Fine particulate organic matter dynamics

The ingestion of particles by collector-gatherers and filter-feeders acts both as retention and utilization, as some fraction of the ingested material is digested and metabolized by the animal consumer. There does not appear to be any estimate of the potential magnitude of this effect owing to benthic collector-gatherers. However, filterfeeders typically consume only a small fraction of transported particles, < 1% of annual transport in one estimate (Webster 1983). Thus, microorganisms, mainly bacteria, appear to be responsible for most of the breakdown and remineralization of the organic C of FPOM that occurs within stream ecosystems. However, this interpretation may reflect inadequate study of benthic detritivores. Rosi-Marshall and Wallace (2002) estimated considerable ingestion of amorphous detritus by macroinvertebrates in a mid-order river in North Carolina, but their impact on system-level degradation of benthic FPOM is largely unknown.

Unlike CPOM with its abundance of direct measurements of breakdown rates, the utilization of FPOM is poorly documented, although microbial utilization can be approximated from measurements of respiration rates. Using laboratory measurements of benthic FPOM respiration, Webster et al. (1999) estimated breakdown rates 0.00104 day1, for a half-life of about 1.8 years. Respiration rates are expected to decline over time as FPOM mass is lost, leaving more refractory material behind, but supporting evidence is weak (Sinsabaugh et al. 1991). Seasonal and latitudinal variation in temperature has a strong influence on respiration rates (Figure 12.7) and thus on the utilization of FPOM (Webster et al. 1999).

FPOM transport distance can be estimated by releasing a known quantity of particles into the stream and measuring water column concentrations at various distances downstream. The decline in particle concentration is fit to an exponential decay equation, and the inverse of the decay coefficient is a measure of average transport distance of a particle before being retained on the streambed. Using corn pollen as a surrogate for FPOM (it is similar in diameter but less dense), Miller and Georgian (1992) estimated mean transport distances of 100-200 m in a second-order stream in New York. Estimated transport distance for natural FPOM labeled with radiocarbon in the Salmon River headwaters of Idaho ranged from 150 to 800 m (Cushing et al. 1993, Newbold et al. 2005). Assuming that particle resuspension occurs every 1.5-3 h and an average transport distance per event of 500 m, Cushing et al. calculate an average downstream transport of 4 —8 km day1. Much greater transport distances, between 3,000 and 10,660 m, were estimated for a sixth-order lowland river, using spores of Lycopodium clavatum with a fluorescent label, and distances were greater under faster currents (Wanner and Pusch 2001). Longer transport distances in larger rivers may reflect fewer opportunities for particle entrapment, whereas the extent of water exchange between surface flows and the hyporheic zone has been shown to correlate with transport distances in smaller streams (Minshall et al. 2000).

Owing to the relative slow rate of utilization of FPOM estimated from respiration measurements, and the relatively long transport distances combined with frequent resuspension, export rather than mineralization appears to be the dominant fate of FPOM from studies of smaller streams and rivers.

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