Ecosystem services in fresh waters depend on a range of different benthic species (Tables 3.1a-3.1e). For example, fish and shellfish yields depend heavily on sustained production of diverse benthic prey species (Huner 1995; New & Valenti 2000). Although only a few of the 390 species of crayfish native to North America are harvested for food, other crayfish species play major roles in ecosystem dynamics by linking sedimentary habitats with overlying waters through burrowing and mixing of sediments, nutrient cycling, breaking down dead organic matter, and grazing on submerged macrophytes (Covich et al. 1999; Hobbs 2001). Benthic invertebrates are essential prey for bottom-feeding fishes and aquatic mammals such as river otters and raccoons. Other freshwater ecosystem services include the breakdown of industrial and residential wastes by microbes and invertebrates (Geber & Bjorklund 2001; DeBruyn & Rasmussen 2002). Fresh waters dilute wastes, provide cooling waters for power generation and other industrial processes, as well as serve demands for recreational swimming, fishing, and boating (Pos-tel & Carpenter 1997). The economic values of fisheries (New & Valenti 2000; Wel-comme 2001) and recreational activities in fresh waters are well documented (Loomis 2000). Moreover, managers rely on monitoring services provided by benthic invertebrates by measuring changes in benthic species' presence and abundance to quantify indicators of water quality (Johnson et al. 1992; Clements & Newman 2002). These benthic species integrate local impacts over various time scales and provide important information on concentrations of dissolved oxygen, nutrients, and specific toxins. All these services are important for managerial decisions regarding water allocations (Pos-tel & Carpenter 1997; Strange et al. 1999).
Benthic ecosystem services are sometimes considered a free resource (Mitsch & Gosselink 1993; Barbier et al. 1994; Acharya 2000). For example, clean drinking water supplies are derived from natural watersheds (Watson & Lawrence 2003). This essential ecosystem service is well studied by ecologists, economists, and environmental engineers. Clean drinking water can be naturally sustainable because of the role played by benthic species that carry out biofiltration, detoxification, and numerous processes that break down organic wastes in rivers, lakes, and groundwaters. Without this recycling by diverse microbes and benthic invertebrates, organic matter accumulates and leads to deoxygenation (through microbial respiration), which then causes rapid deterioration in water quality and often results in fish kills. The effectiveness of this natural "self-cleaning" ecosystem service is limited by the quantity, type, and rates of organic waste inputs that can be processed biologically under specific flow conditions and retention times (see Giller et al., Chapter 6). Thus, anthropogenic threats and influences
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