Lake trout—induced spatial variation in the benthic invertebrates of Yellowstone Lake

Invasive predators can induce trophic cascades in the open water of lakes; however, much less is known about their effect on benthic invertebrates, which inhabit the lake bottom, or benthic zone. Lake trout (Salvelinus namaycush) were introduced to Yellowstone Lake, Wyoming, and reduced the Yellowstone cutthroat trout (Oncorhynchus clakrii bouvieri) population. We predicted that lake trout indirectly reduced predation of benthic invertebrates through cutthroat trout. To estimate how the benthic invertebrate assemblages differed under cutthroat trout– versus lake trout– dominated food webs, we collected benthic invertebrate samples from two areas of Yellowstone Lake in 2004 using a Ponar sampler and compared them with stomach contents from cutthroat trout. Cutthroat trout selectively ate benthic invertebrates with the largest body sizes. The amphipod genus, Gammarus, had the highest biomass of all benthic invertebrates. Gammarus biomass was higher in West Thumb (6,000 mg/m² [0.02 oz/ft²]) where lake trout dominated and lower in South Arm (3,160 mg/m² [0.01 oz/ ft²]) where cutthroat trout dominated (p = 0.01). Additionally, individual body mass of Gammarus was greater in West Thumb (1.6 mg/individual [0.000056 oz/individual]) than in South Arm (1.1 mg/individual [0.000039 oz/individual; p = 0.01). Our results suggest that lake trout predation on cutthroat trout indirectly reduced predation on Gammarus in West Thumb, leading to a relative increase in the local Gammarus biomass and body mass. Monitoring the benthos of Yellowstone Lake may allow managers to understand the food web dynamics at higher trophic levels.