- Large dams and their respective reservoirs can provide renewable energy and water security, but also profoundly alter riverine ecosystems. In the Pacific Northwest, dams and reservoirs cause discontinuities in river networks that have been particularly problematic for anadromous fishes. As barriers to the upstream and downstream migration of anadromous fishes, including threatened Chinook Salmon and Steelhead, these dams and reservoirs are fundamentally altering the population sizes and life history tactics of these species.
Changes in reservoir management, including reservoir draining, have been implemented to improve the downstream passage conditions for salmon juveniles. Improving our understanding of how such reservoir management may affect reservoir ecology and cohorts of salmon over time will be critical in evaluating its feasibility in salmon recovery efforts.
Reservoir draining as a management action could be expected to result in dramatic cross-community effects and the lasting in-reservoir consequences of brief draining are unknown. Here, I explore food webs and trophic ecology of large reservoirs in the Pacific Northwest of North America. Specifically, I explore 1) how juvenile Chinook Salmon are growing as they use reservoirs for rearing habitat; 2) how seasonal draining of these systems may change reservoir food web relationships and alter reservoir fish community structure; and 3) how other reservoir communities
(i.e. zooplankton) may be more resilient to draining. Surprisingly, draining appears to strongly impact the fish community, especially higher trophic levels and introduced species, but does not appear to dramatically alter water quality or lower trophic levels outside of the draining period.
During each annual draining event, I did document a brief spike in ammonium export, likely related to a short-lived sag in dissolved oxygen. This spike only occurred deep in the descending arm of draining and was not associated with apparent differences in nutrients upon refill. An apparent increase in light attenuation in Fall Creek Reservoir may be related to fine suspended sediments, but otherwise observed water quality was consistent with nearby reference reservoirs. Phytoplankton community structure and observed productivity, as with zooplankton, was within expected bounds based on reference reservoirs.
In Chapter 1, I present the theoretical underpinnings for why reservoir management may be ecologically important and review the literature on alternative reservoir drawdown operations. This review found that broad ecological effects might be expected with draining and other water level manipulations. It also appears that the physical characteristics of the system (e.g. substrate), productivity (e.g. trophic state), and seasonality (e.g. time of year manipulation occurs) may be critical in the structure of responses.
In Chapter 2, I present a linked foraging and bioenergetics model (http://growchinook.fw.oregonstate.edu/) to examine juvenile Chinook Salmon growth potential and behavior in reservoirs. This multi-model design and optimization routine (GrowChinook) has broad applications in examining growth potential and predicting habitat use in stratified environments in general. I demonstrate the use of GrowChinook for the spring-summer rearing period in three Willamette River basin reservoirs, Oregon, USA, where juvenile spring Chinook Salmon exhibit a novel life history that includes reservoir-reared juvenile fish that are larger than nearby stream-reared subyearlings. Our simulations support diel vertical migration as a tactic that increases growth potential.
In Chapter 3, I focus on the effects of reservoir management, specifically short-duration seasonal (fall) draining, on community interactions and present
evidence that aquatic food web structure changes with this whole-system manipulation. I evaluate unintended and lagged consequences, including trophic shifts away from piscivory and towards feeding at lower trophic levels for two common piscivorous fishes. Using natural abundances of nitrogen stable isotopes, I observed lower trophic level of feeding for invasive Largemouth Bass (Micropterus salmoides) and native Rainbow Trout (Oncorhynchus mykiss) during the summers following reservoir refilling than in nearby reference reservoirs that were not temporarily drained during fall. Declines in trophic levels of aquatic top predators have been rarely documented outside of controlled laboratory conditions. While useful for assisting outmigration of juvenile salmonids, the temporary draining of a reservoir to riverbed can also result in novel shifts in food web dynamics including reduced piscivory.
In Chapter 4, I analyze 12 years of fish capture data from a screw trap located downstream of Fall Creek Reservoir (Oregon, USA) for changes in timing of passage out of the reservoir and to compare fish species composition pre- and post-draining. A pulse and contraction occurred in the timing of passage for subyearling juvenile Chinook Salmon, with an absence of yearling juveniles in years following draining. This pattern indicated increased export of subyearling juvenile Chinook Salmon with draining operations. Our findings also show that briefly draining the reservoir to streambed results in a reduction of warm-water invasive species in the reservoir after it refills, which could decrease predation and shift competition between invasive and resident riverine-adapted native fishes in the reservoir. Collectively, our findings suggest that this low-cost reservoir management option may improve passage and connectivity for juvenile Chinook Salmon while also decreasing the abundance of invasive fish species in the reservoir and transitioning remaining piscivores to feed on zooplankton and other invertebrates.
In Chapter 5, I provide evidence that annual extreme water level fluctuations may have structured reservoir zooplankton communities so that they are resilient to increased winter draining. I note the importance of zooplankton as a food resource for higher trophic levels, including threatened and endangered salmon species, and the
need to improve our understanding of the structuring mechanisms for reservoir food webs.
In Chapter 6, I conclude with additional observations and recommendations for future research directions that would improve our understanding of trade-offs related to reservoir management, highlighting the crucial need for further evaluations of reservoir draining in other systems and contexts and the importance of planning ahead. As large dams continue to be operated and constructed worldwide, and climate change alters water availability, understanding the community and ecosystem-level effects of reservoir management will be critical to evaluating trade-offs between human water needs, conservation of high value species, and ecosystem services impacted by river fragmentation.