Physical factors influencing fine organic particle transport and deposition in streams

The influence of physical factors on the transport and deposition of fine particulate organic matter (FPOM, 53-106 μm) in streams was investigated using ¹⁴C-labeled natural detritus. Field estimates of mean FPOM transport distance (S(p)) were calculated as the inverse of the longitudinal loss rate (k(p)) of particles. Deposition was determined by standardizing k(p) for depth and velocity and expressed as the mass transfer coefficient, v(dep). S(p) varied by orders of magnitude (7-1000 m) within and among streams. As expected, v(dep) behaved more conservatively than S(p) but still varied by a factor of 18 (0.06-1.10 mm/s). Field-estimated deposition velocities were always less than the quiescent water fall velocity (v(fall)) but no consistent relationship existed between the 2 (r = 0.26, p = 0.53). Variability in S(p) was strongly associated with the cross-sectional area of the transient storage zone (A(s); r = 0.93, p < 0.01) and the uptake length of water (S(w); r = 0.84, p = 0.01). The transfer coefficient was highest in the 2 smallest streams (Q < 15 L/s) but was similar, and unrelated to stream size, among 6 experiments conducted in stream segments where Q exceeded 100 L/s. Variability in FPOM v(dep) was less related to physical characteristics than S(p), although a significant, positive correlation was detected between v(dep) and A(s)/A. Evidence from this study suggests that the mechanisms assumed to govern particle transport in gravitational/hydrodynamic models may not be solely responsible for FPOM deposition in streams and that alternative processes, such as hyporheic filtration and biotic retention, may be important.