Algae are the most important primary producers in running-water ecosystems and because of their sessile nature and short life cycles, their assemblages are used to evaluate stream ecosystem health. Algae are thalloid organisms, bearing chlorophyll a and lacking multicellu-lar gametangia. Algal evolution radiated from a common ancestry to several diverse kingdoms. For example, blue-green algae are classified as bacteria, and dinoflagellate algae as protozoans. Algal taxonomy is based on pigmentation, the chemistry and structure of internal storage products and cell walls, and number and type of flagellae. Five major divisions of algae are common in streams, including the Bacillariophyta (diatoms), Chlorophyta (green algae), Cyanobacteria (blue-green algae), Chrysophyta (yellow-green algae), and Rhodophyta (red algae). Of these, the diatoms, green algae, and cyanobacteria are most prevalent. Assemblages of algae attached to the substrate are referred to as periphyton or aufwuchs. Periphyton attached to submerged substrates is a complex assemblage of algae, bacteria, fungi, and meiofauna bound together with a polysaccharide matrix referred to as biofilm. Algae of the water column are phytoplankton, occurring chiefly in slowly moving lowland rivers as sloughed benthic cells or exports from connected standing waters within the watershed.

Diatoms are extremely abundant in freshwater as well as in saltwater, and typically comprise of majority of species within the periphyton. Generally microscopic, diatoms are brownish-colored single-celled algae constructed of two overlapping siliceous cell walls, or valves, fit together like the halves of a petri dish. Valves are connected to each other by one or more 'girdle' bands. The two valves form the frustule, which is uniquely decorated with pores (punctae), lines (striae), or ribs

(costae). The symmetry of these decorations defines two groups: radially symmetrical centric diatoms and bilaterally symmetrical pennate diatoms. Diatoms may occur individually, in chains, or in colonies, and those with a divided cell wall (raphe) are able to move. In temperate streams, diatoms exhibit two growth blooms: in spring prior to shading by deciduous canopies as water temperatures rise and nutrients are plentiful; and in fall following leaf abscission, when nutrients released from decaying green algae and deciduous litter are available. Diatoms constitute a high-quality, rapid-turnover food resource for macroinvertebrate scrapers and collectors. Representative diatoms common in stream periphyton are shown in Figure 8.

Green algae occur in a variety of habitats, and are distinguished by the number and arrangement of flagella, their method of cell division, and their habitat. In streams, distinctions are made between micro- and macroforms. Macroalgae occurs as a thallus or as filaments. Filamentous forms may be branched or unbranched. Green algae provide attachment sites for diatoms, and are a source of FPOM and photosynthetic oxygen, but are fed upon by few invertebrates.

Blue-green algae, or cyanobacteria, are prokaryotic organisms of ancient lineage which contain the photosyn-thetic pigment phycocyanin, used to capture light for photosynthesis. They occur in a variety of habitats and are one of very few groups of organisms that can convert inert atmospheric nitrogen into an organic form. Bluegreen algae may be filamentous or nonfilamentous, and only filamentous forms with heterocysts are capable of nitrogen fixation in aerobic settings. Several of the heterocyst-containing filamentous taxa, (e.g., Anabaena, Aphanizomenon, and Microcystis) can form dense blooms and produce toxins in warm, nutrient-rich waters. Nitrogen-fixing Nostoc, common in small streams, forms a unique commensal association with the chironomid midge Cricotopus.

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