Riverine food webs are often dominated by allochthonous inputs from terrestrial sources. Coarse particulate organic matter (CPOM) from fallen leaves, dry grasses, and so on, are low in nitrogen. They require nutrients from the stream for their initial degradation. Otherwise, they are not readily available to the food chain. Nutrient availability, and therefore productivity, tends to be higher in hard-water areas (with high calcium and magnesium) than in soft-water regions.
Degradation of the CPOM is accomplished by a coating of fungi and bacteria that adheres to the surface. They break down the cellulose and lignin which cannot be digested by animals. Lignin is degraded mostly by fungi, including species adapted to function in the winter cold, to take advantage of the annual leaf fall. Invertebrates ingest the CPOM, but digest only the attached fungi and bacteria. The CPOM is excreted, and the coating regrows. In repeated cycles of degradation, ingestion, and excretion, the CPOM is gradually broken down into fine particulate organic matter (FPOM) and then to dissolved organic matter (DOM) until it has entered the food chain completely.
The invertebrates that feed on the allochthonous input fall into four groups. Grazers scrape bacteria, fungi, rotifers, and periphyton from surfaces. Examples of grazers are snails and caddis flies. Shredders cut and ingest pieces of leaves and stems which form the CPOM, reducing them in size. They include the stoneflies, amphipods, and crayfish. Collectors and filterers take FPOM from the water. Clams pump water through filtration organs, expending considerable energy. Midge larvae use fanlike collecting structures to ensnare particles. Caddisfly larvae spin long nets to capture FPOM from the current. Most benthic invertebrates simply use bristles on their appendages or mouth-parts. Worms feed on FPOM that has settled to the bottom in more quiescent areas. Predators range from stonefly larvae and dragonfly nymphs to fish and birds of prey. Figure 15.17 shows a schematic food web for this ecosystem. This figure shows three kinds of inputs. Direct light energy fuels the autochthonous primary productivity. Dissolved organics may originate in decay products that run off from the land and provide an allochthonous input for microorganisms. Large organic matter feeds another allochtho-nous input to the cycle, involving the shredders that break the CPOM down to FPOM, ultimately providing food for the scrapers and the filter feeders. Finally, of course, each group of invertebrates is subject to predation by carnivores.
During the day, most invertebrates hide under or between rocks to avoid predation. However, at night many of these will join the current to migrate downstream. Together with FPOM they form drift, which is an important food source for larger collectors and fish. After a lifetime of drifting downstream, many insect larvae may emerge into the air, then fly upstream to lay their eggs, repeating the cycle.
According to the river continuum concept, the balance between allochthanous and autochthanous input changes gradually from small headwaters to large downriver condi-
Light energy and nutrient input
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