Issues in Ecology Number 10 Winter 2003 vascular plants.and bacteria that populate the bottoms of seas freshwater systems are highly adapted to the specific sediment and organic matter conditions of their environment as are many fish species.and do not persist if changes in the type uator an size.or frequency of sediment inputs occur.The fate of these gradients in turn influence organisms is critical to sustaining freshwater ecosystems since dis and both the distributi they are responsible for much of the work of water purification,decomposition,and nutrient cycling. can chano dramatically dow emof dams (BOX 2). Humans have severely altered the natural rates of Utah's Green River.mean mo thly water temper sediment and organic matter supply to aquatic systems, het veen 2 degrees Celsius(C)in winter and 18 dearee C in increasing some inputs while decreasing others (Figure 3) summer before completion of the Flaming Gorge Dam in 1962 Poor agricultural,logging,or construction practices,for After dam closure the annual range of mean monthly wate example,promote high rates of soil erosion.In many areas temperatures below the dam was greatly narrowed,to betweer small streams or wetlands have even been completely 4 C and 9 C.As a result.species richness declined and 18 eliminated through filling.paving.or re-ouingnto artificia genera (that is arouns of related species)of insects were lost channels The U.S.Environmental Protection Agency (EPA other species,notably freshwater shrimp.came to dominate the orts that in one rter of all lakes with sub-standard water ranks of invertebrate animals.Aquatic insects have not recovere ause of impairm despite 20 years of partia tering fror temperature restoration achieve by releasing water from warme reservoir water layers. Wate temperature also dropped in th Colorado River after closure of the Glen Canyon Dam in 1963.and the fo B ty nov 1 cubic meters of sediment build up each year in U.S. (Table 1) in tum cuts off normal sand.silt. d e and gravel supplies to downstream reaches causing streambed erosion Figure 4-Eutrophication from irrigation return flows. ation to月 that both degrades in-channel Photo courtesy the U.S.Geological Survey,South Platte National Water Quality Assessment Program(NAWOA). t the to of an habitat and isolates floodplain and ommonly found much riparian wetlands from the channel further north. during rejuvenating high flows.Channel straightening overgrazing of river and stream banks,and clearing of streamside Nutrient and Chemical Conditions vegetation reduce organic matter inputs and often increase erosion. Natural nutrient and chemical conditions are those that reflect local climate.bedrock,soil,vegetation type.and Temperature and Light topography.Natural water conditions can range from clear nutrient-poor rivers and lakes on crystalline bedrock to much The light and heat properties of a body of water are more chemically enriched and algae-producing freshwaters nflue as by in catchments with organic matter-rich soils or limestone charac r th vity.Water bedrock.This natural regional diversity in watershed characteristics,in turn,sustains high biodiversity. ons,the mela at A condition known as cultural eutrophication occurs ite proces when additional nutrients,chiefly nitrogen and phosphorus.from d human activities enter freshwater ecosystems (Figure 4). fit nd h the living community in a body of water varies from season al species can increase well beyond astern lakes such as Lakes6 Issues in Ecology Number 10 Winter 2003 vascular plants, and bacteria that populate the bottoms of freshwater systems are highly adapted to the specific sediment and organic matter conditions of their environment, as are many fish species, and do not persist if changes in the type, size, or frequency of sediment inputs occur. The fate of these organisms is critical to sustaining freshwater ecosystems since they are responsible for much of the work of water purification, decomposition, and nutrient cycling. Humans have severely altered the natural rates of sediment and organic matter supply to aquatic systems, increasing some inputs while decreasing others (Figure 3). Poor agricultural, logging, or construction practices, for example, promote high rates of soil erosion. In many areas small streams or wetlands have even been completely eliminated through filling, paving, or re-routing into artificial channels. The U.S. Environmental Protection Agency (EPA) reports that in one quarter of all lakes with sub-standard water quality, the cause of impairment is silt entering from agricultural, urban, construction, and other non-point (widely dispersed) sources. Dams alter sediment flows both for the reservoirs behind them and the streams below, silting up the former while starving the latter. By one estimate, another 1.2 billion cubic meters of sediment builds up each year in U. S. reservoirs (Table 1). This sediment capture in turn cuts off normal sand, silt, and gravel supplies to downstream reaches, causing streambed erosion that both degrades in-channel habitat and isolates floodplain and riparian wetlands from the channel during rejuvenating high flows. Channel straightening, overgrazing of river and stream banks, and clearing of streamside vegetation reduce organic matter inputs and often increase erosion. Temperature and Light The light and heat properties of a body of water are influenced by climate and topography as well as by the characteristics of the water body itself: its chemical composition, suspended sediments, and algal productivity. Water temperature directly regulates oxygen concentrations, the metabolic rate of aquatic organisms, and associated life processes such as growth, maturation, and reproduction. The temperature cycle greatly influences the fitness of aquatic plants and animals and, by extension, where species are distributed in the system and how the living community in a body of water varies from season to season. In lakes particularly, the absorption of solar energy and its dissipation as heat are critical to development of temperature gradients between the surface and deeper water layers and also to water circulation patterns. Circulation patterns and temperature gradients in turn influence nutrient cycling, distribution of dissolved oxygen, and both the distribution and behavior of organisms, including game fishes. Water temperature can change dramatically downstream of dams (BOX 2). In Utah’s Green River, mean monthly water temperatures ranged between 2 degrees Celsius (C) in winter and 18 degrees C in summer before completion of the Flaming Gorge Dam in 1962. After dam closure, the annual range of mean monthly water temperatures below the dam was greatly narrowed, to between 4 C and 9 C. As a result, species richness declined and 18 genera (that is, groups of related species) of insects were lost; other species, notably freshwater shrimp, came to dominate the ranks of invertebrate animals. Aquatic insects have not recovered despite 20 years of partial temperature restoration achieved by releasing water from warmer reservoir water layers. Water temperature also dropped in the Colorado River after closure of the Glen Canyon Dam in 1963, and there was a dramatic increase in water clarity. Water clarity now routinely allows visibility to greater than 7 meters, whereas prior to dam closure, the water column was opaque with suspended sediments. The colder, clearer waters have allowed a non￾native trout population to flourish, at the top of an unusual food web more commonly found much further north. Nutrient and Chemical Conditions Natural nutrient and chemical conditions are those that reflect local climate, bedrock, soil, vegetation type, and topography. Natural water conditions can range from clear, nutrient-poor rivers and lakes on crystalline bedrock to much more chemically enriched and algae-producing freshwaters in catchments with organic matter-rich soils or limestone bedrock. This natural regional diversity in watershed characteristics, in turn, sustains high biodiversity. A condition known as cultural eutrophication occurs when additional nutrients, chiefly nitrogen and phosphorus, from human activities enter freshwater ecosystems (Figure 4). The result is a decrease in biodiversity, although productivity of certain algal species can increase well beyond original levels. Midwestern and Eastern lakes such as Lakes Michigan, Huron, Erie, and Figure 4—Eutrophication from irrigation return flows. Photo courtesy the U.S. Geological Survey, South Platte National Water Quality Assessment Program (NAWQA)