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 mu m) in streams was investigated using C-14-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(g); 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(g)/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, map be important.