Variables Associated With Isotopic Niche Size in Fish Communities Across North‐Temperate Lakes

ABSTRACT Larger trophic niches may contribute to the adaptive capacity of food webs by facilitating shifts in resource use during environmental change, but niche size patterns across lakes and fish species remain poorly known. We sampled 14 fish species across 17 Minnesota lakes and used stable isotopes ẟ 13 C (littoral carbon use, hereafter littoral C) and ẟ 15 N (trophic position) to estimate isotopic niche size (SEA b ) for each species in each lake. We tested for relationships between niche size and lake characteristics using common species. We also tested whether niche size differed among and within species and evaluated whether differences could be explained by trophic (e.g., trophic position, littoral C use) or morphological (e.g., total length, length range) variables. Results showed mean niche size varied four‐fold among lakes and was inversely related to lake surface area, total phosphorus, and hypoxic depth. Niches were smaller in larger, more productive lakes with more suitable dissolved oxygen due to reduced littoral C use variation, indicating fish populations focus on more similar habitats along these gradients. Niche size varied over three‐fold among species. Niches were largest in bluegill ( Lepomis macrochirus ) and yellow perch ( Perca flavescens ) and smallest in walleye ( Sander vitreus ) and three prey fish species. Niche size showed dome‐shaped quadratic relationships with length, trophic position, and littoral C across all populations, but species identity explained more variation. Length range had a positive effect on niche size within all species. Niche size and potential flexibility in resource use is greatest in small, unproductive lakes with more extensive hypoxic conditions, in species with intermediate trophic positions, and in populations with large length ranges. These results suggest populations contribute differently to the adaptive capacity of food webs depending on lake, trophic, and morphological characteristics. Overall, our study demonstrates how niche size changes predictably across lakes and species, an important step for understanding lake food web dynamics and managing fish populations.