Building seawalls to sustain intertidal biodiversity in altered and urbanized estuaries

Professor Gee Chapman and Dr David Blockley


Background
New seawall in Sydney Harbour

Altered shorelines, particularly the construction of long stretches of seawalls, have major ecological impacts. Although they provide habitat for intertidal organisms, many intertidal animals in NSW do not (and possibly cannot) live on seawalls as they are currently constructed. Seawalls differ from natural rocky shores in important ways. First, around Sydney, they tend to be steep, often vertical. Second, seawalls have fewer cracks, crevices and overhangs compared to rocky shores and some habitats (e.g. rock pools) are completely missing. These factors are known to affect the distributions and abundances of intertidal organisms. Vertical seawalls have a compressed intertidal area (1 – 2 m) compared to natural gently sloping shores (10s of metres) in NSW. This crowds many species, that do not normally live in close proximity, into very small areas, which may affect associations between mobile species (potentially competing for the same resources), between mobile animals and sessile organisms which potentially compete for space, but also provide habitat for many animals or between different species of sessile organisms.

Many coastal estuaries in NSW (e.g. Tuggerah Lakes) have relatively few natural rocky shores and the shoreline is predominantly vegetated. Nevertheless, in association with residential development around the shores of the lakes, there has been considerable loss of native vegetation and increasing numbers of “hard” structures, including many seawalls. Development and change of natural shores in these lakes will increase and such development adds new intertidal habitat into these estuaries, which may be colonised by native and exotic species not normally widespread in such estuaries.

Seawalls probably have very large impacts on adjacent vegetated and unvegetated soft sediments, especially in estuaries with little tidal flushing. These habitats may be important for many species, including those in the recreational and commercial fisheries. Walls can change water-flow and large amounts of macro-algae and seagrass wrack can accumulate against existing seawalls, where they sink onto the sediment, rot and smell. Accumulations of vegetation can make sediments anoxic, potentially result in the formation of toxic concentrations of ammonia and sulphur in sediments, which can make the sediments depauperate. The role of seawalls in causing wrack to accumulate and adjacent sediments and their biota to change has not been investigated (although local impacts have been documented for one coastal lake in NSW).

Aims

Organisations responsible for building and maintaining seawalls are keen to have more reliable and robust information about effects of seawalls on estuarine biodiversity. They have thus joined with the University of Sydney to do the research necessary to provide the ecological information to underpin their environmental decisions. This project brings together as collaborating partners many of the bodies who have responsibility for construction and repair of seawalls and the management of estuaries in NSW. This includes representatives of local government and state agencies as well as an independent consultant ecologist. These are:

  • Sydney Ports Corporation
  • Hornsby Shire Council
  • Mosman Council
  • North Sydney Council
  • Wyong Shire Council
  • NSW Maritime
  • NSW Department of Primary Industries (Fisheries)
  • Department of Environment and Climate Change
  • Bio-Analysis: Marine, Estuarine & Freshwater Ecology

Specifically, this research will:

  1. Identify which types of seawalls support the most natural intertidal biodiversity
  2. Relate differences in biodiversity to structural features and construction of seawalls
  3. Identify effects of seawalls on ecological processes crucial for the long-term maintenance of biodiversity
  4. Using seawalls as experimental sites, experimentally evaluate the value to biodiversity of different forms of construction
  5. Identify impacts of seawalls on adjacent soft sediments and seagrass
  6. Experimentally trial the use of road diversion devices as seawall “mimics” to evaluate their impact prior to construction
  7. Ensure all relevant data are provided to local government and other relevant State agencies not directly involved in the project to enhance broader ecological knowledge for future decision-making
Importance
Seawall constructed with rockpools to attract animals

People who build new structures or who redesign and repair old seawalls have options about methods of construction, even within constraints of cost, safety and engineering. Some methods of constructing seawalls seem more “environmentally friendly” than others, but data are sparse and patchy. Regulatory authorities (Local Government engineers, environmental officers and planners, with State Government Regulatory Agencies, e.g. NSW Maritime, NSW DPI (Fisheries)) have responsibility for deciding whether to permit the construction of new walls and appropriate designs for repairing old walls in ways to minimize their impact on local biodiversity. Government Agencies (NSW DPI (Fisheries), DECC) are responsible for developing plans of management to protect estuarine biodiversity, but cannot do so adequately without understanding how shoreline development alters ecological patterns and processes in the estuary itself.

Results

Results will be posted as they become available