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 SpartinaASSESSMENT OF THE POTENTIAL CONSEQUENCES OF LARGE-SCALE ERADICATION OFSPARTINA ANGLICA FROM THE TAMAR ESTUARY, TASMANIAM. SHEEHAN 1 AND J.C. ELLISON 21Department <strong>of</strong> Primary Industries, PO Box 3100, Bendigo DC, Victoria, Australia; mat<strong>the</strong>w.sheehan@dpi.vic.gov.au2School <strong>of</strong> Geography and Environmental Studies, University <strong>of</strong> Tasmania, Locked bag 1376, Launceston, Tasmania;Joanna.Ellison@utas.edu.auSpartina anglica is a vigorous exotic perennial salt marsh grass typically inhabiting <strong>the</strong> upperintertidal zone <strong>of</strong> temperate estuaries. Following its introduction into <strong>the</strong> Tamar Estuary, Tasmania,patterns <strong>of</strong> sediment deposition and erosion have been significantly altered. This paper presents <strong>the</strong>background to an interdisciplinary approach used to determine <strong>the</strong> potential impacts <strong>of</strong> <strong>the</strong> widescale eradication <strong>of</strong> S. anglica, from what is now Australia’s largest infestation. Analysis <strong>of</strong> <strong>the</strong>seinterdisciplinary lines <strong>of</strong> inquiry have provided a greater understanding <strong>of</strong> <strong>the</strong> biogeomorphologicalresponses to restoration attempts within intertidal zones and have provided a sound basis on whichto formulate and implement future management <strong>of</strong> Spartina.Keywords: Spartina anglica, Estuaries, salt marsh, intertidal zone, geomorphology, sward, sedimentation,erosion, accretion, eradication, surveyingINTRODUCTIONSpartina anglica, (rice grass or cordgrass) is a vigorousexotic perennial salt marsh grass typically inhabiting<strong>the</strong> upper intertidal zone <strong>of</strong> temperate estuaries. Spartinaanglica influences marsh development through <strong>the</strong> ability <strong>of</strong><strong>the</strong> canopy to promote sediment deposition (Shi et al. 1995),and <strong>the</strong> ability <strong>of</strong> <strong>the</strong> dense rhizomes and roots to increase<strong>the</strong> erosion resistance <strong>of</strong> <strong>the</strong> substrate (Brown 1998a; Brownet al. 1998b; Van Eerdt 1985). These properties, along withits ability to establish and spread rapidly are largely <strong>the</strong>reason for its deliberate introduction to temperate estuariesthroughout <strong>the</strong> world (Chapman 1960; Lee and Partridge1983; Ranwell 1967).Accelerated accretion rates following <strong>the</strong> establishment<strong>of</strong> S. anglica have been observed in <strong>the</strong> Ne<strong>the</strong>rlands (VanEerdt 1985), China (Chung 1990), New Zealand (Lee andPartridge 1983) and in Poole Harbour, United Kingdom(Long et al. 1999). However, compaction and settling havebeen found to be important components <strong>of</strong> sedimentationthat may, in some instances result in negligible change intopographic height <strong>of</strong> mudbanks (Lee and Partridge 1983).Fur<strong>the</strong>rmore, studies <strong>of</strong> Louisiana marshes suggest that rootgrowth and below-ground accumulation <strong>of</strong> organic matter,ra<strong>the</strong>r than inorganic matter governs <strong>the</strong> maintenance <strong>of</strong> saltmarshes in <strong>the</strong> vertical plane (Hatton 1983; Nyman 1995;Turner 2004).This paper provides <strong>the</strong> background to an interdisciplinaryand precautionary study that was conducted between 2002and 2007 as part <strong>of</strong> a PhD research project aimed at assessing<strong>the</strong> impacts <strong>of</strong> <strong>the</strong> wide scale eradication <strong>of</strong> S. anglica from<strong>the</strong> Tamar Estuary, Tasmania. Methodology and results <strong>of</strong>this study have been reported elsewhere (Sheehan & Ellison,2007; Sheehan, 2008).BackgroundThe Tamar Estuary (Fig. 1) extends some 71 kilometers(km) inland from Bass Strait through to its tidal extent at<strong>the</strong> city <strong>of</strong> Launceston. The Tamar is a semi-diurnal mesotidalsystem with tides ranging from 1.3 meters (m) at LowHead, to 4 m at Launceston. The Tamar is well supplied withsediment from <strong>the</strong> South and North Esk Rivers, which, in <strong>the</strong>narrow and poorly flushed estuary, tend to accumulate as finegrainedsilt deposits in <strong>the</strong> upper reaches <strong>of</strong> <strong>the</strong> system bothwithin <strong>the</strong> channel and on adjacent mudflats and shoals. Theaverage rate <strong>of</strong> siltation has been estimated at 30,000 cubicmeters (m 3 ) per year, though this may vary between 10,000and 90,000 depending on variation in river flow (Foster etal. 1986). Sedimentation in <strong>the</strong> Tamar’s upper reaches hasbeen an issue <strong>of</strong> long standing concern, both for reasons <strong>of</strong>shipping channel maintenance and environmental quality.Introduction <strong>of</strong> SpartinaSpartina anglica was introduced in 1947 at Windermere(Fig. 1) in an attempt to stabilize sediments and safeguardagainst fur<strong>the</strong>r siltation <strong>of</strong> <strong>the</strong> shipping channel at a timewhen commercial water traffic travelled <strong>the</strong> length <strong>of</strong> <strong>the</strong>estuary (Phillips 1975; Pringle 1993; Ranwell 1967). Mudflatsand shoals in <strong>the</strong> upper Tamar were prone to severe erosionduring <strong>the</strong> combination <strong>of</strong> flood tide and north-west winds.It was thought that stabilizing <strong>the</strong> mudflats would promotevertical accretion, defining <strong>the</strong> channel to enhance scour, andhence reducing <strong>the</strong> likelihood <strong>of</strong> fur<strong>the</strong>r siltation.In 1997, surveys showed that S. anglica covered some420 hectares (ha) (4.2 km 2 ) <strong>of</strong> intertidal zone within <strong>the</strong>- 129 -