- I used current water management practices in central and eastern Oregon and Washington as natural experiments to quantify the effects of irrigation water withdrawals on macroinvertebrate community structure and life history strategies. Reduced discharge had direct (e.g. decreased velocity and wetted habitat) and indirect (e.g. increased conductivity and temperature) effects on key environmental determinants of macroinvertebrate communities and life history strategies. In general, macroinvertebrate responses were more strongly related to indirect than direct environmental alterations.
For a lowland river system, the severity of community and population level responses depended on the magnitude and duration of low flow events. However, discharge reductions alone, even when exceeding 90% of ambient levels, had no effect on community composition. Rather, changes in community composition were associated with interacting thresholds of reduced discharge and altered water quality (i.e., increased conductivity and temperature). Similar responses were observed at the population level; growth and development alterations for Brachycentrus occidentalis appeared to reduce fitness during high-intensity, long-duration water withdrawals associated with increased temperature. Winter discharge levels facilitated recovery of macroinvertebrate communities on an annual basis, whereas recovery was not observed after discharge and physicochemical variables returned to predisturbance conditions for only one month.
At a broader spatial scale, I examined whether macroinvertebrate responses to water withdrawals of similar magnitude and duration depended on species traits (e.g., voltinism, thermal preference, size) for 12 rivers spanning an altitudinal gradient. Species traits differed among high, mid, and low elevation reference reaches; however, compositional responses to water withdrawals were similar among rivers found at different elevations. Apart from significant density increases, I was unable to detect compositional responses above and below all 12 diversions, despite discharge reduction exceeding 75% of ambient levels. In contrast, the proportional abundance for 10 of 52 species traits (e.g., multivoltinism, streamlined, swimmers) significantly differed above and below all points of diversion.
Irrigation water withdrawals appear to impact macroinvertebrates through indirect environmental alterations that intensify with the magnitude and duration of water withdrawals. Preserving environmental conditions within natural ranges of variability, especially during low water years, appears critical to mitigating adverse biological responses to water withdrawals.