Aggregation of fine particles at the sediment‐water interface

The presence of a bottom sediment layer agitated by mechanical stirring or by resident organisms (tubificid oligochaetes) significantly increases the rate at which fine (1 μm) cohesive particles are removed from suspension in laboratory columns. Measured rates of particle removal are equivalent to deposition velocities ranging from 0.23 m day −1 to 0.41 m day −1 . These rates are an order of magnitude faster than deposition by gravitational settling or coagulation with larger particles in the water column as observed in experimental controls. It is hypothesized that the increased removal rate is the result of aggregation in a sediment layer at the bed‐water interface characterized by loosely bound (fluffy), porous material hydrodynamically coupled to the water column. According to this hypothesis particle removal occurs when motion of the overlying water or organism activity causes suspended fine particles to collide with and stick to the interfacial sediment. This new hypothesis is supported by the mass and size distribution of tracer particles recovered in cores and sediment traps at a coastal site and by theoretical estimates of interfacial aggregation rates.