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 Spartinagroups affect porewater ammonium and soluble sulfidewithout plants present. The objective <strong>of</strong> <strong>the</strong> secondexperiment was to determine whe<strong>the</strong>r <strong>the</strong>se same speciesinfluenced <strong>the</strong> growth <strong>of</strong> S. alterniflora seedlings. Wepredicted that <strong>the</strong>se three functional groups would havedifferent effects on porewater chemistry resulting indifferential growth <strong>of</strong> S. alterniflora seedlings.METHODSWe conducted microcosm experiments in anenvironmental chamber with a constant temperature <strong>of</strong> 14°Cand a 12-hour light cycle. All organisms and sedimentswere collected from mudflats near <strong>the</strong> edge <strong>of</strong> a hybridSpartina marsh at <strong>the</strong> Elsie Roemer Bird Sanctuary,Alameda Island, San Francisco Bay, California.Microcosms consisted <strong>of</strong> clear polycarbonate tubes (9.3centimeters inside diameter [cm I.D.] x 30 cm height [H])filled with two layers <strong>of</strong> homogenized, defaunated sediment.The lower layer (12 cm) was seived to 500 micrometers(μm) and <strong>the</strong>n frozen for two weeks to kill any organismsthat passed through <strong>the</strong> seive. The surface layer (3 cm),which was seived to 300 μm, but unfrozen, inoculated <strong>the</strong>core with natural micr<strong>of</strong>lora. Following reconstruction, <strong>the</strong>cores acclimated in <strong>the</strong> environmental chamber for threedays. Organic matter (one gram [g] dried, ground Ulva sp.)was added to <strong>the</strong> surface <strong>of</strong> each microcosm one day prior toorganism addition. To simulate tidal inundation, weconstructed an elaborate, automated system that filled eachchamber with sea water (32 parts per thousand [ppt])halfway through <strong>the</strong> light cycle and drained each chamberhalfway through <strong>the</strong> dark cycle each day.In <strong>the</strong> first experiment, we tested <strong>the</strong> effects <strong>of</strong> threefunctional feeding groups (subsurface deposit feeders,surface deposit feeders, and surface grazers) on porewaterammonium and soluble sulfide concentrations. Ourexperimental design consisted <strong>of</strong> a defaunated control andthree single species treatments (n = 6) representing each <strong>of</strong><strong>the</strong> three functional groups: <strong>the</strong> capitellid polychaeteHeteromastus filiformis (sub-surface deposit feeder), <strong>the</strong>nassariid snail Ilyanassa obsoleta (surface grazer) and <strong>the</strong>tellinid clam Macoma petalum (surface deposit feeder) wi<strong>the</strong>ight replicates <strong>of</strong> each treatment (Table 1). The numbers <strong>of</strong>individuals added to each microcosm were equivalent todensities at <strong>the</strong> Elsie Roemer site (see Neira et al. 2005).We extracted porewater from depths <strong>of</strong> 2, 4, and 7 cm usingTable 1: Design for Experiment 1. Density is <strong>the</strong> number <strong>of</strong> individuals forthat taxa per core (68 cm 2 ). N=6 for all treatments.Treatment Functional Group DensityDefaunated control - -Heteromastus filiformis Subsurface deposit feeder 30Ilyanassa obsolete Surface grazer 2Macoma petalum Surface deposit feeder 3a perforated stainless steel sampling probe (Berg andMcGla<strong>the</strong>ry 2001) at <strong>the</strong> termination <strong>of</strong> <strong>the</strong> experiment (14days). Porewater samples were analyzed for ammoniumusing <strong>the</strong> indophenol blue method (Solorzano 1969) and forsulfide using a modification <strong>of</strong> <strong>the</strong> method described byCline (1969). For statistical analyses <strong>of</strong> porewaterparameters, we calculated a mean value for all three depthsfor each replicate and analyzed differences amongtreatments separately for ammonium and sulfide usingANOVA.In <strong>the</strong> second experiment, we used <strong>the</strong> same invertebratetreatments and defaunated control as in <strong>the</strong> first experiment,but added one S. alterniflora seedling to each microcosm.We used nine replicates for defaunated controls and each <strong>of</strong><strong>the</strong> three macroinvertebrate treatments. The seedlings usedin this experiment were germinated in <strong>the</strong> greenhouse usingseeds from inflorescences collected in Willapa Bay,Washington. Thirty days after germination, <strong>the</strong> seedlingswere transferred to environmental chambers and acclimatedto experimental temperature and light regimes while salinity<strong>of</strong> <strong>the</strong> water in sediments was gradually increased from 0 to35 ppt over a period <strong>of</strong> 10 days. Seedlings were transplantedto microcosms after this acclimation period. After 14 days,we measured change in total leaf length (sum <strong>of</strong> length <strong>of</strong>individual leaves) and seedling biomass (aboveground andbelowground). We analyzed differences among treatmentsusing ANOVA.RESULTSFor <strong>the</strong> first experiment, preliminary analysis suggestedthat <strong>the</strong> affects <strong>of</strong> benthic invertebrates on porewaterammonium and sulfide concentrations differed amongfunctional groups. Sulfide concentrations were highest inmicrocosms containing H. filiformis but differences betweentreatments were not significant (p = 0.381; Fig. 1A). Theammonium concentration was highest in <strong>the</strong> control andlowest in microcosms containing M. petalum, but again <strong>the</strong>differences were not significant (p = 0.285; Fig. 1B). Thesurface grazer I. obsoleta had no obvious effects onammonium or sulfide.Overall, in <strong>the</strong> second experiment, seedlingestablishment was relatively poor, and <strong>the</strong>re was visibleyellowing and dehydration <strong>of</strong> leaves in all treatments,particularly microcosms containing H. filiformis. However,our preliminary analysis indicates that <strong>the</strong> surface depositfeeder treatment (M. petalum) had a positive effect onseedling establishment (Fig. 2A, 2B and 2C). In contrast tocontrols that had no significant growth, <strong>the</strong> total leaf lengthincreased approximately seven cm in microcosms containingM. petalum. Post-hoc Tukey tests indicated significantdifferences between M. petalum and H. filiformis (p =0.048). Aboveground and belowground biomass were alsohigher in microcosms containing M. petalum than in o<strong>the</strong>r- 166 -