Invasive cordgrass (Spartina spp.) in south-eastern Australia induces island formation, salt marsh development, and carbon storage David M. Kennedy,1* Teresa Konlechner,1 Elisa Zavadil,1 Michela Mariani,1 Vanessa Wong,2 Daniel Ierodiaconou3 and Peter Macreadie4 1School of Geography and National Centre for Coasts and Climate, The University of Melbourne, Parkville, Victoria, Australia 2School of Earth, Atmosphere and Environment, Monash University, Clayton, Victoria, Australia 3School of Life and Environmental Sciences, Faculty of Science, Engineering and Built Environment, Victoria, Australia 4Deakin University, School of Life and Environmental Sciences, Centre for Integrative Ecology, Faculty of Science, Engineering and Built Environment, Burwood, Australia *Corresponding author. Email: [email protected] Received 2 August 2017 • Revised 21 September 2017 • Accepted 23 September 2017 Abstract Invasive vegetation species can lead to major changes in the geomorphology of coastal systems. Within temperate estuaries in the southern hemisphere, espe- cially Australia and New Zealand, the cordgrass Spartina spp. has become established. These species are highly invasive, and their prolific growth leads to the development of supratidal environments in formerly intertidal and subtidal environments. Here, we quantified the impact of Spartina invasion on the geomorphology and sequestration capacity of carbon in the sediments of Anderson Inlet, Victoria, Australia. Spartina was first introduced to the area in the 1930s to aid in land reclamation and control coastal erosion associated with coastal development. We found that Spartina now dominates the intertidal areas of the Inlet and promotes accretion (18 mm/year) causing the formation of over 108 ha of supratidal islands over the past 100 years. These newly formed islands are calculated to potentially contain over 5.5 million tonnes of CO2 equivalent carbon. Future management of the inlet and other Spartina-dominated environ- ments within Australian presents a dilemma for resource managers; on the one hand, Spartina is highly invasive and can outcompete native tidal marshes, thereby warranting its eradication, but on the other hand it is likely more resilient to rising sea levels and has the potential for carbon sequestration. Whether or not the potential advantages outweigh the significant habitat change that is antici- pated, any management strategies will likely require additional research into costs and benefits of all ecosystem services provided by Spartina including in relation to nutrient cycling, shoreline stabilisation, and biodiversity as well as in response to the longevity of carbon found within the sediments. Keywords ecosystem services; blue carbon; Spartina; salt marsh; sea level rise; invasive species Geographical Research • February 2018 • 56(1): 80–91 80 doi:10.1111/1745-5871.12265 D.M. Kennedy et al., Cordgrass salt marsh 81 Introduction Estuary, Tasmania, Australia, led to accretion of a supratidal marsh over previously bare subtidal Salt marshes are commonly found on the margins mud flats (Hedge & Kriwoken, 2000; Kriwoken of estuaries and are characterised by halophytic & Hedge, 2000). In this study, we explore the vegetation that is inundated by the highest tides impact of invasive grass species (Spartina)within (Allen, 2000). Their evolution is closely related Venus Bay contained within a shallow barrier- to sea level and tidal inundation; however, the estuary in Victoria, Australia. Through subsurface precise elevation of a salt marsh within the tidal coring and aerial photo analysis, we assess the prism is the product of a complex interaction of positives and negative environmental impacts of biophysical factors related to allochthonous and invasive-species driven habitat change. autochthonous sedimentation, vegetative commu- nities, and ground water (Rogers, Saintilan & Regional setting Woodroffe, 2014). Salt marshes are therefore not passive environments, and they interact with wider Venus Bay has an approximate surface area of estuarine environments through both vertical and 3.4 km2 located in the south eastern part of lateral accretion. It is their ability to accrete which Anderson Inlet in West Gippsland, Victoria, is most important for their continued presence Australia (Figure 1). Anderson Inlet is predomi- under rising sea level, rather than simply their nantly submarine, with sediment movement asso- elevation in relation to sea level (Lovelock et al., ciated with migrating tidal channels and 2014; Reef et al., 2017). movement of the tidal deltas at the estuary The main external controls on salt marsh mouth. The estuary is 10 km long, being the wid- development are tidal regimes as well as sediment est (2–3 km) in its central regions. It is separated supplies (Allen, 2000). There must also be from the open ocean by a beach-barrier system sufficient accommodation space for marshes to estimated to be 4,500 years old (Li, Gallagher accrete, with the highest tides being the principle & Finlayson, 2000). The estuary is therefore boundary condition. Rates of accretion on the classified as a partially infilled barrier estuary other hand are determined by rates of sediment according to the scheme of Roy (1984). The supply, which can be derived from both organic coastal plain surrounding the inlet is characterised and inorganic sources (Allen, 2000). Vegetation by three terraces ranging up to 6 m elevation. The is a key element as it can both enhance deposition highest corresponds to the last interglacial period of suspended sediment as well as directly contrib- (c. 125 ka) and the lower two, both <2meleva- ute biomass (Reed, 1995). In fact, tidal marshes are tion, relating to early–mid Holocene higher sea among the most efficient ecological systems for levels (Li et al., 2000). the storage of organic carbon (Duarte et al., The coast of Victoria is microtidal with a semi- 2013; McLeod et al., 2011; Pidgeon, 2009), with diurnal spring range of 1.1 m at Port Philip Heads salt marsh ecosystems ranking the highest in (Port of Melbourne, 2013). The tidal prism with organic carbon storage potential among all coastal Anderson Inlet is 22.66 × 106 m3 (McSweeney, wetland and forested terrestrial ecosystems Kennedy, & Rutherfurd, 2017). The mean signifi- (Ouyang & Lee, 2014). The efficiency of carbon cant wave height for the Victorian coast is 2.4 m accumulation is related to the burial of organic with a period of 8.4 s (Hughes & Heap, 2010). material from accreting sediments and the Modelling of open-ocean waves in Victoria presence of anaerobic conditions, which decrease indicates the mean annual wave height for Cape rates of organic matter decomposition (Hedges & Paterson is 1.8 m (WaterTech, 2004). Wave data Keil, 1995; Kristensen, Ahmed & Devol, 1995; within Anderson Inlet are not recorded. Mean Kristensen et al., 2008). annual maximum and minimum air temperatures As marshes’ evolution is dependent on their for Cape Paterson are 9.6–18.8°C with a mean vegetative ecosystems, changes within these habi- annual rainfall of 939 mm (BoM, 2012). tats can have major geomorphic implications. For Spartina (cordgrass) is a genus of approxi- example in Australia, Western Port, Victoria mately 17 rhizomatous perennial grass species (Rogers, Saintilan & Heijnis, 2005) and Tweed found in temperate estuarine and salt marsh envi- River, New South Wales (Rogers et al., 2014), ronments. Spartina spp. are efficient colonisers of changes in tidal prism associated with sea level rise marine habitats renowned for their ability to stabi- have led to mangrove colonisation of salt marsh lise estuarine surfaces, promote accretion, and the habitat. At lower elevations within the tidal prism, conversion of intertidal flats into supratidal mono- the introduction of Spartina grass in the Tamar cultures. Spartina was widely planted for erosion © 2017 Institute of Australian Geographers 82 Geographical Research • February 2018 • 56(1): 80–91 Figure 1 Venus Bay is located within Anderson Inlet in southern Victoria. (a) Anderson Inlet is one of the most southernmost barrier estuaries on the Australian mainland, occurring on the southern coast of Victoria. (b) Venus Bay (white box) is the area where Spartina grass has the most impact and is the focus of this study control and reclamation in many temperate regions Methods because of these attributes (An et al., 2007; Hacker et al., 2001). The impact of Spartina was evaluated within a Two species of Spartina, Spartina x townsendii 280 ha section of Venus Bay (Figure 1). Airborne and Spartina anglica were introduced to coastal Light Detection and Ranging (LiDAR) data were Victoria (Williamson, 1996). S. x townsendii,a collectedin2007bytheVictorianGovernment’s sterile hybrid of American and English cordgrass Department of Environment and Primary Indus- species, was first introduced to the study site in tries. The surveying was conducted using a LADS the 1930s (Williamson, 1996). The second species, Mk II system coupled with a GEC-Marconi S. anglica, is the fertile product of chromozonal FIN3110 inertial motion sensing system and a dual doubling of S. x townsendii (Hacker et al., 2001). frequency kinematic global positioning system S. anglica was introduced to the study site in the (kGPS). This dataset was processed to produce a 1960s (Williamson, 1996). S. anglica is probably seamless terrestrial–marine mosaic from eleva- the dominant species today, however, the two spe- tions of +10 m to depths of À25 m with a final ras- cies are difficult to distinguish and there