Land–water interface

2016

Beaches and sand dunes

Like many features at the interface of the land and water, beaches are dynamic environments and vary widely in morphology, wave climates, tidal regimes, sedimentology and degree of modification. Australia has 10,685 beaches (Short 2006), which Australian culture values highly for their aesthetic qualities and recreational amenity. However, dune systems behind beaches are generally undervalued by society, despite their close link to beach systems. Dunes support important tracts of native vegetation and provide a buffer against beach erosion, and their fresh watertables act as buffers to saltwater intrusion.

A large proportion of Australia’s beaches are relatively pristine, but others are exposed to pressures ranging in scale from local (e.g. erosion) to global (e.g. sea level rise) (Defeo et al. 2009). Even remote beaches and dunes are increasingly pressured by off-road vehicles, particularly for tourism and recreational fishing. Some beaches around state capitals and coastal population centres are significantly modified through coastal armouring and recreational infrastructure. The introduction of hard engineering structures (e.g. groynes and seawalls; see Artificial structures) directly alters surf-zone processes and sediment dynamics, displaces beach habitat, imposes connectivity barriers for species and can reduce the aesthetic amenity. Sydney claims some of Australia’s most popular beaches (e.g. Bondi Beach), but approximately 23 kilometres of its open coast is modified in some form. Soft engineering practices such as beach nourishment, where eroded sand is replaced from other sources, are often expensive and ongoing, and can also negatively affect invertebrate communities and the organisms that rely on them. Between 2001 and 2011, beach nourishment programs to counter the effects of erosion have been implemented on at least 130 Australian beaches, mostly in urbanised areas (Cooke et al. 2012).

Marine debris is found on beaches primarily near urban population centres, but also increasingly on more rural beaches, depending on local oceanographic conditions (see Marine debris). Mechanical cleaning to remove litter and debris is a common practice on many urban beaches. However, it can physically alter the morphology of the beach, damage fauna, and remove seeds and washed-up seaweed, which can be a resource for other species. High human presence on beaches can disturb the nesting and/or foraging of shorebirds and turtles (see Nursing, roosting and nesting), whereas harvesting can deplete local populations. Invasive species, including dogs and foxes, threaten native fauna and dune vegetation, and are particularly prevalent near major cities.

Dunes have been extensively developed and their vegetation altered, much of which can be attributed to dune stabilisation initiatives in the mid-20th century. Historically, dunes were perceived to require stabilisation, leading to the deliberate introduction of marram grass (Ammophila arenaria) in the south. This has now become a widespread and problematic invasive species, as have sea wheatgrass (Thinopyrum junceiforme) and sea spurge (Euphorbia paralias). These invasive species displace native species, alter dune morphology and potentially disrupt ecological functioning (see Invasive species (terrestrial)). In New South Wales, bitou bush (Chrysanthemoides monilifera) is dominant and widespread, displacing all but a few native species, despite well-funded control programs. Dune condition is generally considered poor in southern mainland Australia (south of Perth and Brisbane), whereas it is highly variable in Tasmania.

Because dunes are undervalued and underfunded, they suffer from uncoordinated and transient management programs that are often ineffectual. Local management campaigns can make temporary progress, but usually have short lifespans, and invasive species return once interest fades. In July 2013, national coordination for Weeds of National Significance was suspended, and, even for these species, there is little funding for effective, long-term invasive species control programs. Such downscaling of management scope is concerning, because deterioration of vegetation in one location can have broader consequences for other locations.

In the short and long terms, beaches will continue to receive pressure from coastal population growth on the landward side, and rising sea levels and associated erosion and coastal armouring on the ocean side. However, some natural beach and dune systems away from human development may have the opportunity to adapt to sea level rise by moving landwards and replacing other coastal habitats. The outlook for dunes is similarly poor. Native diversity on dunes is likely to decline as invasive species proliferate, recreational pressures increase and coastal development continues. Dune and beach management could be improved by better understanding of the ecological impacts of invasive species and other pressures. Accurate assessment of trends in the ecology of beach and dune habitats is limited by the paucity of temporal data, and few studies exist other than for a few select systems.

Rocky shoreline

Rocky shorelines are intertidal habitats that support diverse floral and faunal communities. A significant proportion of natural rocky shorelines near urban centres have been replaced or fragmented by artificial structures and foreshores, with consequent biodiversity loss. For example, approximately half of the Sydney Harbour foreshore is artificial (Chapman & Bulleri 2003). Remaining rocky shores near urban centres are being increasingly pressured by contamination (including contaminated groundwater discharge), stormwater and sewage outflows, invasive species, and recreational use. Although the condition of rocky intertidal shores is widely considered to be declining because of multiple pressures, there are limited long-term datasets available to test this belief.

Establishing protected zones is one method of promoting the resilience and recovery of affected shores, but protection of rocky shores is seldom enforced, and unregulated fishing and harvesting are common. A promising approach to ameliorate effects of artificial structures is to incorporate eco-engineering principles into their design (see Artificial structures), which, in principle, can reduce the impact of natural habitat loss on native species. Issues of contamination also require attention, through reduction of contemporary contaminant inputs (e.g. storm water) and the remediation of historical contaminants.

The outlook for rocky shores is poor, as they continue to be subject to multiple pressures. In the future, climate change will increase as a pressure, with rising sea levels reducing the amount of suitable habitat, and rising daytime maximum air temperatures negatively affecting a larger proportion of species. Effective adaptation and management strategies are required to increase resilience in the face of climate change.

Mudflats and sandbars

Mudflats and sandbars are extensive areas of bedded sediment, characterised by fine or coarse grain size, respectively. These dynamic environments are exposed during low tide and submerged during high tide, providing habitat for infauna, and the shorebirds, crustaceans and fish that consume them. Their communities are important for water-column and interstitial (between-grain) water quality through their role in nutrient cycling. However, many areas are believed to be in good condition because they are  remote from most anthropogenic pressures, except for climate change.

There are little data on the condition of mudflats and sandbars beyond small-scale studies, so high uncertainty surrounds their current state and trend. Although some data on the spatial area of tidal flats in estuaries are available, studies are generally one-off and do not consider habitat quality. Despite their large areas, tidal flats are rarely included in mapping exercises conducted by state agencies.

Bedded sediments are sinks for contaminants, and the health of tidal flat communities near urban, agricultural or industrial activity is often threatened by contaminant inputs, including organic enrichment. Furthermore, altered hydrology because of upstream dams, weirs and barrages can limit both water and sediment inputs, and ultimately lead to habitat loss. Likewise, dredging can alter sediment dynamics, and there is the potential for spoil deposition to create entirely new artificial mudflats.

Several recreational activities threaten the quality of mudflats and sandbars as habitats. Harvesting of bait, crabs and bivalves can directly affect populations of these ecologically important bioturbators (animals or plants that rework soils or sediments). Boating can also affect these habitats, because propellers physically disturb sediment, boat wake increases erosion and shifts grain size towards coarser compositions, and boat noise disrupts foraging shorebirds.

Growing human populations are expected to increase a range of pressures on tidal flats in the short term. In the longer term, tidal flats will be strongly influenced by climate-related factors, such as sea level rise and altered storm surge, and coastal modifications in the wake of these pressures.

Clark GF, Johnston EL (2016). Coasts: Land–water interface. In: Australia state of the environment 2016, Australian Government Department of the Environment and Energy, Canberra, https://soe.environment.gov.au/theme/coasts/topic/2016/land-water-interface-0, DOI 10.4226/94/58b659bdc758b