Stream ecology : structure and function of running waters

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 conditions are 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 including herbivores, predators and parasites. Resources can limit the abundance of consumers, known as bottom-up control, and consumers can be responsible for top-down controls over the abundance of lower trophic levels. The interactions of grazers with algae, predators with their animal prey, and among competing species constitute the primary linkages that collectively bind species together into food webs. 10 Lotic Communities Community structure reflects the forces that determine which and how many species occur together, which species are common and which are rare, and the interactions amongst them. The idea that communities exhibit structure requires that assemblages be more than haphazard collections of those species able to disperse to and survive in an area. It leads us to expect that the same species, in roughly the same abundances, will be found in the same locale as long as environmental conditions do not change greatly, and that similar communities should occur wherever environmental circumstances are comparable. 11 Nutrient Dynamics Nutrient cycling involves the transformation of inorganic compounds into organic forms due to biological uptake, and then back to an inorganic state. In rivers this transformation is affected by the transport of water resulting in the longitudinal displacement of the nutrient cycle, which is explained by the nutrient spiraling concept. Nitrogen and phosphorus spiraling studies in aquatic ecosystem provide information about nutrient limitation and retention by the stream ecosystem. Models of nutrient export help us understand nutrient sources and sinks as well as the influence of land use and human activities on nutrient dynamics. 12 Stream Metabolism Stream metabolism refers to the balance between the organic matter that is produced and the organic matter that is consumed within the ecosystem. Inputs are from primary production and detritus, generated within the stream and imported from upstream and beyond the banks. Carbon inputs are respired or exported downstream, and the relative magnitude of these two processes is a measure of ecosystem efficiency. Information about inputs, storage, and outputs are used to construct mass balances that are used to compare organic matter dynamics among rivers. 13 Models and Concepts in Stream Ecology In recent years several models and concepts have been formulated to explain the functioning of aquatic ecosystems. The River Continuum concept provides a framework to explain energy inputs and consumption from headwaters to river mouth. Nutrient and carbon spiraling models substitute distance for time as a useful measure of process rates, allowing comparisons across stream sizes and types. The dynamics of large rivers was neglected until the development of the flood pulse concept to explain the ecological functioning of river-floodplain interactions. The Riverine Productivity model proposes that autochthonous primary production fuels animal secondary production in large rivers despite the apparent dominance of ecosystem respiration by allochthonous inputs. Conceptualization of river systems within landscapes that influence river processes through a hierarchy of geophysical controls provides an improved understanding of river processes and human impacts. Collectively these models organize and synthesize much of stream ecology, and link back to important themes laid out in "roadmap" chapter 1. 14 River Health in the 21st Century Rivers are threatened by habitat degradation, pollution, invasive species, over-harvest and climate change. Such threats are inevitable because fresh water is a non-substitutable resource and humans now appropriate over half of the available supply. Fortunately we now have a more sophisticated understanding of the status of rivers and better tools for their management. The applied sciences of environmental flows, river restoration and ecosystem-based catchment management provide hope that rivers can be improved through well-focused human actions.