Organic Matter Processing

The importance of organic matter contributions from riparian vegetation to stream ecosystems has been fully appreciated for only about 10 years (Cummins 1974). The manner in which aquatic organisms utilize and process organic matter at different seasons and locations along streams is a current research topic (Cummins 1973, 1975; Cummins and Klug 1979; Anderson and Sedell 1979; Hawkins and Sedell 1981). We briefly summarize here the role of aquatic organisms in continually processing and transforming organic matter from the time it enters the stream (fig. 1).

Figure l.
Schematic diagram depicting the processing of dead organic matter in
headwater streams (redrawn from data in Cummins and Spengler 1978).

Coarse particulate organic matter (CPOM: > 1 mm. diameter), such as leaves, starts leaching DOM once it enters the water. Up to 30% of dry weight may be leached in the first day; deciduous leaves leach faster than coniferous needles (Cummins 1974; Hynes etal . 1974). Fungi and bacteria rapidly colonize the leaves undergoing leaching. Although most of these microbes can metabolize cellulose, only some can use lignin (Cummins and Spengler 1978). Certain aquatic insects such as some stonefly nymphs, midge larvae, cranefly larvae, and caddisfly larvae shred or break down leaves (CPOM) during feeding and are called "shredders" (Cummins 1973). The microorganisms that colonize the leaves are an important source of shredder nutrition.

Shredder and microorganism feeding eventually breaks down CPOM into fine particulate organic matter (FPOM: < 1 mm. diameter). However this process is only one source of FPOM. FPOM may result from: 1) shredder and microorganism feeding on CPOM; 2) physical abrasion of CPOM by stream turbulence; 3) fine particles eroded from streambed algae; 4) fine material washed or blown in from the surrounding watershed; and 5) conversion from DOM by chemical and microbial activity (Cummins 1974). Dissolved organic matter leached from CPOM, plus DOM entering from the watershed, aquatic plants, and microbial excretions, can be partially converted into FPOM. This conversion is accomplished by physical flocculation and microbial assimilation, processes dependent on water turbulence, temperature, pH, and various ionic concentrations (Lush and Hynes 1973).

FPOM is the food for aquatic organisms known as "collectors". These animals obtain FPOM either by gathering it from stream substrate deposits or by filtering it from the flowing water. Deposit feeders include certain midge larvae and mayfly nymphs. Filter feeders have diverse ways of capturing FPOM from the passing water (Wallace and Merritt 1980). Blackfly larvae possess fan-shaped structures on their heads for filtering FPOM and transferring it to their mouths. Some caddisfly larvae construct detailed silk nets capable of sieving out FPOM. The net is often held between small twigs or stones exposed to the current, and the larva hides in a tube just behind. The collected FPOM contains bacteria on its surfaces, which increases the quality of the food for the collector. Particle size is very important to collectors since their mouthparts and sieving devices have specific shapes and openings for obtaining and handling FPOM.

A thin film of algae covers most stream substrates and contributes to instream primary production, especially when light intensity and nutrient concentrations are high. Microscopic diatoms are often the most abundant algal group, but larger filamentous green and blue-green algae are also common. Aquatic organisms known as "scrapers" have well-adapted mouthparts for scraping up and consuming this algal film, which also includes some FPOM and microscopic animals. Scrapers in Sierra streams include many mayfly nymphs, water penny beetles, riffle beetles, and some midge larvae.

Some aquatic invertebrates and vertebrates prey on shredders, collectors, scrapers, and each other; they are known as "predators". Predators in Sierra streams include many stonefly nymphs, dragonfly nymphs, some midge larvae, alderfly larvae, and dobsonfly larvae. Most aquatic insects in streams, even those that are predatory, are potential prey for trout and many nongame fish species.

The amount, kind, and timing of riparian vegetation additions to the stream and the shading provided by streamside plants will determine which feeding groups (shredders, collectors, scrapers, predators) prosper at any site. Thus, the population abundance of stream animals and community composition of the stream ecosystem are dependent on riparian vegetation.