Leschenault Estuary

Posted on Jan 9, 2015


Leschenault EstuaryWaterResourcesCoastAndMarineAquaticBiodiversity

 

Looking across the estuary at sunset (D Postma).

Looking across the estuary at sunset (D Postma).

The Leschenault Estuary is located approximately 150 km south of Perth and is one of the larger waterbodies in southern Western Australia, covering an area of approximately 27 km2. It is about 14 km long, 1.5 to 2.5 km wide, and is very shallow – being 1.2 to 2 m deep. It is the only estuary in the state located behind a shore-parallel dune barrier.

Biological and Physical Values

During the past century the Leschenault Estuary has been significantly modified by human activity. Originally there was only one waterbody called the Leschenault Inlet Estuary. Now the system is divided into two waterbodies:

  • the Leschenault Inlet: once the mouth of the Preston River, this is now a semi-confined waterbody/lagoon that has significant recreational and social importance to the City of Bunbury.
  • the Leschenault Estuary: the larger waterbody, into which the Collie and Preston rivers flow. It is now connected to the ocean via The Cut, a man-made opening through the barrier-dune system directly opposite the mouth of the Collie River, established to manage flood flows after Bunbury Port was built.

The establishment of The Cut isolated the estuary basin from its natural ocean entrance, which was closed to prevent sedimentation in the old Bunbury Port. The Leschenault Inlet is all that remains of the old entrance channel. The figure below shows how the Estuary has been modified for human use over time.

Figure 2.2 Leschenault Estuary

The estuary is likely to be altered further with the planned realignment of the lower Preston River to enable the Bunbury Port area’s expansion. This will affect the present delta and possibly the estuary’s ecology.

Hydrology

  • The estuary lies parallel to the coast with the Collie and Preston rivers discharging at its southern end near The Cut, and the Parkfield Drain discharging at its northern end. This results in a marked gradient in salinity from south to north (Brearley 2005) and unlike other estuaries, there is no simple river-to-sea transition.

Vegetation

  • The Leschenault Estuary’s shores and wetlands peripheral to the estuary support five broad types of fringing vegetation (Semeniuk et al. 2000):
    • saltmarsh, comprising sandfire and rushes
    • estuarine fringing forests, typically of small saltwater sheoak, saltwater paperbark, paperbark, and swamp paperbark
    • shallow water-fringing vegetation
    • sandy-rise vegetation
    • freshwater vegetation in habitat areas close to substantial freshwater input.
  • This peripheral vegetation has been identified as being of state significance (Semeniuk et al. 2000). The estuary supports a diversity of ecologically-important habitats including seagrass beds, tidal mud, sand flats, saltmarshes, fringing sedgelands, heathlands and Melaleuca woodland, with their associated biodiversity (McComb et al. 2001).
  • Within the estuary, the aquatic vegetation is composed of at least three recorded species of seagrass: Halophila ovalis, Ruppia megacarpa and Heterozostera tasmanica, and at least seven species of macroalgae (green and brown): Chaetomorpha sp., Graciaria sp., Ulva sp. and Acetabularia sp., as well as two species of Phaeophyta (Hillman et al. 2000). The distribution and abundance of macrophytes are influenced by depth, substrate and salinity (Semeniuk et al. 2000).
  • Seagrass meadows, along with other aquatic vegetation and mangrove areas, are important nursery habitats in the estuary. Crabs, molluscs, prawns and many fish species (Hodgkins et al. 1979; Schwinghammer 1982) use seagrass meadows as nurseries, many of which are important to the recreational fishing industry (EPA 1993; Semeniuk et al. 2000).

Fish

  • Species of fish commonly found in the estuary are mullet, silver bream, tailor, sea garfish, striped perch, roach, whitebait and anchovy, as well as blue manna crabs and king prawns. Of the 42 species recorded in the 1990s by Potter et al. in the nearshore and shallow waters of the estuary (1997):
    • 20 were marine using the estuary as a nursery area, being defined as marine estuarine opportunists
    • 13 were classed as estuarine, living entirely in the estuarine waters and completing their lifecycles in the estuary (of those seven are represented by marine populations)
    • the most common species were the long-finned goby (Favonigobius lateralis), the hardyhead (Atheronosoma elongata) and gobbleguts (Apogon rueppellii).
  • In the deeper waters of the estuary and Collie River, fish catches include larger species dominated by marine estuarine-opportunists and the Perth herring (Nematalosa vlaminghi) (Potter et al. 2000).

Birds

  • The Leschenault Estuary is an extremely important habitat for avifauna internationally, nationally and regionally. The natural setting of the estuary, its large size, the Leschenault Peninsula and the outlying wetlands provide a heterogeneous array of habitats that attracts a very large number and variety of waterbirds (Raines et al. 2001). Within the estuary itself, the shallow water and associated mudflats during low tide provide a variety of food sources (Cresswell 2000). The area is known to host at any one time up to 5000 permanent and migratory birds (Cresswell 2000). In total 130 bird species  have been recorded on the estuary’s western shore, in the Leschenault Peninsula Conservation Park and the area abutting it alone. A list of species recorded is available in Appendix C of the Leschenault Estuary water quality improvement plan found here.

Bottlenose dolphins (Tursiop sp.)

  • Bottlenose dolphins are known to spend time in the Leschenault Estuary, inlet and lower section of the Collie and Preston rivers. The dolphin deaths in 2009 and more recently in December 2010 are a cause for concern. Maintaining the health of the estuary and its associated waterways is important to ensure a healthy dolphin population.
Zippy leaping Estuary_holly_raudino

Zippy leaping out of the water, Leschenault Estuary (Holly Raudino).

 

Current Condition and Trends

Water Quality

  • Before The Cut, the system was mostly brackish during winter. However, the increased marine exchange through The Cut has
    resulted in the estuary becoming predominantly marine.
  • Salinity concentrations are higher in the estuary’s northern part: in summer this area becomes hypersaline.
  • Reduction of freshwater input as a result of the Wellington and other dams in the catchment restricting flows.
  • Competitive water demand between the environment and human use.
  • Reduction of catchment runoff due to climate change and declining rainfall.
  • Reduction of freshwater flushing from rivers in the estuary has resulted in more prolonged salt wedges. Stratification of the water
    column results in areas becoming hypoxic or anoxic, which leads to the release of nutrients from the bottom sediments and thus
    provides conditions favouring algal blooms and fish death.
  • Nutrient enrichment has led to the loss of seagrass over large areas due to the decrease in water clarity and smothering due to
    increased algae biomass.

Sedimentation

  • Increased sedimentation.
  • Estuary has a high sediment-trapping capacity: this affects aquatic life and the distribution of aquatic plant and associated fauna.
  • Hills et al. (1991) report a higher calcium content in the estuary sediments compared with those from other south-west systems, and this might reduce the potential for phosphorus release from these sediments.
  • Organic enrichment of surface sediment is higher than in the Peel Harvey and comparable with part of the eutrophic Harvey Estuary (McComb et al. 2001).
  • Rate of phosphorus release in the water column is low compared with the Peel Harvey and Swan Canning (McComb et al. 2001).
  • Potential for sediment phosphorus release is believed to be relatively small in the estuary following studies between 1988–90 (McCombe et al. 2001).

Loss of fringing vegetation along the estuary

  • Significant and ongoing loss of fringing vegetation as a consequence of urbanisation and unregulated public access.
  • Between 1941 and 1989 half of the original fringing vegetation was lost (350 ha) via clearing for development.
  • This was reduced by half again following development of the Pelican Point canals and the Lakes Estate as well as the further development of land around Bunbury Port.
  • This ongoing and incremental habitat loss is likely to have a significant impact on the estuary’s ecosystems and the fauna depending on them, in particular waterbirds.
Loss of fringing vegetation and associated pressures from increasing urbanisation along the estuary foreshore (Henderson Photography)

Loss of fringing vegetation and associated pressures from increasing urbanisation along the estuary foreshore (Henderson Photography)

Loss of aquatic vegetation and associated biota

  • Donohue et al. (1994) reported a macrophyte biomass composed of a healthy mix of seagrass and macroalgae, indicating that water quality was good – particularly in comparison with the Peel-Harvey and Swan-Canning systems. However, they also reported fluctuations in the mixture of macroalgae in the northern half, suggesting a response to greater nutrient levels.
  • A detailed comparative study carried out in 2005 by Semeniuk V and C (2005b) using long-term data (1982–87) showed significant changes in the estuary’s ecology:
    • decline in macrophyte (seagrass and macroalgae) total biomass, with seagrass generally in low abundance and at some sites absent within the estuary (compared with previous studies) and Halophila ovalis dominating
    • decrease in biodiversity and invertebrate fauna with fundamental changes in the assemblages of molluscs, polychaetes and crustaceans (mollusc species declined from 31 to three at 21 sampling sites; polychaete species declined from 15 species to six species recorded).
  • This decreasing trend is consistent with a previous study by Wurm and Semeniuk (2000) who also observed a link between the decrease in invertebrates and macrophytes from comparative surveys undertaken in April 1997 and May 1998, and 1982–87.
  • Semeniuk et al. (2000a) suggest that changes in the estuarine biotic assemblages are due to the greater marine influence after The Cut was constructed, medium-term changes in the volume of runoff, and changes in hydrochemistry and nutrient concentrations.

Algal bloom

  • 1988: significant algal bloom with nuisance macroalgae Rhizoclonium (McKenna & Derrington 2005, unpublished) and Cladophora for over a week in Victoria Bay (Waterways Commission 1992). Similar phytoplankton blooms were also recorded in 1987 and 1989 (Waterways Commission 1992).
  • 1994: large summer blooms of dinoflagellates (red tides) and diatoms.
  • Reports of nuisance algae have taken place since 1995 (McKenna & Derrington 2005, unpublished).
  • Chlorophyll-a concentrations from 2000–06 ranged from undetectable limits of <0.0005 mg/L to the extreme value of 0.092 mg/L in the lower Preston River (30 times greater than the recommended ANZECC guideline).
  • In October 2009 a large algal bloom occurred along the estuary’s north and eastern shores, dominated by non-toxic algae Cladophora ruppia, Ulva sp., Rhizoclonium sp. and Enteromorpha sp.
  • Dynophysis acuminata were detected in October and November 2008, and August 2010, above recommended levels of 3 cells/mL (Christine Webb, DoW, pers. comm.).

Acid sulfate soils

  • A significant area around the Leschenault Estuary is underlain by acid sulfate soils, with evidence that some of these may be oxidising. Poor acid sulfate soil management could have grave environmental consequences in the Leschenault Estuary and catchment tributaries.

Social and recreational values

Recreational pursuits on the Estuary include kite-surfing and wind-surfing (J. Hugues-Dit-Ciles).

Recreational pursuits on the Estuary include kite- and wind-surfing (J. Hugues-Dit-Ciles).

  • The estuary is important aesthetically providing an important scenic backdrop for the localities of Leschenault and Australind, as well as a large number of housing developments constructed along its eastern shores and on the floodplains of the lower Collie and Brunswick rivers.
  • The Leschenault Estuary, Leschenault Inlet and their associated foreshores provide a major recreational hub for the population of the Greater Bunbury area (Waterways Commission 1992) as well as visitors to the region (WAWRC 1987). The Leschenault Peninsula Conservation Park, Cathedral Avenue foreshore, Pelican Point and the shores of the lower Collie and Brunswick rivers are at present the key recreational assets abutting the Leschenault Estuary and are well used by local residents and tourists alike, who enjoy a range of public facilities such as playgrounds, picnic tables, walking trails, toilets, barbecues, boat ramps and fishing platforms. The water-related activities include swimming, canoeing, kayaking, kite- and wind-surfing, crabbing and fishing.

Conservation Status

The Leschenault Estuary is a declared waterway under the Waterways and Conservation Act 1976.

Leschenault Estuary and Climate Change

 

Flooding across road adjacent to estuary during severe storm (D Postma).

Flooding across road adjacent to estuary during severe storm (D Postma).

SWCC Strategic Priority

The Leschenault Estuary is identified within SWCC’s NRM Strategy as a first order priority asset under the Aquatic Biodiversity, Water Resources and Coasts and Marine Environment themes.

Projects

 

Source:

 

Header and thumbnail images supplied by Jeff and Sarah Henderson 2011

 

 

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