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1 An Introduction to Fluvial Ecosystems An overview of the diversity of rivers and streams, including some of the causes of this diversity, and some of the consequences. The intent is to provide a roadmap for the individual chapters that follow, rather than define terms and explain principles in any detail. 2 Streamflow Fluvial ecosystems exhibit tremendous variability in the quantity, timing and temporal patterns of river flow, and this profoundly influences their physical, chemical and biological condition. This chapter covers the essentials of hydrology, from the global water cycle to the myriad ways that humans alter water flowpaths and river flow. 3 Fluvial Geomorphology Fluvial geomorphology emphasizes the dynamic interplay between rivers and landscapes in the shaping of river channels and drainage networks. It includes study of the linkages among channel, floodplain, network and catchment and helps make sense of the enormous variety exhibited among fluvial systems, and thus the habitat and environmental conditions experienced by the biota. 4 Streamwater Chemistry The constituents of river water include suspended inorganic matter, dissolved major ions, dissolved nutrients, suspended and dissolved organic matter, gases, and trace metals. River chemistry changes temporally under the multiple influences of seasonal changes in discharge regime, precipitation inputs, and biological activity and usually is greatly altered owing to direct and indirect human influences. 5 The Abiotic Environment The abiotic environment includes all physical and chemical variables that influence the distribution and abundance of organisms. Current, substrate and temperature often are the most important variables in fluvial environments, and all organisms show adaptations that limit them to a subset of conditions. Species differ in the specific conditions under which they thrive, and whether those conditionsare narrow or comparatively broad. 6 Primary Producers Primary producers acquire their energy from sunlight and their materials from nonliving sources. The major autotrophs of running waters include the benthic algae and macrophytes in larger rivers, phytoplankton also can be important. Benthic algae occur in intimate association with heterotrophic microbes within an extracellular matrix, referred to as biofilm. Benthic algae are important in fluvial food webs, especially in headwater and midsized streams, and also influence the benthic habitat and nutrient cycling. 7 Detrital Energy Sources Particulate and dissolved organic matter originating both within the stream and in the surrounding landscape is an important basal resource to fluvial food webs. Detritus-based energy pathways can be particularly important, relative to pathways originating from living primary producers, in small streams shaded by a terrestrial canopy and in large, turbid rivers with extensive floodplains. Recent advances in microbial ecology have greatly expanded our understanding of the synergies between autotrophs and heterotrophs. 8 Trophic Relationships The network of consumers and resources that constitute fluvial food webs is supported by a diverse mix of energy supplies that originate within the stream and beyond its banks. These include the living resources of algae and macrophytes, and the non-living resources of particulate and dissolved organic matter. Microorganisms are important mediators of organic matter availability and there is increasing evidence of their importance as a resource to both small and large consumers. Additionally, energy subsidies in the form of falling terrestrial arthropods and the eggs and carcasses of migrating fish contribute to the support of many stream-dwellers. 9 Species interactions The basal resources of algae and detritus and associated microorganisms sustain higher consumers includin