Flow‐substrate interactions create and mediate leaf litter resource patches in streams

Abstract

1. The roles that streambed geometry, channel morphology, and water velocity play in the retention and subsequent breakdown of leaf litter in small streams were examined by conducting a series of field and laboratory experiments. 2. In the first experiment, conditioned red alder (Alnus rubra Bongard) leaves were released individually in three riffles and three pools in a second-order stream. The transport distance of each leaf was measured. Several channel and streambed variables were measured at each leaf settlement location and compared with a similar number of measurements taken at regular intervals along streambed transects ('reference locations'). Channel features (such as water depth) and substrate variables (including stone height, stone height-to-width ratio, and relative protrusion) were the most important factors in leaf retention. 3. In the second experiment, the role of settlement location and reach type in determining the rate of leaf litter breakdown was examined by placing individual conditioned red alder leaves in exposed and sheltered locations (on the upper and lower edges of the upstream face of streambed stones, respectively) in riffle and pool habitats. After 10 days, percent mass remaining of each leaf was measured. Generally, leaves broke down faster in pools than in riffles. However, the role of exposure in breakdown rate differed between reach types (exposed pool > sheltered pool > sheltered riffle > exposed riffle). 4. In the third experiment, the importance of substrate geometry on leaf litter retention was examined by individually releasing artificial leaves upstream of a series of substrate models of varying shape. Substrates with high-angle upstream faces (were vertical or close to vertical), and that had high aspect ratios (were tall relative to their width), retained leaves more effectively. 5. These results show that streambed morphology is an important factor in leaf litter retention and breakdown. Interactions between substrate and flow characteristics lead to the creation of detrital resource patchiness, and may partition leaf litter inputs between riffles and pools in streams at baseflow conditions.

Ephemeroptera, Plecoptera and Trichoptera assemblages in litter in a mountain stream of the Atlantic Rainforest from Southeastern Brazil

“…In order to colonize leaves in riffles, the macroinvertebrates need adaptations that enable them to remain adhered to the substratum in conditions of strong flow (KOBAYASHI & KAGAYA 2002). HOOVER et al (2006) verified a higher rate of decomposition of litter in pools and suggested that litter in riffles may be accessible only for a limited group of detritivorous macroinvertebrates, a situation that could lead to a decrease in the rates of biological fragmentation in riffles. The smaller oxygenation rates in pools, however, may inhibit the colonization by taxa with high oxygen requirements (KOBAYASHI & KAGAYA 2002).…”

Effects of urban stream burial on organic matter dynamics and reach scale nitrate retention

“…Low organic matter standing stocks in the buried reaches likely resulted from a combination of reduced allochthonous and autochthonous inputs and limited retention. The buried reaches had greatly simplified geomorphology with few in-stream devices to physically retain organic matter for subsequent microbial processing (Hoover et al 2006).…”

The influence of flow and season upon leaf-litter breakdown in monsoonal Hong Kong streams

“…Litter breakdown process is also influenced by hydrological regime, especially current velocity and high-flow events (Bird & Kaushik, 1992;Lepori et al, 2005;Paul et al, 2006). While stream flow alone plays a minor role in determining breakdown directly, its effect seems more usually indirect by way of an influence on biological processes (Woodcock & Huryn, 2005;Ferreira et al, 2006;Hoover et al, 2006). For instance, breakdown rates of two leaf species in an Arizona stream decreased by *20% when flows were reduced by 95%, and this decrease was associated with cooler temperatures as well as lower fungal biomass and macroinvertebrate diversity on the leaves (Muehlbauer et al, 2009).…”