Threats to freshwater systems arise from a myriad of human activities, including channelization, groundwater pumping, diversion, dam building, pollution, human-induced climate change, and overexploitation of natural resources (e.g., Postel & Carpenter 1997; Malmqvist & Rundle 2002). Nearly all major rivers and lakes worldwide have large human population densities associated with them or within their drainage basins, usually sited there with relatively little thought to the availability of potable water. The growth of the human population and the mismatch between population growth and provision of, and accessibility to, water resources is an imminent concern (Cohen 1995). An estimated 1.8 billion people now live under a high degree of water stress in areas with limited supplies of potable water (Vorosmarty et al. 2000). This stress may continue to rise, with a projected population living in these areas estimated to be between 2.8 billion and 3.3 billion by 2025 (Engelman & LeRoy 1993, 1995; Cohen 1995).
Stressors and impacts that force changes in freshwater ecosystems can be classified into four major types of threat (Malmqvist & Rundle 2002): (1) complete ecosystem loss or destruction, (2) physical habitat alteration, (3) water chemistry alterations, and (4) modifications of species composition. Ecosystem loss or destruction is often associated with water withdrawal from the system (e.g., in the Alps, Ward et al. 1999) resulting from rapid urbanization and/or intensification of agriculture, and the associated water demand and lowering of water tables by extraction elsewhere. There is a strong correlation between population size and water withdrawal (Gleick 2001), and irrigation dominates water demand at the global level. Habitat alteration of the freshwater system can occur from both instream activities (including channelization, damming, and draining of wetlands) and catchment-related activities (such as deforestation, poor land use, and alteration of the riparian corridor). Changes in water chemistry result from pollution due to wastewater discharge, diffuse nutrient loading from agriculture runoff, acidification from atmospheric inputs, and the introduction of endocrine disruptors (Malmqvist & Rundle 2002). Introductions of exotic species may be direct or indirect (as discussed below). Extinctions are common, often due to overexploitation of the organisms themselves, habitat destruction (or loss of habitat to invasive species replacement), the loss of functions necessary for some life stage of a particular species, or the loss of a symbiont.
We have identified 14 major threats to the six major services provided by freshwater benthic systems (Figure 6.1). Each threat can impact more than one of the services, and many of these impacts are mediated through the benthos. In reality, each threat can be subdivided into a finer series of threats. For example, hydrologic modification can have effects through a decrease in peak flow, increase in low flows, change in timing of peak flows, changes in the rate of drawdown, and/or a decrease in flow variability, and so on. Each ecosystem service can be affected by several different threats, and different stressors may act synergistically. Eutrophication can increase biotic activity and thereby enhance the effect of metal contamination (for example, the mobility of mercury). Likewise, changes in water chemistry, mechanical disturbances to a system, or changes to the characteristics of the habitat can enhance the probability of successful species invasion (Jenkins & Pimm 2003), which in turn may decrease economic success based on a highly profitable food source for humans. Changes in the competitive balance between species can also ensue. One example of this phenomenon is the replacement of the saw-grass (Cladium jamaicense) communities in the wetlands of the Everglades in Florida, United States, by cattail species (Typha latifolia and T. domingensis) as a result of phosphorous and nitrogen loading from agricultural runoff (Newman et al. 1998). In areas of the 600,000—ha Everglades that have the highest phosphorous enrichment, cattails dominate, but in portions of the Everglades where phosphorous remains low, sawgrass still dominates. This shift in community structure directly resulting from human-caused changes in water chemistry is due to the fact that cattails are better able to assimilate nitrogen and phosphorous and to produce biomass.
Figure 6.1. The interaction between six major ecosystem services, provided by freshwater systems, and fourteen potential threats in the freshwater domain. An explanation of the nature of the services is given in Chapter 3, Tables 3.1a—3.1e. Solid lines indicate the direct links between the major services and the various threats, and the dotted lines indicate links that may be mediated through the benthos.
The stressors described above in Figure 6.1 occur in all types of freshwater ecosystems; however, the magnitude and direction of their effects vary across ecosystems. Lakes and wetlands are susceptible to various stressors due to their slow turnover of water, their potential for accumulation of toxins and metals in their sediments, and their dependence on the quality and quantity of water inputs from inflow streams. The susceptibility of rivers and associated wetlands, on the other hand, is exacerbated by the downstream flow of water (and hence pollutants and sediments) and their longitudinal connectivity (upstream and downstream dispersal migration of many species). Almost any significant activity within a river catchment and throughout its drainage network may have the potential to exert effects for large distances upstream and downstream.
Freshwater ecosystems face different threats in different regions, depending largely on the economic activity and state of development. Water is abundant at high latitudes and in the wet tropics; however, in much of North and East Africa, Australia, and parts of North America, the availability of potable water is relatively scarce. Even in the more temperate countries with relatively high overall annual precipitation, major concentrations of population are often located in areas of lowest rainfall (such as Dublin and London), creating local water deficits that require large-scale engineering projects for water storage and/or transfer, as well as water regulation activities to overcome. Roughly 40 percent of the world's population that live in 80 dry, or partially dry, countries face serious periodic droughts (Cohen 1995); these pressures on water resources will be more pronounced in Africa and South America by 2025 (Vorosmarty et al. 2000). Plans to redirect water from uninhabited areas to population centers will create additional problems. Lakes in the developed world are threatened by eutrophication and lowered water tables due to groundwater abstraction, while in the undeveloped world, overexploitation of fish and invasion from exotic plants (e.g., the water hyacinth Eichhornia crassipes) are more problematic. Destruction of running water habitats is extensive in much of the developed world (because of flood control, drainage, clearing channels for transportation and transport of timber, and dredging), as well as in the developing world (largely due to dam construction and mining; see Covich et al., Chapter 3).
Waste disposal poses significant threats to many systems, as treated and untreated domestic and industrial waste leads to significant levels of eutrophication and to metal and other chemical contamination. Sedimentation and nonpoint source pollution result from changing land use such as deforestation, overgrazing, and intensification of agriculture. The degradation of riparian zones that often accompanies such intensification (as in the Netherlands, for example) also changes benthic ecosystem functions dramatically (Gregory et al. 1991). Even atmospheric pollution impacts aquatic ecosystems, as evidenced by acidification of freshwater systems throughout northern Europe, the northeastern United States, and Canada (Stoddard et al. 1999).
Anthropogenic threats and influences alter the balance of natural regulatory factors in freshwater systems such as energy supply and flow, organic and inorganic matter transport, hydrologic regimes, hydrologic and biogeochemical cycles, and water chemistry (Malmqvist & Rundle 2002). These anthropogenic factors change the structure of freshwater sediment, alter temperature regimes, and cause other environmental conditions to change beyond the normal levels of variation and extremes. Such changes will clearly impact species unless they possess certain traits that confer resistance or resilience to the environmental change.
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