Characterization of carbon and nitrogen stable isotope gradients in the northern Gulf of Alaska using terminal feed stage copepodite-V Neocalanus cristatus

Individual terminal-feeding copepodite-V stage Neocalanus cristatus were collected systematically in the northern Gulf of Alaska (GOA) near 60°N from 1998 to 2004 from which the natural abundance of stable carbon and nitrogen isotopes was measured. The data confirmed the existence of an isotopic cross-shelf gradient such that values low in 13C content are diagnostic of oceanic production from the GOA when measured in organisms taken from Prince William Sound (PWS). The mean 13C/12C cross-shelf gradient of 3–4 delta units was relatively strong, with generally good separation between GOA and Prince William Sound observations, whereas the mean 15N/14N gradient of ∼2 delta units was relatively weak, with frequent overlap between GOA and PWS observations. There was a seasonal 15N/14N increase in the GOA. The 13C/12C values observed in PWS were more consistent over time than those observed in the GOA. Distinctively high 13C/12C values that were similar to those typical of PWS were observed at the continental slope during three of the Mays in the 1998–2004 period. The circulation pattern associated with mesoscale eddies, when they occurred just south of the sampling line based on satellite sea-surface height anomaly data, suggested that cross-shelf flow in the offshore direction drove high slope 13C/12C values. These observations led to positing that high 13C/12C values reflect coastal carbon isotope signatures and diatom blooms. Based on samples from May 1996, the three GOA Neocalanus congeners had concordant isotopic patterns with relatively small systematic species differences confirming that the isotopic patterns observed for N. cristatus apply to other zooplankton.