The breakdown of leaf litter and its conversion into FPOM and DOM are well-studied processes (Section 7.1). Physical abrasion, softening of leaf tissue by wetting and the initial leaching of DOM, colonization by fungi and bacteria, and feeding by consumers all contribute to the conversion of CPOM into FPOM and DOM. Leaf breakdown can occur at very low temperatures, but rates are more rapid as temperature increases, and are influenced by the presence of shredders, availability of nutrients, and whether leaves are buried or exposed. The overriding influence, however, is leaf type (Figure 7.2), which results in a fast-to-slow continuum of leaf breakdown rates. In their synthesis of many studies from 11 streams at the Coweeta Hydrologic Laboratory, the average leaf breakdown rate was found to be 0.0098 day1, which implies the loss of 50% of initial mass after 71 days (Webster et al. 1999).
Transport rates are expected to vary with POM size, flows, and the retentiveness of the stream channel. High discharges will entrain and transport even large particles, as can be seen from the export versus discharge relationship for a headwater stream in North Carolina (Figure 12.8). Based on a small number of direct measurements, however, the distances traveled by CPOM are surprisingly short. Leaves typically are trapped by obstructions within a few meters of their point of entry to the channel (Webster et al. 1994) and often are broken down in place without further transport, although they can move tens of meters in storms. Using rectangles of waterproof paper, Webster et al. estimated an
average distance of about 1.5 m from first entry to the stream, depending mainly on depth and likelihood of encountering an obstacle. Wallace et al. (1995) recorded movements of spray-painted red maple leaves and small pieces of colored plastic transparency sheets for up to 4 years. Although few natural leaves were recovered after about 5 months, no differences were observed between natural and artificial leaves, which, over the 4-year study, moved on average 10-20 m year1. The comparison of leaf breakdown to transport rates makes a strong case that, at least for the small streams where most research has been conducted, CPOM is transformed into other organic matter size classes or mineralized rather than exported.
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