Impact of surface water withdrawal on macroinvertebrate drift

Background/Question/Methods

Summer water withdrawal for irrigated agriculture is a leading use of surface water throughout the world.  Water withdrawals have been found to have important impacts on the density and composition of macroinvertebrate benthic assemblages.  However, little work has examined the impact of water withdrawals on the functional role of macroinvertebrates, including drift, in river ecosystems.  Macroinvertebrate drift plays an important role in river ecosystems influencing the distribution of organisms, food web dynamics, and energetic subsidies from upstream reaches to downstream areas.  We examined the macroinvertebrate drift dynamics in an eastern Oregon river subject to multiple points of surface water withdrawal, which created a gradient of increasing impact.  Drift was measured three times during the summer season to determine whether impacts of water withdrawal changed across the irrigation season.  Sampling was conducted for two consecutive years.  Our objectives were to examine across this gradient of water withdrawal: 1) the relationship between discharge and drift density, 2) the total reach-level amount of drift, and 3) drift composition.

Results/Conclusions

No consistent relationships between discharge and drift density, measured as either the number of individuals/m³ of water or as biomass/m³ of water, were found during any of the sampling periods despite a cumulative reduction in surface water of over 95% across the withdrawal gradient.  However, the total reach-level drift declined markedly across the withdrawal gradient, with only 40% as many individuals and 3% of the biomass drifting past the most impacted site versus the least impacted  site during mid-summer sampling.  In addition, drift assemblage composition changed across the withdrawal gradient and the summer season.  Early in the summer the drift composition was similar among all sites and was dominated by mayfly, caddisfly, and midge larvae.  However, by mid-summer, drift became dominated by non-insects (gastropods and daphnids) and midges at the most impacted sites while continuing to be dominated by mayfly, caddisfly, and midge larvae at the least impacted sites.  In conclusion, we demonstrated that water withdrawals had a strong impact on an important ecosystem function of macroinvertebrates, impacting the total amount of transport of macroinvertebrates in the drift and drift composition.  These impacts have potentially important consequences for the ecosystem dynamics of the river downstream of this area of water withdrawal.