Ecological tracers reveal resource convergence among prey fish species in a large lake ecosystem

Abstract

Summary We measured stable isotopes of carbon (δ13C) and nitrogen (δ15N) and fatty acid profiles in Lake Ontario alewife (Alosa pseudoharengus), rainbow smelt (Osmerus mordax), slimy sculpin (Cottus cognatus) and round goby (Neogobius melanostomus) collected from 1982 to 2008 to investigate how temporal variability in these ecological tracers can relate to ecosystem‐level changes associated with the establishment of highly invasive dreissenid mussels. Prey fish δ15N values remained relatively constant, with only slimy sculpin exhibiting a temporal increase in δ15N. In contrast, δ13C values for alewife, rainbow smelt and, especially, slimy sculpin became less negative over time and were consistent with the benthification of the Lake Ontario food web associated with dreissenids. Principal components analysis revealed higher contributions of 14:0 and 16:1n‐7 fatty acids and increasingly negative δ13C values in older samples in agreement with the greater historical importance of pelagic production for alewife, rainbow smelt and slimy sculpin. Temporal declines in fatty acid unsaturation indices and Σn‐3/Σn‐6 ratios, and also increased 24:0/14:0 ratios for alewife, rainbow smelt and slimy sculpin, indicated the increasing importance of nearshore production pathways for more recently collected fish and resulted in values more similar to those for round goby. These results indicate a temporal convergence of the food niche, whereas food partitioning has historically supported the coexistence of prey fish species in Lake Ontario. This convergence is consistent with changes in food‐web processes associated with the invasion of dreissenid mussels.