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 SpartinaIMPACTS OF SPARTINA INVASIONThe introduction <strong>of</strong> S. anglica to <strong>the</strong> Tamar Estuary hasbrought about a dramatic and rapid change to <strong>the</strong> physiography<strong>of</strong> <strong>the</strong> intertidal zone, illustrated well at a photo pointestablished in 1956 at Paper beach (Fig. 4a and b). The colonization<strong>of</strong> what was essentially a vacant niche has transformed<strong>the</strong> gently grading sandy intertidal zones, as shown in Fig. 5,and hard rock intertidal zones (Fig. 6) into laterally extensivemuddy terraces (Figs. 7 and 8).The ecological impacts <strong>of</strong> Spartina invasion in temperateestuaries <strong>of</strong> South Eastern Australia are poorly understood.Studies <strong>of</strong> benthic macro-invertebrate communities in <strong>the</strong>Little Swanport Estuary, Tasmania suggest that Spartinasignificantly increases macro-invertebrate species richnessand total species abundance when compared to previouslynon-vegetated intertidal areas (Hedge 1997). Additionally,macro-invertebrate communities <strong>of</strong> Spartina marshesshowed remarkable similarity to those <strong>of</strong> native salt marshes(Hedge and Kriwoken 2000). It is suggested <strong>the</strong>refore thatSpartina invasion has provided a niche <strong>of</strong> s<strong>of</strong>t substrate anddense protection, favoring macro-invertebrates and somebirds such as <strong>the</strong> purple swamphen (Porphyrio porphyrio).It is likely that this subsequently caused a displacement <strong>of</strong>species that formerly inhabited or utilized <strong>the</strong> intertidal zone.Species assemblage and utilization <strong>of</strong> <strong>the</strong> intertidal zone priorto Spartina is not well documented, but likely was relativelyrich in fish and bird species.DISCUSSIONThis paper has provided background for <strong>the</strong> researchproject that assessed <strong>the</strong> alteration to <strong>the</strong> intertidal zone <strong>of</strong> <strong>the</strong>Tamar Estuary. Using transect based topographic surveys andcoring, <strong>the</strong> volume <strong>of</strong> material trapped under Spartina wascalculated to be 1,193,441 m 3 , comprised <strong>of</strong> approximately17% Spartina-derived organic matter and 83% silts andclays. Based on historical pr<strong>of</strong>iles, sedimentation rates since<strong>the</strong> introduction <strong>of</strong> S. anglica have been estimated at between8.7 and 52.4 millimeters per year (mm yr-1).From <strong>the</strong> analysis <strong>of</strong> 80 cores from four sites, Spartinatrappedsediment was found to contain levels <strong>of</strong> cadmium,copper, lead and zinc elevated above background levels.However, <strong>the</strong>se generaly were below trigger values <strong>of</strong> <strong>the</strong>ANZECC/ARMCANZ (2000) interim sediment qualityguidelines. It is considered unlikely that released sedimentswould impact on water quality or health <strong>of</strong> biota with respectto trace metals or organic contamination.Behaviour <strong>of</strong> sediment with respect to erosion rates,sediment redeposition and causative hydrodynamics werealso monitored within a test area from which Spartina coverwas removed. It has been demonstrated that <strong>the</strong> eradication<strong>of</strong> S. anglica will result in elevation loss from <strong>the</strong> Spartinamarsh surface at a rate six times greater than in vegetatedmarshes. The study assumed that this elevation loss is causedby liberation <strong>of</strong> sediments. The rate <strong>of</strong> elevation loss orerosion is likely to increase once <strong>the</strong> dead S. anglica root matdecomposes and <strong>the</strong> surface cohesion and sediment-bindingcapacity is diminished. Elevation loss increases by a factor <strong>of</strong>1.06 with every 10 m from <strong>the</strong> high water bank. Erosion ratesin <strong>the</strong> outer 40 m <strong>of</strong> <strong>the</strong> marsh were also significantly greaterthan <strong>the</strong> remainder <strong>of</strong> <strong>the</strong> marsh at both sites, suggesting thata process o<strong>the</strong>r than S. anglica removal is contributing to thatretreat <strong>of</strong> <strong>the</strong> lower marsh.Analysis <strong>of</strong> <strong>the</strong>se interdisciplinary lines <strong>of</strong> inquiryhave allowed for a greater understanding <strong>of</strong> <strong>the</strong>biogeomorphological responses to restoration attempts withinintertidal zones and have provided a sound basis on which t<strong>of</strong>ormulate and implement future management <strong>of</strong> Spartina.Spartina eradication is recommended for <strong>the</strong> lower estuary(type 2 marshes) only, where trapped sediment volumesare significantly smaller and tidal flushing is greatest. Thiswould enable significant areas <strong>of</strong> sand/gravel intertidal zonesto recover and reduce <strong>the</strong> likelihood <strong>of</strong> fur<strong>the</strong>r downstreamexpansion <strong>of</strong> Spartina swards. Retaining type 1 marshes in<strong>the</strong> upper estuary will prevent <strong>the</strong> remobilization <strong>of</strong> sedimentsthat contain <strong>the</strong> highest concentrations <strong>of</strong> contaminants, andwill retain <strong>the</strong> marshes for <strong>the</strong> ecological role <strong>the</strong>y currentlyperform.ACKNOWLEDGMENTSThis study was funded by <strong>the</strong> Australian ResearchCouncil Linkage Grant LP0214145, with support from <strong>the</strong>Department <strong>of</strong> Primary Industries and Water, Tasmania and<strong>the</strong> Rice Grass Advisory Group.REFERENCESANZECC/ARMCANZ, 2000. Australian and New Zealand guidelinesfor fresh and marine water quality. 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