ABSTRACT: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.
KEY WORDS: Fatty acid · Sea urchin · Crab · Macroalgae · Invasive species · Dietary tracer · Food web
Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 391: [53][54][55][56][57][58][59][60][61][62][63] 2009 marker FA or an unusually large quantity of one or more FAs, it may be possible to trace primary producers through multiple trophic levels to higher-order consumers (Hall et al. 2006). Thus FA analysis may be particularly useful for elucidating the role of non-native algal species in the food webs of invaded ecosystems if the non-native species differ from native species in their overall FA composition or in their relative abundance of particular marker FAs.The rocky subtidal ecosystem in Atlantic Canada presents an ideal model system for testing the transfer of FA markers across multiple trophic levels, and for tracking a non-native primary producer through a food web using FA analysis. Historically, this ecosystem has alternated between 2 states: kelp beds (mostly Saccharina longicruris and Laminaria digitata) with a low density of sea urchins Strongylocentrotus droebachiensis, and barren grounds dominated by sea urchins and encrusting coralline algae (Scheibling 1986). Facilitative interactions between recently introduced non-native species have brought about a third state of this ecosystem, in which an invasive green alga, Codium fragile ssp. fragile (previously C. fragile ssp. tomentosoides, Provan et al. 2008; hereafter C. fragile), has replaced kelp as the dominant canopy-forming macroalga off Nova Scotia and in the Gulf of Maine (Harris & Tyrrell 2001, Chapman et al. 2002, Levin et al. 2002. The fate of C. fragile production is largely unknown, as it contains a chemical that may deter herbivores in the wild (Lyons et al. 2007, Lyons &...