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 SpartinaSPARTINA INVASION CHANGES INTERTIDAL ECOSYSTEM METABOLISM IN SANFRANCISCO BAYA.C. TYLER 1,2 AND E.D. GROSHOLZ 11 Department <strong>of</strong> Environmental Science and Policy, University <strong>of</strong> California, Davis, CA 95616;annachristinatyler@gmail.com2 Current address: School <strong>of</strong> Biological and Medical Sciences, Rochester Institute <strong>of</strong> Technology, Rochester, NY 14623In San Francisco Bay, Atlantic smooth cordgrass, Spartina alterniflora, and its hybrids haveinvaded unvegetated mudflats and native marshes formerly dominated by Sarcocorniapacifica and Spartina foliosa. This recent, rapid invasion has dramatically changedecosystem processes and food web structure. We measured sediment fluxes <strong>of</strong> carbondioxide (CO 2 ) to determine microalgal gross primary production (GPP), sediment respirationand net sediment metabolism in native intertidal areas (mudflats, S. pacifica and S. foliosa)and in adjacent areas invaded by hybrid Spartina. Sediment microalgal GPP and microalgalchlorophyll a were substantially higher in all native habitats than in <strong>the</strong> hybrid. At <strong>the</strong> sametime, sediment respiration rates were generally higher in native vegetation than in hybriddominatedhabitats, but substantially lower on <strong>the</strong> mudflats than elsewhere. Net sedimentmetabolism switched from autotrophy to heterotrophy following invasion <strong>of</strong> mudflats.However, <strong>the</strong> higher respiration rate in native vegetation, especially S. pacifica, relative tohybrid areas, suggests slower decomposition <strong>of</strong> hybrid detritus. This result is corroboratedby litterbag decomposition rates and indicates a build-up and/or export <strong>of</strong> refractory organicmatter following hybrid invasion. The switch from a microalgal-dominated system to arefractory detritus-dominated system has clear implications for support <strong>of</strong> higher trophiclevels within <strong>the</strong> intertidal zone <strong>of</strong> San Francisco Bay.Keywords: ecosystem metabolism, carbon cycling, Spartina alterniflora, Sarcocornia sp.,Spartina foliosa, benthic microalgaeINTRODUCTIONOne <strong>of</strong> <strong>the</strong> most serious threats to natural ecosystemsand <strong>the</strong> maintenance <strong>of</strong> ecosystem services is <strong>the</strong> invasion <strong>of</strong>non-native plant species (Drake et al. 1989; Vitousek et al.1997). These threats may include <strong>the</strong> extinction <strong>of</strong> nativespecies, loss <strong>of</strong> functional native diversity, changes innutrient cycling and organic matter storage and loss <strong>of</strong>habitat. One <strong>of</strong> <strong>the</strong> most dramatic invasions in coastalsystems in western North America has been <strong>the</strong> invasion <strong>of</strong>Spartina alterniflora (smooth cordgrass), a native <strong>of</strong> westernAtlantic marshes. This invasion has fostered large-scalealterations in ecosystem processes in commercially andecologically important estuaries in both California andWashington.S. alterniflora was first introduced into San FranciscoBay by <strong>the</strong> U.S. Army Corps <strong>of</strong> Engineers in 1973 for marshrestoration (Faber 2000). Subsequent hybridization with <strong>the</strong>native cordgrass, Spartina foliosa, resulted in a highlysuccessful hybrid population (henceforth hybrid Spartina)that has colonized ~500 acres, in south and central portions<strong>of</strong> <strong>the</strong> Bay (Daehler and Strong 1997; Ayres et al. 2004).Hybrid Spartina occupies a wide tidal range that includeshistorically unvegetated mudflats and native marshes oncedominated by S. foliosa and Sarcocornia pacifica (Callawayand Josselyn 1992).The Spartina invasion has greatly altered <strong>the</strong> cycling <strong>of</strong>carbon (C) in <strong>the</strong> intertidal zone. Hybrid Spartina formsdense clones, with an average aboveground biomass <strong>of</strong> 1.8 ±0.3 kilograms per square meter (kg m -2 ) at <strong>the</strong> end <strong>of</strong> <strong>the</strong>growing season (fall; Tyler et al. 2007). Much <strong>of</strong> <strong>the</strong>aboveground portion senesces during <strong>the</strong> winter, but <strong>the</strong>substantial belowground roots and rhizomes (mean = 4.2 ±0.8 kg m -2 ) persist year-round (Tyler et al. 2007). Thecarbon to nitrogen (C:N) ratio <strong>of</strong> Spartina is higher than <strong>the</strong>native vegetation, particularly when compared tomicroalgae, and <strong>the</strong> slow decomposition <strong>of</strong> this refractorydetritus has resulted in substantial deposits <strong>of</strong> belowgroundorganic matter (Neira et al. 2005). We do not know howoverall ecosystem production and respiration are changed asa result <strong>of</strong> this invasion, but this is clearly important inunderstanding <strong>the</strong> overall impact <strong>of</strong> invasion on estuarineecosystem function.The lower productivity and low stature <strong>of</strong> nativesou<strong>the</strong>rn California salt marsh plants, which results in morelight reaching <strong>the</strong> sediment surface, may promote microalgal- 135 -