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 Spartinanature <strong>of</strong> Spartina tissue (Peterson and Howarth 1987). Of<strong>the</strong> two Spartina tissue types sampled, we were expectingmore constrained IsoSource solutions for standing deadSpartina contributions to bivalve diets since we expecteddead tissue to more closely match <strong>the</strong> detritus available tobivalves (Table 1). The living Spartina tissue samples in thisstudy were newly emerged leaves, whereas <strong>the</strong> deadSpartina samples consisted <strong>of</strong> standing dead Spartina stemsand leaves. Living and dead Spartina biomass were sampledto represent <strong>the</strong> end points <strong>of</strong> a range <strong>of</strong> potential Spartinaisotopic signatures during its life history. The range <strong>of</strong>potential intermediate isotopic values that may occur duringsenescence is apparent as <strong>the</strong> distance between <strong>the</strong>signatures <strong>of</strong> living and dead Spartina (Figs. 1A-C).It may seem contradictory that <strong>the</strong> mixing models in thisstudy indicate <strong>the</strong> use <strong>of</strong> Spartina living tissue when <strong>the</strong>bivalves sampled are sediment dwelling or epifaunaldetritivores. These estimates suggest that as Spartinadecomposes, plant fragments in <strong>the</strong> early stages <strong>of</strong> decaymay be consumed. Early during decay, Spartina would haveits greatest nutritional value and would be at its leastrecalcitrant since C:N ratios more than double from livingtissue to <strong>the</strong> standing dead tissue phase <strong>of</strong> Spartina(Hellquist 2005). Alternatively, <strong>the</strong> use <strong>of</strong> Spartina detritusby <strong>the</strong> bivalves may be <strong>the</strong> result <strong>of</strong> trophic modification <strong>of</strong><strong>the</strong> biomass as it passes through <strong>the</strong> detrital food web(Peterson et al. 1986).The spatial proximity <strong>of</strong> <strong>the</strong> bivalves to <strong>the</strong> edge <strong>of</strong> <strong>the</strong>Spartina meadow probably accounts for <strong>the</strong> higher thanexpected inputs <strong>of</strong> Spartina in <strong>the</strong> bivalve diets. Although asimilar study <strong>of</strong> S. anglica and bivalves did not find arelationship between Spartina consumption and proximity toSpartina (Riera et al. 1999), o<strong>the</strong>r work has linked spatialproximity <strong>of</strong> consumers to Spartina with <strong>the</strong> magnitude <strong>of</strong>Spartina dietary contributions (Peterson et al. 1986; Petersonand Howarth 1987). Local sources <strong>of</strong> organic matter were <strong>of</strong>major dietary importance to <strong>the</strong> consumers sampled in aMassachusetts estuary (Deegan and Garritt 1997). Forexample, consumers in <strong>the</strong> upper estuary relied onfreshwater marsh organic matter and oligohalinephytoplankton, whereas in <strong>the</strong> lower estuary marine sourceswere more important (e.g. benthic diatoms and salt marshvegetation). We sampled bivalves collected from a tidalcreek passing through an extensive Spartina meadow, whereSpartina was <strong>the</strong> only immediate source <strong>of</strong> vascular plantproductivity. It is probably not surprising that <strong>the</strong> bivalveshave such a large potential contribution <strong>of</strong> S. anglica to <strong>the</strong>irdiets. Spartina was also a locally important dietary sourcefor <strong>the</strong> mussel Geukensia demissa when sampled fromwithin a salt marsh tidal channel (Peterson et al. 1985,1986). In Padilla Bay, Washington <strong>the</strong> importance <strong>of</strong> locallyabundant productivity sources also has been described forMytilus edulis (Ruckelshaus et al. 1993).Like Spartina alterniflora in Willapa Bay, Washington(Ruesink et al. 2006), S. anglica produces biomassthroughout <strong>the</strong> growing season that senesceces during <strong>the</strong>autumn and over winter. Thus, S. anglica adds copiousamounts <strong>of</strong> wrack to intertidal habitats. Release <strong>of</strong> biomassinto <strong>the</strong> estuary as a consequence <strong>of</strong> control efforts may alsocontribute to <strong>the</strong> high levels <strong>of</strong> S. anglica in bivalve diets atWest Pass. The West Pass area contains <strong>the</strong> greatestconcentrations <strong>of</strong> S. anglica in Washington (Hacker et al.2001) and has been subjected to intense control efforts. In2003, West Pass and its environs were intensively controlledvia applications <strong>of</strong> herbicide as well as mowing. Thesecontrol treatments kill Spartina during <strong>the</strong> growing seasonand produce Spartina wrack much earlier than typicalsenescence. The less recalcitrant Spartina tissue that diesprematurely due to control activity enters <strong>the</strong> food web up t<strong>of</strong>our to five months earlier than is typical. This plant tissuemay <strong>the</strong>n decay faster during <strong>the</strong> warmer summer and earlyfall months. By March, <strong>the</strong> Spartina isotopic signaturewould be fully incorporated into consumer tissue.The contributions to bivalve diets in this study duringMarch 2003 are subject to some sampling limitations. Thelack <strong>of</strong> samples <strong>of</strong> benthic diatoms and SPOM unfortunatelyexcludes productivity sources that should logicallycontribute to bivalve diets. However, additional stableisotope ratio data from West Pass collected later in 2003using δ 13 C, δ 15 N, and δ 34 S provide additional evidence for<strong>the</strong> consumption <strong>of</strong> S. anglica by bivalves (Hellquist 2005).These data indicate that <strong>the</strong>re are seasonal patterns <strong>of</strong> S.anglica use in bivalve diets (Hellquist 2005). However,during <strong>the</strong>se two additional sampling periods that includesamples <strong>of</strong> sestonic and benthic production, <strong>the</strong> dietary role<strong>of</strong> <strong>the</strong>se sources was unexpectedly inconclusive (Hellquist2005).The calculations <strong>of</strong> IsoSource greatly enhance ourability to estimate source contributions where <strong>the</strong>re are moresources than elements analyzed (Phillips 2001; Phillips andGregg 2003). However, it is crucial to remember that <strong>the</strong>semixing models should be viewed as an index <strong>of</strong> dietaryconsumption, and not discrete point estimates (Ben-Davidand Schell 2001; Phillips and Gregg 2003). The uncertaintyassociated with <strong>the</strong> dietary estimates generated by IsoSource(Table 1) reinforces <strong>the</strong> inexact nature <strong>of</strong> isotopic data incases where <strong>the</strong>re are multiple productivity sources withsimilar isotopic ratios and where consumer omnivory isprevalent.Along with Hahn (2003), Hellquist (2005), and Ruesinket al. (2006), this research illustrates how ecosystemprocesses in Washington estuaries can be altered by invasiveplant species including Zostera japonica, S. anglica, and S.alterniflora. To our knowledge, this study is <strong>the</strong> first use <strong>of</strong>multiple stable isotope ratios to examine trophicrelationships in Puget Sound in relation to invasive Spartina.This study provides evidence that bivalves living in- 157 -