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 C-13/C-12 ratio s to differentiate pelagic food webs and to trace food web interactions bet ween adjacent continental shelf and slope/deep ocean environments off south western British Columbia, Canada. Salinity was used to define shelf or slop e/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 C-13/C-12 ratio of plankton was related to the mean upper layer (0-50 m ) salinity. C-13 abundance was enriched (relative to C-12) in the shelf wat er mass compared with the slope water mass. This enrichment persisted up th e food web from particulate organic matter through three size-classes of zo oplankton to larval fish. The cross-shelf spatial scale separating these fo od webs, as determined from spatial semivariograms of C-13/C-12 and the upp er layer mean salinity, was 40-45 km, similar to the Rossby radius for eddi es at this location (50 km). Larval fish may provide a means to monitor exc hanges of plankton between geographically adjacent food webs if time scales for incorporation of new isotope signatures from diets into tissues are de termined.