Impacts on aquatic invertebrate communities of altered drawdown magnitude and timing of spring refill in a managed reservoir within Voyageurs National Park, Minnesota

Background/Question/Methods We compare the impact of water-level regulation under two different regimes in a quasi before-after control-impact paired (BACIP) study on aquatic invertebrates within Voyageurs National Park, Minnesota (VOYA). Concerns about the effects on aquatic biota of managed lake-level fluctuations at VOYA have been expressed repeatedly since dams were constructed between Rainy Lake and Namakan Reservoir in the early 20^th Century.  Because these waters are on the international border, the International Joint Commission (IJC) regulates reservoir management with “rule curves” that establish upper and lower bounds of water levels.  Under the 1970 rule curve (i.e., first regime), Namakan Reservoir had an average winter drawdown of 2.5 meters, while Rainy Lake averaged a 1.1-meter winter drawdown. In 2000, new rule curves (i.e., second regime) were established, reducing winter drawdown in Namakan from 2.5 to 1.1 meters, as well as allowing this reservoir to be refilled to capacity at the end of May instead of the later part of June.  Rainy Lake was left to operate in a manner similar to that under the 1970 rule curve.  We considered Rainy Lake as the “control site”, Namakan Reservoir as the “impacted site”, and the change of the rule curves in 2000 as the “impact”.  Impacts on any biological community can serve as a proxy for the overall impact of reservoir operation on an aquatic ecosystem, but not all communities are equally useful for determining impacts.  We used littoral benthic macroinvertebrates to assess biotic impacts of a change in water-level management in VOYA.

Results/Conclusions Our major findings were (1) benthic community structure became more similar in Namakan Reservoir and Rainy Lake following the change in the rule curves; (2) this change was largely evident as a reduction in amphipod and chironomid densities in Namakan Reservoir since the new rule curves went into effect; (3) littoral zone macrobenthos showed a greater response at shallow depths (1-2m) than at deeper depths (3-5m).  We discuss the potential consequences for littoral benthic productivity, based on temperature changes resulting from refilling the reservoir in the early spring. We also present our findings in the context of an overall model of ecosystem response to variation in water level fluctuation.