Carbon stable isotopes reveal relative contribution of shelf-slope production to the northern California Current pelagic community

To better delineate the relative contribution of shelf and slope production to the northern California Current (NCC) pelagic community, we examined the cross-shelf distribution of d13C in 19 species of nekton, three species of Cancer decapod larvae, five gelatinous zooplankton, two dominant euphausiid species (Thysanoessa spinifera and Euphausia pacifica), calanoid copepods (Acartia sp. and Pseudocalanus sp.), and particulate organic matter (POM). Results showed 13C enrichment from nearshore shelf sites relative to offshore slope sites at all trophic levels. For POM, a significant trend in d13C with log chlorophyll a (Chl a) was observed, with high Chl a values associated with shelf primary production. Copepods, gelatinous zooplankton, and nekton showed a significant linear decrease in d13C with distance offshore. Nekton and gelatinous zooplankton associated with very nearshore shelf waters (,10 km distance offshore) had the highest d13C values, whereas those off the slope (.20 km offshore) were more depleted in 13C. A comparison of results from nonparametric analysis of the pelagic community data to environmental variables also showed variables associated with the shelf and oceanic waters, with distance offshore, sea surface temperature (10 m in depth), and bottom depth being significant. Because pelagic systems are highly dynamic in space and time, our study indicated that d13C could be used as an indicator of relative nearshore and offshore production across multiple trophic levels, even in active upwelling ecosystems such as the NCC. Continental shelf and oceanic pelagic ecosystems are typically associated with marked differences in primary production (Thomas and Strub 2001) and community composition (Brodeur et al. 2005). However, these ecosystems are also connected through cross-shelf advective currents (Mackas and Coyle 2005) and active movement by larger organisms (Beamish et al. 2005). Delineating these systems and measuring their potential connectivity is problematic because of the dynamics of a fluid environment, organism mobility, and constraints on sampling a large marine ecosystem. Cross-shelf advective processes of coastal upwelling have been the focus of recent investigations, in part because of the potential for hydrographic processes to link coastal and offshore basin ecosystems (McFarlane and McKinnell 2005). They are also studied because of the inherent links between changes in climate and atmospheric forcing and upwelling frequency and intensity. However, the extent to which shelf and oceanic ecosystems are connected at various trophic levels is not well understood. The northern California Current (NCC) ecosystem extends from approximately central Vancouver Island (British Columbia, Canada) to northern California. Between May and September the coastal shelf is characterized as having a shelf band (water depth of ,150 m) of high production and an extensive area of relatively low production offshore (.150 m) (Lentz 1992). Differences between the two environments are also associated with species-specific assemblages of birds (Veit et al. 1996), fish (Brodeur et al. 2005), and zooplankton (Cross and Small 1967; Peterson et al. 1979). These systems therefore represent distinct habitats for transitory species, habitats that may overlap through hydrographic processes such as coastal upwelling and cross-shelf advection of primary and secondary production and through active movement by nekton of higher trophic levels. As a result, the varying 1 To whom correspondence should be addressed. Present address: Center for Marine Environmental Studies (CMES), Ehime University, 2-5 Bunkyo cho, Matsuyama, Ehime, 7908577, Japan.