The downward transport of surface particle production constitutes an important mechanism for carbon sequestration by the ocean. Only a small fraction (≃10%) of the flux that leaves the euphotic zone reaches 1000 m depth because the particulate organic matter is consumed and transformed in the oceanic midwater column. The depth at which this transformation occurs is crucial to estimate carbon sequestration. Description of the particle flux and remineralization with depth below the euphotic zone has previously been limited to empirical relationships that neglect physical and biological mechanisms. Because several particle properties and functions (settling speed, rates of coagulation and consumption) are related to particle size, measurements of particle size spectra provide an important tool to understand particle dynamics. Most of the mechanisms affecting particle dynamics known for the surface layer should continue in the mesopelagic region. We review the different mechanisms and formulate equations describing changes in particle size distributions throughout the water column as a result of particle sinking, coagulation, disaggregation, and bacterial and zooplankton consumption. The resulting model describes the midwater particle population by its mass distribution for the size range 1 μm to 1 cm. Most of the particle data sets have a narrower size range but we suggest that model results are not greatly affected by the lack of data on particles smaller than 200 μm because most of the particle mass is in particles larger than 100 μm, as seen in a full size-range particle spectrum. The combination of this model with a unique particle size spectra data set, obtained during an inter-annual survey at the French JGOFS site in the NW Mediterranean Sea, give an insight into the key processes for particle dynamics in the unknown midwater layers.