Differing Forage Fish Assemblages Influence Trophic Structure in Neighboring Urban Lakes

Abstract

Lakes Sammamish and Washington are two large, urban lakes in close proximity to Seattle, Washington. The two lakes have similar assemblages of apex predators, but differences in lake size, species richness, and forage fish assemblages contribute to contrasting trophic relationships. We used stable isotope and diet analyses to assess lake-specific differences in the trophic structure of fish and invertebrates from these lakes. Estimates of δ¹⁵N-based trophic elevation revealed that the food web in Lake Sammamish was truncated by approximately one complete trophic level relative to that of Lake Washington. The shorter food chain in Lake Sammamish reflected the absence of longfin smelt Spirinchus thaleichthys and minimal consumption of threespine stickleback Gasterosteus aculeatus, emphasizing the importance of these species as primary forage fishes and trophic links between invertebrates and piscivores in Lake Washington. As two highly ¹⁵N-enriched and ¹³C-depleted energy sources, longfin smelt and threespine sticklebacks effectively stretched the isotopic endpoints of the Lake Washington food web by contributing to more pronounced ontogenetic shifts in isotope values for piscivores in Lake Washington than for those in Lake Sammamish.