- The distribution and size of large marine aggregates (;0.5 mm) photographed in situ were compared with the small-particle distribution determined from beam attenuation (660 mm) in a section across the shelf and slope of the northern Gulf of Mexico. On the shelf changes in beam attenuation matched the trends in large particle concentration. Over the slope, beam attenuation profiles resembled the aggregates profiles in the upper 400 m with a small surface mixed layer maximum, a decrease through the upper thermocline to a minimum, and a small intermediate nepheloid layer (INL) in both large and small particles between 300 and 400 m. At the two deepest stations beam attenuation remained low from the bottom of the INL to the seafloor, while concentrations of large particles increased through the lower water column to concentrations greater than in the surface water. The distributions suggest that both large and small particles are resuspended from the upper slope and advected seaward, but on the mid slope this mode of transport occurs mostly as large aggregates. From calculations of settling speeds based on estimates of particle density, it appears that these aggregates are important in both the horizontal and vertical flux of particles along continental margins. 1990.