Food web structure on Georges Bank from stable C, N, and S isotopic compositions

A combination of stable isotopic measurements was used to study food web structure of Georges Bank, an important northwestern Atlantic fishing ground. Particulate, invertebrate, and fish samples were analyzed to test δ 13 C, δ 15 N, and δ 34 S measurements as trophic‐level indicators in offshore systems. Neither sulfur nor carbon measurements proved valuable. Sulfur isotopic compositions showed little change with trophic level, and an apparent diversity of phytoplankton carbon isotopic inputs at the base of the food web complicated use of δ 13 C to estimate trophic position. Nitrogen isotopic distributions were, however, robust measures of trophic position and showed four broad trophic levels; unsampled large top carnivores may represent a fifth trophic level.