freshwater habitats is found in many different forms, but all of them can be classified within one of two basic categories: Spatial Complexity deep or shallow, fast or slow current, sunny or shaded, flat mud/sand or stony bottom, small gravel or large rocks, with or without aquatic vegetation, clear or sediment-rich water, small streams or large rivers, etc. Temporal Complexity - alternation of low flow and high water conditions, seasonal changes between warm and cold water, variation over time in the amount of suspended sediments and available nutrients, channel-constrained or expanded into the floodplain, migration of alluvial channels, etc. In general, the higher the habitat complexity, the greater the biodiversity that can be sustained in the habitat. Because all species have specific habitat requirements, changes in biodiversity can usually be predicted from changes in the existing natural habitat. Documentation of the distribution of species and their association with specific habitat features must be an adjunct to biodiversity assessment.
Biodiversity studies that focus upon just naming and counting the species present rarely generate much public support, but are an important first step in understanding the functioning of a given ecosystem. On the other hand, biodiversity studies which also focus upon linkages among habitatspecies-ecosystem services and people will usually command attention.
Main Characteristics of Natural Freshwater Ecosystems Natural freshwater ecosystems, as all other ecosystems, exhibit a set of physical and chemical conditions, as well as spatial and temporal variability, that together give the ecosystem its character and support the communities and services within. The best bet for conserving freshwater ecosystem processes and biodiversity is to understand and preserve as much as possible the fundamental set of characteristics (physical, chemical, biological, and spatial and temporal variation) that define a particular freshwater ecosystem. Running Waters (Rivers and Streams) Running waters show all possible combinations of flow regimes and structural complexity, including substrate heterogeneity (boulders, cobble, gravel or sand), debris dams, rooted vegetation, and confined or braided channels. This variability results in a large number of different freshwater habitats within rivers and streams, and provide organisms with a range of water velocities to choose from - pools, riffles, rapids, etc. Inputs from terrestrial ecosystems provide food (leaves, fruits) and habitat (sediments, twigs and logs) for aquatic organisms. Five critical components of the flow regime regulate ecological processes in running water ecosystems: the magnitude, frequency, duration, timing, and rate of change of hydrologic conditions. The natural flow of a river varies on time scales of hours, days, seasons, years, and even longer (Poff et al. 1997). 4