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 SpartinaFig. 1. Dual isotope (δ 34 Sand δ 13 C) scatterplots <strong>of</strong> <strong>the</strong> three bivalves andsix producers sampled at West Pass, Island County, Washington. Eachopen circle is an individual consumer. A. Macoma balthica (n=12). B.Mya arenaria (n=7). C. Mytilus sp. (n=7). Producers (mean±SE) are <strong>the</strong>same for each panel: C 3 (pooled values <strong>of</strong> Salicornia virginica and Grindeliaintegrifolia: diamond), Fucus spiralis (closed circle), Zostera marina(square), Distichlis spicata (x), Standing dead Spartina (inverted triangle),Living Spartina leaves (triangle). Source sample sizes range from n=3(Zostera) to n=7 (C 3 plants). Error bars are plotted for SE values > 1 ‰.predominant sources <strong>of</strong> carbon available in <strong>the</strong> seston(Ruckelshaus et al. 1993). However, <strong>the</strong> consumption <strong>of</strong>suspended particulate organic matter (SPOM) by <strong>the</strong>sebivalves at West Pass during additional 2003 samplingperiods was surprisingly inconclusive despite using δ 13 C,δ 15 N and δ 34 S to simultaneously estimate dietarycontributions (Hellquist 2005).Previous trophic studies have provided a variety <strong>of</strong>conclusions about <strong>the</strong> importance <strong>of</strong> detrital Spartina inestuarine food webs. These results differ depending onconsumer trophic status and location. For example, filterfeeders including Geukensia demissa (ribbed mussel;Peterson et al. 1985, 1986; Peterson and Howarth 1987),Crassostrea virginica (oyster; Peterson et al. 1986) and Myaarenaria (Peterson et al. 1986; Peterson and Howarth 1987)have been identified as using Spartina biomass in <strong>the</strong>ir diets.The mud snail (Ilyanassa obsoleta) which is a deposit feederalso appears to use Spartina in Massachusetts and NorthCarolina (Peterson et al. 1986; Peterson and Howarth 1987;Currin et al. 1995). Spartina species also contribute to <strong>the</strong>diets <strong>of</strong> fish in Massachusetts (Peterson et al. 1986) andsou<strong>the</strong>rn California (Kwak and Zedler 1997).However, in a study similar to those above, <strong>the</strong> role <strong>of</strong>vascular plants in <strong>the</strong> diets <strong>of</strong> over 50 consumers wasconsidered minimal or nonexistent in a Mississippi estuary(Sullivan and Moncreiff 1990). Instead, macroalgae andzooplankton appeared to be <strong>the</strong> major dietary components <strong>of</strong><strong>the</strong> consumers sampled. Even Geukensia demissa andCrassostrea virginica shown by Peterson et al. (1986) to useSpartina biomass seemed to have little input from vascularplant productivity (Sullivan and Moncreiff 1990).In France, S. anglica appears to be an unused source <strong>of</strong>productivity for bivalves (Riera et al. 1999). Despite <strong>the</strong>availability <strong>of</strong> S. anglica detrital matter, bivalves (Macomabalthica and Mytilus edulis) relied on suspended particulateorganic matter and benthic diatoms as <strong>the</strong>ir primary dietarycomponents. Mytilus edulis had a diet dominated by 65%phytoplankton and 35% diatoms (Riera et al. 1999) whereas<strong>the</strong> diet <strong>of</strong> M. balthica was estimated to be composed <strong>of</strong>76% benthic diatoms and 24% phytoplankton. However,Jackson et al. (1986) used δ 13 C to estimate <strong>the</strong> dietarycontribution <strong>of</strong> S. anglica to M. balthica and <strong>the</strong>ir datasuggested that 34-50% <strong>of</strong> assimilated biomass consumed byM. balthica was Spartina, for a total estimate <strong>of</strong> 0.2-0.3grams <strong>of</strong> carbon per square meter per year (g C m -2 yr -1 )assimilated.Relatively few studies <strong>of</strong> Puget Sound trophic dynamicshave been conducted (e.g., Simenstad and Wissmar 1985;Ruckelshaus et al. 1993). In sou<strong>the</strong>rn Puget Sound estuariesand littoral beaches, detrital carbon was shown to originateprimarily from Zostera spp., epiphytic algae, andmacroalgae (Simenstad and Wissmar 1985). Our data alsoindicate small contributions to bivalve diets from nativemarsh plants with C 3 isotopic signatures (Table 1).Distichlis spicata, a C 4 salt marsh plant that grows inabundance adjacent to <strong>the</strong> Spartina meadows at West Pass,apparently contributes very little to bivalve diets (Table 1).The small contributions <strong>of</strong> Zostera marina to bivalve diets isprobably a function <strong>of</strong> its low abundance in <strong>the</strong> immediatevicinity <strong>of</strong> <strong>the</strong> sampling location.This study indicates that S. anglica biomass is a locallyimportant component <strong>of</strong> bivalve diets in nor<strong>the</strong>rn PugetSound after being in <strong>the</strong> ecosystem for just over 40 years.The extent that Spartina may contribute to bivalve diets inour study is somewhat unexpected due to <strong>the</strong> recalcitrant- 156 -