Transport and settlement of organic matter in small streams

Abstract

Summary 1. After it enters streams, terrestrially derived organic matter (OM) rapidly absorbs water. Using field and laboratory experiments, we examined how this process affected the buoyancy, settling velocity, transport distance and retention locations of four types of organic matter typically found in Pacific coastal streams (‘flexible’ red alder leaves and three ‘stiff’ particle types – Douglas‐fir needles, red cedar fronds and Douglas‐fir branch pieces). 2. Immersion in water rapidly changed the physical characteristics of alder leaves, Douglas‐fir needles and red cedar fronds, which all reached constant still‐water settling velocities after only a few days of soaking. In contrast, the settling velocity of branch pieces continued to increase for 13 days, eventually reaching much higher values than any other OM type. Dried alder leaves became negatively buoyant after only two days of immersion, while other types took substantially longer (up to 24 days) before the specific gravity of all particles was >1. 3. We released saturated OM particles in an experimental channel and found that all particle types travelled further in a fast, shallow ‘riffle’ than a slow, deep ‘pool’. Comparisons with a passive settlement null model indicated that leaves were retained more rapidly than expected in the riffle (by large protruding stones), while the three stiff particle types travelled further than expected (probably due to turbulent suspension) and were retained when they settled in deeper water between larger stones. In pools, passive settlement appeared to dominate the retention of all OM types, with leaves travelling furthest. 4. These retention patterns corresponded well with those observed when saturated OM particles collected in the field were released in two pools and two riffles in a second‐order coastal stream. 5. When the experimental channel and in‐stream data were combined, the retention rates of the three stiff OM types were closely related to calculated Rouse numbers (Rouse number = particle settling velocity/shear velocity), whereas the retention rate of alder leaves was not. This suggests that different physical mechanisms are responsible for the retention of leaves and stiff OM types in shallow streams.

Mean and turbulent velocity fields near rigid and flexible plants and the implications for deposition

“…The tendency for particles to deposit or resuspend is related to the ratio of the particle settling velocity (w s ) to the shear velocity (u * , which represents the velocity scale of turbulent eddies). In aquatic environments, measured settling velocities of organic floc fall around 0.1 to 0.3 cm s À1 [Droppo, 2004;Droppo et al, 1997] and shear velocities range from 1 to 50 cm s À1 [Fuller et al, 2011;Hoover et al, 2010;Statzner and Mueller, 1989], yielding ratios of w s /u * = 0.002 to 0.3. For the lab experiment, we used the bed friction coefficient (C f = 0.006) measured in previous studies over the same baseboards [White and Nepf, 2007] and the channel velocity (U o , Table 1), to estimate u * ≈ 0.7 cm s À1 .…”

Differences in water depth determine leaf-litter decomposition in streams: implications on impact assessment reliability

“…However, any stream reach has a variety of habitats such as pools, jams, riffles and different substrate sizes with different trapping efficiencies (Hoover et al, 2010) that make the distribution of leaf-litter to be patchy (Webster et al, 2001). Moreover, these mesohabitats also differ in a number of environmental characteristics such as flow velocity and oxygen saturation that affect biological communities (Álvarez-Cabria et al, 2010) and decomposition (Flores et al, 2013).…”

Effects of retention site on breakdown of organic matter in a mountain stream

“…Like most stream variables (Palmer & Poff, 1997), the distribution of coarse particulate organic matter (CPOM) in stream channels is extremely patchy (Cummins et al, 1983;Webster & Meyer, 1997;Webster et al, 2001), as potential retention sites differ greatly in their trapping efficiency (Larrañaga et al, 2003;Hoover et al, 2010). Therefore, depending on the stability of retentive structures and on the lability of materials, CPOM can persist in stream channels from days to hundreds of years (reviewed by Tank et al, 2010).…”