Fraction amorphous detritus
FIGURE 8.1 Relationship between the fractions of invertebrate carbon derived from bacteria and the fraction of amorphous detritus in invertebrate guts. (Reproduced from Hall and Meyer 1998.)
invertebrates derived from 0% to 100% of their C from bacteria and this amount was significantly related to the percent of amorphous detritus in invertebrate guts (Figure 8.1), so it is likely that biofilm was ingested incidentally through the consumption of particles of FPOM and CPOM. The assimilation of bacterial C may also have resulted from consumption of bacterial exopoly-mers secreted within the "slime" and consumed by the grazing of substrate surfaces. Enrichment of stream ecosystems with highly labile dissolved organic carbon (DOC) provides further evidence of a direct connection to higher trophic levels. When the amount of bioavailable C was increased in a small stream by the addition of dextrose, the density, growth, and respiration of epilithic bacteria increased in a treatment reach compared to the control, and growth rates of chironomid larvae also increased (Wilcox et al. 2005).
Figure 8.2 summarizes our best current understanding of the relationships among microbial
production and the meiofauna within stream food webs (Schmid-Araya and Schmid 2000). The number of trophic links in the microbial web is strongly influenced by the small size of its members and especially the size of a bacterial cell, which is about 0.5 ^m. Because few suspension feeders are able to capture particles of this size (Wotton 1994), the ingestion of bacteria by flagellates (~5 ^m) and ciliates (~25 ^m)
provides a microconsumer pathway for this energy to reach larger consumers. On the other hand, bacteria associated with biofilm in a surface microlayer may be ingested directly by ben-thic consumers of surface films and by deposit feeders that pass organic matter and associated microbes directly through their guts. Clearly, the number of trophic transfers between bacterial production and macroinvertebrate consumer can be one or many, with significant consequences for energy dissipation, and this may vary between water column and benthic habitats. It is conceivable that in some habitats the microbial and metazoan webs are linked, and in others the microbial loop is an energy sink, internally dissipating whatever energy is obtained from dissolved and particulate C sources. Elucidation of the magnitude of the "link versus sink'' role of the microbial loop is an important current challenge in lotic ecology.
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