Eco-engineering of modified shorelines recovers wrack subsidies
Graphical abstract
Introduction
Many intertidal communities, including those of tidal flats, mangroves, saltmarshes, sandy beaches and rocky shores, are dependent on accumulations of wrack (stranded phyto-detritus), (Dugan et al., 2003; Orr et al., 2005; Olabarria et al., 2010). Wrack provides food and structural habitat for invertebrates which, in turn, fuel predatory fish and shorebirds, and is an important component of carbon and nutrient cycles (Thompson et al., 2002; Spiller et al., 2010; Dugan et al., 2011b). Wrack can be transported across habitat boundaries by wind and water and may contain detritus from both terrestrial and marine primary producers (Polis et al., 1997; Bartels et al., 2012). The accumulation of wrack on shorelines is dependent on rates of detrital production by donor habitats, transport processes that deliver wrack, as well as structural (e.g. vegetation, cobbles) and physical (e.g. wave energy, tidal regime, shoreline slope) features of the recipient habitat that affect wrack retention (Orr et al., 2005). Hence, human activities that modify the identity and productivity of primary producers, that change coastal circulation and drainage patterns, and/or that alter structural features of shorelines have the potential to have large impacts on wrack pathways and the food webs they support (Kirkman and Kendrick, 1997; Defeo et al., 2009; Dugan et al., 2011a).
Shoreline hardening or coastal armouring (i.e. the construction of artificial structures for coastal defence) is increasing in response to burgeoning human population growth along the coast (Gittman et al., 2015; Firth et al., 2016). It is estimated that in some coastal areas >50% of the natural nearshore habitats have been replaced by coastal defence structures that protect reclaimed land, coastal property and infrastructure from erosion and inundation (Lam et al., 2009; Dafforn et al., 2015; Gittman et al., 2015; Firth et al., 2016). These artificial structures are often placed in transition zones, such as the intertidal, serving as physical barriers to the movement of organisms, energy and materials between terrestrial and aquatic environments (Bishop et al., 2017). Coastal defence structures typically have less habitat complexity as compared to the natural habitats they replace (Bulleri and Chapman, 2010). When placed parallel to the shoreline below the high water mark, their predominately vertical orientation can reduce the width of the intertidal zone (Sobocinski et al., 2010; Heatherington and Bishop 2012; Harris et al., 2014; Heerhartz et al., 2014) and may also impact adjacent marine habitats by increasing current strength and wave reflection (Dugan et al., 2011a). The net effect can be reduced intertidal accumulations of wrack on armoured shorelines (Sobocinski et al., 2010; Heatherington and Bishop, 2012; Harris et al., 2014; Heerhartz et al., 2014) which in turn support reduced abundances and altered assemblages of invertebrates (Dethier et al., 2016), with cascading impacts through higher trophic levels (Sobocinski et al., 2010; Heatherington and Bishop, 2012; Harris et al., 2014; Heerhartz et al., 2014).
Marine eco-engineering seeks to mitigate the impacts of artificial structures to biodiversity (Chapman and Underwood, 2011; Firth et al., 2016; Strain et al., 2017), ecological connectivity (Airoldi et al., 2005; Bishop et al., 2017) and ecosystem functioning (Morris et al., 2017). At smaller scales of millimetres to meters, eco-engineering may include modification of the design of new structures (Chapman and Underwood, 2011; Perkol-Finkel et al., 2017), or increasing the roughness, texture and/or microhabitats of existing structures, through additive (i.e. retrofit with artificial or natural habitats) or subtractive (i.e. grout removal, drilling) processes (Strain et al., 2017). Larger-scale interventions (meters to 100s of meters) typically involve partially or wholly removing the need for artificial structures by planting coastal habitats, including mangroves, saltmarshes, wetlands (Currin et al., 2008; Currin et al., 2010; Gedan et al., 2011) or biogenic reefs (Piazza et al., 2005; Scyphers et al., 2011) that stabilize sediments and/or dampen wave energy. Studies have addressed the efficacy of these interventions in enhancing biodiversity (Chapman et al., 2017; Strain et al., 2017; Toft et al., 2017), but their effects on ecological functions such as connectivity between land and water and the resulting accumulation of wrack remains untested. Where these interventions add structural elements to the intertidal zone, or restore its width, they may enhance retention and accumulation of wrack on intertidal shorelines relative to traditional coastal armouring.
Here we examine the role of 2 eco-engineered intertidal habitats in enhancing the cover and biomass of wrack in a heavily urbanised estuary in southeast Australia. In this region, much of the natural intertidal shoreline has been replaced by seawalls. However at 3 sites the seawalls have been eco-engineered to include man-made rockpools in the low to mid intertidal zone and/or planted saltmarshes in the mid tidal zone (Heath and Moody, 2013). We tested the following hypotheses: 1) on armoured shorelines, wrack accumulations will be confined to the low intertidal zone; 2) armoured shorelines will support less wrack accumulations than the natural shorelines (rockpools, rocky shores, sandy beaches); shorelines that have been eco-engineered will 3) have greater accumulations and retention of wrack than armoured shorelines, and 4) similar accumulations and retention of wrack to analagous natural habitats. This study was conducted across 4 sites with 2 examples of each eco-engineered habitat to assess their importance for enhancing the cover and biomass of wrack in locations that are exposed to different levels of wind and waves.
Section snippets
Study site
The study was done in Kogarah Bay (Fig. 1), a heavily modified embayment of the Georges River estuary, New South Wales, Australia, 15 km south of Sydney city. The Bay is approximately 2300 m in length, 700 m in width and 3 m in depth (Alyazichi et al., 2015), with adjacent land-use a mixture of residential and urban parkland. Kogarah Bay is characterised by semi-diurnal tides, with a spring tidal range of 2 m.
Sampling and wrack deployments were done at 4 sites, across which there were 2
Results
Across all time points, the cover and biomass of terrestrial and marine wrack showed a significant negative relationship with water motion and a weak positive relationship with wind speed (Tables S1-S3). Wrack was predominantly comprised of a mixture of leaves and bark (together 51.61% of total dry weight) and sticks (34.65%) from terrestrial plants, as well as seagrass blades (13.28% from Posidonia australis and Zostera capricorni combined).
Discussion
This is the first study to quantify the effects on organic matter subsidies of eco-engineering interventions to armoured shorelines. Like previous studies (Sobocinski et al., 2010; Heatherington and Bishop, 2012; Harris et al., 2014; Heerhartz et al., 2014), we found smaller wrack accumulations on shorelines with than without coastal armouring. Man-made rockpools and planted saltmarsh increased wrack accumulations on armoured shorelines by enhancing the retention of wrack. Hence,
Acknowledgements
We thank the many people that helped in collecting, and processing the wrack samples, in particular, Ben Lucas and Vinicci Cheng. We thank the Editor and the 2 anonymous reviewers for their comment an earlier draft of the manuscript. We also thank the The Ian Potter Foundation, Harding Miller Foundation, The New South Wales Government Office of Science and Research and the Coastal Node of the NSW Office of Environment and Heritage Adaptation Hub for their financial support. This study was part
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