What generates daily cycles of marine snow?

The recent discovery of daily cycles in the concentration of marine aggregates raises questions regarding the process producing the cycles and whether or not aggregation theory is able to predict them. A model of particle dynamics was used to study these questions. The model incorporated particle aggregation, break-up and sedimentation as well as diel growth and grazing. Three main processes considered to be possible causes for daily cycles of marine aggregates (growth, turbulence and grazing) were tested with the model. The results demonstrated that aggregation theory is able to predict daily cycles of marine aggregates. Neither the daily cycle of growth, nor that of grazing, was able to generate daily cycles alone. However, daily cycles of marine turbulence in the mixed layer caused a clear cyclic behaviour of particulate matter in the model. Therefore, these results suggested that diel variation of turbulence is the best candidate to explain the daily cycles of aggregates observed in the sea.