Summary

Primary producers acquire their energy from sunlight and their materials from nonliving sources. The major autotrophs of running waters are the benthic algae and macrophytes, and phy-toplankton also can be important in larger rivers. Benthic algae occur in intimate association with heterotrophic microbes within an extracellular matrix, referred to as periphyton, biofilm, and sometimes as Aufwuchs. Benthic algae are important in fluvial food webs, especially in headwater and midsized streams, and also influence the benthic habitat and nutrient cycling. Proximate factors that may limit benthic algal communities include light and nutrients which, along with temperature, influence biomass accrual; and disturbance and grazing, which are the factors that lead to algal dislodgement and biomass loss. The importance of any one factor to algal growth depends upon whether some other factor is in even shorter supply, and these environmental conditions vary with location and by season. Light often is the limiting factor in streams shaded by forest cover, as evidenced by lower benthic algal abundance in shaded versus unshaded streams, and by seasonal peaks before leaf-out and after leaves are shed. Phosphorus and N have each been shown to limit algal growth, sometimes in combination, but nutrient limitation can be overridden by low light or intense herbivory. Stream current has opposing effects on benthic algae accrual depending on growth form and architecture and is best described as a "subsidy-stress" response. The flow of water brings continual renewal of gasses and nutrients, and so current benefits algal growth by enhancing nutrient uptake. However, current also exerts a shear stress on benthic algae, which can cause cell sloughing, and high flows disturb and scour the substrate. Different temperatures apparently favor particular algal taxa, with filamentous green algae and cyanobac-teria more abundant in warmer rivers; and growth rate of course increases with increasing temperatures. Substrate is a factor through its interaction with current, in that different areas of a stone may be more or less exposed to flow and smaller particles are more likely to be dislodged. Grazers can significantly limit benthic algae and influence community composition by selectively eliminating certain species and growth forms. The composition and biomass of the benthic algae varies temporally and spatially due to the interaction of these multiple environmental factors.

Macrophytes include flowering plants, bryo-phytes (mosses and liverworts), and some filamentous benthic algae. Macrophytes can be placed in four categories according to their growth form. These are emergent, submersed, and floating-leaved taxa, which are all rooted, and free-floating plants, which usually are not attached to the substrate and often form large mats. Angiosperms 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. Macrophytes are important to fish and invertebrates as habitat and as refuge from predators, and they can increase habitat heterogeneity for aquatic organisms by modifying water velocity and trapping sediments and organic matter. Herbivory on freshwater macro-phytes usually is minor, and most biomass enters the detritus pool where its breakdown and utilization can be rapid.

The phytoplankton consists of cells and colonies of algae and cyanobacteria suspended in the water column and transported by currents. Light, nutrients, flow, and temperature all influence seasonal variation in phytoplankton abundance. Unable to maintain populations in fast-flowing streams, phytoplankton can become abundant in slowly moving rivers and backwaters where their doubling rates exceed downstream losses due to current. In large rivers, light penetration may be < 1 m and so vertical mixing within a deep and turbid water column further limits the opportunity for photosynthesis. When compared with lakes, river phytoplankton populations are much less abundant for a particular nutrient concentration. Although invasive mol-lusks have been observed to dramatically reduce phytoplankton populations in some large rivers, grazing on phytoplankton typically is minor, as zooplankton doubling times are too slow to counter downstream losses. Thus, it appears that downstream export rather than in situ energy processing is the dominant fate of large river phytoplankton production.

Chapter seven

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