Virtually all substrates that receive light, whether in small streams or large rivers, sustain a benthic algal community. Benthic algae support fluvial food webs, remove nutrients from the water column, and can attenuate the current and stabilize sediments, thereby modifying the aquatic habitat (Stevenson 1996, Dodds and Biggs 2002).
Benthic algae can be further categorized by their size. Macroalgae are benthic forms having a mature thallus visible to the naked eye, in comparison with smaller microalgae that cannot be distinguished without a microscope. Algae also can be categorized by whether they grow on stones (epilithon), sediments (epipelon), sand (epipsammon), wood (epixylon), or other plants (epiphyton). Epipelic and epipsammic taxa form films or mats on silt and mud bottoms, and typically are motile and easily swept away by an increase in current. Because of their motility, filter paper and glass slides placed on the sediment surface are readily colonized, thus serving as one sampling technique. Epiphytic taxa occur on macrophytes, particularly angiosperms, where epiphytic coating can be detrimental to the host plant. Unlike epipelic species, epiphytic and epi-lithic taxa usually are firmly attached by mucilaginous secretions or via a basal cell and stalk, and thus are less likely to be dislodged by the current.
Some algal species are in contact with the substrate along the entire cell wall, colony, or filamentous system. This growth form is termed adpressed, and contrasts with erect forms in which only a basal cell or basal mucilage contacts the substrate. As a consequence of this variety in growth form and lifestyle, a close look at a benthic algal community reveals much structural diversity (Figure 6.1).
Diatoms, green algae, and cyanobacteria typically contribute the majority of species found within the periphyton, although red algae, chry-sophyceans, and tribophyceans may also occur
(Graham and Wilcox 2000). The prevalence of diatoms is apparent from Patrick's (1961) cell counts from glass slides placed in three US rivers, Moore's (1972) study of the epipelon of a southern Ontario stream, and Chudyba's (1965) study of epiphytes of Cladophora glomerata in the Skawa River, Poland (Table 6.1). In an extensive survey of the macroalgae of 1,000 stream reaches in North America, Sheath and Cole (1992) recorded 259 taxa of macroalgae, of which 35% were green algae, 24% were cyano-bacteria, 21% were diatoms and other chryso-phytes, and 20% were red algae. Many diatoms do not form mats, gelatinous colonies, or filaments, and so would be underrepresented in a survey of visible macroalgae.
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