Evidence for lasting alterations to aquatic food webs with short-duration reservoir draining - Fig 2

Trophic levels of key taxa (top) and δ15N ratios for Largemouth Bass and Rainbow Trout (bottom). (A) Reference reservoir food chain compared to the (B) altered reservoir food chain observed in the previously drained reservoir. Additional species found in Willamette Basin reservoirs but not pictured here include: bullhead catfish, Largescale Sucker and sculpin (detritivores); dace, mosquitofish, Redside Shiner, and Threespine Stickleback (invertivores); Bull Trout, crappie, Northern Pikeminnow, Walleye and Yellow Perch (juvenile invertivores, adult piscivores). (C and D) δ15N ratios increase naturally with increasing size of fish in reference reservoirs. The threshold for piscivory is indicated by a red dashed line. The grey dotted vertical line indicates the 150 mm threshold, above which fish diets are predominantly piscivorous. Although both species are capable of consuming fish as prey when smaller than 150 mm, this size threshold was calculated as the intersection of these (C and D) reference ontogenetic trajectories and the predicted piscivory isotopic ratio. (E and F) δ15N values of large (>150 mm) (E) Largemouth Bass and (F) Rainbow Trout in Fall Creek Reservoir (shown in orange) are lower than in other reservoirs (shown in gray; S1 Table). Fall Creek experienced riverbed draining while other reservoirs maintained conservation pools. Lower δ15N values indicate that consumers are feeding lower on the food chain.