Primary producers

In this and the following chapter we examine the sources of energy to lotic food webs. Autotrophs or primary producers are organisms that acquire their energy from sunlight and materials from nonliving sources. Algae, higher plants, and some bacteria and protists are important autotrophs in running waters. Heterotrophs obtain energy and materials by consuming living or dead organic matter. All animals of course are heterotrophic, but so also are fungi and many protists and bacteria that gain nourishment through the processing of dead organic matter and often make that organic matter more nutrient rich and more accessible to other consumers. Together, these autotrophs and microbial heterotrophs constitute the basal energy resources that support higher trophic levels in lotic food webs. The major autotrophs of running waters include large plants, referred to as macrophytes, and various small producers including individual cells, colonies, and filamentous growth forms. Algae suspended in the water column are referred to as phytoplankton; those attached to substrates are referred to as benthic algae or periphyton.

Macrophytes include aquatic angiosperms, bryophytes (mosses and liverworts), and some members of the benthic algae when they become large (filaments of the green alga Cladophora have been estimated to reach 6 m). Angiosperms generally require moderate depths and slow currents, and so are most common in springs, rivers of intermediate size, and along the margins and in backwaters of larger rivers. Bryophytes are restricted in distribution, but can be abundant in cool climates and in shaded headwater streams. Benthic algae occur on virtually all surfaces within rivers, typically in intimate association with heterotrophic microbes and an extracellular matrix, to which the all-inclusive terms periphyton, biofilm, or Aufwuchs apply. Phytoplankton are unable to maintain populations in fast-flowing streams, but can become abundant in slowly moving rivers and backwaters where their doubling rates exceed downstream losses due to current. Thus, according to a somewhat idealized view of the longitudinal profile of a river system, benthic algae and occasional bryophytes predominate in headwater and upper stream sections, and benthic algae become more abundant farther downstream where the river widens and hence is less shaded by streamside vegetation. Macrophytes occur mainly in midsized rivers and along the margins of larger rivers, and substantial phytoplankton populations develop only in large, lowland rivers (Figure 1.7). Because benthic algae are found in nearly all running waters and often are important in fluvial food webs, they have been the main focus of study

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