African Journal of Marine Science 2012, 34(3): 313–322 Printed in South Africa — All rights reserved Copyright © NISC (Pty) Ltd AFRICAN JOURNAL OF MARINE SCIENCE ISSN 1814-232X EISSN 1814-2338 http://dx.doi.org/ 10.2989/1814232X.2012.725279 African Journal of Marine Science is co-published by NISC (Pty) Ltd and Taylor & Francis Plant traits and spread of the invasive salt marsh grass, Spartina alterniflora Loisel., in the Great Brak Estuary, South Africa JB Adams 1 *, A Grobler 1 , C Rowe 1 , T Riddin 1 , TG Bornman 2 and DR Ayres 3 1 Department of Botany, Nelson Mandela Metropolitan University, PO Box 7700, Port Elizabeth 6031, South Africa 2 South African Environmental Observation Network, Elwandle Node, 18 Somerset Street, Grahamstown 6140, South Africa 3 Evolution and Ecology, 2320 Storer Hall, University of California, Davis, One Shields Avenue, Davis, CA 95616, USA * Corresponding author, e-mail: janine.adams@nmmu.ac.za Spartina alterniflora Loisel., widely recognised as an aggressive invader of estuaries and salt marshes around the world, was discovered growing in the temporarily open/closed Great Brak Estuary on the southern Cape coast of South Africa in 2004. This is the first record of this invasive plant in Africa as well as its first occurrence in an estuary that closes to the sea. Plant traits and sediment characteristics were measured in 2009 and 2011 and found to be comparable to those reported elsewhere. Prior to the 2011 sampling, S. alterniflora stands had been flooded for almost eight months. As a result, sediment redox potential (−268 + 4 mV) was significantly lower in 2011. Sediments were mostly clay in 2009 (71 ± 0.01%) compared to a predominance of sand in 2011 (40 ± 0.02%). These differences were related to the artificial breaching of the estuary one month prior to sampling in March 2011. The grass currently occupies 1.1 ha in the salt marsh, sandflat and mudflat habitats of the estuary where its cover is expanding at a rate of 0.162 ha y –1 . Individual stands numbered about 12 in 2006, but have increased to 24 in 2011. These stands are expanding laterally at 0.9 m y –1 although the long period of inundation during 2010 reduced this to 0.6 m y –1 . Expansion is due to vegetative spread as an analysis of the sediment seed bank showed no S. alterniflora seeds and very few salt marsh seeds (1 132 seeds m –2 ). If left unchecked, S. alterniflora has the potential to replace 42.9 ha or 41% of the total estuary habitat in the Great Brak Estuary, but also has the potential to invade other estuaries in South Africa, especially those with extensive intertidal habitat and containing S. maritima (19 estuaries in total). This study illustrates the adaptive potential of this invasive marsh plant and indicates the possibility of invasion in seasonally closed estuaries in other locations around the world. Keywords: biomass, closed estuary, cordgrass, expansion rate, habitat loss, intertidal habitat Spartina alterniflora is native to the Atlantic and Gulf coasts of North America, occurring as far south as northern Argentina. It is a major invasive grass along the USA Pacific coast, in China and New Zealand (Ayres et al. 1999, Chung 2006). The ecological impact that this species has had on the estuarine environment now overshadows the engineering benefits (e.g. An et al. 2007). It invades open intertidal mudflats, converting them into dense monospecific marshes and results in direct habitat loss (Hedge et al. 2003, Wang et al. 2006). Spartina alterniflora invasions also cause a trophic shift from an algal- to a detritus-based foodweb, owing to belowground biomass of S. alterniflora being five times larger than aboveground biomass (Simenstad and Thom 1995, Levin et al. 2006). As a result, richness and diversity of fish, and shore and wading birds are reduced (Callaway and Josselyn 1992, Daehler and Strong 1996, Neira et al. 2006, Wang et al. 2008). In Willapa Bay, Washington, USA, the introduction and spread of S. alterniflora resulted in a 20% reduction in habitat for aquatic birds. Besides habitat loss, there have also been numerous reports on changes in benthic community abundance and composition (Levin et al. 2006, Neira et al. 2006) and altered trophic function (Chen et al. 2009). In South Africa, Spartina maritima (Curtis) Fernald occurs in 19 of the larger estuaries that are permanently open to the sea (Adams et al. 1999). It forms an important habitat for invertebrates such as the salt marsh crab Sesarma catenata. Worldwide, this species occupies a wide and discontinuous range from Western Europe to North, East and southern Africa. Pierce (1982) reported that S. maritima may have been introduced into South Africa based on Marchant and Goodman’s (1969) proposal that this species had a tropical origin since plants growing in warmer regions were more vigorous than those from temperate regions. However, the origin of this species in South Africa is still unknown (Yannic et al. 2004). Spartina maritima has been replaced extensively in Britain by Spartina anglica (Mobberley 1956, Marchant 1967, Marchant and Goodman 1969, Ayres and Strong 2001). Both S. alterniflora and S. anglica are capable of spreading rapidly due their fertile seeds (Zedler and Kercher 2004), unlike S. maritima which rarely sets seed (Marchant and Goodman 1969). Of the 19 estuaries in South Africa in which S. maritima occurs, intertidal salt marsh area totals 1 264.22 ha (van Niekerk and Turpie 2012), which could be potentially lost through hybridisation with S. alterniflora (Strong and Ayres 2009) and expansion into existing intertidal salt marsh. 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