Proceedings of the Third International Conference on Invasive ...

Chapter 3: Ecosystem Effects <strong>of</strong> Invasive Spartina<strong>Proceedings</strong> <strong>of</strong> <strong>the</strong> <strong>Third</strong> <strong>International</strong> <strong>Conference</strong> on Invasive SpartinaAs noted for introduced Zostera japonica (Hahn 2003),<strong>the</strong> invasion <strong>of</strong> S. anglica could potentially alter detritaldecomposition rates and detrital quality within estuariesdominated by native eelgrass, Zostera marina. LikeSpartina alterniflora that has higher C:N ratios than Z.marina (Ruesink et al. 2006), mudflats colonized by S.anglica have higher C:N ratios than those with nativeproducers (Hellquist 2005). Changes in detrital availabilityand quality may influence <strong>the</strong> feeding patterns <strong>of</strong> residentdetrital consumers. The S. anglica invasion provides aunique opportunity to understand <strong>the</strong> trophic integration <strong>of</strong>an invasive producer in an estuarine ecosystem.Estuarine food webs are complex due to <strong>the</strong>predominance <strong>of</strong> detrital pathways, <strong>the</strong> abundance <strong>of</strong>potential productivity sources for consumption, omnivory,spatial heterogenity, and opportunistic feeding <strong>of</strong> consumersthat may change throughout <strong>the</strong> course <strong>of</strong> <strong>the</strong> year (Deeganand Garritt 1997; Riera et al. 1999). The use <strong>of</strong> multiplestable isotopes (especially δ 13 C and δ 34 S) can providevaluable insight into how Spartina spp. may contribute to<strong>the</strong> diets <strong>of</strong> estuarine consumers (Peterson et al. 1986;Peterson and Howarth 1987; Deegan and Garritt 1997;Valiela et al. 2000; Connolly et al. 2004). For example,13 C/ 12 C isotope ratios can distinguish C 3 from C 4 vegetationor marine algae from terrestrial C 3 vegetation (Fry and Sherr1984, Deegan and Garritt 1997). In marine studies, 34 S/ 32 Sisotope ratios are especially useful to distinguish sources <strong>of</strong>productivity because SO 2– 4 and HS – that are used by plantshave distinct isotopic signatures (Fry et al. 1982; Trust andFry 1992; Connolly et al. 2004).Due to <strong>the</strong> consistency <strong>of</strong> isotopic signatures fromproducers to consumers, trophic relationships can bediscerned from stable isotopic data that o<strong>the</strong>rwise mayremain virtually unknown (Fry and Sherr 1984; Peterson etal. 1985). Once isotopic data <strong>of</strong> consumers and potentialdietary producers is obtained, stable isotopic mixing modelscan be used to estimate <strong>the</strong> proportion <strong>of</strong> sources (producers)that contribute to <strong>the</strong> composition <strong>of</strong> a mixture (consumers;Phillips 2001).We present data that describe <strong>the</strong> potential contributions<strong>of</strong> Spartina biomass to <strong>the</strong> diets <strong>of</strong> three bivalves in nor<strong>the</strong>rnPuget Sound through <strong>the</strong> use <strong>of</strong> δ 13 C and δ 34 S stable isotoperatios and mixing models calculated by <strong>the</strong> computerapplication IsoSource (Phillips and Gregg 2003). As adeposit feeder that scours surface sediments for organicmatter, we expected that <strong>the</strong> clam Macoma balthica wouldhave a Spartina contribution in its diet. For <strong>the</strong> filter feedersMya arenaria (s<strong>of</strong>t-shelled clam) and Mytilus sp. (mussel),we expected Spartina contributions to be minimal or entirelyabsent.MATERIALS AND METHODSSamples were collected in March 2003 at West Pass(Camano Island, Island County, Washington; 48º 15’ 19” N,122º 24’ 56” W). The West Pass and English Boomshoreline is an extensive area <strong>of</strong> mudflat fringed by nativesalt marsh and Spartina meadows located in south SkagitBay, just north <strong>of</strong> <strong>the</strong> original introduction site <strong>of</strong> Spartinaanglica near Stanwood, Washington. This area is dominatedby approximately 100 ha <strong>of</strong> S. anglica meadows that growalong mudflats adjacent to <strong>the</strong> West Pass channel. Producerswere randomly collected along transects and included C 3emergent vascular vegetation (Grindelia integrifolia andSalicornia virginica), submergent vascular vegetation(Zostera marina), macroalgae (Fucus spiralis), and C 4vascular plants. C 4 plants included S. anglica (living leafand standing dead tissue) and <strong>the</strong> native salt marsh grassDistichlis spicata.Three bivalve species (Macoma balthica, Mya arenaria,and Mytilus sp.) were chosen based on <strong>the</strong>ir feeding patternsand co-occurrence with Spartina. Macoma is a surfacedeposit feeder, Mya a filter feeder, and Mytilus is anepifaunal filter feeder (Incze et al. 1982; Dame 1996). BothMacoma and Mya burrow in sediments immediately adjacentto Spartina meadows or burrow among Spartina roots alongtidal channels. Mytilus was found along Spartina meadowstypically using Spartina root masses and stems as anchoringsubstrate. Bivalves and Spartina were randomly sampled on<strong>the</strong> same transect at <strong>the</strong> edge <strong>of</strong> a large tidal channel thatpasses through <strong>the</strong> largest Spartina meadow at West Pass.Following collection, bivalves were held for 24 hours toallow gut contents to clear. Plant and animal samples werefrozen, cleaned to remove sediment, epiphytes, andcarbonates (10% HCl acid washes) and dried in a dryingoven. Samples were ground into a fine powder prior toisotopic analysis. For Macoma all visceral mass tissue wasused for analysis, whereas for Mya and Mytilus adductormuscles were dissected for analysis. Sample sizes forMacoma, Mya, and Mytilus were 12, seven, and sevenindividuals respectively. Producer sample sizes ranged fromthree to seven collections.Samples were analyzed for δ 13 C at <strong>the</strong> Idaho StableIsotope Laboratory, University <strong>of</strong> Idaho, Moscow, Idaho.Continuous-flow stable isotopic analyses were conducted ona Finnigan-MAT, Delta+ isotope ratio mass spectrometer(Thermo Finnigan, Thermo Electron: Waltham,Massachusetts). Samples were flash-combusted in a NC2500 elemental analyzer (CE Instruments, Thermo Electron:Waltham, Massachusetts) interfaced with a Conflo II. Stableisotope ratios are <strong>the</strong> ratio (R) <strong>of</strong> 13 C/ 12 C or 34 S/ 32 Sexpressed in standard delta (δ) notation values in parts permille (‰) where δ 13 C or δ 34 S = [(R sample /R standard)-1 ]*1000.The δ 13 C and δ 15 N reference standard was acetanilide (δ 13 C= -26.07‰ ± 0.11‰; δ 15 N = -0.33 ± 0.17‰). Spinach wasused as an internal standard (δ 13 C = -26.2‰ ± 0.2‰).Sample analysis <strong>of</strong> δ 34 S was conducted by Iso-Analytical Limited (Sandbach, Cheshire, United Kingdom).The reference standard was NBS-127 (barium sulfateδ 34 S V-CDT = 20.3‰; IAEA, Vienna, Austria). Calibration and- 154 -