Fatty acids tracers for native and invasive macroalgae in an experimental food web

We assessed the potential of fatty acid (FA) markers for tracing primary production from an invasive green alga (Codium fragile ssp. fragile) and a native kelp (Saccharina longicruris) through 2 trophic levels in an experimental food web: a primary consumer, the green sea urchin Strongylocentrotus droebachiensis, and 2 secondary consumers, the native rock crab Cancer irroratus and invasive green crab Carcinus maenas. Sea urchins fed the 2 algal species had distinct gonadal FA compositions, and contained markers of each alga. Crabs of each species were then fed the gonads of sea urchins which had been fed either S. longicruris or C. fragile, and their hepatopancreases were analyzed. We were able to distinguish crabs of both species with C. fragile at the base of the food chain using the marker FAs 16:3n-3 and 18:3n-3. The overall FA composition of the hepatopancreas differed with diet in the green crab but not the rock crab. Our results suggest that 16:3n-3 may be a useful marker for tracing C. fragile production in grazers and some secondary consumers in rocky subtidal habitats in the Northwest Atlantic, but signal attenuation with each trophic transfer will limit the utility of this approach in higher consumers.