The Decomposition of Coarse Particulate Organic Matter

The fate of CPOM is best known for autumn-shed leaves, which are major energy inputs to many small, forested streams. The topic is reviewed by Webster and Benfield (1986), and

TABLE 7.1 Sources of organic matter (OM) to fluvial ecosystems.

Sources of input

Comments

Coarse particulate organic matter (CPOM)

• Leaves and needles

• Macrophytes during dieback*

• Other plant parts (flowers, fruit, pollen)

• Other animal parts (feces and carcasses)

Fine particulate organic matter (FPOM)

• Breakdown of CPOM

• Feces of small consumers

• From DOM by microbial uptake

• From DOM by physical-chemical processes

• Sloughing of organic layers

• Forest floor litter and soil

• Stream bank and channel

Dissolved organic matter (DOM)

• Groundwater

• Subsurface or interflow

• Leachate from detritus of terrestrial origin

• Throughfall

• Extracellular release and leachate from algae*

• Extracellular release and leachate from macrophytes*

Major input in woodland streams, typically pulsed seasonally Locally important

May be major biomass component, very slowly utilized Little information available Little information available

Major input where leaf fall or macrophytes provide CPOM Important transformation of CPOM Organic microlayers on stones and other surfaces Flocculation and adsorption, probably less important than microbial uptake route Of local importance, may show temporal pulses Little information available

Influenced by storms causing increased channel width and inundation of floodplain, affected by overland versus subsurface flow Little known, likely related to storms

Major input, relatively constant over time, often highly refractory More important during storms

Possibly important during storms causing overland flow

Major input, pulsed depending upon leaf fall

Smaller input, dependent on contact of precipitation and clouds with canopy Of local importance, may show seasonal and diel pulses Of local importance, may show seasonal and diel pulses

Much OM originates outside the stream reach where it is measured. Some (sources marked with an asterisk) is produced by photosynthesis within the stream and subsequently enters the pools of dissolved or particulate OM.

methods of study are described by Boulton and Boon (1991) and Graca et al. (2005). The breakdown of macrophytes is similar to that of leaves of terrestrial origin, although some minor differences are noted below. The breakdown of woody material is, not surprisingly, much slower than that of leaves, and is of lesser importance to higher trophic levels. Other sources of CPOM that enter heterotrophic pathways in running waters and may be locally or seasonally important, such as flower parts, animal feces, and carcasses of large animals, have received less study.

Once CPOM enters streams it undergoes a breakdown process or is exported (Webster et al. 1999). Studies of OM breakdown start with the source material, often using leaves picked from riparian trees just prior to abscission, and follow its disappearance over time. As the process advances, leaves release solutes and are colonized by microorganisms and invertebrates, which enhance fragmentation and mineralization (the conversion of organic C compounds into inorganic carbon dioxide [CO2]). The original leaf is transformed into several products including microbial and shredder biomass, FPOM, DOM, nutrients, and CO2 (Gessner et al. 1999). DOM and FPOM can undergo further microbial degradation or be transported downstream.

The loss of leaf mass over time is approximately log-linear (Figure 7.1), although some data have been interpreted as linear or as consisting of two or more distinct stages. Webster and Ben-field (1986) argue that a simple exponential model provides a general description of the breakdown process,

where Wt = dry mass at time t, Wi = initial dry mass, and t is time, measured in days. The statistic k (in units days1), which is the slope of the plot of the natural logarithm of leaf mass versus time, provides a single measure of breakdown rate.

The rate of leaf breakdown is determined by intrinsic chemical and structural differences

Particulate Organic Matter

Time (week)

FIGURE 7.1 Leaf dry mass remaining (as %) from alder (*) and willow (O) leaf packs in an experiment conducted in a Black Forest stream, Germany. Error bars represent 95% confidence intervals. (Reproduced from Hieber and Gessner 2002.)

Time (week)

FIGURE 7.1 Leaf dry mass remaining (as %) from alder (*) and willow (O) leaf packs in an experiment conducted in a Black Forest stream, Germany. Error bars represent 95% confidence intervals. (Reproduced from Hieber and Gessner 2002.)

among leaves, a number of environmental variables, and the feeding activity of detritivores. Petersen and Cummins (1974) suggested a continuum of decomposition rates from slow to fast, based on the breakdown of leaves from six deciduous tree species in a small Michigan stream. They also recognized that this variation in leaf decomposition rates, which they termed a ''processing continuum,'' had important consequences for invertebrate consumers by extending the time interval over which microbially colonized leaf litter was available. The wide variation in the breakdown rate of the leaves of different plant species has now been amply documented (Figure 7.2). Nonwoody plant leaves decompose much more quickly, on average, than do leaves of woody plants (mean half-lives in Figure 7.2 are approximately 65 days and 100-150 days, respectively). Submerged and floating macrophytes are among the fastest to decay, presumably because they contain the least amount of support tissue and often the highest concentration of potentially limiting elements such as nitrogen (N) and phosphorus (P).

A number of environmental factors also influence breakdown rate. Although leaf breakdown can occur at near-zero temperatures (Short et al. 1980), breakdown rates generally are faster at warmer temperatures (Abelho et al. 2005). Faster breakdown also occurs in more nutrient-rich systems, apparently due to the greater availability of N. Laboratory studies typically show acceleration of leaf breakdown in response to N addition (Meyer and Johnson 1983). Higher nitrate concentrations partly explain higher leaf decomposition rates in streams in logged catchments (Benfield et al. 2001). Rosemond et al. (2002) found that variation in leaf breakdown rates along a stream in Costa Rica correlated with a natural gradient in P concentrations. Experimental addition of N and P to a stream at the Coweeta Hydrologic Laboratory in the southern Appalachians enhanced the loss of wood mass and increased microbial respiration and fungal biomass (Gulis et al. 2004). However,

FIGURE 7.2 The breakdown rates for various woody and nonwoody plants, based on 596 estimates compiled from field studies in all types of freshwater ecosystems. Means ± 1 standard error are shown, and the variation is due to (at least) effects of site, technique, and numerous environmental variables. The number of individual rate estimates is shown in parentheses. (Reproduced from Webster and Benfield 1986.)

FIGURE 7.2 The breakdown rates for various woody and nonwoody plants, based on 596 estimates compiled from field studies in all types of freshwater ecosystems. Means ± 1 standard error are shown, and the variation is due to (at least) effects of site, technique, and numerous environmental variables. The number of individual rate estimates is shown in parentheses. (Reproduced from Webster and Benfield 1986.)

nutrient additions to a stream in the Caribou National Forest in southeast Idaho did not affect leaf breakdown, suggesting that microorganisms were not nutrient limited due to relatively high ambient nutrient concentrations (Royer and Minshall 2001). Low pH retards decomposition by inhibiting the activity of microorganisms and invertebrates (Dangles et al. 2004a). Hydrologic fluctuations can cause abrasion and fragmentation, which may expose more surface area to microbial action (Benfield et al. 2001), and burial, which can reduce microbial activity by reducing the availability of oxygen (Sponseller and Benfield 2001). Metal pollution can decrease decomposition rates by negatively affecting shredders and microorganisms (Niyogi et al. 2001, Duarte et al. 2004, Carlisle and Clements 2005).

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