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Is the invasive cordgrass, Spartina anglica, becoming integrated within invertebrate food webs in north Puget Sound, WA, USA?
Hellquist, C.*,1, Nelson, S. 1, Rhodes, B. 1, Lee, R. 1, Black, R.1, 1 Washington State University, Pullman, WA
ABSTRACT- Spartina anglica is an invasive estuarine cordgrass that has colonized approximately 400 ha of intertidal habitats in north Puget Sound, WA. This invasion has resulted in the extensive colonization of some mudflats while other similar mudflats may have little or no Spartina present. This distribution pattern has created opportunities to examine trophic relationships following colonization of a non-native species. We examined estuarine trophic relationships by sampling the stable isotopic composition (13C, 15N, and 34S) of phytoplankton, zooplankton, benthic invertebrates, macroalgae, and vascular plants. Initial data using 15N and 13C indicate that invertebrate isotopic ratios vary between sites and species. Macoma balthica (Bivalvia) collected within roots of S. anglica has a C signature (13C -16‰) that is relatively similar to S. anglica (13C -13‰). In bivariate isotopic scatterplots with 15N, Macoma balthica is placed closer to Spartina than it is to other bivalves. This placement may indicate the use of some Spartina-derived carbon. However, Macoma nasuta (Bivalvia) and Mytilus edulis (Bivalvia) collected among Spartina have coinciding isotopic signatures (13C -21; 15N 9-10‰) that are different from those of Spartina (13C -13‰; 15N‰). Isotopic signatures of Macoma nasuta and Mytilus for 15N more closely resemble published values for phytoplankton (ca. 13C -21; 15N 9-10‰). These data suggest that despite living among the roots of small Spartina clones (Macoma nasuta) or living attached to the base of Spartina stems (Mytilus), these bivalves are not using Spartina productivity for nutrition.
Key words: trophic, estuaries, Spartina anglica, interactions