Fine particulate organic matter

Fine particulate organic matter (FPOM) is generated within streams from the breakdown of leaf litter and other CPOM, and in some circumstances fecal pellets can be very abundant (Section 7.2.1). In addition, considerable amounts of FPOM enter the channel as a product of the breakdown of vascular plant material from the surrounding terrestrial landscape. FPOM inputs are low during base flow and increase during storms and seasonal high flows (Hedges et al. 1986, Mulholland 1997a), as rising flows entrain particles from stream banks and side channels.

Many studies document variation in the concentration of suspended FPOM with season, surrounding vegetation, and stream flow. A comparison of 31 streams and medium-sized rivers in North America, Europe, the Arctic, and Antarctica found POM to range between 0.14 and 15.30 mg L 1 (Golladay 1997). In streams and rivers of eastern United States, POM estimates ranged between 0.5 and 52 mg L 1 (Webster et al. 1995); in undisturbed forested catchments, mean annual concentrations often are < 2mg L 1 (Fisher and Likens 1973, Naiman and Sedell 1979). Forested streams frequently exhibit higher FPOM concentrations that non-forested streams, as was observed in a comparison of native forest to pasture sites in a New Zealand stream (Young and Huryn 1999), and in the above-mentioned 31 streams from different regions (Golladay 1997). However, high FPOM concentrations also are seen in low gradient streams flowing through agricultural or multiple-use catchments (Malmqvist et al. 1978), and especially in larger lowland rivers (Thames, Ber-rie 1972b; South Platte, Colorado, Ward 1974). POM concentrations show a positive relationship with regional precipitation, probably due to the influence of precipitation on terrestrial production and thus on the supply of CPOM. Floodplains can be a large source of organic matter to rivers, and higher POM concentrations usually are observed in low gradient streams during inundation (Golladay 1997).

FPOM concentrations are influenced by changes in particle availability, which is influenced primarily by terrestrial production and seasonal changes in biological processes within the stream ecosystem; and by discharge, which varies both seasonally and unpredictably. In streams of the eastern United States during normal flows, FPOM concentrations are higher during spring and summer than during autumn and winter (Figure 12.4), probably because lower biological activity during the colder months results in less instream particle generation (Webster et al. 1995). If the particle supply is relatively constant, then an increase in discharge will cause a dilution effect, even if the total amount of FPOM in transport is greater. In Bear Brook, New Hampshire, POM concentrations were highest in summer but transport was greater in winter (Fisher and Likens 1973), and so dilution is responsible for low winter concentrations. However, in southern Appalachian streams dilution is not the sole explanation for lower winter concentrations, because transport is highest in spring and summer (Webster and Golladay 1984).

FIGURE 12.4 Seasonal variation in FPOM concentrations in a forested headwater stream in the southern Appalachians. Error bars show 95% confidence intervals. (Reproduced from Webster and Golladay 1984.)

An increase in streamflow in response to a rainstorm results in a corresponding increase in particle concentrations as POM generated during low flows and stored in depositional areas is entrained by rising stream levels (Fisher and Likens 1973, Meyer and Likens 1979). This indicates that the major pool of POM lies in areas already wet or adjacent to the stream's wetted perimeter, where FPOM accumulates during low flows. Inputs of POM from outside of the stream also are likely to be greatest during the rising limb of the hydrograph, due to the erosive effects of rainfall on soil and stream bank litter and generation of flow in previously dry channels. During the descending limb, water enters the stream principally by subsurface flow, and carries little or no POM. Thus, concentrations are highest on the rising limb of the hydrograph (Figure 12.5) and then decline due to exhaustion of the particle supply and dilution of the entrained material.

Just as depletion of benthic POM contributes to concentration differences between rising and falling limbs of the hydrograph, time elapsed since the last storm and seasonal differences in particle generation interact with discharge to determine POM peaks during storms. In a forested headwater stream studied by Wallace et al.

FIGURE 12.5 Changes in discharge, FPOC concentrations, and FPOC transport during a summer storm in a small forested catchment in New Hampshire. Note that FPOC concentrations peak on the rising limb of the hydrograph, indicating rapid entrainment of small particulates. A second hydrograph peak resulted in a much smaller FPOC concentration peak, evidence that washout rapidly depletes the available FPOC supply. — indicates discharge; O—O denotes FPOC concentration and A... A denotes FPOC export. (Reproduced from Bilby and Likens 1979.)

ooooooooooo

6 June 7 June 8 June 9 June

FIGURE 12.5 Changes in discharge, FPOC concentrations, and FPOC transport during a summer storm in a small forested catchment in New Hampshire. Note that FPOC concentrations peak on the rising limb of the hydrograph, indicating rapid entrainment of small particulates. A second hydrograph peak resulted in a much smaller FPOC concentration peak, evidence that washout rapidly depletes the available FPOC supply. — indicates discharge; O—O denotes FPOC concentration and A... A denotes FPOC export. (Reproduced from Bilby and Likens 1979.)

(1982b), POM concentrations increased greatly with rising discharge during the first autumn storm, apparently due to the availability of abundant FPOM generated by autumn leaf fall, coupled with a lengthy prior period of low flow. A winter storm resulted in a smaller increase, which Wallace et al. attributed to depletion of benthic FPOM by washout during prior fall storms. In larger rivers, the effect of individual storms on POM concentrations is less pronounced, and seasonal variation in flows and floodplain inundation is more important. As is found in smaller systems, POM concentrations in large rivers are highest on the rising limb of the hydrograph, and decline thereafter (Thurman 1985).

Because POM concentrations usually are higher when discharge is greater, most FPOM is transported during episodic and seasonal floods, and thus flow conditions that occur during only a small fraction of the annual discharge cycle can account for a very large fraction of annual transport. High discharges representing just 1% of the annual discharge regime accounted for 20% of water export and fully 70% of annual export of

FPOM from a small stream at Hubbard Brook Experimental Forest (Bilby and Likens 1980, Bilby 1981). Some 75-80% of POM transport in small Coweeta streams occurred during storms (Webster et al. 1990), demonstrating the importance of accurately sampling these episodic events. In fact, unless sampling is continuous or captures high flows very thoroughly, total transport may be seriously underestimated.

0 0

Post a comment