Stable carbon isotopes as pelagic food web tracers in adjacent shelf and slope regions off British Columbia, Canada

Surveys were conducted in spring 1992 to examine the use of 13C/12C ratios to differentiate pelagic food webs and to trace food web interactions between adjacent continental shelf and slope/deep ocean environments off southwestern British Columbia, Canada. Salinity was used to define shelf or slope/deep ocean water masses and their productivity conditions because eddies and meanders at the shelf break were observed to draw water off the shelf. The 13C/12C ratio of plankton was related to the mean upper layer (0-50 m)Salinity. 13C abundance was enriched (relative to 12C) in the shelf water mass compared with the slope water mass. This enrichment persisted up the food web from particulate organic matter through three size-classes of zooplankton to larval fish. The cross-shelf spatial scale separating these food webs, as determined from spatial semivariograms of 13C/12C and the upper layer mean salinity, was 40-45 km, similar to the Rossby radius for eddies at this location (50 km). Larval fish may provide a means to monitor exchanges of plankton between geographically adjacent food webs if time scales for incorporation of new isotope signatures from diets into tissues are determined.