Contribution of a pelagic specialist (cisco, <i>Coregonus artedi</i>) to fish community isotopic niche space varies along lake phosphorus gradients

Species can stabilize food webs by contributing to different energy pathways. Cisco are important for pelagic carbon (PC) pathways in lakes, and we predicted high PC use would cause cisco to contribute more to isotopic niche size of the fish community relative to other species. We used δ 13 C and δ 15 N to estimate PC use and trophic position for estimates of isotopic niche size for fish communities in 17 Minnesota (USA) lakes. Cisco used more PC than 20 other fish species tested resulting in a six-fold larger contribution to community niche space compared to other species. Cisco contribution to community niche space was negatively related to fish community PC use, which in turn was positively related to lake total phosphorus. Yellow perch overlapped the most onto the cisco niche, with lowest overlap in oligotrophic lakes where yellow perch consumed less PC. Thus, cisco have high, inflexible reliance on PC and are most unique and contribute most to community niche space in oligotrophic lakes, highlighting the importance of preserving cisco habitat in these systems.