The transport of organic matter in natural waters depends on the size and suspended mass fraction of flocs, which are agglomerations of organic and inorganic particles. Prevailing theory links floc size and floc fraction primarily to turbulence and particle concentration. Hypothetically, the size and mass fraction of flocs should decrease as turbulence increases, and they should increase as suspended particle concentration increases.

To test these hypotheses, we examined floc properties in a coastal bottom boundary layer south of Martha's Vineyard, Massachusetts. For three weeks in September, 2005, we monitored the size distribution of particles ranging in diameter from micrometers to centimeters. At the same time, we measured waves and currents, which allowed us to estimate boundary shear stress, and the optical properties of the suspension, which allowed us to characterize particle concentration and composition.

Results show that the maximum values and ranges of floc size and floc fraction decrease with increasing shear stress and particle concentration. These observations are consistent with the hypothesized role of turbulence in limiting floc size and floc packaging, but they are inconsistent with the hypothesized role of particle concentration. Instead, particle composition apparently plays a major role in determining floc size and floc mass fraction, with the presence of organic material encouraging the growth of large and abundant flocs.