Elsevier

Ecological Engineering

Volume 112, March 2018, Pages 26-33
Ecological Engineering

Eco-engineering of modified shorelines recovers wrack subsidies

https://doi.org/10.1016/j.ecoleng.2017.12.009Get rights and content

Highlights

  • Seawalls act as barriers to land-sea organic matter (wrack) subsidies.

  • Intertidal placement of seawalls eliminates high-intertidal wrack accumulation.

  • Less wrack is also retained in front of seawalls than on unmodified shorelines.

  • Man-made rockpools and saltmarshes enhance wrack retention on modified shorelines.

  • Eco-engineering may reinstate wrack subsidies to modified shorelines.

Abstract

Wrack (stranded phyto-detritus) from terrestrial and marine sources is an important source of carbon and nutrients for many intertidal habitats. In urbanised estuaries, seawalls may act as a barrier to the transport of wrack between terrestrial and marine habitats and, where they reduce the width and habitat complexity of the intertidal zone, negatively impact on wrack accumulation and retention on estuarine shorelines. We assessed differences in the accumulation of wrack between natural (sandy beach, rocky reef and rockpools) and armoured stretches of shoreline, and the efficacy of eco-engineering interventions (man-made rockpools and planted saltmarshes) in enhancing the accumulation and retention of wrack. Surveys conducted at 4 sites in Kogarah Bay, New South Wales, Australia, on 3 dates during autumn-winter 2016 revealed that on natural shorelines most wrack accumulated at the high intertidal mark. Placement of seawalls restricted wrack accumulation to the low intertidal zone, in front of the seawall. The cover of wrack was significantly less in the low intertidal zone in front of seawalls than either the high or low tidal elevations of the natural sandy beaches and rockpools. The man-made rockpools in the low tidal zone had higher cover and biomass of marine and terrestrial wrack than the low intertidal zone of the seawalls, and both these and the planted saltmarshes also supported the accumulation of wrack at the mid intertidal elevation. Experimental deployments of marked seagrass wrack revealed that the eco-engineered habitats enhanced wrack retention over a 7 day period compared with the seawalls. However, the artificial rockpools also retaining more wrack than the equivalent natural habitat. Our results suggest that the structural complexity of eco-engineered habitats can be effective at trapping and retaining wrack, with potential flow-on effects to benthic assemblages through increased food and shelter resources. When designing eco-engineering interventions for modified shorelines, scientists and managers should consider not only their impacts on biodiversity but also on key ecosystem functions.

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

References (51)

  • Y.M. Alyazichi et al.

    Source identification and assessment of sediment contamination of trace metals in Kogarah Bay, NSW, Australia

    Environ. Monit. Assess.

    (2015)
  • P. Bartels et al.

    Reciprocal subsidies between freshwater and terrestrial ecosystems structure consumer resource dynamics

    Ecology

    (2012)
  • M.J. Bishop et al.

    Non-additive, identity-dependent effects of detrital species mixing on soft-sediment communities

    Oikos

    (2008)
  • M.J. Bishop et al.

    Context-specific effects of the identity of detrital mixtures on invertebrate communities

    Ecolo. Evol.

    (2013)
  • M.J. Bishop et al.

    Morphological traits and density of foundation species modulate a facilitation cascade in Australian mangroves

    Ecology

    (2013)
  • C.M. Bozek et al.

    Impacts of seawalls on saltmarsh plant communities in the Great Bay Estuary, New Hampshire, USA

    Wetlands Ecol. Manag.

    (2005)
  • F. Bulleri et al.

    The introduction of coastal infrastructure as a driver of change in marine environments

    J. Appl. Ecol.

    (2010)
  • M. Chapman et al.

    Use of seagrass wrack in restoring disturbed Australian saltmarshes

    Ecol. Manag. Restor.

    (2004)
  • M. Chapman et al.

    An assessment of the current usage of ecological engineering and reconciliation ecology in managing alterations to habitats in urban estuaries

    Ecol. Eng.

    (2017)
  • C.A. Currin et al.

    Utilization of a citizen monitoring protocol to assess the structure and function of natural and stabilized fringing salt marshes in North Carolina

    Wetlands Ecol. Manag.

    (2008)
  • C.A. Currin et al.

    Developing alternative shoreline armoring strategies: the living shoreline approach in North Carolina

  • K.A. Dafforn et al.

    Marine urbanization: an ecological framework for designing multifunctional artificial structures

    Front. Ecol. Environ.

    (2015)
  • J.E. Dugan et al.

    Marine macrophyte wrack inputs and dissolved nutrients in beach sands

    Estuaries Coasts

    (2011)
  • L.B. Firth et al.

    Ocean sprawl: challenges and opportunities for biodiversity management in a changing world

    Oceanogr. Mar. Biol. Annu. Rev.

    (2016)
  • K.B. Gedan et al.

    The present and future role of coastal wetland vegetation in protecting shorelines: answering recent challenges to the paradigm

    Clim. Change

    (2011)
  • Cited by (13)

    • Heat budget model facilitates exploration of thermal ecology on urban shoreline infrastructure

      2021, Ecological Engineering
      Citation Excerpt :

      In many urban harbors including San Diego, Seattle, Singapore, Sydney and Hong Kong, greater than 50% of the shorefront area has been hardened with infrastructure, and this trend is expected to continue as a result of growing rates of coastal urbanization (Davis et al., 2002; Chapman and Bulleri, 2003; Dugan et al., 2008; Lam et al., 2009; Siemenstad et al., 2011; Morley, et al., 2012; Gittman et al., 2015; Dong et al., 2016; Patrick et al., 2016). In response to well-documented detrimental effects of traditional shoreline infrastructure on coastal biodiversity and species abundance (Chapman, 2003; Bulleri and Chapman, 2010; Dugan et al., 2011; Gittman et al., 2016; Bishop et al., 2017; Mayer-Pinto et al., 2017), eco-engineering approaches have been increasingly used to protect coastal societies while also providing habitat for estuarine and marine species (Borsje et al., 2011; Chapman and Underwood, 2011; Dafforn et al., 2015a, Mayer-Pinto et al., 2018; Sutton-Grier et al., 2015; Hall et al., 2018; Strain et al., 2018a). While even enhanced (gray-green hybrid) infrastructure is generally not comparable to unaltered ecosystems in its ecological value, the addition of temperature-mitigating and water retaining features that provide shade, shelter, and microhabitat complexity has been associated with positive effects on intertidal species diversity and abundance on seawalls, bulkheads, and other coastal defense structures (Chapman and Blockley, 2009; Martins et al., 2010; Firth et al., 2014; Perkol-Finkel and Sella, 2015; Loke and Todd, 2016).

    • Eco-engineered mangroves provide complex but functionally divergent niches for estuarine species compared to natural mangroves

      2021, Ecological Engineering
      Citation Excerpt :

      Marine wrack is commonly trapped by the above-ground vegetative structures of mangroves (Hogarth, 2007) and rocky shorelines and the accumulation of wrack in estuarine systems is dependent on the physical structure of recipient habitat (Orr et al., 2005). Rock-fillets are structurally complex barriers that are likely to have similar accumulative effects to other complex artificial structures (man-made rock-pools that can accumulate 1.5–3 times more wrack than that of planted saltmarsh and armoured coastal sites; Strain et al., 2018a, 2018b). Higher structural complexity of eco-engineered rock-fillets could have reduced the rate of loss of wrack in a similar manner thus providing a highly abundant ephemeral trophic subsidy.

    View all citing articles on Scopus
    View full text