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 Spartinacorrection standards were IAEA S-1 (silver sulfide δ 34 S V-CDT= -0.3‰) and Iso-Analytical IA-R025 (barium sulfateδ 34 S V-CDT = +8.53‰). IA-R025 (barium sulfate δ 34 S V-CDT =+8.53‰) and whale baleen (δ 34 S V-CDT = +16.30‰) wereused as internal standards.The IsoSource isotopic mixing model analysisapplication (www.epa.gov/wed/pages/models.htm; Phillipsand Gregg 2003) was used to estimate <strong>the</strong> feasiblecontributions <strong>of</strong> S. anglica and six o<strong>the</strong>r potential sources to<strong>the</strong> diets <strong>of</strong> Macoma, Mya, and Mytilus. IsoSource calculates<strong>the</strong> ranges <strong>of</strong> all possible source contributions when <strong>the</strong>number <strong>of</strong> sources placed in <strong>the</strong> mixing model exceeds n+1,where n is <strong>the</strong> number <strong>of</strong> isotopic tracers (e.g. δ 13 C or δ 34 S;Phillips and Gregg 2003). Therefore, IsoSource allowssource contributions to a mixture to be estimated regardless<strong>of</strong> <strong>the</strong> number <strong>of</strong> sources and isotopic tracers available. In<strong>the</strong>se circumstances unique solutions cannot be obtained dueto <strong>the</strong> use <strong>of</strong> large numbers <strong>of</strong> sources in <strong>the</strong> mixing model(Phillips and Gregg 2003). Since unique solutions cannot bedetermined, IsoSource output provides ranges <strong>of</strong> all possiblesource contributions to <strong>the</strong> mixture based on mass balancecalculations (Table 1; Phillips and Gregg 2003). The morenarrow <strong>the</strong> range <strong>of</strong> <strong>the</strong>se solutions (e.g 30%-50% vs. 10-70%), <strong>the</strong> more reliable <strong>the</strong> interpretation <strong>of</strong> sourcecontributions to <strong>the</strong> mixture can be. Thus, IsoSource is veryuseful for studies <strong>of</strong> trophic relationships where numerousfood sources could exist for a consumer. IsoSource has beenapplied to a variety <strong>of</strong> trophic studies inside and outside <strong>of</strong>estuaries (e.g. Felicetti et al. 2003; Melville and Connolly2003; Ben-David et al. 2004; Newsome et al. 2004).The carbon isotope ratios <strong>of</strong> <strong>the</strong> bivalves were adjustedby a 1‰ fractionation factor based on Peterson and Fry(1987), Vander Zanden and Rasmussen (1999), andMcCutchan et al. (2003). Sulfur appears to have little to n<strong>of</strong>ractionation associated with its movement through trophiclevels. Due to <strong>the</strong> scarcity <strong>of</strong> sulfur fractionation data(McCutchan et al. 2003), <strong>the</strong> typical assumption that <strong>the</strong>re isminimal fractionation <strong>of</strong> sulfur was applied (Peterson andHowarth 1987). A biomass contribution increment <strong>of</strong> 1%and a tolerance interval <strong>of</strong> 0.11‰ was used for all IsoSourceanalyses to insure that <strong>the</strong> full range <strong>of</strong> feasible solutionswas generated (Table 1).RESULTSMacoma (n=12) clustered close to <strong>the</strong> outer edge <strong>of</strong>mixing polygon in close proximity to living and deadSpartina tissue (Figure 1A). Only living tissue <strong>of</strong> Spartinawas considered to be a component <strong>of</strong> <strong>the</strong> diet <strong>of</strong> Macoma inall solutions <strong>of</strong> <strong>the</strong> mixing model with contributions rangingfrom 37-60% (Table 1). Five <strong>of</strong> <strong>the</strong> six productivity sourcessampled could be absent in <strong>the</strong> diet <strong>of</strong> Macoma (Table 1).Four Mya clustered near <strong>the</strong> center <strong>of</strong> <strong>the</strong> mixingpolygon while three o<strong>the</strong>r individuals clustered close toSpartina (Figure 1B). IsoSource indicated that Mya couldreceive 40-59% <strong>of</strong> its diet from living Spartina tissue andTable 1. IsoSource results. Ranges <strong>of</strong> feasible biomass source contributionsfor <strong>the</strong> each bivalve summarized from 34,854 mass balanced mixingmodel solutions (Macoma balthica), 23,996 solutions (Mya arenaria), and56,083 solutions (Mytilus sp.). Each distribution is defined by <strong>the</strong> minimumand maximum values as well as <strong>the</strong> 1 st and 99 th percentiles. The percentilesrepresent 98% <strong>of</strong> all <strong>the</strong> possible IsoSource solutions. The meanpercent <strong>of</strong> each source distribution is cited to show central tendency <strong>of</strong> <strong>the</strong>distribution only and should not be considered a point estimate <strong>of</strong> <strong>the</strong>source contribution to <strong>the</strong> diet <strong>of</strong> <strong>the</strong> bivalve.Source Consumer Biomass %Minimum 1% Mean 99% MaximumSpartina (living leaves) Macoma 37 41 52 59 60Mya 40 44 53 58 59Mytilus 19 24 36 43 44Spartina (standing dead) Macoma 0 0 15 38 46Mya 0 0 8 27 35Mytilus 0 0 11 36 46Distichlis spicata Macoma 0 0 10 26 31Mya 0 0 6 19 24Mytilus 0 0 8 25 32C 3 Vascular Plants Macoma 0 0 5 15 17Salicornia virginica Mya 3 5 14 23 26Grindelia integrifolia Mytilus 5 8 20 32 35Zostera marina Macoma 0 0 9 28 33Mya 0 0 10 32 40Mytilus 0 0 13 42 52Macroalgae Macoma 0 0 9 26 31Fucus spiralis Mya 0 0 9 30 38Mytilus 0 0 12 40 503-26% <strong>of</strong> its diet from C 3 vegetation. All o<strong>the</strong>r sources hadwide ranges <strong>of</strong> potential producer contributions that includedzero as likely dietary contributions (Table 1).Living Spartina biomass and C 3 vegetation were <strong>the</strong>only dietary sources that IsoSource included within all <strong>of</strong> <strong>the</strong>feasible solutions for Mytilus. Living Spartina couldcontribute 19-44% to <strong>the</strong> diet <strong>of</strong> Mytilus, whereas C 3vegetation may contribute 5-35% <strong>of</strong> its diet. All o<strong>the</strong>rpotential sources included 0% as <strong>the</strong> most frequent feasiblecontribution (Table 1). Six <strong>of</strong> <strong>the</strong> seven individuals <strong>of</strong>Mytilus clustered in <strong>the</strong> center <strong>of</strong> <strong>the</strong> mixing polygon whileone individual was more depleted in its δ 34 S signature(Fig. 1C).DISCUSSIONOur mixing models present evidence that Spartinaanglica is consumed by bivalves. The use <strong>of</strong> Spartina byMacoma is particularly convincing based on <strong>the</strong> constrainedmixing model solution and <strong>the</strong> proximity <strong>of</strong> individuals to<strong>the</strong> isotopic signatures <strong>of</strong> living and dead Spartina (Fig. 1A).The mixing model for Mya is also convincing, although <strong>the</strong>δ 13 C values tend to be more depleted than those <strong>of</strong> Macomapotentially indicating smaller Spartina contributions to itsdiet compared to Macoma. With <strong>the</strong> exception <strong>of</strong> oneoutlying individual, Mytilus had <strong>the</strong> most consistent isotopicsignatures for δ 13 C and δ 34 S, and <strong>the</strong> lowest ranges <strong>of</strong>potential Spartina contributions. As a suspension feedingmussel, Mytilus should be drawing its food from <strong>the</strong>- 155 -