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 SpartinaA. LIGHTmmol photons m -2 s -1B. BENTHIC CHLOROPHYLL amg Chl a m -225002000150010005000250200150100500MF HYB FOL HYB SAR HYBMF HYB FOL HYB SAR HYBFig. 3. Light measured at <strong>the</strong> sediment surface (3A) and benthicchlorophyll a (3B) measured in mudflats (MF), S. foliosa (FOL), S.pacifica (SAR) and adjacent hybrid areas. Stars represent significantdifferences between areas (p < 0.05) based on ANOVA.during flooding, which can be 25–49% (Holmes and Mahall1982; Pinckney and Zingmark 1993). In spite <strong>of</strong> <strong>the</strong>limitations <strong>of</strong> our current GPP estimates, <strong>the</strong>re is a clear andconsistent decline in microalgal abundance. This decline hasvery important ramifications for higher trophic levels.Infaunal invertebrate abundance and diversity declined 75%upon invasion <strong>of</strong> mudflats at <strong>the</strong> Alameda site (Neira et al.2005). While <strong>the</strong>re are a number <strong>of</strong> factors that maycontribute to this loss, including a lack <strong>of</strong> available space,slower water velocities, and predation (Neira et al. 2005),<strong>the</strong> availability <strong>of</strong> microalgae as a food source is also a keycontributor (Levin et al. 2006; Grosholz et al. 2009).The massive increase in belowground biomass andorganic matter that occurs when mudflats are invaded bySpartina is <strong>the</strong> likely cause <strong>of</strong> higher respiration rates inhybrid habitats. Both microbial and root respiration shouldbe higher in invaded areas. However, higher respirationrates in native vegetation relative to hybrid areas are moredifficult to interpret. We did not observe an increase inbelowground biomass in hybrid habitats relative to S. foliosaor S. pacifica. Therefore, root respiration is likely to besimilar among <strong>the</strong>se three habitats. However, in separatelitterbag experiments, we found that both S. foliosa and S.pacifica decompose more rapidly than hybrid Spartina(Tyler, unpub. data). This is consistent with <strong>the</strong> higher C:Nratio <strong>of</strong> hybrid Spartina relative to native vegetation.Because <strong>the</strong> hybrid detritus is refractory and <strong>the</strong>reby a poorqualityfood source for microbes and invertebrates, wewould expect <strong>the</strong> low sediment respiration rates that we didindeed observe. When decomposition rates are low, organicmatter will remain intact and will build up in <strong>the</strong> system overtime. The build up <strong>of</strong> this detritus is also supported by <strong>the</strong>finding that Spartina detritus does not contribute to highertrophic levels as readily as native plant detritus (Brusati2004; Brusati and Grosholz 2007, 2008).The difference in net sediment metabolism <strong>of</strong> hybridsediments relative to S. foliosa and S. pacifica is drivenlargely by <strong>the</strong> high respiration rates found in native habitats.Hybrid invasion acts to decrease <strong>the</strong> overall metabolic rateand create an excess inventory <strong>of</strong> organic matter. Themudflats, however, switched from net autotrophy to ne<strong>the</strong>terotrophy upon invasion, due to higher respiration andlower microalgal photosyn<strong>the</strong>sis. In this case, <strong>the</strong> invasionacts to eliminate a valuable food source (microalgae) andreplace it with a poor quality food source (Spartina detritus).Future refinements to this model <strong>of</strong> ecosystemmetabolism following Spartina hybrid invasion will includevascular plant production, above-ground decomposition <strong>of</strong>vascular plant detritus and <strong>the</strong> adjustments to microalgalGPP discussed above. From this, we will be able toestimate, on an annual basis, <strong>the</strong> very significant impact <strong>of</strong>invasive Spartina on ecosystem function in San FranciscoBay. At this point, we can conclude that this invasion hasresulted in <strong>the</strong> replacement <strong>of</strong> mudflats and marshescontaining abundant microalgae and bioavailable detritus bya habitat with lower microalgal productivity and refractorydetritus. This change has very important ramifications forhigher trophic levels and for <strong>the</strong> health <strong>of</strong> <strong>the</strong> estuary as awhole.ACKNOWLEDGMENTSFunding for this work came from a NSF BiocomplexityGrant (DEB-0083583). We are particularly grateful to U.Mahl, R. Blake, C. Sorte, N. Christensen and C. Love forfield and laboratory assistance and to <strong>the</strong> C. Goldman lab for<strong>the</strong> use <strong>of</strong> lab facilities.REFERENCESAyres, D.R., D.L. Smith, K. Zaremba, S. Klohr, and D.R. Strong.2004. Spread <strong>of</strong> exotic cordgrasses and hybrids (Spartina sp.) in<strong>the</strong> tidal marshes <strong>of</strong> San Francisco Bay, California, USA.Biological Invasions 6:221-231.Brusati, E.D. 2004. Effects <strong>of</strong> native and hybrid cordgrass onbenthic invertebrate communities and food webs. Ph.D.Dissertation, University <strong>of</strong> California, Davis, Davis, California,USA.Brusati, E.D., and E.D. Grosholz. 2007. Effect <strong>of</strong> native andinvasive cordgrass on Macoma petalum density, growth, and- 138 -