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 Spartinaproductivity that is generally greater than that <strong>of</strong> <strong>the</strong> vascularplants (Zedler 1984). In contrast, primary production inAtlantic and Gulf coast marshes is generally dominated by S.alterniflora (Zedler 1980). Studies <strong>of</strong> salt marsh food webson both <strong>the</strong> Atlantic and Pacific coasts show that microalgaemay contribute up to 50% <strong>of</strong> <strong>the</strong> C assimilated byinvertebrates and higher trophic levels (Page 1995; Deeganand Garritt 1997; Kwak and Zedler 1997; Page 1997). Theimpact <strong>of</strong> Spartina invasion on microalgal productivitywithin vegetated marshes <strong>of</strong> San Francisco Bay is not wellunderstood. However, because algae are more readilyassimilated than detrital Spartina, <strong>the</strong> invasion <strong>of</strong> Spartinamay ultimately reduce <strong>the</strong> availability <strong>of</strong> primary productionto higher trophic levels (i.e., consumers).The results presented here are a preliminary analysis <strong>of</strong><strong>the</strong> impact <strong>of</strong> invasive Spartina on microalgal production,sediment respiration and net sediment metabolism. Futureanalysis <strong>of</strong> <strong>the</strong>se data, in combination with detailed estimates<strong>of</strong> vascular plant production and decomposition rates willultimately give us a more complete understanding <strong>of</strong> howSpartina influences overall ecosystem functioning in <strong>the</strong>intertidal areas <strong>of</strong> San Francisco Bay.METHODSSediment CO 2 fluxes and benthic chlorophyll a (Chl a)were measured in native- and hybrid-dominated areas at foursites in South San Francisco Bay in December 2003 andMarch, June and September 2004 (Fig. 1). Two sites,Cogswell marsh (Hayward Regional Shoreline) and SanMateo marsh, represent areas formerly dominated bypickleweed, S. pacifica. The Elsie Roemer Bird Sanctuaryon Alameda Island and <strong>the</strong> San Lorenzo marsh at Robert’sLanding are mudflats currently being invaded by hybridSpartina. Also at San Lorenzo, areas <strong>of</strong> native S. foliosa areoverrun with hybrid Spartina.Sediment CO 2 fluxes were measured in polycarbonatecores as described in (Neubauer et al. 2000). Briefly,polycarbonate cores (9.3 centimeters inside diameter [cmI.D.]) were inserted 5 cm into <strong>the</strong> sediment (headspaceapproximately 800 cubic centimeters [cm 3 ]). Incurrent andexcurrent tubes were fitted into <strong>the</strong> sealed lid and connectedto a LiCor LI820 Infrared CO 2 Gas Analyzer. Air flow wasmaintained at approximately 500 millileters per minute (mlmin -1 ) using a small electric air pump. Prior to eachsampling period <strong>the</strong> Gas Analyzer was calibrated using CO 2standards (Scott Specialty Gases, 0 parts per million [ppm]and 1,007 ppm CO 2 ). All measurements were conductedbetween approximately 10 am and 2 pm during low tide.Each core was darkened using plastic pots coated withaluminum foil for a minimum <strong>of</strong> 30 minutes prior to makingmeasurements. We recorded CO 2 concentrations in <strong>the</strong> coreevery two seconds for 4-6 minutes in <strong>the</strong> dark and <strong>the</strong>nremoved <strong>the</strong> darkening pot. After waiting ano<strong>the</strong>r 4-6minutes, <strong>the</strong> CO 2 concentrations in <strong>the</strong> light were recorded.SBOSMHBAIALARLAOLACOGALCFig. 1. South San Francisco Bay showing sampling sites: Elsie RoemerBird Sanctuary (ALA), Robert’s Landing (RLA), Cogswell (COG) andSan Mateo (SMH).Three replicates were performed in each area at each site.Fluxes were estimated based on <strong>the</strong> slope <strong>of</strong> <strong>the</strong> change inconcentration over time using <strong>the</strong> equation:dC VJ = •dt Awhere J is <strong>the</strong> flux rate in micromoles per square meter perhour (μmol m -2 h -1 ), A is <strong>the</strong> core area, V is <strong>the</strong> headspacevolume, C is <strong>the</strong> concentration <strong>of</strong> CO 2 and t is time.Light at <strong>the</strong> sediment surface and at <strong>the</strong> top <strong>of</strong> <strong>the</strong>canopy was recorded every two minutes during <strong>the</strong> samplingperiod using a Li-Cor model LI1400 meter with a 4πspherical quantum sensor. Sediment temperature adjacent toeach core was measured using an analog soil temperatureprobe. Following each set <strong>of</strong> measurements two small (1.1cm I.D. x 0.5 cm depth) cores were taken for Chl a analysis.These samples were kept frozen until sonication <strong>of</strong> <strong>the</strong>sediment with cold 90% acetone, overnight extraction in <strong>the</strong>freezer and standard spectrophotometric measurement.Hourly gross primary production (GPP) was calculatedfrom <strong>the</strong> difference between CO 2 fluxes in <strong>the</strong> light and <strong>the</strong>dark by assuming that sediment respiration was <strong>the</strong> same in<strong>the</strong> light and <strong>the</strong> dark. Daily GPP was calculated bymultiplying hourly GPP by <strong>the</strong> numbers <strong>of</strong> hours <strong>of</strong> light on<strong>the</strong> day <strong>of</strong> measurement. Net daily sediment metabolism was- 136 -