Stable isotope signatures from plant and animal tissue can be used to infer the source of nutrients in aquatic environments. All plants and animals contain two atomic forms of the element nitrogen (N), 14N and to a lesser extent the heavier form 15N. By measuring the ratio of 15N to 14N in dried tissue, and comparing this value to a world-wide standard, the stable isotope ratio of nitrogen, or d15N in a plant can be determined. The d15N of estuarine plants reflects that of their nutrient source. The source of nitrogen used by a plant can be traced by comparing the d15N signatures of the plant to the signatures of possible sources. This technique is particularly effective at tracing the influence of effluent discharges. Sewage and prawn farm effluent typically have very elevated d15N signatures, consequently plants that use nitrogen derived from these sources also have elevated d15N signatures. The extent of influence of a discharge can be traced by sampling and analysing plants from increasing distances from the effluent discharge.

FRC Environmental has used stable isotope analysis to trace nutrient sources in a variety of projects including the assessment of sewage plumes and monitoring of aquaculture facilities.
Recent projects include:

• Southwest Brisbane Wastewater Augmentation Environmental Impact Study - Aquatic Ecology, Brisbane City Council.
  
  FRC Environmental evaluated the impacts of upgrading the Oxley STP on the surrounding estuarine ecosystem. As part of this assessment the isotopic signature of mangroves and deployed algae were analysed to determine the current extent of sewage influence.


• Sandgate STP Upgrading EIS, Brisbane City Council.
  
  FRC Environmental evaluated the potential impacts of the proposed upgrading of the Sandgate Water Treatment Plant on Cabbage Tree Creek and nearby Moreton Bay. Isotopic studies indicated an influence of sewage discharge in close proximity to the discharge.


• Logan - Nerang Diffuse Loads Study, South-East Queensland Wastewater Management Strategy Study.
  
  In phase 1 of this task FRC undertook a synoptic survey of d15N and d13C signatures of key flora, fauna, sediments and soils to provide a preliminary assessment of significant nitrogen and carbon sources. The survey provided an overview of the condition of each of the catchments in the Logan Nerang Study area, and identified areas that are influenced by sewage, effluent from other activities, and areas of high catchment disturbance.


• Pioneer Bay Monitoring Program - Seagrass Isotope Studies, Whitsunday Shire Council.
  
  FRC were commissioned to design and undertake a monitoring program to assess the effects of sewage effluent discharge on the seagrass and coral communities of the Whitsunday coast. This program is now in its fifth year. Stable isotopes are used to trace the geographic extent of effluent 'enrichment' within Pioneer Bay.


• Baseline Survey and Monitoring of Estuarine Ecosytems, Bowen, Hamilton Prawn Farm, 1999 - 2003.
  
  FRC is undertaking the assessment and monitoring of the environmental impacts of this prawn farm on the surrounding estuarine ecosystem. Benthic invertebrates, cover of epiphytic algae on mangrove roots, seagrass distribution and the d15N isotopic signature of seagrass, mangroves and algae in the vicinity of the effluent discharge and in nearby control creeks are monitored.


• Coombabah Lake and Creek Environmental Inventory - Nutrient Pathways Study and Benthic Fauna Study, Gold Coast City Council.
  
  FRC was responsible for the design and implementation of a stable isotope study using aquatic macrophytes, to determine the sources of nutrient input to the estuarine reaches of the Coombabah creek system, incorporating Coombabah Lake.


• Ninds Creek Environmental Monitoring Program: Isotope Analysis
  
  This report presents results of nitrogen isotope analysis of mangrove leaves collected in March 2002 up and downstream of the Johnstone Sewage Treatment Plant on Ninds Creek, and from control sites in nearby Sandfly Creek. This study was undertaken on behalf of the Johnstone Shire Council and is a component of the Environmental Impact Monitoring Program (EIMP) for the Johnstone Shire Sewage Treatment Plant (STP).


• Finfish and Macro-invertebrate Survey of Cooroy Creek, Cooroy, Noosa Council.
  
  The planned expansion of the Cooroy Waste Water Treatment Plant required the design and conduct of a fin fish and macro-invertebrate survey for the Cooroy - Six Mile Creek system. The survey focused on the detection of possible rare and endangered species; and the use of C and N isotopes to trace the source of nutrients in both water and plant tissue. An assessment of the current level of effluent-related impact was completed and recommendations made relating to water quality and effluent treatment.