Floating wetlands

Council's Coasts & Catchments team explain the floating wetland to pupils from North Curl Curl

Public School at the Project's controlled environment facility at Curl Curl Lagoon - 26 March 2021

Floating Wetlands Pilot Project

The following describes the concept of introducing floating wetlands to improve the health of the Curl Curl Lagoon and Greendale Creek ecosystem, the purpose of this pilot project, what it was able to achieve, and what further action is now planned.

What benefits would a floating wetland bring?

A permanent floating wetland in Curl Curl Lagoon could provide bird habitat and a safe nesting area as well as filter and clean the currently heavily polluted water.

Working to restore the degraded natural environmental ecosystem would involve members of the local community and instil pride in place. The community could see a future in which restoration is possible and that innovative solutions that include native vegetation can play a part in regeneration.

What action can be taken to improve water quality?

Biological treatment is possible by harnessing the natural ability of plants and microbes to absorb nutrients (such as phosphorus and nitrogen) and break down contaminants through biological processes known as bioremediation.

What role can wetlands play?

Wetlands can improve water quality in Intermittently Opening and Closing Lakes and Lagoons (ICOLLs), managing runoff, wastewater and industrial contaminants.

Wetlands enable natural processes to biologically filter water as it passes through shallow areas of dense aquatic vegetation and permeable bottom soils. By planting wetland plants in a platform that floats on the surface of the water, the roots and microbes take up sediment that then becomes part of the plant material and biomass.

This process would reduce turbidity and increase the level of oxygen in the Lagoon's water, enabling the ecosystem to support more plant and animal life.

What is this project's current status?

The project ran between November 2019 and June 2021 and was able to demonstrate that a floating bed of wetland plants nourished only by water taken from Curl Curl Lagoon could remain healthy after 8 months and produce a measurable reduction in water nutrients.

How did the project progress?

Since 1980 Greendale Creek and Curl Curl Lagoon in John Fisher Park at Curl Curl NSW have been the subject of an extensive programme of environmental restoration and rehabilitation led by Sydney’s Northern Beaches Council and assisted by community groups including Curl Curl Lagoon Friends Inc.

In recent years Council’s Coasts & Catchments team has been interested in proving the concept of using floating beds of wetlands plants anchored in the main water body to help improve water quality.

In November 2019 Council and Lagoon Friends received a $20,000 grant under the Commonwealth Department of Science, Industry, Environment & Resource’s Communities Environment Program (CEP) to conduct a pilot project to establish whether the floating wetlands concept could help achieve the desired outcome of improving water quality. Member for Warringah in the Australian Parliament Ms. Zali Steggall OAM, MP was instrumental in inviting Curl Curl Lagoon Friends to apply for the CEP grant.

Council staff led the scientific and technical elements of the project while Lagoon Friends took responsibility for community participation, project management, and administration of the CEP grant.

A controlled growing environment comprising an above-ground 5000L poly water tank, suspended platform holding a growing medium, cage, crane, winch, air lift aeration pump, water transfer pump and associated equipment and services was established on a site within the grounds of the Council operated Curl Curl Community Nursery adjacent to the Lagoon. CEP grant funds assisted in the establishment of this facility.

In October 2020 water was pumped from the Lagoon into the tank and the first test cycle commenced. By May 2021 it was demonstrated that species Carex appressa could survive on nothing more than sunlight, air and the nutrients present in the water sourced from the Lagoon.

Scientific testing of water samples taken daily over a 14 day period in March 2021 revealed an encouraging general net decrease in nutrient concentration.

While this pilot project had originally intended to compare the growth and nutrient reduction performance of a number of species, ultimately one test cycle only could be completed during the grant period. But with the necessary infrastructure now fully operational, further testing can continue into the future provided the costs of water testing can be met.

During the course of the project, progress updates were published on Council’s and Lagoon Friends’ websites and social media channels. On-site open days were attended by local school children and the general public. A working model of the production air lift water pump constructed from transparent Perspex pipe proved to be an excellent educational aid and an object of interest to all those who attended the public demonstrations.

The first growing trial has demonstrated that a garden of wetland plants floating in water drawn from Curl Curl lagoon can survive and improve the water quality. As a final step, the project purchased a small floating bed to see whether the same result can be replicated in a field trial in a section of Greendale Creek.

In conclusion, the combination of CEP grant funds, physical resources and manpower from Council, volunteer effort from Lagoon Friends, and the support of Warringah and Manly’s MPs produced what the team considers a good outcome against the project’s original objectives and a solid platform on which improvements to the water quality of Curl Curl Lagoon will continue to be developed.

What does the controlled growing environment look like?

Controlled growing facility adjacent to Cur Curl Lagoon showing 5000L aquaculture tank, growing platform with established plants temporarily suspended above tank, air lift aeration pump, electrical equipment enclosures and security fencing.

How were the nutrient uptake / removal tests run?

A test cycle comprised setting up certain predefined conditions in the tank, selecting a plant species, monitoring plant development, taking water samples for a defined period, having those samples analysed, then planning the next test cycle.

While this pilot project had intended to conduct and compare the results from more than one cycle, one trial only was able to be completed during the grant period.

For Trial #1 the objective was to illustrate and assess the effectiveness of the wetland unit comprising selected plants (sedge Carex appressa), their root system and the microbial biofilm on the surface of the root system.

A time series of water sampling was undertaken. The floating wetland was removed from tank using the jib crane and the water sampled for nutrients. The tank was then filled with water pumped from the lagoon. This “fresh” volume of lagoon water was sampled for nutrient concentration. The wetland was then lowered into the tank and water sampled for nutrients. From this point onwards, water from the tank was sampled roughly every 24 hours or when practical, for the following 14 days.

Samples were filtered through 45um syringe filters and frozen. On completion of the 14 days of sampling, frozen samples were packed on dry ice and shipped over night to the designated laboratory for processing and analysis. Nutrient analysis was conducted at the Ecochemistry laboratory at Australian National University and urea was analysed at the Laboratory for Inorganic Chemistry (Nutrients), Forensic and Scientific Services at Queensland Health.

How well did the plants survive?

Carex development after 7 months (Oct 20 - May 21) in controlled growing environment

What results did nutrient uptake / removal testing show?

Results were encouraging in that a general net decrease in nutrient concentration can be seen over time. Not only did the plants survive the water exchange process but also the doubling in water salinity that accompanied that exchange.

With the assumption that a newly filled lagoon is enriched in nutrients, the plants also tolerated this change of environment, from nutrient depleted water, (6 month old lagoon water sitting in the tank supported growth) to a more nutrient rich and saltier environment. These are the environmental and physical challenges a full scale wetland would experience in Curl Curl Lagoon.

What role does the air lift pump play?

The role of the air lift pump is to aerate and circulate the water in which wetlands plants are growing. It is an optional but valuable inclusion in the controlled system to assist and improve the water filtration and purification action of the growing plants. By design, an air lift pump is low energy input (e.g. solar).

An air lift pump uses slightly compressed air from a small, low power electric air pump to generate bubbles at the base of a cylinder immersed in a body of water. The rising movement of the bubbles aerates the water and transfers it vertically then horizontally to a target area elsewhere in the water body.

To demonstrate the operating principles of the air lift pump the project commissioned a working scale model made from transparent Perspex piping. This dynamic model proved to be an excellent educational aid and an object of interest to both adults and children who have attended the public demonstrations.

Unfortunately, an initial attempt to economise by constructing the model from materials purchased for this purpose proved too difficult, leading the project to then commission construction of a model by specialists.

Council's Coasts & Catchments team demonstrate a transparent educational working model of an airlift pump.

An airlift pump is installed in the controlled growing environment and a second unit will be installed

in the first field trial site.

Testing in the natural environment

The final activity in the pilot project has been the purchase of a small floating wetlands module to see if the results achieved in the controlled growing environment can now be replicated in a section of Greendale Creek.

The target location for the bed is adjacent to the main pedestrian bridge at the boundary between the Creek and Lagoon where the water depth is relatively constant and the interest of Park users will be readily attracted.

The installation, which is currently in progress, will include an air lift pump powered by an enclosed air pump connected to mains power from the duck pond viewing platform.

Once established, it may be possible to monitor the biodiversity effect around the root structure with the aid of a small underwater camera. Also, it will be interesting to see the response of the bird population to the introduction of a floating garden: will they try to occupy the bed and what would that mean for the garden’s viability?

Target site in Greendale Creek for first field trial of a mini floating wetlands platform