Proceedings of the Third International Conference on Invasive ...

<strong>Proceedings</strong> <strong>of</strong> <strong>the</strong> <strong>Third</strong> <strong>International</strong> <strong>Conference</strong> on Invasive SpartinaChapter 3: Ecosystem Effects <strong>of</strong> Invasive SpartinaTHE ROLE OF SPARTINA ANGLICA PRODUCTION IN BIVALVE DIETS IN NORTHERNPUGET SOUND, WA, USAC.E. HELLQUIST 1 AND R.A. BLACKSchool <strong>of</strong> Biological Sciences, Washington State University, Pullman, WA 991641 Current address: Department <strong>of</strong> Biological Sciences, State University <strong>of</strong> New York, Oswego, NY 13126;eric.hellquist@oswego.eduThe importance <strong>of</strong> salt marsh productivity to coastal food webs is a question <strong>of</strong> ecological andeconomic importance. In nor<strong>the</strong>rn Puget Sound, Washington, USA, a relatively new source <strong>of</strong>production has become prominent with <strong>the</strong> establishment <strong>of</strong> invasive Spartina anglica (Poaceae:English cordgrass). Spartina anglica has converted native coastal mudflat communities that hadlittle or no emergent vascular vegetation into expansive cordgrass meadows. One consequence <strong>of</strong>Spartina productivity on invaded mudflats may be altered trophic patterns. Three bivalves withdifferent feeding modes (Macoma balthica, Mya arenaria, and Mytilus sp.) that are commonlyfound at <strong>the</strong> edges <strong>of</strong> Spartina meadows were selected to investigate whe<strong>the</strong>r Spartina iscontributing to bivalve diets. We compared <strong>the</strong> stable isotope ratios (δ 13 C and δ 34 S) <strong>of</strong> <strong>the</strong> bivalvesto potential food sources including macroalgae, Spartina, and o<strong>the</strong>r vascular plants. We estimated<strong>the</strong> feasible contributions <strong>of</strong> production to bivalve diets during March 2003 by analyzing <strong>the</strong> results<strong>of</strong> multiple source, mass-balanced linear mixing models as calculated by IsoSource. Our estimatesindicate that Spartina biomass may comprise 37-60% <strong>of</strong> <strong>the</strong> diet <strong>of</strong> Macoma, while dead Spartinabiomass contributes 0-46%. For Mya arenaria a filter-feeding clam, 40-59% <strong>of</strong> its diet may containSpartina biomass, while 0-35% <strong>of</strong> its diet may consist <strong>of</strong> dead Spartina biomass. Mytilus sp., afilter-feeding mussel, had 19-44% <strong>of</strong> its diet originating from Spartina biomass while dead Spartinamay be 0-46% <strong>of</strong> its diet. Spartina consumption by bivalves is consistent with previous isotopicstudies. Although Spartina biomass is considered recalcitrant, <strong>the</strong> immediate proximity <strong>of</strong> <strong>the</strong>consumers to vast quantities <strong>of</strong> Spartina productivity may best explain <strong>the</strong> prevalence <strong>of</strong> Spartina inbivalve diets while o<strong>the</strong>r potential sources have minor estimated contributions. This study providesan initial examination <strong>of</strong> how <strong>the</strong> biomass <strong>of</strong> an invasive plant species is becoming integrated intoestuarine trophic webs.Keywords: Spartina anglica, stable isotopes, mixing models, IsoSource, Macoma, Mytilus, MyaINTRODUCTIONEstuarine salt marshes intercept nutrients and biomassfrom uplands and also export nutrients and biomass intonearshore coastal ecosystems (Deegan and Garritt 1997;Tealand Howes 2000; Valiela et al. 2000). The importance <strong>of</strong> saltmarsh productivity in estuarine ecosystems has been acentral research question with ramifications foreconomically important coastal fisheries (Peterson et al.1985, 1986; Teal and Howes 2000; Valiela et al. 2000).Few studies have addressed <strong>the</strong> role <strong>of</strong> invasive speciesin altering ecosystem processes in marine and estuarineenvironments (Ruiz et al. 1997, Grosholz 2002) despite <strong>the</strong>wealth <strong>of</strong> research examining how invasive species alterlandscapes, nutrient cycling, and species interactions (Macket al. 2000 and references <strong>the</strong>rein). Spartina anglica C. E.Hubbard (Poaceae; English cordgrass) was introduced intonor<strong>the</strong>rn Puget Sound, Washington in <strong>the</strong> early 1960s inSnohomish County (Hacker et al. 2001; Hellquist 2005).Spartina anglica covers nearly 300 solid hectares (ha) <strong>of</strong>Puget Sound intertidal habitat (Hedge et al. 2003). Spartinaanglica colonizes s<strong>of</strong>t sediments and is capable <strong>of</strong> rapidlyconverting mudflat habitats into elevated Spartina meadows.Habitat conversion by Spartina is <strong>of</strong> great concernecologically, economically, and aes<strong>the</strong>tically (Hacker et al.2001; Hedge et al. 2003).There are no native maritime species <strong>of</strong> Spartina in <strong>the</strong>Pacific Northwest <strong>of</strong> North America and thus mudflatecosystems <strong>of</strong> Puget Sound have developed without lowintertidal meadows <strong>of</strong> emergent C 4 vegetation (i.e. S.anglica) along <strong>the</strong>ir periphery. Generally, <strong>the</strong> high carboncontents <strong>of</strong> C 4 plants have been considered to be <strong>of</strong> lownutritional quality (Caswell et al. 1973). Extensive meadows<strong>of</strong> Spartina that colonize mudflats represent a large subsidy<strong>of</strong> low-quality productivity for consumers. At Alice Bay,Washington, S. anglica meadows had over 10 times <strong>the</strong>aboveground biomass as uninvaded mudflat (Hellquist2005). In Willapa Bay, Washington, colonization <strong>of</strong> Zosterajaponica and Spartina alterniflora has increased primaryproductivity by more than 50% in intertidal mudflats(Ruesink et al. 2006).- 153 -