Measurements of δ13C, δ15N, and C/N on diatom-bound organic matter were made over the Holocene and Last Glacial Maximum (LGM) from three sediment cores in the Southern Ocean, one each from the Atlantic, Indian, and Pacific sectors. The site in the Scotia Sea (Atlantic sector) differed considerably from the other two sites by having markedly lower δ13C, more variable δ15N and C/N ratios, and a sedimentary diatom assemblage that was never dominated by Fragilariopsis kerguelensis. Although environmental parameters certainly have a strong impact on the isotope ratios, δ13C is also correlated to the proportion of F. kerguelensis in the three cores investigated here (r 2 = 0.8). Extreme values of δ13, δ15N, and C/N at the Last Glacial Maximum were also related to the abundance of winter stages of Eucampia antárctica. These results suggest that diatom specific isotope records should be interpreted in conjunction with information on the species composition of the samples. Copyright 2008 by the American Geophysical Union.