in the water column (apparent marine snow sinking rate) was calculated. Throughout the study, marine snow was always present and usually abundant (average 125 l −1). The water-column particulate matter load ranged from 8 to 18 mg l −1. The vertical flux of particulate matter through the water column was large (average 74 g m −2 day −1). The average vertical flux was roughly equal to the sedimentation rate estimated from geochronology studies of sediment deposition in Cape Lookout Bight. More than 90% of the variation in the vertical flux of particulate matter through the water column was due to variations in the flux of marine snow. Variations in the vertical flux of marine snow were, however, not significantly correlated to the abundance of marine snow. Measured sinking rates ranged from around 10 to about 150 m day −1, increased with aggregate size, and varied little between days. The calculated apparent sinking rates of marine snow varied a great deal between days and were not similar to the measured sinking rates. The distribution and orientation of marine snow in the water column photographs appeared to delineate small (mm to cm diameter) eddies. Eddies in a random selection of water column photographs were measured. The upper limit to aggregate size appeared to have been set by the size of small turbulent eddies in the water column. The apparent sinking rate of marine snow, the total vertical flux and the flux of marine snow were all significantly correlated ( r>0.97) to the ratio of aggregate dry weight and eddy diameter. The data suggest that marine snow sinking rates were modified by turbulence in the water column.