Deposition, benthic residence, and resuspension of fine organic particles in a mountain stream

Abstract

We estimated deposition and resuspension rates of natural particulate organic matter (POM) in Bloomington Creek, Idaho, a mountain stream flowing at 225 L s™1. POM was collected from the water column, fractionated into two size classes— very fine POM (VFPOM, 15-52 µm) and fine POM (FPOM, 53-106 µm), and radiolabeled by using 14C-dimethylsulfate. The labeled particles in each size class and a conservative tracer were released to the stream in metered pulses and then sampled from the water column at six stations extending 1 km downstream for 4 d. Deposition and resuspension rates were estimated by fitting a one-dimensional advection-dispersion model to 14C-concentrations measured during and after release. Model-estimated deposition velocities were 0.12 (0.09-0.16, 95% confidence interval) and 0.18 (0.10-0.31) mm s−1 for VFPOM and FPOM, respectively. There was some (~0.05 mm s21) additional short-term (~20 min) detention of VFPOM and FPOM that may have been related to transient storage. For VFPOM, 34% of deposited particles resuspended after a mean residence time of 13 (6.9-25) h, and the remainder resuspended with a residence time of 7.5 (2.9-19) d. For FPOM, these estimates were 17%, 2.4 (1.0-4.9) h, and 2.6 (1.7-4.0) d, respectively. The weighted mean residence times and downstream velocities of particle migration were 5.1 d and 150 m d−1 for VFPOM, and 2.2 d and 230 m d−1 for FPOM. The migration velocities suggest that a significant fraction of particles exported from headwater streams travel long distances and can reach larger riverine or marine environments before mineralization.