Abstract
Water-resource managers are challenged to balance growing water demand with protecting aquatic ecosystems and biodiversity. Management decisions can benefit from improved understanding of water-withdrawal impacts on hydrologic regimes and ecological assemblages. This study used ecological limit functions for fish groups within the Tennessee and Cumberland River basins to predict species richness responses under simulated constant-rate (CR) and percent-of-flow (POF) withdrawals and for different minimum flow level protections. Streamflow characteristics (SFC) and richness were generally less sensitive to POF withdrawals than CR withdrawals among sites, fish groups, and ecoregions. Species richness generally declined with increasing withdrawals, but responses were variable depending on site-specific departures of SFCs from reference conditions, drainage area, fish group, ecoregion, and minimum flow level. Under POF withdrawals, 10% and 20% daily flow reductions often resulted in loss of <1 species and/or ≤5% richness among fish groups. Median ecological withdrawal thresholds ranged from 3.5–31% for POF withdrawals and from 0.01–0.92 m3/s for CR withdrawals across fish groups and ecoregions. Application of minimum flow level cutoffs often resulted in damping effects on SFC and richness responses, indicating that protection of low streamflows may mitigate hydrologic alteration and fish species richness loss related to water withdrawals. Site-specific and regionally summarized responses of flow regimes and fish assemblages under alternative withdrawal strategies in this study may be useful in informing water-management decisions regarding streamflow allocation and maintaining ecological flows.
Highlights
The natural flow regime is considered a master variable in determining lotic ecosystem processes [1].It is well documented that alterations of streamflow regimes pose significant threats to riverine ecosystems and biodiversity worldwide [2,3,4,5,6]
We investigated the following general hypotheses related to hydrologic and ecological impacts of CR compared to POF withdrawals: (1) Streamflow characteristics (SFC) should be less sensitive to POF withdrawal scenarios than to CR withdrawals, SFCs associated with flow ratios, frequency, variability, and timing of flows; (2) resulting predicted fish species richness should generally decrease with increased water withdrawals regardless of CR or POF withdrawals; and (3) application of minimum flow levels (MFL) should generally dampen patterns of change of SFCs, species richness, and withdrawal threshold responses regardless of CR or POF withdrawals
Results of this study indicate that increasing water-withdrawal rates, regardless of the withdrawal strategy, will likely result in increased loss of fish species richness across a variety of streams and ecoregions
Summary
The natural flow regime is considered a master variable in determining lotic ecosystem processes [1]. It is well documented that alterations of streamflow regimes pose significant threats to riverine ecosystems and biodiversity worldwide [2,3,4,5,6]. Many studies have shown that aquatic communities can be negatively impacted by alteration of the magnitude, timing, and/or duration of flows to which they are adapted [2,4,5,6,7,8]. Water withdrawals for domestic, agricultural, and industrial purposes can directly reduce streamflow quantity and disrupt the natural timing and variability of flows [9]. With increasing human population and intensifying land-use practices, global water withdrawals have already increased dramatically over the past 50 years [10] and are expected to continue [11].
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