Trophic relationships of two species of grebe on a prairie lake based on stable isotope analysis

We used stable isotope analysis of carbon and nitrogen to investigate the trophic relationships of Western ( Aechmophorus occidentalis (Lawrence)) and Red-necked ( Podiceps grisegena (Boddaert)) Grebes from Lake Wabamun, a large prairie lake in north-central Alberta, Canada. We collected tissue samples (i.e., pectoralis muscles and primary feathers) from 42 Western and 22 Red-necked Grebes (adults and juveniles) that had died in captivity after an oil spill on Lake Wabamun on 3 August, 2005. We also collected fish and invertebrates, focusing on taxa known to serve as prey for grebes. Based on comparisons of 15 N enrichment in muscle tissue, Western Grebes, northern pike ( Esox lucius Linnaeus), yellow perch ( Perca flavescens (Mitchill)), and lake whitefish ( Coregonus clupeaformis (Mitchill)) were top consumers in Lake Wabamun, as were Red-necked Grebes. Adult Western Grebe muscle was more enriched with 15 N than adult Red-necked Grebe muscle by 1.5‰, indicating that Western Grebes foraged at a slightly higher trophic position on Lake Wabamun than Red-necked Grebes. Red-necked Grebe muscles and feathers were also less depleted in 13 C than Western Grebe muscle, indicating that Red-necked Grebes likely spent more time foraging in the littoral zone than Western Grebes. There were no differences in muscle or feather isotopic signals between juvenile and adult Western Grebes, suggesting that both age groups fed on similar prey items and in similar habitats. Muscle tissues and primary feathers of larger Red-necked Grebes were more enriched in both 15 N and 13 C than smaller grebes likely because larger grebes consume more or larger fish.