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 SpartinaIMPACTS OF BENTHIC INVERTEBRATES ON SEDIMENT POREWATER AMMONIUM ANDSULFIDE:CONSEQUENCES FOR SPARTINA SEEDLING GROWTHU.H. MAHL 1,2 ,A.C.TYLER 1,3 AND E.D. GROSHOLZ 11 Department <strong>of</strong> Environmental Science and Policy, University <strong>of</strong> California, Davis, CA 956162 Current address: Department <strong>of</strong> Biology, University <strong>of</strong> Notre Dame, South Bend, IN 46556; umahl@nd.edu3 Current address: School <strong>of</strong> Biological and Medical Sciences, Rochester Institute <strong>of</strong> Technology, Rochester, NY 14623Since its introduction in <strong>the</strong> 1970s, Spartina alterniflora and its hybrids have rapidly invaded <strong>the</strong>intertidal zone <strong>of</strong> San Francisco Bay. We know relatively little about <strong>the</strong> biotic and abiotic factorsthat have ei<strong>the</strong>r facilitated or hindered this invasion. In its native range, nitrogen availability andsediment anoxia are known to limit <strong>the</strong> growth <strong>of</strong> S. alterniflora. Benthic invertebrates may alterporewater concentrations <strong>of</strong> both ammonium and soluble sulfide, and <strong>the</strong>reby indirectly influence S.alterniflora success. In order to better understand how <strong>the</strong> resident macr<strong>of</strong>aunal invertebratecommunity may impact <strong>the</strong> expansion <strong>of</strong> S. alterniflora into <strong>the</strong> intertidal zone, we examined howcommon invertebrates influence concentrations <strong>of</strong> ammonium and soluble sulfide in sedimentporewater. We conducted laboratory microcosm experiments to determine how species from threefunctional feeding groups <strong>of</strong> benthic invertebrates (subsurface deposit feeders, surface depositfeeders and surface grazers) affect porewater in unvegetated areas adjacent to Spartina-invadedareas. In a separate experiment, we examined <strong>the</strong> growth <strong>of</strong> S. alterniflora seedlings in <strong>the</strong> presence<strong>of</strong> individual invertebrates. Relative to microcosms without invertebrates, where porewaterammonium was fairly high (greater than 400 micromoles per liter [μM]), we found slightly lowerporewater ammonium in microcosms with Heteromastus filiformis (subsurface deposit feeder) orMacoma petalum (surface deposit feeder). Porewater sulfide was slightly higher in <strong>the</strong> presence <strong>of</strong>H. filiformis only. The significantly greater growth <strong>of</strong> S. alterniflora seedlings in <strong>the</strong> presence <strong>of</strong> M.petalum than in <strong>the</strong> presence <strong>of</strong> H. filiformis suggests that both high sulfide and high ammoniummay have been detrimental to seedling success. Thus, invertebrates may indirectly influence <strong>the</strong>success <strong>of</strong> S. alterniflora seedlings by altering porewater chemistry.Keywords: ammonium, benthic, functional group, invasive species, porewater, Spartina, sulfideINTRODUCTIONAmong <strong>the</strong> greatest threats to natural ecosystems areinvasions by non-native species (Drake et al. 1989;Vitousek et al. 1997). However, <strong>the</strong> mechanisms thatdetermine <strong>the</strong> success or failure <strong>of</strong> new invasions are <strong>of</strong>tenpoorly understood. The invasion <strong>of</strong> Pacific Coast estuariesby Spartina alterniflora and its hybrids (S. alterniflora xnative Spartina foliosa) provides some insight into howSpartina can rapidly change benthic communities throughaltering physico-chemical properties <strong>of</strong> <strong>the</strong> habitat. Theshift from native mudflats and relatively diverse uppermarsh habitats to dense Spartina meadows followinginvasion has pr<strong>of</strong>ound effects on <strong>the</strong> functional diversity <strong>of</strong>benthic invertebrates, nutrient cycling, and o<strong>the</strong>r physicalprocesses (Neira et al. 2005, 2006; Levin et al. 2006).However, we still have little information on what biotic andabiotic factors may contribute to <strong>the</strong> great success <strong>of</strong>Spartina in <strong>the</strong> intertidal zone <strong>of</strong> San Francisco Bay.Determining which factors most strongly influence seedlingestablishment will greatly improve our understanding <strong>of</strong> <strong>the</strong>dynamics <strong>of</strong> <strong>the</strong> invasion.Previous work has shown that nutrient limitation andanoxia affect <strong>the</strong> growth and survival <strong>of</strong> native S.alterniflora in Atlantic coast salt marshes (e.g., Gallagher1975; King et al. 1982; DeLaune et al. 1983; Dai andWiegert 1997) and that nutrient limitation may be especiallysevere during <strong>the</strong> early stages <strong>of</strong> marsh development in bothnative and invaded marshes (Tyler et al. 2003, 2007).Benthic invertebrates can significantly impact <strong>the</strong>concentration <strong>of</strong> solutes in porewater and nutrient cyclingthrough bioturbation, burrow ventilation and consumption <strong>of</strong>organic matter (e.g. Aller 1982, Peterson and Heck 1999;Christensen et al. 2000). However, research has not beendone to determine whe<strong>the</strong>r <strong>the</strong> effects <strong>of</strong> benthiccommunities on <strong>the</strong> concentration <strong>of</strong> nutrients (e.g.,ammonium) and toxic metabolites (e.g., soluble sulfide) inporewater can affect <strong>the</strong> success <strong>of</strong> invasive plants like S.alterniflora.As part <strong>of</strong> an investigation linking benthic communitystructure to <strong>the</strong> establishment <strong>of</strong> S. alterniflora seedlings, weperformed two pilot microcosm experiments. The objective<strong>of</strong> <strong>the</strong> first experiment was to determine whe<strong>the</strong>r commonbenthic invertebrates from three different functional feeding- 165 -