Particulate and DOM originating both within the stream and in the surrounding landscape is an important basal resource to fluvial food webs. Detritus-based energy pathways can be particularly important, relative to pathways originating from living primary producers, in small streams shaded by a terrestrial canopy and in large, turbid rivers with extensive floodplains. Coarse, fine, and dissolved OM originate from a myriad of sources. Leaves, fruits, and other plant products that fall or are transported by wind and gravity into the stream are major CPOM inputs, and the carcasses and feces of insects and larger animals also contribute. Most FPOM originates from the fragmentation of larger particles, particularly terrestrial vegetation, and is likewise transported into the channel or is produced by the breakdown of larger particles within the stream. Soil water is usually the major source of DOM, which is a heterogeneous pool of molecules of widely varying bioavailability. The quality and accessibility of nonliving OM as a basal resource typically depends on the presence of bacteria or fungi whose degradative activity can alter palatability or accessibility of OM to consumers. Microorganisms are critical mediators of OM pathways, aiding in the processing of POM and uptake of DOM, and markedly increasing the energy available to consumers both small and large.

Autumn-shed leaves are a primary CPOM input to forested streams in temperate regions, and their decay serves as the primary model of CPOM processing. Leaves, which serve as the OM or C substrate, quickly release DOM when wetted, and soon are colonized by microorganisms and invertebrates, which enhance fragmentation and mineralization. The original leaf is transformed into several products including mi-crobial and shredder biomass, FPOM, DOM, nutrients, and CO2. Leaf breakdown rates vary considerably and as a consequence the supply of CPOM to the stream food web, although pulsed seasonally, is less so than would be the case if all leaves had similar breakdown rates. Temperature, oxygen availability, and nutrient supply are key environmental variables that influence the decomposition process. Colonization by microorganisms, particularly fungi, is critical to leaf decomposition as well as the leaf's nutritional value, although bacteria play a greater role as particle size diminishes and the presence of leaf-shredding invertebrates is also important. When detritivorous invertebrates are excluded, the breakdown process is significantly slowed.

FPOM is an amorphous collection of particles < 1 mm, originating from instream CPOM breakdown, sloughed cells of algae, invertebrate fecal pellets, and fragments derived from the terrestrial environment. The uptake of DOM within biofilms provides another avenue for FPOM production. Black fly larvae, by consuming bacteria and very small FPOM and producing fecal pellets that are larger than some of the material that they ingest, illustrate yet another pathway for the generation of organic particles. The sources, processing, and eventual fate of FPOM are less well accounted for than is the case for CPOM. Although the breakdown of CPOM within the stream is the best-studied pathway, it seems likely that FPOM originates from many other sources. Because of the small size of FPOM, bacteria likely are more important than fungi in microbial processing.

DOM typically is the largest pool of organic C in running waters and is incorporated into POM due primarily to uptake by microorganisms. Soil and groundwater are major pathways of DOM transport from terrestrial vegetation and wetlands into river water, whereas leachate from leaves and from extracellular release by algae and higher plants can be temporally important instream sources. Because it comprises a hetero geneous mix of bioavailable, refractory, and perhaps inhibitory compounds, total DOM is a poor predictor of microbial metabolism. DOM is removed from streamwater by both abiotic and biotic processes, and DOM uptake appears to be greatest in biofilms, which are symbiotic assemblages of bacteria, fungi, and algae that interact both synergistically and antagonistically in their demand for C and nutrients. Because biofilms are embedded in a polysaccharide matrix, exoen-zymes and exudates resulting from cell lysis and extracellular release are held within the matrix by low diffusion rates, allowing microorganisms to benefit from available compounds and possibly permitting bacteria to shift energy from enzyme production to cell growth. Biofilms can develop in the dark, in which case the algal contribution is negligible, but heterotrophic microorganisms benefit most when algal production within the biofilm is vigorous. Biofilm development on substrates and to a lesser degree on suspended POM depends greatly on the presence of labile DOM compounds and in some circumstances on nutrient availability as well. LMW DOM such as glucose can greatly stimulate uptake and growth by microorganisms, and at present the most effective assay of DOM bioavail-ability is via microorganism response.

Particulate and DOM are important energy sources in almost all lotic ecosystems and frequently can be the dominant energy source. Detritus and the associated microbial biomass, along with algae and other primary producers, form the basal resources for the various trophic levels occupied by invertebrates, fishes, and other consumers in stream food webs. We turn now to the diversity of consumers and the feeding adaptations that govern their effectiveness with the diverse producer and detrital resources described in this and the preceding chapter.

Chapter eight

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