Introduced Lake Trout Produced a Four-Level Trophic Cascade in Yellowstone Lake

—Introduction of lake trout Salvelinus namaycush into a system can add a trophic level, potentially affecting organisms at lower trophic levels. Similar to many lakes and reservoirs in the western United States, lake trout were introduced into Yellowstone Lake, Wyoming. Previous studies showed that lake trout reduced the population and altered the size structure of native Yellowstone cutthroat trout Oncorhynchus clarkii bouvieri in Yellowstone Lake, but we sought to determine the degree to which lake trout predation changed lower trophic levels. We predicted that the structure of lower trophic levels would change in conformance with trophic cascade theory because Yellowstone cutthroat trout consume zooplankton. We compared zooplankton and phytoplankton assemblages between the period when Yellowstone cutthroat trout were abundant and the period after they declined. As predicted by trophic cascade theory, zooplankton biomass shifted from being dominated by copepods before lake trout introduction to being dominated by cladocerans after lake trout introduction, with zooplankton body lengths 17% longer after introduction. Vertical water clarity increased by 1.6 m because of a twofold decrease in chlorophyll a and a threeto sevenfold decrease in phytoplankton biovolume. Thus, the introduction of lake trout and subsequent decline of Yellowstone cutthroat trout likely altered lower trophic levels in Yellowstone Lake. Trophic cascades may be common in western U.S. lakes and reservoirs where native salmonids are present and where lake trout have been introduced. Lake trout Salvelinus namaycush are introduced predators in many lakes and reservoirs in the western United States (Martinez et al. 2009). Lake trout are apex predators (Post et al. 2000) and can cause native and nonnative fishes to decline (Martinez et al. 2009). For example, lake trout reduced the populations of native Lahontan cutthroat trout Oncorhynchus clarkii henshawi in Lake Tahoe, California, and bull trout Salvelinus confluentus in Lake McDonald, Glacier National Park, Montana. However, the degree to which lower trophic levels are affected by the introduction of lake trout is largely unknown. Introduction of lake trout into a system may add a trophic level to a food web, potentially producing a four-level trophic cascade (Fretwell 1987; Post and Takimoto 2007). Research has focused on the functional controls of the common twoand three-level trophic cascades (Borer et al. 2005; Halpern et al. 2005; Shurin and Seabloom 2005). Four-level trophic cascades have seldom been included in meta-analyses, probably because they are more difficult to observe in longer food chains as top-down effects from predators attenuate down the food chain (Shurin et al. 2002). Additionally, the cumulative probability of strong interactions among four trophic levels is lower than that for two or three trophic levels. Nonetheless, nonexperimental four-level trophic cascades have been observed. For example, overfishing and removal of Atlantic cod Gadus morhua, which served as the fourth trophic level of the Scotian Shelf food web, increased phytoplankton standing stocks (Frank et al. 2005). Characteristics of the plankton assemblages depend on the number of trophic levels in the ecosystem (e.g., Carpenter et al. 1987, 2001). The addition of piscivores reduces planktivorous fish abundance, causing largebodied zooplankton to dominate the herbivore assemblage. The biomass of zooplankton and large zooplankton species (length . 1 mm) is higher in piscivore-dominated lakes because of lower predation * Corresponding author: [email protected] 1 Present address: Wyoming Natural Diversity Database, University of Wyoming, Laramie, Wyoming, 82071-3381, USA. Received August 19, 2009; accepted May 12, 2010 Published online September 29, 201