FPOM originating from leaf decomposition

Feeding by leaf-shredding insects produces FPOM not only by fragmenting larger particles into smaller ones but also through the production of feces. With assimilation efficiencies ~ 10-20% and ingestion rates in the range of the animal's mass per day (Golladay et al. 1983), many consumers of CPOM produce copious amounts of feces. Coprophagy has been reported to be a dietary mainstay in some instances (Hynes 1970) and an important supplement in others

(Wotton 1980). Some stream-dwelling invertebrates such as amphipods and isopods produce pellets enclosed in a peritrophic membrane, while most insects apparently produce feces that are less discrete and more variable in size (Shepard and Minshall 1981). Ladle and Griffiths (1980) provide a pictorial description and commentary on size, shape, texture, cohesiveness, and so forth. Most such particles appear to vary between 100 and 1,000 |im in longest dimension, but since a correlation exists between size of particles and the organisms that produced them, the smallest invertebrates probably produce even smaller fecal particles.

Consumers differ in how they attack leaf material, which can affect the appearance of resultant FPOM. Larvae of the crane fly Tipula and many limnephilid caddis flies eat all parts of the leaf, both mesophyll and venation, while the peltoperlid stonefly Talloperla cornelia avoids venation and concentrate mainly on mesophyll, cuticle, and epidermal cells (Ward and Woods 1986). Fecal pellets from Tipula visually resembled macerated leaf fragments and were similar to source material in lignin, hemicellulose, and cellulose content. If one included the nonin-gested fragments, resultant FPOM even more faithfully resembled its source. In contrast, T. cornelia produced a macerated FPOM in which lignin content was substantially reduced, especially from leaves with highest initial concentrations. Cellulose was also reduced, while hemicellulose either remained similar or increased.

In addition to their role in changing the palat-ability and nutritional content of CPOM, aquatic fungi generate substantial amounts of FPOM in the form of asexual spores called conidia. Gess-ner and Chauvet (1997) determined that nearly half of fungal production is allocated to the production of conidia, which are released into the water and so available to fine-particle feeders. Maximum production of conidia occurred shortly after leaf colonization.

Because the most readily assimilated material is likely to be processed in the steps prior to FPOM production (Figure 7.3), much of what remains is likely to be quite refractory. This is born out by the finding that the respiration rate associated with native detritus was much lower than that of conditioned and mechanically ground oak and hickory leaves (Ward and Cummins 1979). As FPOM is decomposed and reduced in size, one might expect particles to become more refractory to microbial action and lower in nutritional value. A study of the chemical composition and microbial activity of FPOM in relation to particle size supports this expectation (Peters et al. 1989). As particle size decreased from 500 to 10 |im, organic content declined while lignin and cellulose content increased.

Black fly larvae ingest FPOM and DOM and produce larger particulate material in the form of fecal pellets (Wotton et al. 1998). Hershey et al. (1996) observed a 28% increase in the AFDM of FPOM and an alteration of the particle size distribution downstream of a filtering black fly aggregation. Each larva can produce on average 575 pellets per day, and so in dense aggregations (~ 600,000 m2 within a 40 m reach of an Arctic tundra stream), the daily production of pellets was estimated at 1.3-9-2 x 109. About one third of this material deposits on the stream-bed and is available to other benthic consumers, thus larval black flies represent an important link between FPOM, DOM, and consumers in benthic communities (Wotton et al. 1998, Mal-mqvist et al. 2001).

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