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Table of Contents

GEODATE

Volume 35 Issue 1 2022

GEODATE

Volume 31, Issue 1, 2022

PORTERS CREEK WETLAND: ECOSYSTEM AT RISK

Lorraine Chaffer, Geography Education Consultant, Lakehaven, NSW.

Freshwater wetlands are among the world’s most biologically productive ecosystems and play an important role in maintaining the health of downstream river systems and estuaries. Undervalued at a global scale, it is estimated that 35% of wetlands were lost in the five decades since 1970 with the rate of loss accelerating in recent years.

Porters Creek Wetland (PCW) is a small ecologically significant freshwater wetland on the NSW Central Coast. The 5 km2 wetland occupies a shallow basin at the southern end of the Porters Creek Catchment, a sub-catchment of the Tuggerah Lakes system. It is the largest remaining freshwater wetland in the Central Coast Local Government Area (LGA). PCW plays a major role in filtering runoff before it enters the Wyong River and ultimately Tuggerah Lake, it mitigates flooding in the lower Wyong River and supports a large diversity of plant and species. Over time, agriculture, population growth, urbanisation and infrastructure projects have threatened the ecological integrity of the wetland, necessitating significant management interventions. 

Ecological Integrity  refers to the ability of an ecosystem to maintain its natural ecological values including its biodiversity and functioning

Ecosystems that can maintain their biodiversity and functioning, without human management, have higher ecological integrity.

                         FIGURE 1: Location of Porters Creek Wetland 

                        Source: Author

                        FIGURE 2: The value of Porters Creek Wetland

                        Source: Author

Ecosystem functioning

Hydrology

Porters Creek Wetland lies in a wide shallow basin between the suburb of Watanobbi and Warnervale Aircraft Landing Area (Warnervale Airport). Most of the catchment has a low gradient, with the highest elevation just 131 metres. An average rainfall of 1200 mm, weather events such as East Coast Lows and cycles of drought and flood contribute to variations in the hydrology of the wetland. The basin quickly fills with water after rain and becomes deeply inundated during floods. In dry weather PCW slowly dries, some areas drying out completely during prolonged drought. The underlying alluvial loam soil assists water retention. 

FIGURE 3: Catchment boundaries

Source: Figure created for PCW Education Resource (Wetland Web CD) by Wyong Shire Council. 

In the past, catchment runoff flowed overland and seeped into the soil, gradually making its way into the wetland across a broad front. Channel flow came from three creeks in the catchment, Buttonderry Ck, Porters Cr, and Woongarrah Cr. This natural system has been increasingly interrupted by human activity, with urban development on greenfield sites and the channelisation of stormwater flow in particular, increasing runoff into the wetland. 

Biodiversity

Porters Creek is rich in biodiversity, supporting 200 kinds of plants in vegetation communities that transition from the fringe of the wetland to the deepest billabongs. At the edges of the wetland where it is mostly dry, prickly paperbarks and Woollybutts dominate. Further into the wetland, wetter conditions favour laced paperbarks and swamp mahogany with an understory of small plants such as river buttercups and knotweeds. At the billabongs, where it is almost always wet, water loving plants such as reeds, rushes and sedges thrive along with a variety of submerged and floating species like pondweed and Azolla. Several native species are listed as vulnerable under the Threatened Species Conservation Act 1995 including an Angophera and a Melaleuca. The Swamp Mahogany is recognised as a keystone species in the tall paperbark forest and the Swamp Mahogany’s ecosystem is classified as endangered and in need of protection by The Office of Environment and Heritage.

Keystone species – species that play a critical role in ecological communities by determining the types and numbers of various other species in an ecosystem.

Five plant communities have been identified, each the basis of complex food webs for the 60 kinds of birds, 25 kinds of mammals, 16 kinds of frogs and lizards, snakes, crayfish. spiders and insects that inhabit the wetland. Some species rely on seasonal sources of food such as the winter flowering plants that attract squirrel gliders and birds. The wetland is a traditional feeding and resting ground for Australian and overseas migratory bird species such as Swifts and during droughts is a refuge for many local species. Two vulnerable mammals, the squirrel glider and mouse eared fishing bat, and the vulnerable wallum froglet, inhabit the wetland. The resilience of the wetland is dependent on maintaining its biodiversity.

Resilience – The ability of ecosystems to tolerate and recover from ecological disturbance 

FIGURE 4: Plant communities in PCW

Source: Illustration created for Central Coast Council e-book publications

FIGURE 5: Biodiversity

Source: Author

Human impacts and future threats

In the 1800’s the valleys, and floodplains, including the fringes of Porters Creek Wetland, were cleared and fenced for cattle grazing. The fencing, and channels built to drain farmland, changed the flow of water into the wetland although the overall impact at this time was limited as water still entered the wetland across a broad area. Farming reduced the size of the wetland and increased its vulnerability to edge effects such as exotic species invasion, excess stormwater and increased sediment and nutrient loads. Since the 1980’s farmland has been increasingly rezoned urban, although some grazing, horticulture and hobby farms remain. 

The major threats to Porters Creek Wetland are rapid population growth and urban development driven by state government policy. Porters Creek Wetland is downstream from one of the fastest urban growth regions in NSW. Over the past 40 years the predominantly rural suburb of Warnervale has become the six urban release suburbs of Halloran, Wadalba, Hamlyn Terrace, Woongarah, Wallarah and Warnervale. NSW Government targets propose a further 41,500 houses for the Central Coast LGA by 2036 with a large proportion of these to be built within the Tuggerah Lakes and Porters Creek catchments.

FIGURE 6: Urbanisation; Greenfield locations transformed into housing estates at Warnervale

Source: Author

Changing hydrology and biodiversity loss

The increase of non-porous urban surfaces has changed the hydrology of the catchment in two essential ways.

  1. Less rain infiltrates the ground resulting in less ground water for natural environments, including creeks and wetlands, during dry periods. 
  2. The urban stormwater system increases the amount of rain that is flushed into natural streams and wetlands overloading and degrading them with nutrients, sediment and water.

Runoff from housing, industrial subdivisions and transport infrastructure is concentrated into the locations where stormwater channels empty into the wetland. Excess water at these locations has caused the dieback of trees such as the Woollybutt, that do not like to be permanently “wet” and weaken tree root support. Dead and fallen trees open up the wetland canopy encouraging invasive plant species to establish and reducing the food and habitat for native fauna such as possums and squirrel gliders. The Main Northern Railway and the Sydney Newcastle Freeway dissect the edges of the wetland, further concentrating runoff into locations such as the drainage channels under the railway line at Watanobbi. 

Water flows directly downhill at Watanobbi, speeding into the wetland through two stormwater outlets, delivering weeds, litter, nutrients, and sediment. The effects extend deep into the wetland where weeds such as Sagitaria and Wandering Jew have overtaken native species and exotic species such as Mosquito Fish are impacting native fish and frogs. 

FIGURE 7: Stormwater impact at Fishburn Drain, Watanobbi

Source: Author

FIGURE 8: Sagitaria invasion

Source: Author

Other threats to the size and functioning of PCW include:

  • noise and unburnt aircraft fuel, the removal of flight restrictions and the potential expansion of the Warnervale Aircraft Landing Ground into the wetland
  • expanding industrial development at the Wyong North and Warnervale Industrial Parks, and the construction of Warnervale Town Centre. 

FIGURE 9: Warnervale Aircraft Landing Ground. View across Warnervale Aircraft Landing Ground towards PCW. Warnervale Industrial Park on the right.

Source: Central Coast Council; https://www.centralcoast.nsw.gov.au

Nature and Rate of Change

Change in the 1800’s was limited to reducing the wetlands size with some change to hydrology. These changes, whilst increasing the vulnerability of the wetland, did not significantly alter the functioning of the ecosystem. Since the l97O’s when urban release development accelerated, the rate of change has been rapid and tree dieback substantial. The transformation from greenfield to urban land use continues at a rapid pace under the NSW Government’s Central Coast Regional Plan 2036. Edge effects such as weed invasion, feral animals, bushfires, and nutrient loading have had a slower more insidious impact on biodiversity and water quality in the wetland and further downstream.  

The Tuggerah Lakes

The ecological functioning of Porters Creek Wetland is critically important for the future of the Tuggerah Lakes ecosystem. The environmental processes that occur in ecosystems such as PCW ensure good quality water downstream. Wetland plants and microorganisms absorb nutrients such as nitrogen, phosphorus and heavy metals and slow water flow, allowing sediment to settle. Shallow, slow-moving or still water bodies allow UV rays to kill bacteria and pathogens. In the absence of natural wetlands, engineering solutions are costly, hard to maintain and do not guarantee water quality downstream. 

Over the past 200 years, the Tuggerah Lake Catchment has lost an estimated 50 per cent of its wetlands through draining and clearing for rural, urban, and industrial development. The Expert Panel Report from the Tuggerah Lakes Water Quality Independent Review into water quality in Tuggerah Lakes states that runoff from catchments is one of the most significant threats to water quality in the Tuggerah Lakes and more specifically that:

  • changes to hydrology from urban development was the largest risk to the future health of the Porters Creek Wetland and,
  • poor erosion and sediment control is a key risk to water quality from planned future development

Can effective management restore the ecological integrity of Porters Creek Wetland? 

Explore this question in Part 2: Porters Creek Wetland. Management and Protection. 

Student activities

1. Describe the location and spatial extent of PCW.

2. Use a digital tool such as the Central Coast Council Mapping Tool at https://maps.centralcoast.nsw.gov.au/public/, Google Earth, Google Maps or SixMaps e-topo maps to make close observations about the wetland, its catchment and surrounding development. Create a sketch map with an appropriate scale to summarise your observations.

3. Define the following concepts in your own words.
a. Ecological integrity
b. Ecosystem functioning
c. Vulnerability
d. Resilience

3. Why is the biodiversity of Porters Creek Wetland important?

4. Explain the ecological link between the functioning of Porters Creek Wetland and water quality in the Tuggerah Lakes.

5. Create a consequence chart to illustrate how urban development impacts on Porters Creek Wetland. You will find a consequence chart template here
https://globaleducation.edu.au/templates/consequences-chart.html

6. Suggest ways in which government bodies and developers can reduce the impact of urban development on downstream waterways.

7. ‘The rate of change in Porters Creek Wetland Catchment is a greater threat than the nature of the change’.
a. Explain the terms ‘rate of change’ and ‘nature of change’.
b. Respond to the statement and justify your position.

8. Research how wetlands are legally protected. Consider protections at global, national, and local scales.

Suggested resources

Central Coast Council. Tuggerah Lakes Estuaries and Case Studies eBook available at

https://loveourwaterways.centralcoast.nsw.gov.au/learn/educational-resources/tuggerah-lakes-estuary-projects-case-studies-ebook

Central Coast Council. Wetlands e-book available at

https://loveourwaterways.centralcoast.nsw.gov.au/learn/educational-resources/wetlands-ebook

NSW EDU. Blue Planet. Case study Integrated Water Cycle Management available at

http://www.blueplanet.nsw.edu.au/sitefiles/blueplanetnsweduau/porters_creek_entire_case_study.pdf

References

Central Coast Council. Love our waterways available at

https://loveourwaterways.centralcoast.nsw.gov.au/learn/explore-waterways/explore-our-pathways

Central Coast Council. Tuggerah Lakes Estuary Management Plan available at

https://loveourwaterways.centralcoast.nsw.gov.au/learn/health-our-estuary/tuggerah-lakes-estuary-management-plan

Central Coast Council. Wetlands An essential filter for our lakes available at

https://www.centralcoast.nsw.gov.au/council/news/media-releases/wetlands-essential-filter-lakes

NSW Government. Tuggerah Lakes Water Quality Independent Review available at

https://www.environment.nsw.gov.au/-/media/OEH/Corporate-Site/Documents/Water/Estuaries/tuggerah-lakes-expert-panel-final-report.pdf

Rumbalara Environmental Education Centre 2021. Porters Creek Wetland Google site available at  https://sites.google.com/education.nsw.gov.au/porters-creek-wetland/home

Sciencing. How Do Wetlands Purify Water? Available at

https://sciencing.com/do-wetlands-purify-water-7585568.html

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